Cara Therapeutics (CARA) 10-K 1 Mar 22 2021 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 24, 2022	Jun. 30, 2021
Document And Entity Information [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
Entity File Number	001-36279
Entity Registrant Name	CARA THERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	75-3175693
Entity Address, Address Line One	4 Stamford Plaza
Entity Address, Address Line Two	107 Elm Street
Entity Address, Address Line Three	9th Floor
Entity Address, City or Town	Stamford
Entity Address, State or Province	CT
Entity Address, Postal Zip Code	06902
City Area Code	203
Local Phone Number	406-3700
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	CARA
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 695,807,241
Entity Common Stock, Shares Outstanding		53,513,518
Auditor Name	Ernst & Young LLP
Auditor Firm ID	42
Auditor Location	Stamford, Connecticut
Entity Central Index Key	0001346830
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 13,453	$ 31,683
Marketable securities	153,582	149,242
Income tax receivable	697	1,507
Other receivables	455	557
Inventory, net	2,584
Prepaid expenses	2,519	12,076
Total current assets	173,290	195,065
Operating lease right-of-use assets	2,973	4,279
Marketable securities, non-current	69,754	70,565
Property and equipment, net	631	840
Restricted cash	408	408
Total assets	247,056	271,157
Current liabilities:
Accounts payable and accrued expenses	15,861	16,881
Operating lease liabilities, current	1,755	1,602
Total current liabilities	17,616	18,483
Operating lease liabilities, non-current	1,918	3,673
Commitments and contingencies (Note 18)
Stockholders' equity:
Preferred stock; $0.001 par value; 5,000,000 shares authorized at December 31, 2021 and December 31, 2020, zero shares issued and outstanding at December 31, 2021 and December 31, 2020
Common stock; $0.001 par value; 100,000,000 shares authorized at December 31, 2021 and December 31, 2020, 53,480,812 shares and 49,872,213 shares issued and outstanding at December 31, 2021 and December 31, 2020, respectively	53	50
Additional paid-in capital	708,585	641,195
Accumulated deficit	(480,758)	(392,317)
Accumulated other comprehensive (loss) income	(358)	73
Total stockholders' equity	227,522	249,001
Total liabilities and stockholders' equity	$ 247,056	$ 271,157

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
BALANCE SHEETS
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	53,480,812	49,872,213
Common stock, shares outstanding	53,480,812	49,872,213

Statements of Comprehensive (Lo

Statements of Comprehensive (Loss) Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue:
Total revenue	$ 23,028	$ 135,082	$ 19,886
Operating expenses:
Research and development	82,701	107,851	113,820
General and administrative	29,410	21,846	17,745
Total operating expenses	112,111	129,697	131,565
Operating (loss) income	(89,083)	5,385	(111,679)
Other income, net	642	2,334	4,490
(Loss) income before income tax benefit	(88,441)	7,719	(107,189)
Income tax benefit	0	691	816
Net (loss) income	$ (88,441)	$ 8,410	$ (106,373)
Net (loss) income per share:
Basic	$ (1.74)	$ 0.18	$ (2.49)
Diluted	$ (1.74)	$ 0.18	$ (2.49)
Weighted average shares:
Basic	50,718,765	47,413,250	42,669,333
Diluted	50,718,765	47,915,030	42,669,333
Other comprehensive (loss) income, net of tax of $0:
Change in unrealized (losses) gains on available-for-sale marketable securities	$ (431)	$ (97)	$ 284
Total comprehensive (loss) income	(88,872)	8,313	(106,089)
License and Milestone Fees
Revenue:
Total revenue	21,223	134,439	19,746
Collaborative Revenue
Revenue:
Total revenue	706
Commercial Supply Revenue
Revenue:
Total revenue	701
Clinical Compound Revenue
Revenue:
Total revenue	$ 398	$ 643	$ 140

Statements of Comprehensive (_2

Statements of Comprehensive (Loss) Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
STATEMENTS OF COMPREHENSIVE (LOSS) INCOME
Other comprehensive (loss) income, net of tax	$ 0	$ 0	$ 0

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income	Total
Balance, Value at Dec. 31, 2018	$ 39	$ 428,059	$ (294,354)	$ (114)	$ 133,630
Balance, Shares at Dec. 31, 2018	39,547,558
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, value	$ 7	136,491			136,498
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, Shares	6,325,000
Issuance of common stock upon entry into License Agreement with Enterise Biopharma, Inc. ($34.42 per share)		4,000			4,000
Issuance of common stock upon entry into License Agreement with Enterise Biopharma, Inc. ($34.42 per share), shares	170,793
Stock-based compensation expense		10,587			10,587
Shares issued upon exercise of stock options, value	$ 1	6,105			6,106
Shares issued upon exercise of stock options, shares	555,847
Shares issued upon vesting of restricted stock units, value		1,784			1,784
Shares issued upon vesting of restricted stock units, shares	110,832
Shares issued for consulting services, value		197			197
Shares issued for consulting services, shares	10,195
Net (loss) income			(106,373)		(106,373)
Other comprehensive income (loss)				284	284
Balance, Value at Dec. 31, 2019	$ 47	587,223	(400,727)	170	186,713
Balance, Shares at Dec. 31, 2019	46,720,225
Sale of common stock under license agreement with Vifor, value	$ 3	38,446			38,449
Sale of common stock under license agreement with Vifor, shares	2,939,552
Stock-based compensation expense		12,486			12,486
Shares issued upon exercise of stock options, value		691			691
Shares issued upon exercise of stock options, shares	55,852
Shares issued upon vesting of restricted stock units, value		2,349			2,349
Shares issued upon vesting of restricted stock units, shares	156,584
Net (loss) income			8,410		8,410
Other comprehensive income (loss)				(97)	(97)
Balance, Value at Dec. 31, 2020	$ 50	641,195	(392,317)	73	249,001
Balance, Shares at Dec. 31, 2020	49,872,213
Sale of common stock under license agreement with Vifor, value	$ 3	44,966			44,969
Sale of common stock under license agreement with Vifor, shares	3,282,391
Stock-based compensation expense		17,850			17,850
Shares issued upon exercise of stock options, value		1,639			$ 1,639
Shares issued upon exercise of stock options, shares	136,787				136,787
Shares issued upon vesting of restricted stock units, value		2,935			$ 2,935
Shares issued upon vesting of restricted stock units, shares	189,421
Net (loss) income			(88,441)		(88,441)
Other comprehensive income (loss)				(431)	(431)
Balance, Value at Dec. 31, 2021	$ 53	$ 708,585	$ (480,758)	$ (358)	$ 227,522
Balance, Shares at Dec. 31, 2021	53,480,812

Statements of Stockholders' E_2

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020
Follow-on Public Offering
Price per share	$ 23
Underwriting discounts and commissions and offering expenses	$ 8,977
License Agreements [Member] | Enteris Biopharma, Inc. [Member]
Price per share	$ 34.42
License Agreements [Member] | Vifor International Ltd. [Member]
Price per share		$ 15.23	$ 17.0094

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net (loss) income	$ (88,441)	$ 8,410	$ (106,373)
Adjustments to reconcile net (loss) income to net cash used in operating activities:
Stock-based compensation expense	20,785	14,835	12,568
Depreciation and amortization	248	209	198
Amortization expense component of lease expense	1,306	806	601
Noncash expense related to oral formulation license agreement			4,000
Amortization (accretion) of available-for-sale marketable securities, net	861	154	(1,381)
Realized gain on sale of available-for-sale marketable securities	(39)	(272)
Realized gain on sale of property and equipment	(70)	0	0
Deferred revenue		(22,262)	(19,747)
Changes in operating assets and liabilities:
Income tax receivable	810	(691)	(152)
Other receivables	102	414	(45)
Inventory, net	(2,584)
Prepaid expenses	9,557	(3,213)	(4,058)
Accounts payable and accrued expenses	(1,020)	(2,784)	6,043
Operating lease liabilities	(1,602)	(1,093)	(879)
Net cash used in operating activities	(60,087)	(5,487)	(109,225)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	173,484	144,320	253,584
Proceeds from redemptions of available-for-sale marketable securities, at par	20,500	27,035	2,000
Proceeds from sale of available-for-sale marketable securities	10,029	41,600	0
Purchases of available-for-sale marketable securities	(208,795)	(232,881)	(286,082)
Proceeds from sale of property and equipment	70
Purchases of property and equipment	(39)	(349)	(18)
Net cash used in investing activities	(4,751)	(20,275)	(30,516)
Financing activities
Proceeds from the sale of common stock under license agreement with Vifor	44,969	38,449
Proceeds from the sale of common stock in a follow-on public offering, net of issuance costs			136,498
Proceeds from the exercise of stock options	1,639	691	6,106
Net cash provided by financing activities	46,608	39,140	142,604
Net (decrease) increase in cash, cash equivalents and restricted cash	(18,230)	13,378	2,863
Cash, cash equivalents and restricted cash at beginning of period	32,091	18,713	15,850
Cash, cash equivalents and restricted cash at end of period	$ 13,861	$ 32,091	18,713
Noncash investing and financing activities
Shares of common stock issued in connection with oral formulation license agreement			4,000
Shares of common stock issued in exchange for consulting services			$ 197

Business

Business	12 Months Ended
	Dec. 31, 2021
Basis of Presentation
Business	1. Business Cara Therapeutics, Inc., or the Company, is an early commercial-stage biopharmaceutical corporation formed on July 2, 2004. The Company is leading a new treatment paradigm to improve the lives of patients suffering from pruritus. The Company’s primary activities to date have been organizing and staffing the Company, developing its lead product and product candidates, including conducting preclinical studies and clinical trials of difelikefalin-based product candidates, and raising capital. On August 23, 2021, the Company received U.S. Food and Drug Administration, or FDA, approval for KORSUVA TM Collaboration and Licensing Agreements) As of December 31, 2021, the Company has raised aggregate net proceeds of approximately $519,600 from several rounds of equity financing, including its initial public offering, or IPO, which closed in February 2014 and four follow-on public offerings of common stock, which closed in July 2019, July 2018, April 2017 and August 2015, respectively, and the issuance of convertible preferred stock and debt prior to the IPO. The Company had also received approximately $224,900 under its license and supply agreements for difelikefalin, primarily with Vifor, Vifor Fresenius Medical Care Renal Pharma Ltd., or VFMCRP, Maruishi Pharmaceutical Co. Ltd., or Maruishi, and Chong Kun Dang Pharmaceutical Corp., or CKDP, and an earlier product candidate for which development efforts ceased in 2007. In October 2021, the Company received net proceeds of $44,969 from the issuance and sale of 3,282,391 shares of the Company’s common stock to Vifor in connection with U.S. regulatory approval for KORSUVA injection in August 2021. Additionally, in October 2020, the Company received net proceeds of $38,449 from the issuance and sale of 2,939,552 shares of the Company’s common stock to Vifor in connection with the Company’s license agreement with Vifor. Furthermore, in May 2018, the Company received net proceeds of $14,556 from the issuance and sale of 1,174,827 shares of the Company’s common stock to Vifor in connection with the Company’s license agreement with VFMCRP (see Notes 10, Shareholders’ Equity Collaboration and Licensing Agreements As of December 31, 2021, the Company had unrestricted cash and cash equivalents and marketable securities of $236,789 and an accumulated deficit of $480,758. The Company has incurred substantial net losses and negative cash flows from operating activities in nearly every fiscal period since inception and expects this trend to continue for the foreseeable future. The Company recognized a net loss of $88,441 and had net cash used in operating activities of $60,087 for the year ended December 31, 2021. The Company is subject to risks common to other life science companies including, but not limited to, uncertainty of product development and commercialization, lack of marketing and sales history, development by its competitors of new technological innovations, dependence on key personnel, market acceptance of products, product liability, protection of proprietary technology, ability to raise additional financing, and compliance with the FDA and other government regulations. If the Company does not successfully commercialize KORSUVA injection or any of its other product candidates, it will be unable to generate additional recurring product revenue or achieve profitability.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with generally-accepted accounting principles in the United States, or GAAP, requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, as of the date of the financial statements as well as the reported amounts of revenues and expenses during the reporting period. The more significant estimates include the fair value of marketable securities that are classified as level 2 of the fair value hierarchy, the amount and periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, related party accounts receivable, inventory valuation and related reserves, the determination of prepaid research and development, or R&D, clinical costs and accrued research projects, the amount of non-cash compensation costs related to share-based payments to employees and non-employees, the incremental borrowing rate used in lease calculations and the likelihood of realization of deferred tax assets. The ongoing COVID-19 pandemic has interrupted business operations across the globe. Estimates and assumptions about future events and their effects cannot be determined with certainty and therefore require the exercise of judgment. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the reported amounts of assets and liabilities or the disclosure of contingent assets and liabilities. These estimates, however, may change as new events occur and additional information is obtained, and are recognized in the financial statements as soon as they become known. Actual results could differ materially from the Company’s estimates and assumptions. Concentration of Credit Risk Financial instruments, which potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents, marketable securities, and related party accounts receivable. The Company invests its cash reserves in money market funds or high-quality marketable securities in accordance with its investment policy. The stated objectives of its investment policy are to preserve capital, provide liquidity consistent with forecasted cash flow requirements, maintain appropriate diversification and generate returns relative to these investment objectives and prevailing market conditions. The Company’s investment policy includes guidelines on acceptable investment securities, limits interest-bearing security investments to certain types of debt and money market instruments issued by the U.S. government and institutions with investment grade credit ratings and places restrictions on maturities and concentration by asset class and issuer. The Company’s cash and cash equivalents and marketable securities are held by three major financial institutions. In accordance with the Company’s policies, the Company monitors exposure with its counterparties. The Company also maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The percentages of revenue recognized from license agreement partners of the Company in the years ended December 31, 2021, 2020 and 2019 are included in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Revenue ​ ​ Year Ended December 31, ​ 2021 2020 2019 License Agreement Partner: ​ VFMCRP 67 % 17 % 99 % Vifor 25 % 83 % — % ​ For the years ended December 31, 2021, 2020 and 2019, no additional license agreement partners or customers accounted for more than 10% of the Company’s revenue. Concentration of Suppliers The Company relies on three suppliers to manufacture KORSUVA injection active pharmaceutical ingredient, or API, finished drug product, and finished goods. If any of the Company’s suppliers were to limit or terminate production or otherwise fail to meet the quality or delivery requirements to satisfy the supply commitments, the process of locating and qualifying alternate sources would require up to two years , during which time production could be delayed. Such delays could have a material adverse effect on the Company’s business, financial position, and results of operations. Cash and Cash Equivalents Cash and cash equivalents include cash on hand, demand deposits, deposits with banks and highly liquid money market funds with holdings of cash and other investments with original maturities of three months or less. Marketable Securities On January 1, 2020, the Company adopted Accounting Standards Update, or ASU, No. 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments The Company deems certain of its investments to be marketable securities if the investment, or in the case of money market funds, the securities underlying the money market fund, meet the definition of a debt security in Accounting Standards Codification, or ASC, section 320-10-20. The Company considers its marketable securities to be available-for-sale, which are its only financial instruments that are within the scope of ASU 2016-13 as of December 31, 2021. The Company’s investments in marketable securities, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper, and municipal bonds are highly rated by Moody’s and S&P and have maturities primarily of less than one year but no longer than three years. Accordingly, credit risk associated with the Company’s available-for-sale debt security portfolio is mitigated. ASU 2016-13 modifies the prior other-than-temporary impairment model for available-for-sale debt securities by requiring (1) estimating expected credit losses (the portion of the amortized cost basis of a financial asset that the Company does not expect to collect) only when the fair value is below the amortized cost of the asset; (2) recording a credit loss without regard to the length of time a security has been in an unrealized loss position; (3) limiting the measurement of the credit loss to the difference between the security’s amortized cost basis and its fair value; and (4) presenting credit losses as an allowance rather than as a write-down, which will allow the Company to record reversals of credit losses in current period net income, a practice that was previously prohibited. ​ The Company reviews each of its available-for-sale marketable securities for unrealized losses (declines in fair value below its amortized cost basis) at each balance sheet date presented in its financial statements and whenever events or changes in circumstances indicate that the amortized cost basis of an asset may not be recoverable. In accordance with the adoption of ASU 2016-13, the Company is required to determine whether any portion of the unrealized loss for any available-for-sale debt security is due to a credit loss, and if so, to measure the amount of the credit loss. ​ The Company will rely on both qualitative and quantitative factors to determine whether the unrealized loss for each available-for-sale debt security at any balance sheet date is due to a credit loss. Qualitative factors may include a credit downgrade, severity of the decline in fair value below amortized cost and other adverse conditions related specifically to the security, as well as the intent to sell the security, or whether the Company will more likely than not be required to sell the security before recovery of its amortized cost basis. The Company’s assessment of whether a security is impaired could change in the future due to new developments or changes in assumptions related to any particular security. If material qualitative factors indicate that a credit loss has occurred, the Company will determine the magnitude of that credit loss using a discounted cash flow model or other quantitative method. If the Company intends to sell the security or it is more likely than not that the Company will be forced to sell the security before recovery of the amortized cost of the security, the entire unrealized loss is deemed to be a credit loss, which is recognized in net (loss) income. Otherwise, the portion of the unrealized loss that is due to a credit loss will be recorded as an allowance for credit loss, which will offset the balance of marketable securities and as credit loss expense within other income, net. The portion of the unrealized loss that is not due to a credit loss as well as all unrealized gains will be recorded in Accumulated Other Comprehensive Income (Loss). There was no cumulative effect adjustment as a result of the adoption of ASU 2016-13 on January 1, 2020 (see Note 3, Available-for-Sale Marketable Securities Fair Value Measurements) ​ Accrued interest receivables are excluded from the Company’s amortized cost bases for its available-for-sale marketable securities and are included within Other receivables. The Company’s policy is to not measure an allowance for credit losses on accrued interest receivable balances at each reporting period since it elects to write off uncollectible accrued interest receivable balances as credit loss expense in a timely manner, which is by maturity date for all categories of its debt securities. Fair Value of Financial Instruments The Company applies fair value accounting for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. The Company defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities which are required to be recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions and credit risks. The Company’s financial instruments consist of cash, cash equivalents, available-for-sale marketable securities, prepaid expenses, restricted cash, accounts payable and accrued liabilities. The fair values of cash, cash equivalents, prepaid expenses, restricted cash, accounts payable and accrued liabilities approximate their carrying values due to the short-term nature of these financial instruments. Available-for-sale marketable securities are reported as marketable securities at their fair values, based upon pricing of securities with the same or similar investment characteristics as provided by third-party pricing services, as described below. The valuation techniques used by the Company are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. The Company classifies its investments in a fair value hierarchy that is intended to increase consistency and comparability in fair value measurements and related disclosures. The fair value hierarchy is divided into three levels based on the source of inputs as follows: ● Level 1 – Observable inputs – quoted prices in active markets for identical assets and liabilities. ● Level 2 – Observable inputs other than the quoted prices in active markets for identical assets and liabilities – such as quoted prices for similar instruments, quoted prices for identical or similar instruments in inactive markets, or other inputs that are observable or can be corroborated by observable market data. ● Level 3 – Unobservable inputs – includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions. The Company records transfers between levels in the hierarchy by assuming that the transfer occurred at the end of the quarter or year-to-date period. Valuation Techniques - Level 2 Inputs The Company estimates the fair values of its financial instruments categorized as level 2 in the fair value hierarchy, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper and municipal bonds, by taking into consideration valuations obtained from third-party pricing services. The pricing services use industry standard valuation models, including both income- and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, benchmark yields, issuer credit spreads, benchmark securities, and other observable inputs. The Company obtains a single price for each financial instrument and does not adjust the prices obtained from the pricing service. The Company validates the prices provided by its third-party pricing services by reviewing their pricing methods, obtaining market values from other pricing sources and comparing them to the share prices presented by the third-party pricing services. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by its pricing services as of December 31, 2021 or December 31, 2020. Inventory, net Inventories are stated at the lower of cost or net realizable value. The Company determines the cost of inventory using first-in, first-out, or FIFO, method. The Company capitalizes inventory costs associated with the Company’s products prior to regulatory approval, when, based on management’s judgment, future commercialization is considered probable and the future economic benefit is expected to be realized; otherwise, such costs are expensed. The determination to capitalize inventory costs is based on various factors, including status and expectations of the regulatory approval process, any known safety or efficacy concerns, potential labeling restrictions, and any other impediments to obtaining regulatory approval. As KORSUVA injection is the Company’s first commercial product and probability could not be established prior to regulatory approval on August 23, 2021, all inventory costs prior to regulatory approval were expensed as incurred. The Company periodically analyzes its inventory levels to identify inventory that may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and writes down such inventories as appropriate. In addition, the Company’s products are subject to strict quality control and monitoring which the Company performs throughout the manufacturing process. If certain batches or units of product no longer meet quality specifications or become obsolete due to expiration, the Company records a charge to write down such unmarketable inventory to its estimated realizable value. Property and Equipment, net Property and equipment (consisting of computer and office equipment, furniture and fixtures and leasehold improvements) are stated at cost, net of accumulated depreciation and amortization of leasehold improvements. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets. Leasehold improvements are amortized over the lesser of their useful lives or the life of the lease. ​ ​ ​ Asset Category Useful Lives Computer and office equipment 5 years Furniture and fixtures 7 years Leasehold improvements lesser of useful life of asset or life of lease (Stamford - 7 years) ​ The Company reviews the recorded values of property and equipment for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset or group of assets may not be fully recoverable. Revenue Recognition Under ASU 2014-09, Revenue from Contracts with Customers (Topic 606) The Company has entered into agreements to license its intellectual property, or IP, related to difelikefalin to develop, manufacture and/or commercialize drug products. These agreements typically contain multiple performance obligations, including licenses of IP and R&D services. Payments to the Company under these agreements may include nonrefundable license fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company receives its share of the net profits from the sale of KORSUVA injection in the U.S. through its license agreement with Vifor. The Company has adopted a policy to recognize revenue net of tax withholdings, as applicable. The Company identifies agreements as contracts that create enforceable rights and obligations when the agreement is approved by the parties, identifies the rights of the parties and the payment terms, has commercial substance and it is probable that the Company will collect the consideration to which it will be entitled in exchange for the goods and services that will be transferred to the customer. The counterparty is considered to be a customer when it has contracted with the Company to obtain goods and services that are the output of the Company’s ordinary activities (i.e., development of pharmaceutical products) in exchange for consideration. A performance obligation is a promise to transfer distinct goods or services to a customer. Performance obligations that are both capable of being distinct and distinct within the context of the contract are considered to be separate performance obligations. Performance obligations are capable of being distinct if the counterparty is able to benefit from the good or service on its own or together with other resources that are readily available to it. Performance obligations are distinct within the context of the contract when each performance obligation is separately identifiable from each other; i.e., the Company is not using the goods or services as inputs to produce or deliver the combined output or outputs specified by the customer; one or more of the goods or services does not significantly modify or customize one of the other goods or services in the contract; and goods or services are not highly interdependent or not highly interrelated. Performance obligations that are not distinct are accounted for as a single performance obligation over the period that goods or services are transferred to the customer. The determination of whether performance obligations in a contract are distinct may require significant judgment. The transaction price is the amount of consideration that the Company expects to be entitled to in exchange for transferring promised goods or services to the customer based on the contract terms at inception of a contract. There is a constraint on inclusion of variable consideration related to licenses of IP, such as milestone payments or sales-based royalty payments, in the transaction price if there is uncertainty at inception of the contract as to whether such consideration will be recognized in the future because it is probable that there will be a significant reversal of revenue in the future when the uncertainty is resolved. The determination of whether or not it is probable that a significant reversal of revenue will occur in the future depends on the likelihood and magnitude of the reversal. Factors that could increase the likelihood or magnitude of a reversal of revenue include (a) the susceptibility of the amount of consideration to factors outside the entity’s influence, such as the outcome of clinical trials, the timing of initiation of clinical trials by the counterparty and the approval of drug product candidates by regulatory agencies, (b) situations in which the uncertainty is not expected to be resolved for a long period of time, and (c) level of the Company’s experience in the field. When it becomes probable that events will occur, for which variable consideration was constrained at inception of the contract, the Company allocates the related consideration to the separate performance obligations in the same manner as described below. At inception of a contract, the Company allocates the transaction price to the distinct performance obligations based upon their relative standalone selling prices. Standalone selling price is the price at which an entity would sell a promised good or service separately to a customer. The best evidence of standalone selling price is an observable price of a good or service when sold separately by an entity in similar circumstances to similar customers. Since the Company typically does not have such evidence, it estimates standalone selling price so that the amount that is allocated to each performance obligation equals the amount that the Company expects to receive for transferring goods or services. The methods that the Company uses to make such estimates include (1) the adjusted market assessment approach, under which the Company forecasts and analyzes difelikefalin in the appropriate market, the phase of clinical development as well as considering recent similar license arrangements within the same phase of clinical development, therapeutic area, type of agreement, etc. and (2) the expected cost of satisfying the performance obligations plus a margin, or the expected cost plus a margin approach. The Company recognizes revenue when, or as, it satisfies a performance obligation by transferring a promised good or service to a customer and the customer obtains control of the good or service. Revenue related to the grant of a license that is a distinct performance obligation and that is deemed to be functional IP is recognized at the point in time that the Company has the right to payment for the license, the customer has legal title to the license and can direct the use of the license (for example, to grant sublicenses), the customer has the significant risks and rewards of ownership of the license and the customer has accepted the asset (license) by signing the license agreement. Recognition of revenue related to R&D services that are a distinct performance obligation or that are combined with granting of a license as a single performance obligation is deferred at inception of a contract and is recognized as those services are performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. Milestone payments are considered to be variable consideration and are not included in the transaction price at inception of the contract if it is uncertain that the milestone will be achieved. Rather, when it becomes probable that the milestone will be achieved and, therefore, there will not be a significant reversal of revenue in future periods, the respective amount to be earned is included in the transaction price, allocated to the distinct performance obligations based on their relative standalone selling price and recognized as revenue, as described above. Sales milestones and sales-based royalty payments related to a license of IP are recognized as revenue when the respective sales occur. License and Milestone Fees License and milestone fees include upfront and milestone payments associated with the Company’s license agreements with Vifor, VFMCRP, Maruishi and CKDP. All upfront and milestone payments associated with the license agreements with Vifor, VFMCRP and CKDP are recognized as license and milestone fees since they contain only one performance obligation. Upfront and milestone payments associated with the license agreement with Maruishi are allocated between License and milestone fees and Collaborative revenue based on the relative standalone selling prices determined at contract inception (see Note 13, Revenue Recognition Collaborative Revenue Collaborative revenue includes milestone payments associated with the Company’s license agreement with Maruishi that were allocated to the R&D services performance obligation (see Note 13, Revenue Recognition Commercial Supply Revenue Commercial supply revenue includes sales of KORSUVA injection commercial product to Vifor, which ultimately will act as the principal in the net profit-sharing arrangement between the two parties upon expected commercial launch in April 2022, which then sells to third parties in the U.S. Commercial supply revenue is recognized when Vifor obtains control of the Company’s commercial product, which occurs at a point in time, typically upon receipt of KORSUVA injection by Vifor, and generally occurs after the commercial product has passed all quality testing required for acceptance by Vifor. The Company calculates its commercial supply revenue based on its cost of goods sold, or COGS, plus an agreed upon margin. Clinical Compound Revenue Clinical compound revenue includes sales of clinical compound to Vifor (prior to FDA approval), VFMCRP, and Maruishi for the years ended December 31, 2021, 2020 and 2019. The Company recognizes revenue on clinical compound sales when control has transferred to Vifor, VFMCRP and Maruishi, which occurs at a point in time, typically upon receipt of the clinical compound, and generally occurs after the clinical compound has passed all quality testing required for acceptance. The sales of clinical compound are reimbursed at COGS plus an agreed upon margin. Cost of Goods Sold (COGS) COGS includes costs related to sales of the Company’s commercial product, KORSUVA injection, to Vifor. Costs related to the sales of KORSUVA injection are generally recognized upon receipt of shipment by Vifor. The Company’s COGS for KORSUVA injection includes the cost of producing commercial product that correspond with commercial supply revenue, such as third-party supply and overhead costs, as well as certain period costs related to freight, packaging, stability, and quality testing. For the year ended December 31, 2021, no COGS was recognized as all inventory costs prior to regulatory approval on August 23, 2021 were expensed as incurred. Research and Development (R&D) Expenses R&D costs are charged to expense as incurred. Costs incurred under agreements with third parties are charged to expense as incurred in accordance with the specific contractual performance terms of such agreements. R&D expenses include, among other costs, compensation and other personnel-related costs, including consultant costs, and costs to conduct clinical trials using clinical research organizations, or CROs, which include upfront, milestone and monthly expenses as well as reimbursement for pass through costs. The amount of clinical trial expense recognized in any period varies depending on the duration and progress of each clinical trial, including the required level of patient enrollment, the rate at which patients actually enroll in and drop-out of the clinical trial, and the number of sites involved in the trial as well as the activities to be performed by the sites each period. R&D costs also include costs to manufacture product candidates and clinical supplies, laboratory supplies costs, facility-related costs and stock-based compensation for R&D personnel. Non-refundable R&D advance payments are deferred and capitalized as prepaid R&D expense. The capitalized amounts are expensed as the related goods are delivered or services are performed. As of December 31, 2021 and 2020, the Company recorded $1,481 and $11,286 as prepaid R&D expense, respectively. General and Administrative (G&A) Expenses G&A costs are charged to expense as incurred. G&A expenses consist primarily of salaries and other related costs, including stock-based compensation, for personnel in executive, finance, accounting, legal, business development, information technology and human resources functions. Other costs include facility costs not otherwise included in R&D expenses, legal fees, insurance costs, investor relations costs, patent costs and fees for accounting and consulting services. As noted in Note 12, Collaboration and Licensing Agreements ​ Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Deferred income tax assets are reduced, as necessary, by a valuation allowance when management determines it is more likely than not that some or all of the tax benefits will not be realized. There were no material uncertain tax positions taken as of December 31, 2021 and 2020. The Company does not have any interest or penalties accrued related to tax positions as it does not have any unrecognized tax benefits. In the event the Company determines that accrual of interest or penalties are necessary in the future, the amount will be presented as a component of interest expense. Stock-Based Compensation The Company grants stock options to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as compensation for services performed. All share-based payments, including grants of stock options, are recognized based on their grant date fair values. The grant date fair value of stock options is estimated using the Black-Scholes option valuation model. Using this model, fair value is calculated based on assumptions with respect to (i) the fair value or market price of the Company’s common stock on the grant date; (ii) expected volatility of the Company’s common stock price, (iii) the periods of time over which employees and members of the Company’s Board of Directors or non-employee consultants are expected to hold their options prior to exercise (expected term), (iv) expected dividend yield on the Company’s common stock, and (v) risk-free interest rates. The Company’s common stock has been traded on a public exchange only since January 31, 2014. Since that time, exercises of stock options have been limited due to various factors, including fluctuations in the Company’s stock price to below the exercise prices of awards and blackout periods during which exercises are not allowed, among others. Therefore, the Company believes that as of December 31, 2021, it does not have sufficient company-specific information available to determine the expected term based on its historical data. As a result, the expected term of stock options granted is determined using the average of the vesting period and term (6.25 years), an accepted method for the Company’s option grants under the SEC’s Staff Accounting Bulletin No. 110, Share-Based Payment. The Company calculates the expected volatility using company-specific trading activity of its common stock over the option’s expected term. The expected dividend yield is zero as the Company has never paid dividends and does not currently anticipate paying any in the foreseeable future. Risk-free interest rates are based on quoted U.S. Treasury rates for securities with maturities approximating the option’s expected term. The Company’s policy is to account for forfeitures of share-based payments as they occur. Compensation cost for all share-based payments granted with service-based graded vesting schedules is recognized using the straight-line method over the requisite service period. (Loss) Income Per Share The Company computes basic net (loss) income per share by dividing net (loss) income by the weighted average number of shares of common stock outstanding. Diluted net (loss) income per share includes the potential dilutive effect of common stock equivalents as if such securities were converted or exercised during the period, when the effect is dilutive. Common stock equivalents may include outstanding stock options and restricted stock units, which are included under the treasury stock method when dilutive. For each of the years ended December 31, 2021 and 2019, the Company excluded the effects of potentially dilutive shares that were outstanding during those respective periods f

Available-for-Sale Marketable S

Available-for-Sale Marketable Securities	12 Months Ended
	Dec. 31, 2021
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Marketable Securities	3. Available-for-Sale Marketable Securities As of December 31, 2021 and 2020, the Company’s available-for-sale marketable securities consisted of debt securities issued by the U.S. Treasury, U.S. government-sponsored entities and investment grade institutions as well as municipal bonds. The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2021 and 2020: As of December 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 11,573 ​ $ — ​ $ (3) ​ $ 11,570 U.S. government agency obligations ​ 17,020 ​ — ​ (45) ​ 16,975 Corporate bonds ​ 66,495 ​ — ​ (171) ​ 66,324 Commercial paper ​ ​ 106,914 ​ ​ 5 ​ ​ (31) ​ ​ 106,888 Municipal bonds ​ 21,692 ​ — ​ (113) ​ 21,579 Total available-for-sale marketable securities ​ $ 223,694 ​ $ 5 ​ $ (363) ​ $ 223,336 ​ As of December 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 20,710 ​ $ 41 ​ $ (1) ​ $ 20,750 U.S. government agency obligations ​ 22,125 ​ 4 ​ (1) ​ 22,128 Corporate bonds ​ 49,080 ​ 61 ​ (23) ​ 49,118 Commercial paper ​ 116,139 ​ 5 ​ (17) ​ 116,127 Municipal bonds ​ ​ 11,680 ​ ​ 12 ​ ​ (8) ​ ​ 11,684 Total available-for-sale marketable securities ​ $ 219,734 ​ $ 123 ​ $ (50) ​ $ 219,807 ​ The following tables summarize the fair value and gross unrealized losses of the Company’s available-for-sale marketable securities by investment category and disaggregated by the length of time that individual debt securities have been in a continuous unrealized loss position as of December 31, 2021 and 2020: As of December 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Less than 12 Months ​ 12 Months or Greater ​ Total ​ ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Value Losses Value Losses Value Losses U.S. Treasury securities ​ $ 11,570 ​ $ (3) ​ $ — ​ $ — ​ $ 11,570 ​ $ (3) U.S. government agency obligations ​ 9,456 ​ (45) ​ — ​ — ​ 9,456 ​ (45) Corporate bonds ​ ​ 62,704 ​ ​ (170) ​ ​ 2,020 ​ ​ (1) ​ ​ 64,724 ​ ​ (171) Commercial paper ​ ​ 52,163 ​ ​ (31) ​ ​ — ​ ​ — ​ ​ 52,163 ​ ​ (31) Municipal bonds ​ 20,562 ​ (105) ​ 1,017 ​ (8) ​ 21,579 ​ (113) Total ​ $ 156,455 ​ $ (354) ​ $ 3,037 ​ $ (9) ​ $ 159,492 ​ $ (363) ​ As of December 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Less than 12 Months ​ 12 Months or Greater ​ Total ​ ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Value Losses Value Losses Value Losses U.S. Treasury securities ​ $ 12,682 ​ $ (1) ​ $ — ​ $ — ​ $ 12,682 ​ $ (1) U.S. government agency obligations ​ ​ 2,500 ​ ​ (1) ​ ​ — ​ ​ — ​ ​ 2,500 ​ ​ (1) Corporate bonds ​ 23,553 ​ (23) ​ — ​ — ​ 23,553 ​ (23) Commercial paper ​ 68,897 ​ (17) ​ — ​ — ​ 68,897 ​ (17) Municipal bonds ​ 6,259 ​ (8) ​ — ​ — ​ 6,259 ​ (8) Total ​ $ 113,891 ​ $ (50) ​ $ — ​ $ — ​ $ 113,891 ​ $ (50) ​ As of December 31, 2021 and 2020, respectively, no allowance for credit losses were recognized on the Company’s available-for-sale debt securities as no portion of the unrealized losses associated with those securities were due to credit losses. The information that the Company considered in reaching the conclusion that an allowance for credit losses was not necessary for the following categories of securities is as follows: ​ As of December 31, 2021 and 2020, the Company held a total of 58 out of 76 positions and 30 out of 59 positions, respectively, that were in an unrealized loss position, two of which had been in an unrealized loss position for 12 months or greater. Unrealized losses individually and in aggregate, including any in an unrealized loss position for 12 months or greater, were not considered to be material for each respective period. Based on the Company’s review of these securities, the Company believes that the cost basis of its available-for-sale marketable securities is recoverable. U.S. Treasury and U.S. government agency obligations. Corporate bonds, commercial paper, and municipal bonds. The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2021, the Company’s marketable debt securities mature at various dates through November 2024. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2021 ​ As of December 31, 2020 Contractual maturity Amortized Cost Fair Value Amortized Cost Fair Value Less than one year ​ $ 153,631 ​ $ 153,582 ​ $ 149,164 ​ $ 149,242 One year to three years ​ 70,063 ​ 69,754 ​ 70,570 ​ 70,565 Total ​ $ 223,694 ​ $ 223,336 ​ $ 219,734 ​ $ 219,807 ​ All available-for-sale marketable securities are classified as Marketable securities, current or Marketable securities, non-current depending on the contractual maturity date of the individual available-for-sale security. Other income, net includes interest and dividends, accretion/amortization of discounts/premiums, realized gains and losses on sales of securities and credit loss expense due to declines in the fair value of securities, if any. The cost of securities sold is based on the specific identification method. During the year ended December 31, 2021, the Company sold certain shares of its available-for-sale debt securities with a total fair value of $10,029, which resulted in realized gains of $39. During the year ended December 31, 2020, the Company sold certain shares of its available-for-sale debt securities with a total fair value of $41,600, which resulted in realized gains of $272. There were no sales of available-for-sale marketable securities during the year ended December 31, 2019. As of December 31, 2021 and 2020, accrued interest receivables on our available-for-sale debt securities were $455 and $311, respectively.

Accumulated Other Comprehensive

Accumulated Other Comprehensive (Loss) Income	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Accumulated Other Comprehensive (Loss) Income	4. Accumulated Other Comprehensive (Loss) Income The following table summarizes the changes in accumulated other comprehensive (loss) income, net of tax, from unrealized gains (losses) on available-for-sale marketable securities, the Company’s only component of accumulated other comprehensive (loss) income, for the years ended December 31, 2021, 2020 and 2019. ​ ​ ​ ​ ​ Total Accumulated ​ ​ Other Comprehensive ​ ​ (Loss) Income Balance, December 31, 2018 ​ $ (114) Other comprehensive income before reclassifications ​ 284 Amount reclassified from accumulated other comprehensive loss ​ — Net current period other comprehensive income ​ 284 Balance, December 31, 2019 ​ 170 Other comprehensive income before reclassifications ​ 175 Amount reclassified from accumulated other comprehensive income ​ (272) Net current period other comprehensive loss ​ (97) Balance, December 31, 2020 ​ $ 73 Other comprehensive loss before reclassifications ​ (392) Amount reclassified from accumulated other comprehensive income ​ (39) Net current period other comprehensive loss ​ (431) Balance, December 31, 2021 ​ $ (358) ​ Amounts reclassified out of accumulated other comprehensive (loss) income into net (loss) income are determined by specific identification. The reclassifications out of accumulated other comprehensive (loss) income and into net (loss) income were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Affected Line Item in the Component of Accumulated Other ​ Year Ended December 31, ​ Statements of Comprehensive (Loss) Income 2021 2020 2019 Comprehensive (Loss) Income Unrealized gains (losses) on available-for-sale marketable securities: ​ ​ ​ Realized gains on sales of securities ​ $ 39 ​ $ 272 ​ $ — Other income, net Income tax effect ​ — ​ — ​ — Benefit from income taxes Realized gains on sales of securities, net of tax ​ $ 39 ​ $ 272 ​ $ — ​ ​ ​

Inventory, net

Inventory, net	12 Months Ended
	Dec. 31, 2021
Inventory Disclosure [Abstract]
Inventory, net	5. Inventory, net Inventories consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Raw materials ​ $ 927 ​ $ — Work-in-process ​ 1,657 ​ — Total ​ $ 2,584 ​ $ — ​ As of December 31, 2021, inventory balances include inventory costs subsequent to regulatory approval of KORSUVA injection on August 23, 2021. For the year ended December 31, 2021, no COGS was recognized as all inventory sold during 2021 pertained to inventory costs incurred prior to regulatory approval on August 23, 2021, which were expensed as incurred. There were no write-downs of commercial supply inventory during the year ended December 31, 2021.

Prepaid Expenses

Prepaid Expenses	12 Months Ended
	Dec. 31, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses	6 As of December 31, 2021, the amount of prepaid expenses was $2,519, consisting of $1,481 of prepaid R&D clinical costs, $369 of prepaid insurance and $669 of other costs. As of December 31, 2020, the amount of prepaid expenses was $12,076, consisting of $11,286 of prepaid R&D clinical costs, $223 of prepaid insurance and $567 of other costs.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, net	7. Property and Equipment, net Property and equipment, net consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 Computer and office equipment ​ $ 239 ​ $ 211 Laboratory equipment ​ — ​ 628 Furniture and fixtures ​ 330 ​ 330 Leasehold improvements ​ 1,223 ​ 1,212 ​ ​ $ 1,792 ​ $ 2,381 Less accumulated depreciation and amortization ​ 1,161 ​ 1,541 Property and equipment, net ​ $ 631 ​ $ 840 ​ Depreciation and amortization expense included in R&D expense and G&A expense was $248, $209 and $198 for the years ended December 31, 2021, 2020 and 2019, respectively. During the year ended December 31, 2021, the Company sold laboratory equipment that was fully depreciated, which resulted in gains on sales of property and equipment of $70. There were no gains or losses on sales of property and equipment during the years ended December 31, 2020 and 2019.

Restricted Cash

Restricted Cash	12 Months Ended
	Dec. 31, 2021
Restricted Cash [Abstract]
Restricted Cash	8 The Company is required to maintain a stand-by letter of credit as a security deposit under its leases for its office space in Stamford, Connecticut (refer to Note 18, Commitments and Contingencies: Leases As of December 31, 2021 and 2020, the Company had $408 of restricted cash related to the Stamford lease in long-term assets. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Balance Sheets that sum to the total of the same such amounts shown in the Statements of Cash Flows. ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Cash and cash equivalents ​ $ 13,453 ​ $ 31,683 Restricted cash, long-term assets ​ 408 ​ 408 Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows ​ $ 13,861 ​ $ 32,091 ​

Accounts Payable and Accrued Ex

Accounts Payable and Accrued Expenses	12 Months Ended
	Dec. 31, 2021
Payables And Accruals [Abstract]
Accounts Payable and Accrued Expenses	9 Accounts payable and accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Accounts payable ​ $ 5,625 ​ $ 4,893 Accrued research projects ​ 4,648 ​ 6,194 Accrued compensation and benefits ​ 4,959 ​ 4,955 Accrued professional fees and other ​ 629 ​ 839 Total ​ $ 15,861 ​ $ 16,881 ​

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Stockholders' Equity	10. Stockholders’ Equity The Company’s Board of Directors has authorized 100,000,000 shares of the Company’s common stock, par value $0.001 per share, and 5,000,000 shares of undesignated preferred stock, par value $0.001 per share, that may be issued from time to time by the Board of Directors of the Company in one or more series. As of December 31, 2021, there were 53,480,812 shares of common stock outstanding Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the Board of Directors, subject to the preferential rights of the holders of preferred stock, if any. In October 2021, the Company issued 3,282,391 shares of its common stock to Vifor in connection with the milestone earned for the U.S. regulatory approval of KORSUVA injection in August 2021 (see Note 12, Collaboration and Licensing Agreements ​ In August 2021, as a result of the achievement of certain performance targets, an aggregate of 44,002 performance-based restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In June 2021, as a result of the completion of the one-year vesting period, an aggregate of 36,000 restricted stock units of members of the Board of Directors vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In February and March 2021, as a result of the achievement of certain performance targets, an aggregate of 76,750 performance-based restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In February 2021, as a result of the completion of the first year of the three-year vesting period, an aggregate of 32,669 time-based restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In December 2020, as a result of the achievement of a performance target, an aggregate of 36,750 restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In October 2020, the Company issued 2,939,552 shares of its common stock to Vifor in connection with the license agreement entered into with Vifor (see Note 12, Collaboration and Licensing Agreements ​ Pursuant to the stock purchase agreement with Vifor, or the Vifor Purchase Agreement, Vifor will not, and will not cause any direct or indirect affiliate to, during the period beginning on October 15, 2020 and ending at the close of business on October 15, 2022, or the Restricted Period, (i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of common stock of the Company or any securities convertible into or exercisable or exchangeable for common stock of the Company (including without limitation, common stock or such other securities which may be deemed to be beneficially owned by Vifor in accordance with the rules and regulations of the SEC and securities which may be issued upon exercise of a stock option or warrant) owned by Vifor as of the date hereof or acquired prior to the end of the Restricted Period (collectively with the common stock, referred to as the Lock-Up Securities), except any such sale, option or contract by and between Vifor and one of its affiliates (including Vifor Pharma Group Ltd. or VFMCRP), (ii) enter into any hedging, swap or other agreement or transaction that transfers, in whole or in part, any of the economic consequences of ownership of the Lock-Up Securities, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Lock-Up Securities, in cash or otherwise, (iii) make any demand for or exercise any right with respect to the registration of any Lock-Up Securities, or (iv) publicly disclose the intention to do any of the foregoing. Under the Vifor Purchase Agreement, the parties also agreed that, in certain circumstances, upon the request of Vifor, the parties will enter into a registration rights agreement prior to the end of the Restricted Period that would provide Vifor (or its affiliate transferee) customary registration rights with respect to the shares of common stock issued pursuant to the Vifor Purchase Agreement following the expiration of the Restricted Period. ​ In June 2020, as a result of the completion of the one-year vesting period, an aggregate of 24,000 restricted stock units of members of the Board of Directors vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In April and June 2020, as a result of the achievement of certain performance targets, an aggregate of 95,834 restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation ​ In December 2019, as a result of the achievement of a clinical performance target, restricted stock units of various executive officers vested and were converted into 36,666 shares of the Company’s common stock (see Note 14, Stock-Based Compensation In August 2019, the Company entered into a Non-Exclusive License Agreement, or the Enteris License Agreement, with Enteris Biopharma, Inc., or Enteris (see Note 18, Commitments and Contingencies into a Common Stock Purchase Agreement, or the Enteris Purchase Agreement, with Enteris and its affiliate, EBP Holdco LLC, collectively referred to as Purchaser, pursuant to which the Company issued and sold to Purchaser 170,793 shares of its common stock in a private placement in satisfaction of the $4,000 portion of the upfront fee payable in shares of the Company’s common stock pursuant to the Enteris License Agreement, and for no additional consideration, based on a purchase price of $23.42 per share, which was equal to the 30-day volume weighted average price of the Company’s common stock on August 20, 2019. In addition, if the Company exercises its right, but not obligation, to terminate its obligation to pay any royalties under the Enteris License Agreement in exchange for a lump sum payment in cash, or the Royalty Buyout, it may elect to make 50% of the payment in stock by issuing additional shares of the Company’s common stock valued at the 30-day volume weighted average price of the Company’s common stock as of such exercise. The Company did not exercise its Royalty Buyout right and such right expired in August 2021. Pursuant to its obligations under the Enteris Purchase Agreement, the Company effected the registration and sale of the shares issued and sold to Purchaser thereunder in accordance with the applicable requirements of the Securities Act of 1933, as amended, or the Securities Act, through the filing of an automatic shelf registration statement on Form S-3ASR (File No. 333-233666) with the SEC on September 9, 2019. In addition, the Enteris Purchase Agreement includes customary representations, warranties and covenants by the Company (see Note 18, Commitments and Contingencies In July 2019, the Company entered into an underwriting agreement with J.P. Morgan Securities LLC and Jefferies LLC, as representatives of the several underwriters named therein, relating to the issuance and sale by the Company of 6,325,000 shares of its common stock, which included the exercise of the underwriters’ option to purchase 825,000 additional shares of common stock, at a public offering price of $23.00 per share. The Company closed this offering on July 29, 2019, including the full exercise of the underwriters’ option to purchase 825,000 additional shares of common stock. The Company received net proceeds of $136,498, after deducting $8,977 of underwriting discounts and commissions and offering expenses. This offering was made pursuant to the Company’s Shelf Registration Statement on Form S-3 (File No. 333-230333), or the Shelf Registration Statement, filed with the SEC on March 15, 2019 and declared effective on April 4, 2019, and a related prospectus supplement dated July 24, 2019, which was filed with the SEC on July 25, 2019. The Shelf Registration Statement provides for aggregate offerings of up to $300,000 of common stock, preferred stock, debt securities, warrants or any combination thereof. The securities registered under the Shelf Registration Statement include unsold securities that had been registered under the Company’s previous shelf registration statement (File No. 333-216657) that was declared effective on March 24, 2017. In May 2019, as a result of the achievement of a clinical performance target, an aggregate of 74,166 restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 14, Stock-Based Compensation In March 2019, the Company entered into a consulting agreement with an existing stockholder. In accordance with the agreement, the stockholder provided various consulting services to the Company in exchange for 10,195 unregistered shares of the Company’s common stock. The closing price of the Company’s common stock on March 20, 2019, or the Effective Date of the consulting agreement, was $19.37 per share. The services provided by the consultant were performed during the six-month period following the Effective Date. During the year ended December 31, 2019, stock-based compensation expense of $197 was recognized in the Statements of Comprehensive (Loss) Income, all of which related to G&A expense.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	11. Fair Value Measurements The following tables summarize the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2021 and 2020 and by level within the fair value hierarchy: Fair value measurement as of December 31, 2021: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted prices in ​ Significant other ​ Significant Financial assets ​ ​ ​ ​ active markets for ​ observable ​ unobservable ​ ​ ​ ​ ​ identical assets ​ inputs ​ inputs Type of Instrument Total (Level 1) (Level 2) (Level 3) Cash and cash equivalents: ​ ​ ​ ​ Money market funds and checking accounts ​ $ 13,453 ​ $ 13,453 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ ​ ​ ​ U.S. Treasury securities ​ 11,570 ​ — ​ 11,570 ​ — U.S. government agency obligations ​ 16,975 ​ — ​ 16,975 ​ — Corporate bonds ​ 66,324 ​ — ​ 66,324 ​ — Commercial paper ​ 106,888 ​ — ​ 106,888 ​ — Municipal bonds ​ 21,579 ​ — ​ 21,579 ​ — Restricted cash: ​ ​ ​ ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 237,197 ​ $ 13,861 ​ $ 223,336 ​ $ — ​ Fair value measurement as of December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted prices in ​ Significant other ​ Significant Financial assets ​ ​ ​ ​ active markets for ​ observable ​ unobservable ​ ​ ​ ​ ​ identical assets ​ inputs ​ inputs Type of Instrument Total (Level 1) (Level 2) (Level 3) Cash and cash equivalents: ​ ​ ​ ​ Money market funds and checking accounts ​ $ 31,683 ​ $ 31,683 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ U.S. Treasury securities ​ ​ 20,750 ​ ​ — ​ ​ 20,750 ​ ​ — U.S. government agency obligations ​ 22,128 ​ — ​ 22,128 ​ — Corporate bonds ​ 49,118 ​ — ​ 49,118 ​ — Commercial paper ​ 116,127 ​ — ​ 116,127 ​ — Municipal bonds ​ ​ 11,684 ​ ​ — ​ ​ 11,684 ​ ​ — Restricted cash: ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 251,898 ​ $ 32,091 ​ $ 219,807 ​ $ — ​ There were no purchases, sales or maturities of Level 3 financial assets and no unrealized gains or losses related to Level 3 available-for-sale marketable securities for the years ended December 31, 2021, 2020 and 2019. There were no transfers of financial assets between Levels 1, 2, or 3 classifications during the years ended December 31, 2021 and 2020.

Collaboration and Licensing Agr

Collaboration and Licensing Agreements	12 Months Ended
	Dec. 31, 2021
Basis of Presentation
Collaboration and Licensing Agreements	12. Collaboration and Licensing Agreements Vifor (International) Ltd. (Vifor) In October 2020, the Company entered into a license agreement with Vifor, or the Vifor Agreement, under which the Company granted Vifor an exclusive license solely in the United States to use, distribute, offer for sale, promote, sell and otherwise commercialize difelikefalin injection for all therapeutic uses relating to the inhibition, prevention or treatment of itch associated with pruritus in hemodialysis and peritoneal dialysis patients in the United States. Under the Vifor Agreement, the Company retains all rights with respect to the clinical development of, and activities to gain regulatory approvals of, difelikefalin injection in the United States. The Joint Commercialization Committee, or JCC, have the responsibility for overall coordination and oversight of Vifor (and any affiliates or sublicensees). The Company’s membership on the JCC is at its sole discretion and is not its obligation. ​ The Vifor Agreement provides full commercialization rights in dialysis clinics to Vifor in the United States under a profit-sharing arrangement. Pursuant to the profit-sharing arrangement, the Company will generally be entitled to 60% of the net profits (as defined in the Vifor Agreement) from sales of difelikefalin injection in the United States (excluding sales to Fresenius Medical Center dialysis clinics, compensation for which is governed by the VFMCRP Agreement) and Vifor is entitled to 40% of such net profits, subject to potential temporary adjustment in future years based on certain conditions. Under the Vifor Agreement, in consideration of Vifor’s conduct of the marketing, promotion, selling and distribution of difelikefalin injection in the United States, the Company will pay a marketing and distribution fee to Vifor based on the level of annual net sales. This fee will be deducted from product sales in calculating the net profits that are subject to the profit-sharing arrangement under the Vifor Agreement. ​ The Company has identified one performance obligation under ASC 606: granting of the license to Vifor. No other performance obligations were identified in the Vifor Agreement (see Note 13, Revenue Recognition Under the terms of the Vifor Agreement, the Company received from Vifor an upfront payment of $100,000 and an additional payment of $50,000 for the purchase of an aggregate of 2,939,552 shares of the Company’s common stock at a price of $17.0094 per share, which represents a premium over a pre-determined average closing price of the Company’s common stock. The purchase of the Company’s common stock was governed by a separate stock purchase agreement, or the Vifor Stock Purchase Agreement. The excess of the stock purchase price over the cost of the Vifor shares at the closing price of the Company’s common stock on the purchase date of $11,551 was added to the upfront payment for accounting purposes. ​ In addition, pursuant to the Vifor Agreement, the Company is eligible to receive payments of up to $240,000 upon the achievement of certain sales-based milestones. ​ At inception of the Vifor Agreement, the transaction price of $111,551 was allocated entirely to the one performance obligation, as described above, and was recorded as license and milestone fees revenue during the year ended December 31, 2020. Any future sales milestones are constrained from the transaction price at inception since there is a significant uncertainty as to whether these milestones would be achieved. These sales milestones will be recognized as revenue if, and when, such sales transactions occur in the future. After U.S. regulatory approval of KORSUVA injection in August 2021, the Company received an additional $50,000 in October 2021 for the purchase of an aggregate of 3,282,391 shares of the Company’s common stock at a price of $15.23 per share, which represents a 20% premium to the 30-day trailing average price of the Company’s common stock as of the date of the achievement of the milestone. The purchase of the Company’s common stock was governed by the Vifor Stock Purchase Agreement. The excess of the stock purchase price over the cost of the purchased shares at the closing price of the Company’s common stock on the date of the achievement of the milestone of $5,031 was included as license and milestone fees revenue for accounting purposes for the year ended December 31, 2021. The license also requires Vifor to promote and take orders for difelikefalin injection, or Licensed Product, throughout the United States, including coordinating with VFMCRP promotional activities to FMC U.S. Dialysis Clinics which are subject to the Company’s rights under the VFMCRP Agreement. The license also allows Vifor to grant sub-licenses, which, in certain cases, requires the Company’s prior written consent. The Company retains the rights to import, distribute, promote, sell and otherwise commercialize the Licensed Product on an exclusive basis outside of the Field either in or outside of the United States. The Company retains the rights to make and have made the Licensed Product, on a non-exclusive basis, in the United States for commercial sale of the Licensed Product for use for all therapeutic uses to prevent, inhibit or treat itch associated with pruritus in hemodialysis and peritoneal-dialysis patients, or the Field, anywhere in the world and for supply of Licensed Product to Vifor under the terms of a supply agreement, or the Vifor Supply Agreement, which was executed in September 2021. The supply price is the Company’s COGS, as calculated under GAAP, plus an agreed upon margin. The Vifor Supply Agreement will co-terminate with the Vifor Agreement. The Vifor Supply Agreement is accounted for as a customer option that is not a material right because the selling price of the Licensed Product under the Vifor Supply Agreement is the Company’s COGS plus an agreed upon margin, which is commensurate with the “COGS plus” model that the Company would charge other parties under similar agreements (the standalone selling price) and not at a discount. Therefore, the sale of commercial supply to Vifor is not a performance obligation under the Vifor Agreement but rather the Vifor Supply Agreement is a separate agreement from the Vifor Agreement. The only performance obligation under the Vifor Supply Agreement is the delivery of the Licensed Product to Vifor for commercialization. Revenue from the sale of the Licensed Product to Vifor will be recognized in the Company’s Statements of Comprehensive (Loss) Income as sales of the Licensed Product occur. The Company had commercial supply revenue of $701 for the year ended December 31, 2021 with no associated COGS since all inventory costs prior to regulatory approval on August 23, 2021 were expensed as incurred (see Note 2, Summary of Significant Accounting Policies – Inventories Cost of Goods Sold ​ The Vifor Agreement will continue in effect until its expiration upon the cessation of commercial sale of difelikefalin injection in the United States by Vifor and its affiliates and sublicensees, or until the earlier termination of the Vifor Agreement. The Vifor Agreement may be terminated earlier by either party for material breach that is not cured within 60 days, bankruptcy by either party and by both parties upon mutual written consent. The Company may terminate the Vifor Agreement if Vifor challenges the validity of any licensed patent rights, except if such patent challenge results from the Company’s action against Vifor for infringement of any licensed patent in the United States. In addition, upon the earlier of (1) the acceptance for filing of an NDA covering Licensed Product filed with the FDA (after completion of the Phase 3 program) or (2) the third anniversary of the Effective Date, the Vifor Agreement may be terminated by Vifor in its entirety upon written notice to the Company. Such termination will be effective twelve months following the date of such notice ​ Vifor Fresenius Medical Care Renal Pharma Ltd. (VFMCRP) In May 2018, the Company entered into a license agreement, or the VFMCRP Agreement, with VFMCRP under which the Company granted VFMCRP an exclusive, royalty-bearing license, or the VFMCRP License, to seek regulatory approval to commercialize, import, export, use, distribute, offer for sale, promote, sell and otherwise commercialize the Licensed Product in the Field worldwide (excluding the United States, Japan and South Korea), or the Territory. VFMCRP cannot perform development activities on their own unless specifically allocated to VFMCRP by the Joint Development Committee, or JDC, and Joint Steering Committee, or JSC. The Company’s membership on the JSC or JDC is at its sole discretion and is not its obligation. The Company is responsible, at its own cost, to undertake clinical and non-clinical development, or the R&D services. The Company is also responsible to provide all content and subject matter expertise required for registration with the European Medicines Agency, or EMA, in the European Union, or the EU, that will be needed by VFMCRP for such registration, including participation in regulatory meetings, as needed. VFMCRP will contribute, at its own cost, its clinical development expertise as reasonably useful for such development activities, such as preparing the clinical results that the Company presents to it in a format acceptable to the EMA to obtain marketing approval in the EU. The Company has identified two performance obligations under ASC 606: (1) granting of the VFMCRP License and (2) the R&D services. The Company has determined that these two performance obligations are not capable of being distinct (i.e., do not have standalone value for VFMCRP) because VFMCRP cannot benefit (derive potential cash flows) from either one on its own or together with other resources that are readily available to it since VFMCRP is relying on the Company’s expertise in investigating chronic kidney disease-associated pruritus, or CKD-aP, and its know-how obtained from multiple years of pre-clinical and clinical development, and years of interactions with the FDA which other companies or CROs would not have. The VFMCRP License does not provide benefit to VFMCRP until and unless the Company conducts the pivotal clinical trials and other supportive trials in CKD-aP to gather sufficient clinical data for VFMCRP to obtain marketing approval in the Territory. Furthermore, VFMCRP does not have the right to perform development activities on its own unless specifically allocated by the JDC or JSC. The two identified performance obligations are also not distinct within the context of the contract, (i.e., are not separately identifiable from each other) because of the nature of the promise within the context of the contract. The nature of the promise is to transfer a combined deliverable to VFMCRP based on the agreement (to support the ability of VFMCRP to commercialize the Licensed Product) and the Company determined that the VFMCRP License and the R&D services are inputs rather than a transfer of each of these goods and services individually. In addition, the two identified performance obligations are highly interrelated and interdependent because satisfaction of both performance obligations is required for VFMCRP to derive benefit from the VFMCRP Agreement for commercialization of the Licensed Product in the Territory. Therefore, the two performance obligations are not distinct from each other and are accounted for as a single performance obligation. Upon entry into the VFMCRP Agreement, VFMCRP made a non-refundable, non-creditable $50,000 upfront payment to the Company and Vifor purchased 1,174,827 shares of the Company’s common stock, or the Vifor Shares, for $20,000 at a price of $17.024 per share, which represents a premium over a pre-determined average closing price of the Company’s common stock. The purchase of the Company’s common stock was governed by a separate stock purchase agreement. The excess of the stock purchase price over the cost of the Vifor Shares at the closing price of the Company’s common stock on the purchase date of $5,444 was added to the upfront payment for accounting purposes. At inception of the VFMCRP Agreement, the transaction price of $55,444 was allocated entirely to the one combined performance obligation, as described above, and was initially recorded as deferred revenue. License and milestone revenue was recognized proportionately as the R&D services were conducted (i.e., prior to submission of an NDA). After U.S. regulatory approval of KORSUVA injection in August 2021, the Company was entitled to receive a $15,000 regulatory milestone payment which was received in October 2021, and was recorded as license and milestone fees revenue for the year ended December 31, 2021, based on the identification of one combined performance obligation at contract inception. The Company is eligible to receive from VFMCRP additional regulatory and commercial milestone payments in the aggregate of up to $455,000, consisting of up to $15,000 in regulatory milestones and up to $440,000 in tiered commercial milestones, all of which are sales related. The Company is also eligible to receive tiered double-digit royalty payments based on annual net sales, as defined in the VFMCRP Agreement, of difelikefalin injection in the licensed territories. The Company retains full commercialization rights for difelikefalin injection for the treatment of CKD-aP in the United States except in the dialysis clinics of Fresenius Medical Care North America, or FMCNA, where VFMCRP and the Company will promote difelikefalin injection under a profit-sharing arrangement (subject to the terms and conditions of the VFMCRP Agreement) based on net FMCNA clinic sales recorded by the Company. At inception of the VFMCRP Agreement, there was significant uncertainty as to whether marketing approval would be obtained in the Territory for the Licensed Product. Therefore, at that time, there was a significant probability that any potential revenue from sales of the Licensed Product that would be included in the transaction price would be reversed when the uncertainty is resolved. Consequently, any sales royalties and sales milestones are constrained from the transaction price at inception of the VFMCRP Agreement and will be recognized as revenue if, and when, such sales transactions occur in the future. The license also requires VFMCRP to promote and take orders in the U.S. for sale by the Company to FMC U.S. Dialysis Clinics and allows VFMCRP to grant sub-licenses, which, in certain cases, requires the Company’s prior written consent. The Company retains the rights to import, distribute, promote, sell and otherwise commercialize the Licensed Product outside of the Field and outside of the Territory. The Company retains the rights to make and have made the Licensed Product in the Territory for commercial sale by VFMCRP in the Field in or outside the Territory and for supply of Licensed Product to VFMCRP under the terms of a supply agreement, or the VFMCRP Supply Agreement, which was executed in May 2020. The supply price is the Company’s COGS, as calculated under U.S. GAAP, plus an agreed upon margin. The VFMCRP Supply Agreement will co-terminate with the VFMCRP Agreement. The VFMCRP Supply Agreement is accounted for as a customer option that is not a material right because the selling price of the Licensed Product under the VFMCRP Supply Agreement is the Company’s COGS plus an agreed upon margin, which is commensurate with the “COGS plus” model that the Company would charge other parties under similar agreements (the standalone selling price) and not at a discount. Therefore, the sale of clinical compound to VFMCRP is not a performance obligation under the VFMCRP Agreement but rather the VFMCRP Supply Agreement is a separate agreement from the VFMCRP Agreement. The only performance obligation under the VFMCRP Supply Agreement is the delivery of the Licensed Product to VFMCRP for commercialization. Revenue from the sale of the Licensed Product to VFMCRP will be recognized in the Company’s Statements of Comprehensive (Loss) Income as sales of the Licensed Product occur. During the years ended 2021 and 2020, the Company recognized clinical compound revenue of $361 and $115, respectively, from the sale of clinical compound to VFMCRP and as a result, the Company incurred R&D expense of $343 and $108 during the respective periods. The VFMCRP Agreement terminates upon the expiration of all royalty terms with respect to the Licensed Products, which expire on a Product-by-Product and country-by-country basis, at the latest of (a) the expiration of all patent rights licensed to VFMCRP covering such Licensed Product; (b) the expiration of all regulatory and data exclusivity applicable to such Licensed Product in such country and (c) the tenth anniversary of the first commercial sale of such Product in such country. The VFMCRP Agreement may be terminated earlier by either party for material breach that is not cured within 60 days, bankruptcy by either party and by both parties upon mutual written consent. The Company may terminate the VFMCRP Agreement if VFMCRP challenges the validity of any licensed patent rights, except if such patent challenge results from the Company’s action against VFMCRP for infringement of any licensed patent in the Territory. In addition, upon the earlier of (1) the acceptance for filing of an NDA covering Licensed Product filed with the FDA (after completion of the Phase 3 program) or (2) the third anniversary of the Effective Date, the VFMCRP Agreement may be terminated by VFMCRP in its entirety or with respect to any countries within the Territory upon written notice to the Company. Such termination will be effective twelve months following the date of such notice. If the VFMCRP Agreement terminates early for any reason stated above, VFMCRP’s licenses will terminate, VFMCRP’s rights to use the Company’s confidential information and the Company’s know-how will revert to the Company and VFMCRP will assign and transfer to the Company all right, title and interest in all regulatory applications (IND’s and NDA’s), regulatory approval applications and regulatory approvals in the Territory covering Licensed Product. Maruishi Pharmaceutical Co., Ltd. (Maruishi) In April 2013, the Company entered into a license agreement with Maruishi, or the Maruishi Agreement, under which the Company granted Maruishi an exclusive license to develop, manufacture, and commercialize drug products containing difelikefalin for acute pain and/or uremic pruritus in Japan. Maruishi has the right to grant sub-licenses in Japan, which entitles the Company to receive sub-license fees, net of prior payments made by Maruishi to the Company. Under the Maruishi Agreement, the Company and Maruishi are required to use commercially reasonable efforts, at their own expense, to develop, obtain regulatory approval for and commercialize difelikefalin in the United States and Japan, respectively. In addition, the Company provided Maruishi specific clinical development services for difelikefalin used in Maruishi’s field of use. Under the Maruishi Agreement, the Company identified two performance obligations in accordance with ASC 606: (1) the license; and (2) the R&D services specific to the uremic pruritus field of use (specified as Phase 1 and proof-of-concept clinical trials), both of which were determined to have standalone value. The Company determined that these performance obligations had standalone value due to the fact that Maruishi obtained the right to develop the compound on its own and the Company was specifically contracted to perform specific R&D services as noted above. The Company believes that these early stage R&D services performed by the Company did not require any specific expertise or know-how, but rather could have been completed by outside third parties, therefore providing standalone value to Maruishi. Under the terms of the Maruishi Agreement, the Company is eligible to receive milestone payments upon the achievement of defined clinical and regulatory events as well as tiered, low double-digit royalties with respect to any sales of the licensed product sold in Japan by Maruishi, if any, and share in any sub-license fees. During the years ended December 31, 2021, 2020 and 2019, the Company recognized clinical compound revenue of $37, $528 and $140, respectively, from the sale of clinical compound to Maruishi. The Company incurred R&D expense related to the Maruishi Agreement of $33, $476, and $126 (all related to the cost of clinical compound sold to Maruishi) during the years ended December 31, 2021, 2020 and 2019, respectively. Chong Kun Dang Pharmaceutical Corporation (CKDP) In April 2012, the Company entered into a license agreement, or the CKDP Agreement with CKDP in South Korea, under which the Company granted CKDP an exclusive license to develop, manufacture and commercialize drug products containing difelikefalin in South Korea. The Company and CKDP are each required to use commercially reasonable efforts, at their respective expense, to develop, obtain regulatory approval for and commercialize difelikefalin in the United States and South Korea, respectively. The Company identified the granting of the license as its only performance obligation under the CKDP Agreement. Under the terms of the CKDP Agreement, the Company is eligible to receive milestone payments upon the achievement of defined clinical and regulatory events as well as tiered royalties, with percentages ranging from the high single digits to the high teens, based on net sales of products containing difelikefalin in South Korea, if any, and share in any sub-license fees.

Revenue Recognition

Revenue Recognition	12 Months Ended
	Dec. 31, 2021
Revenue Recognition [Abstract]
Revenue Recognition	13. Revenue Recognition The Company currently recognizes revenue in accordance with ASC 606, as amended, for the Vifor, VFMCRP, Maruishi and CKDP agreements (see Note 12 , Collaboration and Licensing Agreements For the year ended December 31, 2021, the Company had license and collaboration agreements with Vifor, VFMCRP, Maruishi and CKDP. The following table provides amounts included in the Company’s Statements of Comprehensive (Loss) Income as revenue for the years ended December 31, 2021, 2020 and 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 License and milestone fees ​ ​ ​ ​ ​ ​ ​ ​ ​ VFMCRP ​ $ 15,000 ​ $ 22,262 ​ $ 19,746 Vifor ​ ​ 5,031 ​ ​ 111,551 ​ ​ — Maruishi ​ ​ 1,192 ​ ​ — ​ ​ — CKDP ​ ​ — ​ ​ 626 ​ ​ — Total license and milestone fees ​ $ 21,223 ​ $ 134,439 ​ $ 19,746 Collaborative revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ Maruishi ​ $ 706 ​ $ — ​ $ — Total Collaborative revenue ​ $ 706 ​ $ — ​ $ — Commercial supply revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ Vifor (KORSUVA injection) ​ $ 701 ​ $ — ​ $ — Total commercial supply revenue ​ $ 701 ​ $ — ​ $ — Clinical compound revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ VFMCRP (difelikefalin injection) ​ $ 361 ​ $ 115 ​ $ — Maruishi ​ ​ 37 ​ ​ 528 ​ ​ 140 Total clinical compound revenue ​ $ 398 ​ $ 643 ​ $ 140 ​ Contract balances As of December 31, 2021 and 2020, there were no material balances of receivables, and no other assets or deferred revenue related to the Vifor, VFMCRP, Maruishi and CKDP agreements. The Company routinely assesses the creditworthiness of its license and collaboration partners. The Company has not experienced any losses related to receivables from its license or collaboration partners during the years ended December 31, 2021, 2020, or 2019. Performance obligations Under the Vifor Agreement, the Company’s only performance obligation is granting a license to allow Vifor to commercialize difelikefalin in the United States, which occurred at inception of the contract in October 2020 (see Note 12, Collaboration and Licensing Agreements Under the Vifor Supply Agreement, the Company’s only performance obligation is the delivery of KORSUVA injection to Vifor in accordance with the receipt of purchase orders. Under the VFMCRP Agreement, the Company’s performance obligations of granting a license to allow VFMCRP to commercialize difelikefalin injection worldwide, except in the United States, Japan and South Korea, which occurred at inception of the contract in May 2018, and performing R&D services by the Company to obtain sufficient clinical data which will be shared with VFMCRP to allow them to receive regulatory approval to sell difelikefalin in the licensed territory, are not distinct, and are accounted for as a single performance obligation during the period that the R&D services are rendered (see Note 12, Collaboration and Licensing Agreements The Company’s distinct performance obligations under the Maruishi Agreement include transfer of the license to the Company’s IP, which allowed Maruishi to develop and commercialize difelikefalin, for acute pain and uremic pruritus indications in Japan, which occurred at inception of the contract in 2013, and performance of R&D services, which occurred from 2013 to 2015, as those services were rendered. The Company agreed to conduct limited work on an oral tablet formulation of difelikefalin and to conduct Phase 1 and proof-of-concept Phase 2 clinical trials of an intravenous formulation of difelikefalin to be used to treat patients with uremic pruritus. The Company agreed to transfer the data and information from such development to Maruishi for its efforts to obtain regulatory approval in Japan. These activities are referred to as R&D services (see Note 12, Collaboration and Licensing Agreements The Company’s only performance obligation under the supply agreement with Maruishi is to deliver clinical compound to Maruishi in accordance with the receipt of purchase orders. The Company’s only performance obligation under the VFMCRP Supply Agreement is to deliver difelikefalin injection to VFMCRP in accordance with the receipt of purchase orders. Under the CKDP Agreement, the Company’s only performance obligation is to transfer the license to the Company’s IP related to difelikefalin. Upon execution of the Vifor, VFMCRP, Maruishi and CKDP agreements, the Company received a single fixed payment from each counterparty in exchange for granting the respective licenses and performing its other obligations (if applicable). In addition, each of the counterparties made an equity investment in the Company’s common stock. Transaction price allocated to the remaining performance obligations At inception of the Vifor Agreement, the entire transaction price of $111,551 was allocated to the one performance obligation, as described above. For the year ended December 31, 2020, the entire $111,551 was recognized as license and milestone fees revenue as the license was granted to Vifor in October 2020. As of December 31, 2021, there were no remaining performance obligations under the Vifor Agreement. The Company is eligible to receive milestone payments in the future. At inception of the VFMCRP Agreement, the entire transaction price of $55,444 was allocated to the one combined performance obligation, as described above. As of December 31, 2021, there were no remaining performance obligations and the entire $55,444 has been recognized as license and milestone fees revenue based on the percentage of R&D services that has been completed since the inception of the VFMCRP Agreement. The Company is eligible to receive milestone payments and sales royalties in the future. As of December 31, 2021, there were no remaining performance obligations under either the Maruishi Agreement or the CKDP Agreement, although the Company is eligible to receive milestone payments and sales royalties in the future. Significant judgments In applying ASC 606, as amended, to its four contracts, the Company made the following judgments that significantly affect the timing and amount of revenue recognition: 1. Determination of the number of distinct performance obligations in a contract The VFMCRP Agreement contains one combined performance obligation, which includes the Company’s two performance obligations to grant a license to VFMCRP and conduct R&D services. Both of those performance obligations are inputs to the promise, within the context of the contract, to transfer a combined output for which VFMCRP has contracted (the ability of VFMCRP to commercialize the Licensed Product) (see Note 12, Collaboration and Licensing Agreements The Maruishi Agreement contains two distinct performance obligations: the granting of the license and the promise to deliver defined R&D services. Under the Maruishi Agreement, the license and the R&D services represent distinct goods or services from each other because Maruishi is able to benefit from the license on its own or together with other resources that are readily available to it (i.e., capable of being distinct). Maruishi’s ability to benefit from the license without the R&D services is indicated by its ability to conduct clinical trials of difelikefalin on its own and by the provision in the Maruishi Agreement whereby if the Company suspends or discontinues its development activity, the Company will provide information regarding its development efforts up to that point so that Maruishi may continue development and commercialization of the product in Japan. Therefore, the R&D services do not significantly affect Maruishi’s ability to use and benefit from the license. In addition, the Company’s promise in the Maruishi contract to transfer the license is separately identifiable from the promise to provide defined R&D services (i.e., distinct within the context of the contract) because the Company is not using the goods or services as inputs to produce or deliver the combined output or outputs specified by the customer. The combined output specified by Maruishi is its right to conduct development activities related to difelikefalin in Japan, which could result in regulatory approval in Japan. That right is derived from the Company’s grant of the license. Maruishi is conducting clinical trials on its own and does not require the R&D services provided by the Company. Furthermore, the R&D services do not significantly modify or customize the license and vice versa. Finally, the license and R&D services are not highly interdependent or highly interrelated because the Company is able to fulfill its promise to transfer the initial license independently from its promise to subsequently provide the R&D services, which Maruishi can obtain on its own. The only performance obligation in the Vifor and CKDP agreements is the granting of the license. The only performance obligation in the Vifor Supply Agreement is the delivery of KORSUVA injection to Vifor. 2. Determination of the transaction price, including whether any variable consideration is included at inception of the contract The transaction price is the amount of consideration that the Company expects to be entitled to in exchange for transferring promised goods or services to the customer. The transaction price must be determined at inception of a contract and may include amounts of variable consideration. However, there is a constraint on inclusion of variable consideration, such as milestone payments or sales-based royalty payments, in the transaction price related to licenses of IP, if there is uncertainty at inception of the contract as to whether such consideration will be recognized in the future (see Note 2, Summary of Significant Accounting Policies: Revenue Recognition The decision as to whether or not it is probable that a significant reversal of revenue will occur in the future, depends on the likelihood and magnitude of the reversal and is highly susceptible to factors outside the entity’s influence (for example, the Company cannot determine the outcome of clinical trials; the Company cannot determine if or when they or the counterparty will initiate or complete clinical trials; and the Company’s ability to obtain regulatory approval is difficult). In addition, the uncertainty is not expected to be resolved for a long period of time (in the order of years) and finally, the Company has limited experience in the field. Therefore, at inception of the Vifor, VFMCRP, Maruishi and CKDP agreements, milestones and sales-based royalty payments were not included in the transaction price based on the factors noted above. Under the Vifor Agreement, the one performance obligation was satisfied when the license was granted to Vifor in October 2020, and as a result, $111,551 (including the upfront payment of $100,000 and the premium on the common stock purchased by Vifor of $11,551) was recognized as license and milestone fees revenue during the year ended December 31, 2020. The remaining potential consideration was considered to be variable consideration and was constrained at inception of the contract, which includes regulatory approvals and sales milestones (see Note 12, Collaboration and Licensing Agreements Under the VFMCRP Agreement, the single combined performance obligation was satisfied as the R&D services were rendered and the transaction price (including the upfront payment of $50,000 and the premium on the common stock purchased by VFMCRP of $5,444) was recognized as revenue as the R&D services were performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. The remaining potential consideration was considered to be variable consideration and was constrained at inception of the contract, which includes regulatory approvals and sales milestones and sales royalties (see Note 12, Collaboration and Licensing Agreements All performance obligations under the Maruishi Agreement and the CKDP Agreement were satisfied by the end of 2015. In the future, any milestone event will be recognized in accordance with Note 2, Significant Accounting Polices: Revenue Recognition, Under the Maruishi Agreement, the transaction price includes only the non-refundable and non-creditable upfront license fee of $15,337, including the premium of $337 from the sale of Company stock to Maruishi, that was paid to the Company at inception of the contract. The remaining potential consideration was considered to be variable consideration and was constrained at inception of the contract, including an aggregate of up to $10,500, which the Company is eligible to receive upon achievement of clinical development and regulatory milestones, a one-time sales milestone of one billion Yen when a certain sales level is attained; a mid-double-digit percentage of all non-royalty payments received by Maruishi from its sub-licensees, if any; and tiered royalties based on net sales of products containing difelikefalin in Japan, if any, with minimum royalty rates in the low double digits and maximum royalty rates in the low twenties. Under the CKDP Agreement, the transaction price includes only the non-refundable and non-creditable upfront license fee of $646, including the premium of $83 from the sale of Company stock to CKDP, that was paid to the Company at inception of the contract. The remaining consideration was considered to be variable consideration and was constrained at inception of the contract, including an aggregate of up to $3,750, which the Company is eligible to earn upon achievement of clinical development and regulatory milestones. The Company is also eligible to receive a mid-double-digit percentage of all non-royalty payments received by CKDP from its sub-licensees, if any, and tiered royalties ranging from the high single digits to the high teens based on net sales of products containing difelikefalin in South Korea, if any. 3. Determination of the estimate of the standalone selling price of performance obligations In order to recognize revenue under ASC 606, as amended, for contracts for which more than one distinct performance obligation has been identified, the Company must allocate the transaction price to the performance obligations based upon their standalone selling prices. The best evidence of standalone selling price is an observable price of a good or service when sold separately by an entity in similar circumstances to similar customers. If such evidence is not available, standalone selling price should be estimated so that the amount that is allocated to each performance obligation equals the amount that the entity expects to receive for transferring goods or services. The Company has identified more than one performance obligation only in the Maruishi Agreement. Since evidence based on observable prices is not available for the performance obligations under the Maruishi Agreement, the Company considered market conditions and entity-specific factors, including those contemplated in negotiating the agreements, as well as certain internally developed estimates. At inception of the Maruishi Agreement, the Company determined the estimate of standalone selling price for the license performance obligation by using the adjusted market assessment approach. Under this method, the Company forecasted and analyzed difelikefalin in the Japanese market, the phase of clinical development as well as considered recent similar license arrangements within the same phase of clinical development, therapeutic area, type of agreement, etc. To estimate the standalone selling price of the R&D services, the Company forecasted its expected costs of satisfying that performance obligation and added a margin for that service. 4. Determination of the method of allocation of the transaction price to the distinct performance obligations At inception of the Maruishi Agreement, the Company allocated the transaction price of $15,337 between the two performance obligations based on their relative standalone selling prices, determined as described above. The Company determined that the license and the R&D services had estimated standalone selling prices of $10,200 and $6,200, respectively. The resulting percentage allocations were applied to the $15,337 of total transaction price, which resulted in $9,637 being allocated to the license performance obligation, which was recognized immediately as license revenue, while $5,700 was allocated to the R&D services performance obligation. The amount allocated to the R&D services performance obligation was initially recorded as deferred revenue and was recognized as collaborative revenue as the R&D services were provided through July 2015. Since the Vifor, VFMCRP and CKDP 5. Determination of the timing of revenue recognition for contracts Revenue should be recognized when, or as, an entity satisfies a performance obligation by transferring a promised good or service to a customer; i.e., when the customer obtains control of the good or service. The licenses granted to Vifor, Maruishi and CKDP are being accounted for as distinct performance obligations. As discussed below, the licenses relate to functional IP for which revenue is recognized at a point in time – in the case of these three license agreements, the point in time is at inception of the contract because the customer obtained control of the license at that point. The licenses grant Vifor, Maruishi and CKDP the right to use the Company’s IP relating to difelikefalin as it existed at the point in time that the licenses were granted. That IP has significant standalone functionality as it provides the customer with the ability to perform a function or task, such as to manufacture difelikefalin, conduct clinical trials, or commercialize difelikefalin, and is considered to be functional IP. During the license periods, the Company is continuing to develop and advance difelikefalin by conducting clinical trials. Those development efforts are for its own benefit and do not substantively change the significant standalone functionality of the licensed IP granted to Vifor, Maruishi or CKDP. Therefore, the Company’s ongoing development efforts do not significantly affect the IP’s utility to which Vifor, Maruishi or CKDP have rights. Furthermore, if the Company abandons its development efforts, Vifor, Maruishi or CKDP may still continue to develop difelikefalin in their respective countries. The R&D services performance obligation under the Maruishi Agreement represents a separate performance obligation. The R&D services were provided to Maruishi by the Company from inception of the agreement in 2013 through the third quarter of 2015, at which time the Company had fulfilled its promise related to the R&D services. Revenue related to the R&D services performance obligation was recognized as services were performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. Similarly, under the VFMCRP Agreement, revenue related to the single distinct performance obligation, which includes both granting of the license and performance of the R&D services, was recognized as the R&D services were performed, based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. As of December 31, 2021, there is no remaining amount of the transaction price to be recognized as license and milestone fees revenue. Under the Vifor Supply Agreement, revenue from the sales of KORSUVA injection are recognized in the Company’s Statements of Comprehensive (Loss) Income as sales of KORSUVA injection occurs and control is transferred to Vifor. 6. Determination of consideration as variable consideration, including factors related to inclusion in the transaction price at inception of the contract and timing of recognition as revenue. The Vifor, VFMCRP, Maruishi and CKDP agreements contain potential payments related to achievement of defined milestone events and royalties (excluding Vifor) upon net sales of future products, which are considered to be variable consideration because of the uncertainty of occurrence of any of those events specified in those agreements at inception of the agreements. Therefore, those potential payments were not included in the transaction price at the inception of the agreements. Revenue related to achievement of milestone events is recognized when the Company has determined that it is probable that a milestone event will be achieved and there will not be a significant reversal of revenue in future periods. Upon probability of achievement of a milestone event, the most likely amount of variable consideration is included in the transaction price. Subsequent changes to the transaction price, after contract initiation, are allocated to the performance obligations in the contract on the same basis as at contract inception. Revenue for variable consideration is recognized in the same manner (point in time or over time) as for the performance obligations to which the payment amounts were allocated. The Maruishi Agreement and the CKDP Agreement specify that certain development milestones will be achieved at pre-specified defined phases of a clinical trial (such as initiation or completion or other pre-specified time during a clinical trial as specified in the agreements). After U.S. regulatory approval of KORSUVA injection in August 2021, the Company received an additional $50,000 milestone payment in October 2021 from Vifor for the purchase of an aggregate of 3,282,391 shares of the Company’s common stock at a price of $15.23 per share, which represents a 20% premium to the 30-day trailing average price of the Company’s common stock as of the date of the achievement of the milestone. The excess of the stock purchase price over the cost of the purchased shares at the closing price of the Company’s common stock on the date of the achievement of the milestone of $5,031 was included as license and milestone fees revenue for the year ended December 31, 2021, as the variable consideration was deemed probable upon the FDA approval in August 2021. After U.S. regulatory approval of KORSUVA injection in August 2021, the Company received a $15,000 milestone payment in October 2021 from VFMCRP, which was recorded as license and milestone fees revenue for the year ended December 31, 2021, as the variable consideration was deemed probable upon the FDA approval in August 2021. In January 2021, the criteria for revenue recognition for a milestone event set forth in the Maruishi Agreement was achieved, and the Company recorded $1,192 as license and milestone fees revenue and $706 as collaboration revenue based on the relative standalone selling prices described above at contract inception for the year ended December 31, 2021. In May 2021, the Company received the $1,898 payment (after contractual foreign currency exchange adjustments) from Maruishi for the milestone event achieved. During the year ended December 31, 2020, the criteria for revenue recognition for a milestone event set forth in the CKDP Agreement was achieved, and the Company recorded $626 (net of South Korean taxes) as license and milestone fees revenue. No milestone events were probable of occurrence or achieved during the years ended December 31, 2021 and 2019. Sublicense payments Vifor’s, VFMCRP’s, Maruishi’s and CKDP’s right to grant sub-licenses is explicitly stated in their respective license agreements. The amount of any potential sub-license fees to be received by the Company, which is based on a formula if applicable to that respective agreement, is considered to be variable consideration and is constrained from inclusion in the transaction price at inception of the contract since at that time it was probable that there would be a reversal of such revenue in the future because the Company did not know if a sublicense would be granted in the future. Sales-based Royalty Payments The VFMCRP Agreement, CKDP Agreement and Maruishi Agreement each allow the Company to earn sales-based royalty payments in exchange for a license of intellectual property. In that case, the Company will recognize revenue for a sales-based royalty only when (or as) the later of the following events occurs: a. The subsequent sale or usage occurs. b. The performance obligation to which some or all of the sales-based royalty has been allocated has been satisfied (or partially satisfied). Since the sale (item a, above) occurs after the license was delivered (item b, above), the sales-based royalty exception, to exclude such royalty payments from the transaction price, applies to the overall revenue stream. Therefore, sales-based royalty payments are recognized as revenue when the customer’s sales occur. To date, no royalties have been earned or were otherwise due to the Company.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2021
Disclosure Of Compensation Related Costs Share Based Payments [Abstract]
Stock-Based Compensation	14. Stock-Based Compensation 2019 Inducement Plan In October 2019, the Company’s Board of Directors adopted the 2019 Inducement Plan, or the 2019 Plan, which is a non-stockholder approved stock plan adopted pursuant to the “inducement exception” provided under Nasdaq Listing Rule 5635(c)(4), or Rule 5635, for the purpose of awarding (i) non-statutory stock options, (ii) restricted stock awards, (iii) restricted stock unit awards, (iv) other stock awards (collectively, the Inducement Awards) to new employees of the Company, as inducement material to such new employees entering into employment with the Company. On November 20, 2019, the Company filed a Registration Statement on Form S-8 with the SEC covering the offering of up to 300,000 shares of its common stock, par value $0.001, pursuant to the Company’s 2019 Plan. No stock options were granted under the 2019 Inducement Plan during the years ended December 31, 2021 and 2020. During the year ended December 31, 2019, the Company granted 47,500 stock options under the 2019 Plan to new employees. Initial grants of Inducement Awards made to employees vest as to 25% on the first anniversary of the date of grant and the balance ratably over the next 36 months and subsequent grants vest monthly over a period of four years from the grant date. 2014 Equity Incentive Plan The Company’s 2014 Equity Incentive Plan, or the 2014 Plan, is administered by the Company’s Board of Directors or a duly authorized committee thereof, referred to as the Plan administrator. The 2014 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of equity compensation, collectively referred to as Stock Awards. Additionally, the 2014 Plan provides for the grant of performance cash awards. Incentive stock options may be granted only to employees. All other awards may be granted to employees, including officers, non-employee directors, and consultants. No incentive stock options may be granted under the 2014 Plan after the tenth anniversary of the effective date of the 2014 Plan. Stock Awards granted under the 2014 Plan vest at the rate specified by the Plan administrator. Initial grants of Stock Awards made to employees and non-employee consultants generally vest as to 25% on the first anniversary of the date of grant and the balance ratably over the next 36 months and subsequent grants vest monthly over a period of four years from the grant date. Stock options initially granted to members of the Company’s Board of Directors vest over a period of three years in equal quarterly installments from the date of the grant, subject to the option holder’s continued service as a Director through such date. Subsequent grants to Directors that are made automatically at Annual Meetings of Stockholders vest fully on the earlier of the first anniversary of the date of grant and the next Annual Meeting of Stockholders. The Plan administrator determines the term of Stock Awards granted under the 2014 Plan up to a maximum of ten years. The aggregate number of shares of the Company’s common stock reserved for issuance under the 2014 Plan has automatically increased on January 1 of each year, beginning on January 1, 2015 and will continue to increase on January 1 of each year through and including January 1, 2024, by 3% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. On January 1, 2022, the aggregate number of shares of common stock that may be issued pursuant to Stock Awards under the 2014 Plan automatically increased from 8,984,679 to 10,589,103. The maximum number of shares that may be issued pursuant to the exercise of incentive stock options under the 2014 Plan is 30,000,000 shares. Restricted Stock Units On December 17, 2021, the Compensation Committee of the Company’s Board of Directors approved and granted a total of 63,573 time-based restricted stock units to certain executive officers under the 2014 Plan with a grant date fair value of $12.45 per share. The restricted stock units vest in two equal installments on December 15, 2022 and June 15, 2023. As a result, the Company recognizes compensation expense associated with these restricted stock units ratably over the 18-month vesting period following the grant date. For the year ended December 31, 2021, the Company recognized $20 of stock compensation expense, with $8 recorded in R&D expense and $12 in G&A expense. As of December 31, 2021, none of the 63,573 restricted stock units were vested or settled in shares of the Company’s common stock. On October 29, 2021, the Compensation Committee of the Company’s Board of Directors also approved and granted 147,942 time-based restricted stock units in connection with the appointment of the Company’s new Chief Executive Officer, or CEO, under the 2014 Plan with a grant date fair value of $16.83 per share. The first tranche of 142,000 restricted stock units vests 25% on the first anniversary of the date of grant and the balance quarterly over the next 36 months. The second tranche of 5,942 restricted stock units fully vests on March 31, 2022. As a result, the Company recognizes compensation expense associated with these two restricted stock unit tranches ratably over their respective vesting periods following the grant date. For the year ended December 31, 2021, stock compensation expense of $144 was recognized in G&A expense. As of December 31, 2021, none of the 147,942 restricted stock units were vested or settled in shares of the Company’s common stock. Pursuant to the Company’s non-employee director compensation policy, an aggregate of 43,200 restricted stock units were granted to non-employee directors on June 3, 2021, the date of the Company’s 2021 Annual Meeting of Stockholders, under the 2014 Plan with a grant date fair value of $13.06 per share. The restricted stock units will vest on the earlier of (i) June 3, 2022 and (ii) immediately prior to the Company’s next Annual Meeting of Stockholders following the grant date, subject to the recipient’s continued service through such date. As a result, the Company recognizes compensation expense associated with these restricted stock units ratably over the one-year vesting period following the grant date. For the year ended December 31, 2021, stock compensation expense of $326 was recognized in G&A expense. As of December 31, 2021, none of the 43,200 restricted stock units were vested or settled in shares of the Company’s common stock. On March 30, 2021, the Compensation Committee of the Company’s Board of Directors approved and granted a total of 176,000 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $20.59 per share. Vesting of the restricted stock units is contingent on the achievement of certain performance targets related to clinical and regulatory milestones, subject to the recipient’s continuous service through each performance target. Recognition of compensation expense associated with these awards begins when, and to the extent, the performance criteria is probable of achievement and the employee has met the service conditions. In August 2021, performance targets relating to 44,002 restricted stock units had been achieved and thus restricted stock units vested and the awards were settled in shares of common stock. For the year ended December 31, 2021, the Company recognized $906 of stock compensation expense, with $329 recorded in R&D expense and $577 recorded in G&A expense. As of December 31, 2021, 44,002 of the 176,000 restricted stock units vested and were settled in shares of the Company’s common stock. Additionally on March 30, 2021, the Compensation Committee of the Company’s Board of Directors also approved and granted a total of 100,000 time-based restricted stock units to certain executive officers under the 2014 Plan with a grant date fair value of $20.59 per share. The restricted stock units vest in three equal installments annually from the date of the grant. As a result, the Company recognizes compensation expense associated with these restricted stock units ratably over the three-year vesting period following the grant date. For the year ended December 31, 2021, the Company recognized $592 of stock compensation expense, with $166 recorded in R&D expense and $426 in G&A expense. G&A amounts recorded in the period included $75 of stock compensation expense relating to the modification of restricted stock units on November 1, 2021 (see Stock Award Modifications Pursuant to the Company’s non-employee director compensation policy, an aggregate of 36,000 restricted stock units were granted to non-employee directors on June 4, 2020, the date of the Company’s 2020 Annual Meeting of Stockholders, under the 2014 Plan with a grant date fair value of $15.62 per share. The restricted stock units fully vested on June 3, 2021. As a result, the Company has recognized compensation expense associated with these restricted stock units ratably over the one-year vesting period following the grant date. For the years ended December 31, 2021 and 2020, $239 and $323, respectively, of stock compensation expense relating to these restricted stock units was recognized in G&A expense. All of the 36,000 restricted stock units vested and were settled in shares of the Company’s common stock as of December 31, 2021. In February 2020, the Compensation Committee of the Company’s Board of Directors approved and granted a total of 138,000 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $16.36 per share. Vesting of the restricted stock units is contingent on the achievement of certain performance targets related to clinical and regulatory milestones, subject to the recipient’s continuous service through each performance target. Recognition of compensation expense associated with these awards begins when, and to the extent, the performance criteria is probable of achievement and the employee has met the service conditions. In February and March 2021, performance targets relating to 36,750 and 40,000 restricted stock units, respectively, had been achieved and thus restricted stock units vested and the awards were settled in shares of common stock. For the year ended December 31, 2021, the Company recognized $1,256 of stock compensation expense relating to the vesting of these restricted stock units, with $524 recorded in R&D expense and $732 in G&A expense. As a result of the achievement of a performance target relating to its NDA filing in December 2020, the Company recognized $601 of stock compensation expense, with $196 recorded in R&D expense and $405 in G&A expense for the year ended December 31, 2020. As of December 31, 2021, 113,500 of the 138,000 restricted stock units vested and were settled in shares of the Company’s common stock. Additionally in February 2020, the Compensation Committee of the Company’s Board of Directors also approved and granted a total of 98,000 time-based restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $16.36 per share. The restricted stock units vest in three equal installments annually from the date of the grant. As a result, the Company recognizes compensation expense associated with these restricted stock units ratably over the three-year vesting period following the grant date. In February 2021, 32,669 of these restricted stock units vested and were settled in shares of the Company’s common stock in satisfaction of the first year of vesting. For the year ended December 31, 2021, the Company recognized $607 of stock compensation expense, with $174 recorded in R&D expense and $433 recorded in G&A expense. G&A amounts for 2021 included $73 of stock compensation expense relating to the modification of restricted stock units on November 1, 2021 (see Stock Award Modifications Pursuant to the terms of the Company’s non-employee director compensation policy, an aggregate of 24,000 restricted stock units were granted to non-employee directors on June 4, 2019, the date of the Company’s 2019 Annual Meeting of Stockholders, under the 2014 Plan with a grant date fair value of $20.47 per share. As a result, the Company recognized compensation expense associated with these restricted stock units ratably over the one-year vesting period following the grant date and were fully vested in June 2020. For the years ended December 31, 2020 and 2019, the Company recognized $205 and $287, respectively, of stock compensation expense relating to these restricted stock units in G&A expense. As of December 31, 2021, all of the 24,000 restricted stock units vested and were settled in shares of the Company’s common stock. In March 2019, the Compensation Committee of the Company’s Board of Directors approved and granted a total of 215,000 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $16.10 per share. Vesting of the restricted stock units was contingent on the achievement of certain performance targets related to clinical milestones, subject to the recipient’s continuous service through the vesting events. Recognition of compensation expense associated with these awards begins when, and to the extent, the performance criteria is probable of achievement and the employee has met the service conditions. In April and June 2020, performance targets relating to 65,834 and 30,000 restricted stock units, respectively, had been achieved and thus such restricted stock units vested, and the awards were settled in shares common shares common A summary of restricted stock unit activity related to employees and non-employee members of the Company’s Board of Directors as of and for the year ended December 31, 2021 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ Number of ​ Average Grant ​ Units Date Fair Value Outstanding, December 31, 2020 235,250 ​ $ 16.25 Awarded 530,715 ​ 17.95 Vested and released (189,421) ​ 17.20 Outstanding, December 31, 2021 576,544 ​ $ 17.50 Restricted stock units exercisable (vested and deferred), December 31, 2021 — ​ ​ ​ ​ Stock Options A summary of the Company’s stock option activity related to employees, non-employee members of the Board of Directors and non-employee consultants for the 2019 Plan and the 2014 Plan as of and for the year ended December 31, 2021 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ Number of ​ Average Exercise ​ Intrinsic ​ Shares Price Value Outstanding, December 31, 2020 5,469,393 ​ $ 15.02 ​ Granted 1,422,750 ​ 17.82 ​ Exercised (136,787) ​ 11.98 ​ Forfeited (239,457) ​ 18.01 ​ Expired (3,619) ​ 24.93 ​ Outstanding, December 31, 2021 6,512,280 ​ $ 15.58 ​ $ 3,537 Weighted average remaining contractual life as of December 31, 2021 (in years) 6.88 ​ ​ Options exercisable, December 31, 2021 4,120,917 ​ $ 14.73 ​ $ 3,525 Weighted average remaining contractual life as of December 31, 2021 (in years) 5.79 ​ ​ Options vested and expected to vest as of December 31, 2021 6,449,787 ​ $ 15.39 ​ $ 3,537 Weighted average remaining contractual life as of December 31, 2021 (in years) 6.06 ​ ​ ​ The total fair value of options vested during the years ended December 31, 2021, 2020 and 2019 was $12,844, $12,819, and $10,074, respectively. The intrinsic value of options exercised during the years ended December 31, 2021, 2020 and 2019 was $965, $152, and $5,741, respectively. During the years ended December 31, 2021, 2020 and 2019, the Company granted 1,422,750, 1,377,850 and 1,324,000 stock options, respectively, to employees and non-employee members of the Board of Directors. There were no options granted to nonemployee consultants during the years ended December 31, 2021, 2020 and 2019. The fair values of the stock options granted to those groups were estimated using the Black-Scholes option valuation model with the following ranges of assumptions (see Note 2, Summary of Significant Accounting Policies – Stock-Based Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 2019 Risk-free interest rate 0.66% - 1.41 % 0.35% - 1.57 % 1.55% - 2.62 % Expected volatility 71.6% - 83.5 % 71.8% - 74.8 % 71.1% - 75.2 % Expected dividend yield 0 % 0 % 0 % Expected life of employee and Board options (in years) 6.25 6.25 6.25 ​ ​ The weighted average grant date fair value of options granted to employees and non-employee members of the Board of Directors for their Board service during the years ended December 31, 2021, 2020 and 2019 was $12.00, $10.33, and $11.67, respectively. During the years ended December 31, 2021, 2020 and 2019, the Company recognized compensation expense relating to stock options as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Research and development ​ $ 7,126 ​ $ 6,765 ​ $ 5,206 General and administrative ​ 9,569 ​ 4,943 ​ 5,094 Total stock option expense ​ $ 16,695 ​ $ 11,708 ​ $ 10,300 ​ The following were excluded from the table above as they are not related to stock options: compensation expense for i) the vesting of executives’ restricted stock units for $1,201, $1,432 and $604 in R&D expense for the years ended December 31, 2021, 2020 and 2019, respectively, and $2,324, $1,167 and $1,180 in G&A expense for the years ended December 31, 2021, 2020 and 2019, respectively; ii) compensation expense relating to the Board of Directors’ restricted stock units for $565, $528 and $287 in G&A expense for the years ended December 31, 2021, 2020 and 2019, respectively; and iii) the issuance of common stock relating to the consulting agreement for $197 in G&A expense for the year ended December 31, 2019. As of December 31, 2021, the total compensation expense relating to unvested options granted to employees and non-employee members of the Board of Directors that had not yet been recognized was $24,418, which is expected to be realized over a weighted average period of 2.58 years. The Company will issue shares upon exercise of options from common stock reserved. The Company does not expect to realize any tax benefits from its stock option activity or the recognition of stock-based compensation expense because the Company currently has net operating losses and has a full valuation allowance against its deferred tax assets. Accordingly, no amounts related to excess tax benefits have been reported in cash flows from operations or cash flows from financing activities for the years ended December 31, 2021, 2020 and 2019. Stock Award Modifications In November 2021, the Company and the former President and CEO mutually agreed to a transition from CEO to a consulting role through June 30, 2022, if not terminated earlier per the terms of the consulting agreement. As a result, the Company modified the terms of its former CEO’s outstanding Stock Awards to (1) automatically vest any unvested stock options or time-based restricted stock units that would have vested in the twelve month period following the end of the consulting period if continuous service is achieved with the Company during such twelve-month period; (2) extend the period during which the vested stock options may be exercised through the earlier of (i) eighteen months following the separation date (November 8, 2021); or (ii) the original expiration date applicable to each of the stock options, unless terminated earlier in accordance with the 2014 Plan, if continuous service is achieved with the Company; and (3) extend the period in which performance-based vesting milestones for restricted stock units may be achieved through March 31, 2022, if continuous service is achieved with the Company. ​ The Company determined that vested Stock Awards which had modifications due to the extension of the exercise period were Type 1 modifications pursuant to ASC 718 because those Stock Awards would have vested before and after the modification. Acceleration of vesting for the Stock Awards that would have vested in the twelve-month period following the consulting term was determined to be a Type 3 modification requiring stock compensation expense pursuant to ASC 718 because absent the modification terms, those Stock Awards would have been forfeited as of the last day that the former CEO provided continuous service as a consultant. Performance-based restricted stock units were not considered probable of achieving performance targets on the modification date, or as of December 31, 2021, which resulted in no stock compensation expense being recorded on these performance-based restricted stock units. ​ During the year ended December 31, 2021, total incremental stock compensation expense relating to modifications of stock options and time-based restricted stock units of the former CEO were $5,056, which is included in G&A expense. Of that total amount, $4,908 is included in G&A expense in the stock option compensation expense table above.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes The Company’s benefit from income taxes is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 2019 Current: ​ ​ ​ Federal ​ $ — ​ $ — ​ $ — State ​ — ​ (691) ​ (816) ​ ​ — ​ (691) ​ (816) Deferred: ​ ​ ​ Federal ​ — ​ — ​ — State ​ — ​ — ​ — ​ ​ — ​ — ​ — Benefit from income taxes ​ $ — ​ $ (691) ​ $ (816) ​ The Company’s tax benefits relate to state R&D tax credits exchanged for cash. The State of Connecticut provides companies with the opportunity to exchange certain R&D credit carryforwards for cash in exchange for foregoing the carryforward of the R&D credit. The program provides for such exchange of the R&D credits at a rate of 65% of the annual R&D credit, as defined. Because the Company’s revenue in 2020 exceeded $70,000, it is not eligible to exchange its 2021 R&D tax credits for cash, therefore there was no benefit from income taxes for the year ended December 31, 2021. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 2019 Income taxes using U.S. federal statutory rate 21.00 % 21.00 % 21.00 % State income taxes, net of federal benefit 7.84 % (58.68) % (1.99) % Tax Cuts and Jobs Act 0.00 % 0.00 % 0.00 % Impact of R&D tax credit on effective tax rate 2.87 % (52.06) % 4.34 % Stock option shortfalls and cancellations (4.74) % 0.35 % (0.17) % Permanent items and other (1.29) % (5.08) % 0.36 % Change in valuation allowance (25.69) % 84.61 % (22.76) % Provision to return 0.01 % 0.92 % (0.02) % Non-taxable revenue 0.00 % 0.00 % 0.00 % ​ 0.00 % (8.94) % 0.76 % ​ Significant components of the Company’s deferred tax assets and liabilities are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 Valuation allowance $ (143,388) $ (120,666) ​ ​ ​ ​ ​ ​ ​ Net operating loss carryforwards ​ ​ 113,636 ​ ​ 93,507 Federal and state tax credits ​ 24,428 ​ 19,982 Deferred revenue ​ — ​ — Stock-based compensation expense ​ 5,023 ​ 6,732 Other ​ 1,268 ​ 2,414 Deferred tax assets ​ 144,355 ​ 122,635 ​ ​ ​ ​ ​ ​ ​ Other ​ (967) ​ (1,969) Deferred tax liabilities: ​ (967) ​ (1,969) ​ ​ ​ ​ ​ Net deferred tax asset: ​ $ — ​ $ — ​ A 100% valuation allowance has been recorded on the deferred tax asset as of December 31, 2021 and 2020 because management believes it is more likely than not that the asset will not be realized. The change in the valuation allowance during 2021 and 2020 was an increase of $22,722 and $6,530, respectively. The financial statements reflect expected future tax consequences of such positions presuming the taxing authorities possess full knowledge of the position and all relevant facts. As of December 31, 2021 and 2020, the Company had no unrecognized tax benefits or related interest and penalties accrued. In the event the Company determines that accrual of interest or penalties are necessary in the future, the amount will be presented as a component of income tax expense. At December 31, 2021, the Company had federal and state net operating loss, or NOL, carryforwards of $443,134 and $347,313, respectively. The federal and state tax loss carryforwards will begin to expire in 2026 and 2027, respectively, unless previously utilized. The federal NOLs arising in 2018 and forward have an unlimited carryforward period and losses from 2018-2020 may be carried back five years due to the Coronavirus Aid, Relief, and Economic Security Act of 2020, or the CARES Act. The Company conducted a 382 analysis in the first quarter of 2021. This analysis showed a limited change of ownership had occurred, and thus the full amount of the Company’s NOL carryforwards and R&D tax credits could be utilized annually in the future to offset taxable income or tax, respectively. The Company also had federal and state R&D tax credit carryforwards of $19,864 and $5,516, respectively. The federal credits will begin expiring in 2025 unless previously utilized. The Connecticut credit carryforwards have no expiration period. Because of the NOL and research credit carryforwards, tax years 2006 through 2021 remain open to U.S. federal and state tax examinations. In March 2020, former President Trump signed into law the CARES Act (H.R. 748), which was further expanded with the signing of the Consolidation Appropriations Act of 2021 (H.R. 133) on December 27, 2020. The CARES Act (and December expansion) includes a variety of economic and tax relief measures intended to stimulate the economy, including loans for small businesses, payroll tax credits/deferrals, and corporate income tax relief. Due to the Company’s history of tax loss carryforwards and full valuation allowance, the CARES Act did not have a significant effect to the income tax provision, as the corporate income tax relief was directed towards cash taxpayers.

Net (Loss) Income Per Share

Net (Loss) Income Per Share	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Net (Loss) Income Per Share	16. Net (Loss) Income per Share The Company computes net (loss) income per share in accordance with ASC 260-10, Earnings per Share Summary of Significant Accounting Policies – (Loss) Income per Share The denominators used in the net (loss) income per share computations are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Basic: Weighted average common shares outstanding 50,718,765 47,413,250 42,669,333 Diluted: Weighted average common shares outstanding - Basic 50,718,765 47,413,250 42,669,333 Common stock equivalents* — 501,780 — Denominator for diluted net (loss) income per share 50,718,765 47,915,030 42,669,333 * For the year end December 31, 2020, common stock equivalents include dilutive stock options and restricted stock units. For the years ended December 31, 2021 and 2019, no amounts were considered as their effects would have been anti-dilutive due to net losses for those periods. Basic and diluted net (loss) income per share is computed as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Net (loss) income - basic and diluted ​ $ (88,441) ​ $ 8,410 ​ $ (106,373) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ 50,718,765 ​ 47,413,250 ​ 42,669,333 Effect of dilutive securities: ​ ​ ​ ​ ​ ​ Stock options ​ ​ — ​ ​ 481,254 ​ ​ — Restricted stock units ​ ​ — ​ ​ 20,526 ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 50,718,765 ​ ​ 47,915,030 ​ ​ 42,669,333 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net (loss) income per share: ​ ​ ​ Basic ​ $ (1.74) ​ $ 0.18 ​ $ (2.49) Diluted ​ $ (1.74) ​ $ 0.18 ​ $ (2.49) ​ As of December 31, 2021, 6,512,280 stock options and 576,544 restricted stock units were outstanding, which could potentially dilute basic earnings per share in the future, but were not included in the computation of diluted net loss per share because to do so would have been anti-dilutive as a result of the net loss for the period. As of December 31, 2020, 5,469,393 stock options and 235,250 restricted stock units were outstanding, which could potentially dilute basic earnings per share in the future. 481,254 of these outstanding stock options and 20,526 of these restricted stock units were considered dilutive and included in the computation of diluted net income per share for the year ended December 31, 2020. As of December 31, 2019, 4,450,517 stock options and 119,834 restricted stock units were outstanding, which could potentially dilute basic earnings per share in the future, but were not included in the computation of diluted net loss per share because to do so would have been anti-dilutive as a result of the net loss for the period.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2021
Postemployment Benefits [Abstract]
Employee Benefit Plan	17. Employee Benefit Plan The Company’s defined contribution retirement plan complies with Section 401(k) of the Internal Revenue Code. All employees over the age of 21 are eligible to participate in the plan at the beginning of the next calendar month after three consecutive months of service . Employees are able to defer a portion of their pay into the plan on the first day of the quarter on or after the day all age and service requirements have been met. All eligible employees receive an employer contribution equal to 3% of their salary up to the annual IRS limit. During the years ended December 31, 2021, 2020 and 2019, employer contributions to the plan were $460 , $349 and $279 , respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	18. Commitments and Contingencies License Agreement with Enteris Biopharma, Inc. In August 2019, the Company entered into the Enteris License Agreement, pursuant to which Enteris granted to the Company a non-exclusive, royalty-bearing license, including the right to grant sublicenses, under certain proprietary technology and patent rights related to or covering formulations for oral delivery of peptide active pharmaceutical ingredients with functional excipients to enhance permeability and/or solubility, known as Enteris’s Peptelligence ® technology, to develop, manufacture and commercialize products using such technology worldwide, excluding Japan and South Korea. As consideration for the licensed rights under the Enteris License Agreement, the Company paid an upfront fee equal to $8,000 , consisting of $4,000 in cash and $4,000 in shares of the Company’s common stock pursuant to the Enteris Purchase Agreement (see Note 10, Stockholders’ Equity ). As a result, the Company recognized R&D expense of $8,000 related to the Enteris License Agreement during the year ended December 31, 2019. The Company is also obligated, pursuant to the Enteris License Agreement, to pay Enteris (1) milestone payments upon the achievement of certain development, regulatory and commercial milestones and (2) low-single digit royalty percentages on net sales of licensed products, subject to reductions in specified circumstances. Until the second anniversary of the entry into the Enteris License Agreement, the Company has the right, but not the obligation, to terminate the Royalty Buyout. The Company did not exercise its Royalty Buyout right and such right expired in August 2021. During the years ended December 31, 2021 and 2020, the Company paid $15,000 and $5,000 , respectively, to Enteris for milestones earned in relation to the Enteris License Agreement. As a result, the Company recognized $15,000 and $5,000 of R&D expense related to the Enteris License Agreement during the years ended December 31, 2021 and 2020, respectively. The Enteris License Agreement will expire on a country-by-country, licensed product-by-licensed product basis upon the later of (1) the expiration (or invalidation) of all valid claims in licensed patent rights that cover such product in such country, (2) the end of the calendar quarter in which generic competition (as defined in the Enteris License Agreement) occurs for such product in such country and (3) ten years from the first commercial sale of such product. Either party may terminate the Enteris License Agreement upon written notice if the other party has failed to remedy a material breach within 60 days (or 30 days in the case of a material breach of a payment obligation). Enteris may terminate the Enteris License Agreement upon 30 days ’ written notice to the Company if the Company or any of its affiliates formally challenge the validity of any licensed patent rights or assists a third party in doing so. The Company may terminate the Enteris License Agreement for any reason or no reason (a) prior to receipt of first regulatory approval for a licensed product in the United States for any indication upon 30 days ’ prior written notice to Enteris or (b) on or after receipt of first regulatory approval for a licensed product in the United States for any indication upon 60 days ’ prior written notice to Enteris. Manufacturing Agreement with Patheon UK Limited In July 2021, the Company entered into an API Commercial Supply Agreement with Polypeptide Laboratories S.A., or PPL, that defines each party’s responsibilities with respect to PPL’s manufacture and supply of the active pharmaceutical ingredient difelikefalin, or API, for the difelikefalin injection product candidate. Under the API Commercial Supply Agreement, PPL shall manufacture API at its facility for sale and supply to the Company, in the amounts as set forth in purchase orders to be provided by the Company. The Company will be required to purchase its requirements of API for each year of the term of the agreement, based on internal forecasts. The API Commercial Supply Agreement will continue until the fifth anniversary of the approval by the FDA of the new drug application for KORSUVA injection, unless the API Commercial Supply Agreement is earlier terminated, and will automatically be extended for successive five-year periods unless either party gives notice to the other party of its intention to terminate. In July 2019, the Company entered into a Master Manufacturing Services Agreement, or MSA, with Patheon UK Limited, or Patheon. The MSA governs the general terms under which Patheon, or one of its affiliates, will provide non-exclusive manufacturing services to the Company for the drug products specified by the Company from time to time. Pursuant to the MSA, the Company has agreed to order from Patheon at least a certain percentage of its commercial requirements for a product under a related Product Agreement. Each Product Agreement that the Company may enter into from time to time will be governed by the terms of the MSA, unless expressly modified in such Product Agreement. The MSA has an initial term ending December 31, 2023, and will automatically renew after the initial term for successive terms of two years each if there is a Product Agreement in effect, unless either party gives notice of its intention to terminate the MSA at least 18 months prior to the end of the then current term. Either party may terminate the MSA or a Product Agreement upon written notice if the other party (1) has failed to remedy a material breach within a specified time or (2) is declared insolvent or bankrupt, voluntarily files a petition of bankruptcy or assigns such agreement for the benefit of creditors. The Company may terminate a Product Agreement (a) upon 90 days’ prior written notice if any governmental agency takes any action that prevents the Company from selling the relevant product in the relevant territory, (b) upon six months’ prior written notice if it does not intend to order manufacturing services due to a product’s discontinuance in the market, or (c) upon 90 days’ prior written notice if it determines that the manufacture or supply of a product likely infringes third-party rights. Patheon may terminate the MSA or a Product Agreement (i) upon six months’ prior written notice if the Company assigns such agreement to an assignee that is unacceptable to Patheon for certain reasons, or (ii) upon 30 days’ prior written notice if, after the first year of commercial sales, the Company forecasts zero volume for 12 months. The MSA contains, among other provisions, customary representations and warranties by the parties, a grant to Patheon of certain limited license rights to the Company’s intellectual property in connection with Patheon’s performance of the services under the MSA, certain indemnification rights in favor of both parties, limitations of liability and customary confidentiality provisions. Also in July 2019, the Company entered into two related Product Agreements under the MSA, one with each of Patheon and Patheon Manufacturing Services LLC, or Patheon Greenville, to govern the terms and conditions of the manufacture of commercial supplies of difelikefalin injection, the Company’s lead product candidate. Pursuant to the Product Agreements, Patheon and Patheon Greenville will manufacture commercial supplies of difelikefalin injection at the Monza, Italy and Greenville, North Carolina manufacturing sites, respectively, from API supplied by the Company. Patheon and Patheon Greenville will be responsible for supplying the other required raw materials and packaging components and will also provide supportive manufacturing services such as quality control testing for raw materials, packaging components and finished product. Leases The Company’s Stamford Lease had an initial 7-year December 2023 In connection with the signing of the Stamford Lease, the Company entered into a standby letter of credit agreement which serves as a security deposit for the Premises. The standby letter of credit is automatically renewed annually through November 2023. This standby letter of credit is secured with restricted cash in a money market account (refer to Note 8, Restricted Cash In June 2020, the Company entered into an amendment to the Stamford Lease to add additional office space, or the Lease Amendment. The term of the Lease Amendment began when renovation of the additional space was completed and the Company took possession of the additional space in October 2020, or the Amendment Commencement Date, and ends on December 31, 2023. The Lease Amendment is also renewable for one five-year term (see Note 2 - Summary of Significant Accounting Policies – Leases The rent for the Lease Amendment is at market rate as of the signing of the Lease Amendment. The Lease Amendment requires monthly lease payments, including rent escalations, during the lease term. The Company began paying rent for the Lease Amendment on the Amendment Commencement Date. In October 2020, the Company recorded an operating lease liability of $1,934 for the Lease Amendment as the sum of the present value of the future minimum lease payments over the term for the new lease. The Company also recorded a corresponding ROU asset of $1,934, as no lease incentives were identified in the Lease Amendment. Under ASC 842, lease expenses on the Stamford lease and Lease Amendment are recognized on a straight-line basis over the lease term. As a result, $1,624, $1,116 and $937 of operating lease cost, or lease expense, was recognized for the years ended December 31, 2021, 2020 and 2019, respectively, consisting of $1,137 relating to R&D lease expense and $487 relating to G&A lease expense for the Stamford Lease and Lease Amendment in the 2021 period, $781 relating to R&D lease expense and $335 relating to G&A lease expense for the Stamford Lease and Lease Amendment in the 2020 period, and $656 relating to R&D lease expense and $281 relating to G&A lease expense for the Stamford Lease in the 2019 period. Other information related to the Stamford Lease and Lease Amendment was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 ​ Cash paid for amounts included in the measurement of lease liabilities: ​ ​ ​ ​ Operating cash outflows relating to operating leases ​ $ 1,921 ​ $ 1,403 ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ — ​ $ 1,934 ​ Remaining lease term - operating leases (years) ​ 2.0 ​ 3.0 ​ Discount rate - operating leases ​ ​ 7.0 % ​ 7.0 % ​ Future minimum lease payments under the non-cancellable operating leases for the Stamford lease and the Lease Amendment, as well as a reconciliation of these undiscounted cash flows to the operating lease liabilities as of December 31, 2021, were as follows: ​ ​ ​ ​ Year Ending December 31, ​ ​ 2022 $ 1,957 2023 ​ 1,991 Total future minimum lease payments, undiscounted ​ 3,948 Less imputed interest ​ (275) Total ​ $ 3,673 ​ ​ ​ ​ Operating lease liabilities reported as of December 31, 2021: ​ Operating lease liabilities - current ​ $ 1,755 Operating lease liabilities - non-current ​ 1,918 Total ​ $ 3,673 ​ ​ ​

Legal Matters

Legal Matters	12 Months Ended
	Dec. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Legal Matters	19. Legal Matters From time to time, the Company may become subject to arbitration, litigation or claims arising in the ordinary course of its business. The Company is not currently a party to any arbitration or legal proceeding that, if determined adversely to the Company, would have a material adverse effect on its business, operating results or financial condition. The results of any future claims or proceedings cannot be predicted with certainty, and regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2021
Related Party Transactions [Abstract]
Related Party Transactions	20. Related Party Transactions As of December 31, 2021, Vifor owned 7,396,770 , or 13.8% , of the Company’s common stock. Both Vifor and VFMCRP are considered related parties as of December 31, 2021 (see Note 12, Collaboration and Licensing Agreements ). Sales of KORSUVA injection to Vifor for $701 were included within Commercial supply revenue on the Company’s Statement of Comprehensive (Loss) Income for the year ended December 31, 2021. Sales of clinical compound to VFMCRP for $361 were included within Clinical compound revenue on the Company’s Statement of Comprehensive (Loss) Income for the year ended December 31, 2021.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	21. Subsequent Events On March 1, 2022, the Company filed a universal shelf registration statement, or the Shelf Registration Statement, which provides for aggregate offerings of up to $300,000 of common stock, preferred stock, debt securities, warrants or any combination thereof. The Shelf Registration Statement has not yet been declared effective by the Securities and Exchange Commission. The securities registered under the Shelf Registration Statement include $154,525 of unsold securities that had been registered under the Company’s previous Registration Statement on Form S-3 (File No. 333-230333) that was declared effective on April 4, 2019. The Company may offer additional securities under its Shelf Registration Statement, when declared effective, from time to time in response to market conditions or other circumstances if it believes such a plan of financing is in the best interests of its stockholders. On March 1, 2022, the Company entered into an open market sales agreement, or the Sales Agreement, with Jefferies LLC, as sales agent, pursuant to which it may, subject to the effectiveness of the Shelf Registration Statement, from time to time, issue and sell common stock with an aggregate value of up to $80,000 in an at-the-market offering. Jefferies is acting as sole sales agent for any sales made under the Sales Agreement for a 3% commission on gross proceeds. The common stock will be sold at prevailing market prices at the time of the sale, and, as a result, prices may vary. Unless otherwise terminated earlier, the Sales Agreement continues until all shares available under the Sales Agreement have been sold. ​ ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with generally-accepted accounting principles in the United States, or GAAP, requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, as of the date of the financial statements as well as the reported amounts of revenues and expenses during the reporting period. The more significant estimates include the fair value of marketable securities that are classified as level 2 of the fair value hierarchy, the amount and periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, related party accounts receivable, inventory valuation and related reserves, the determination of prepaid research and development, or R&D, clinical costs and accrued research projects, the amount of non-cash compensation costs related to share-based payments to employees and non-employees, the incremental borrowing rate used in lease calculations and the likelihood of realization of deferred tax assets. The ongoing COVID-19 pandemic has interrupted business operations across the globe. Estimates and assumptions about future events and their effects cannot be determined with certainty and therefore require the exercise of judgment. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the reported amounts of assets and liabilities or the disclosure of contingent assets and liabilities. These estimates, however, may change as new events occur and additional information is obtained, and are recognized in the financial statements as soon as they become known. Actual results could differ materially from the Company’s estimates and assumptions.
Concentration of Credit Risk and Suppliers	Concentration of Credit Risk Financial instruments, which potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents, marketable securities, and related party accounts receivable. The Company invests its cash reserves in money market funds or high-quality marketable securities in accordance with its investment policy. The stated objectives of its investment policy are to preserve capital, provide liquidity consistent with forecasted cash flow requirements, maintain appropriate diversification and generate returns relative to these investment objectives and prevailing market conditions. The Company’s investment policy includes guidelines on acceptable investment securities, limits interest-bearing security investments to certain types of debt and money market instruments issued by the U.S. government and institutions with investment grade credit ratings and places restrictions on maturities and concentration by asset class and issuer. The Company’s cash and cash equivalents and marketable securities are held by three major financial institutions. In accordance with the Company’s policies, the Company monitors exposure with its counterparties. The Company also maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The percentages of revenue recognized from license agreement partners of the Company in the years ended December 31, 2021, 2020 and 2019 are included in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Revenue ​ ​ Year Ended December 31, ​ 2021 2020 2019 License Agreement Partner: ​ VFMCRP 67 % 17 % 99 % Vifor 25 % 83 % — % ​ For the years ended December 31, 2021, 2020 and 2019, no additional license agreement partners or customers accounted for more than 10% of the Company’s revenue. Concentration of Suppliers The Company relies on three suppliers to manufacture KORSUVA injection active pharmaceutical ingredient, or API, finished drug product, and finished goods. If any of the Company’s suppliers were to limit or terminate production or otherwise fail to meet the quality or delivery requirements to satisfy the supply commitments, the process of locating and qualifying alternate sources would require up to two years , during which time production could be delayed. Such delays could have a material adverse effect on the Company’s business, financial position, and results of operations.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash on hand, demand deposits, deposits with banks and highly liquid money market funds with holdings of cash and other investments with original maturities of three months or less.
Marketable Securities	Marketable Securities On January 1, 2020, the Company adopted Accounting Standards Update, or ASU, No. 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments The Company deems certain of its investments to be marketable securities if the investment, or in the case of money market funds, the securities underlying the money market fund, meet the definition of a debt security in Accounting Standards Codification, or ASC, section 320-10-20. The Company considers its marketable securities to be available-for-sale, which are its only financial instruments that are within the scope of ASU 2016-13 as of December 31, 2021. The Company’s investments in marketable securities, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper, and municipal bonds are highly rated by Moody’s and S&P and have maturities primarily of less than one year but no longer than three years. Accordingly, credit risk associated with the Company’s available-for-sale debt security portfolio is mitigated. ASU 2016-13 modifies the prior other-than-temporary impairment model for available-for-sale debt securities by requiring (1) estimating expected credit losses (the portion of the amortized cost basis of a financial asset that the Company does not expect to collect) only when the fair value is below the amortized cost of the asset; (2) recording a credit loss without regard to the length of time a security has been in an unrealized loss position; (3) limiting the measurement of the credit loss to the difference between the security’s amortized cost basis and its fair value; and (4) presenting credit losses as an allowance rather than as a write-down, which will allow the Company to record reversals of credit losses in current period net income, a practice that was previously prohibited. ​ The Company reviews each of its available-for-sale marketable securities for unrealized losses (declines in fair value below its amortized cost basis) at each balance sheet date presented in its financial statements and whenever events or changes in circumstances indicate that the amortized cost basis of an asset may not be recoverable. In accordance with the adoption of ASU 2016-13, the Company is required to determine whether any portion of the unrealized loss for any available-for-sale debt security is due to a credit loss, and if so, to measure the amount of the credit loss. ​ The Company will rely on both qualitative and quantitative factors to determine whether the unrealized loss for each available-for-sale debt security at any balance sheet date is due to a credit loss. Qualitative factors may include a credit downgrade, severity of the decline in fair value below amortized cost and other adverse conditions related specifically to the security, as well as the intent to sell the security, or whether the Company will more likely than not be required to sell the security before recovery of its amortized cost basis. The Company’s assessment of whether a security is impaired could change in the future due to new developments or changes in assumptions related to any particular security. If material qualitative factors indicate that a credit loss has occurred, the Company will determine the magnitude of that credit loss using a discounted cash flow model or other quantitative method. If the Company intends to sell the security or it is more likely than not that the Company will be forced to sell the security before recovery of the amortized cost of the security, the entire unrealized loss is deemed to be a credit loss, which is recognized in net (loss) income. Otherwise, the portion of the unrealized loss that is due to a credit loss will be recorded as an allowance for credit loss, which will offset the balance of marketable securities and as credit loss expense within other income, net. The portion of the unrealized loss that is not due to a credit loss as well as all unrealized gains will be recorded in Accumulated Other Comprehensive Income (Loss). There was no cumulative effect adjustment as a result of the adoption of ASU 2016-13 on January 1, 2020 (see Note 3, Available-for-Sale Marketable Securities Fair Value Measurements) ​ Accrued interest receivables are excluded from the Company’s amortized cost bases for its available-for-sale marketable securities and are included within Other receivables. The Company’s policy is to not measure an allowance for credit losses on accrued interest receivable balances at each reporting period since it elects to write off uncollectible accrued interest receivable balances as credit loss expense in a timely manner, which is by maturity date for all categories of its debt securities.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company applies fair value accounting for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. The Company defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities which are required to be recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions and credit risks. The Company’s financial instruments consist of cash, cash equivalents, available-for-sale marketable securities, prepaid expenses, restricted cash, accounts payable and accrued liabilities. The fair values of cash, cash equivalents, prepaid expenses, restricted cash, accounts payable and accrued liabilities approximate their carrying values due to the short-term nature of these financial instruments. Available-for-sale marketable securities are reported as marketable securities at their fair values, based upon pricing of securities with the same or similar investment characteristics as provided by third-party pricing services, as described below. The valuation techniques used by the Company are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. The Company classifies its investments in a fair value hierarchy that is intended to increase consistency and comparability in fair value measurements and related disclosures. The fair value hierarchy is divided into three levels based on the source of inputs as follows: ● Level 1 – Observable inputs – quoted prices in active markets for identical assets and liabilities. ● Level 2 – Observable inputs other than the quoted prices in active markets for identical assets and liabilities – such as quoted prices for similar instruments, quoted prices for identical or similar instruments in inactive markets, or other inputs that are observable or can be corroborated by observable market data. ● Level 3 – Unobservable inputs – includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions. The Company records transfers between levels in the hierarchy by assuming that the transfer occurred at the end of the quarter or year-to-date period. Valuation Techniques - Level 2 Inputs The Company estimates the fair values of its financial instruments categorized as level 2 in the fair value hierarchy, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper and municipal bonds, by taking into consideration valuations obtained from third-party pricing services. The pricing services use industry standard valuation models, including both income- and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, benchmark yields, issuer credit spreads, benchmark securities, and other observable inputs. The Company obtains a single price for each financial instrument and does not adjust the prices obtained from the pricing service. The Company validates the prices provided by its third-party pricing services by reviewing their pricing methods, obtaining market values from other pricing sources and comparing them to the share prices presented by the third-party pricing services. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by its pricing services as of December 31, 2021 or December 31, 2020.
Inventory, net	Inventory, net Inventories are stated at the lower of cost or net realizable value. The Company determines the cost of inventory using first-in, first-out, or FIFO, method. The Company capitalizes inventory costs associated with the Company’s products prior to regulatory approval, when, based on management’s judgment, future commercialization is considered probable and the future economic benefit is expected to be realized; otherwise, such costs are expensed. The determination to capitalize inventory costs is based on various factors, including status and expectations of the regulatory approval process, any known safety or efficacy concerns, potential labeling restrictions, and any other impediments to obtaining regulatory approval. As KORSUVA injection is the Company’s first commercial product and probability could not be established prior to regulatory approval on August 23, 2021, all inventory costs prior to regulatory approval were expensed as incurred. The Company periodically analyzes its inventory levels to identify inventory that may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and writes down such inventories as appropriate. In addition, the Company’s products are subject to strict quality control and monitoring which the Company performs throughout the manufacturing process. If certain batches or units of product no longer meet quality specifications or become obsolete due to expiration, the Company records a charge to write down such unmarketable inventory to its estimated realizable value.
Property and Equipment, net	Property and Equipment, net Property and equipment (consisting of computer and office equipment, furniture and fixtures and leasehold improvements) are stated at cost, net of accumulated depreciation and amortization of leasehold improvements. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets. Leasehold improvements are amortized over the lesser of their useful lives or the life of the lease. ​ ​ ​ Asset Category Useful Lives Computer and office equipment 5 years Furniture and fixtures 7 years Leasehold improvements lesser of useful life of asset or life of lease (Stamford - 7 years) ​ The Company reviews the recorded values of property and equipment for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset or group of assets may not be fully recoverable.
Revenue Recognition	Revenue Recognition Under ASU 2014-09, Revenue from Contracts with Customers (Topic 606) The Company has entered into agreements to license its intellectual property, or IP, related to difelikefalin to develop, manufacture and/or commercialize drug products. These agreements typically contain multiple performance obligations, including licenses of IP and R&D services. Payments to the Company under these agreements may include nonrefundable license fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company receives its share of the net profits from the sale of KORSUVA injection in the U.S. through its license agreement with Vifor. The Company has adopted a policy to recognize revenue net of tax withholdings, as applicable. The Company identifies agreements as contracts that create enforceable rights and obligations when the agreement is approved by the parties, identifies the rights of the parties and the payment terms, has commercial substance and it is probable that the Company will collect the consideration to which it will be entitled in exchange for the goods and services that will be transferred to the customer. The counterparty is considered to be a customer when it has contracted with the Company to obtain goods and services that are the output of the Company’s ordinary activities (i.e., development of pharmaceutical products) in exchange for consideration. A performance obligation is a promise to transfer distinct goods or services to a customer. Performance obligations that are both capable of being distinct and distinct within the context of the contract are considered to be separate performance obligations. Performance obligations are capable of being distinct if the counterparty is able to benefit from the good or service on its own or together with other resources that are readily available to it. Performance obligations are distinct within the context of the contract when each performance obligation is separately identifiable from each other; i.e., the Company is not using the goods or services as inputs to produce or deliver the combined output or outputs specified by the customer; one or more of the goods or services does not significantly modify or customize one of the other goods or services in the contract; and goods or services are not highly interdependent or not highly interrelated. Performance obligations that are not distinct are accounted for as a single performance obligation over the period that goods or services are transferred to the customer. The determination of whether performance obligations in a contract are distinct may require significant judgment. The transaction price is the amount of consideration that the Company expects to be entitled to in exchange for transferring promised goods or services to the customer based on the contract terms at inception of a contract. There is a constraint on inclusion of variable consideration related to licenses of IP, such as milestone payments or sales-based royalty payments, in the transaction price if there is uncertainty at inception of the contract as to whether such consideration will be recognized in the future because it is probable that there will be a significant reversal of revenue in the future when the uncertainty is resolved. The determination of whether or not it is probable that a significant reversal of revenue will occur in the future depends on the likelihood and magnitude of the reversal. Factors that could increase the likelihood or magnitude of a reversal of revenue include (a) the susceptibility of the amount of consideration to factors outside the entity’s influence, such as the outcome of clinical trials, the timing of initiation of clinical trials by the counterparty and the approval of drug product candidates by regulatory agencies, (b) situations in which the uncertainty is not expected to be resolved for a long period of time, and (c) level of the Company’s experience in the field. When it becomes probable that events will occur, for which variable consideration was constrained at inception of the contract, the Company allocates the related consideration to the separate performance obligations in the same manner as described below. At inception of a contract, the Company allocates the transaction price to the distinct performance obligations based upon their relative standalone selling prices. Standalone selling price is the price at which an entity would sell a promised good or service separately to a customer. The best evidence of standalone selling price is an observable price of a good or service when sold separately by an entity in similar circumstances to similar customers. Since the Company typically does not have such evidence, it estimates standalone selling price so that the amount that is allocated to each performance obligation equals the amount that the Company expects to receive for transferring goods or services. The methods that the Company uses to make such estimates include (1) the adjusted market assessment approach, under which the Company forecasts and analyzes difelikefalin in the appropriate market, the phase of clinical development as well as considering recent similar license arrangements within the same phase of clinical development, therapeutic area, type of agreement, etc. and (2) the expected cost of satisfying the performance obligations plus a margin, or the expected cost plus a margin approach. The Company recognizes revenue when, or as, it satisfies a performance obligation by transferring a promised good or service to a customer and the customer obtains control of the good or service. Revenue related to the grant of a license that is a distinct performance obligation and that is deemed to be functional IP is recognized at the point in time that the Company has the right to payment for the license, the customer has legal title to the license and can direct the use of the license (for example, to grant sublicenses), the customer has the significant risks and rewards of ownership of the license and the customer has accepted the asset (license) by signing the license agreement. Recognition of revenue related to R&D services that are a distinct performance obligation or that are combined with granting of a license as a single performance obligation is deferred at inception of a contract and is recognized as those services are performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. Milestone payments are considered to be variable consideration and are not included in the transaction price at inception of the contract if it is uncertain that the milestone will be achieved. Rather, when it becomes probable that the milestone will be achieved and, therefore, there will not be a significant reversal of revenue in future periods, the respective amount to be earned is included in the transaction price, allocated to the distinct performance obligations based on their relative standalone selling price and recognized as revenue, as described above. Sales milestones and sales-based royalty payments related to a license of IP are recognized as revenue when the respective sales occur. License and Milestone Fees License and milestone fees include upfront and milestone payments associated with the Company’s license agreements with Vifor, VFMCRP, Maruishi and CKDP. All upfront and milestone payments associated with the license agreements with Vifor, VFMCRP and CKDP are recognized as license and milestone fees since they contain only one performance obligation. Upfront and milestone payments associated with the license agreement with Maruishi are allocated between License and milestone fees and Collaborative revenue based on the relative standalone selling prices determined at contract inception (see Note 13, Revenue Recognition Collaborative Revenue Collaborative revenue includes milestone payments associated with the Company’s license agreement with Maruishi that were allocated to the R&D services performance obligation (see Note 13, Revenue Recognition Commercial Supply Revenue Commercial supply revenue includes sales of KORSUVA injection commercial product to Vifor, which ultimately will act as the principal in the net profit-sharing arrangement between the two parties upon expected commercial launch in April 2022, which then sells to third parties in the U.S. Commercial supply revenue is recognized when Vifor obtains control of the Company’s commercial product, which occurs at a point in time, typically upon receipt of KORSUVA injection by Vifor, and generally occurs after the commercial product has passed all quality testing required for acceptance by Vifor. The Company calculates its commercial supply revenue based on its cost of goods sold, or COGS, plus an agreed upon margin. Clinical Compound Revenue Clinical compound revenue includes sales of clinical compound to Vifor (prior to FDA approval), VFMCRP, and Maruishi for the years ended December 31, 2021, 2020 and 2019. The Company recognizes revenue on clinical compound sales when control has transferred to Vifor, VFMCRP and Maruishi, which occurs at a point in time, typically upon receipt of the clinical compound, and generally occurs after the clinical compound has passed all quality testing required for acceptance. The sales of clinical compound are reimbursed at COGS plus an agreed upon margin.
Cost of Goods Sold (COGS)	Cost of Goods Sold (COGS) COGS includes costs related to sales of the Company’s commercial product, KORSUVA injection, to Vifor. Costs related to the sales of KORSUVA injection are generally recognized upon receipt of shipment by Vifor. The Company’s COGS for KORSUVA injection includes the cost of producing commercial product that correspond with commercial supply revenue, such as third-party supply and overhead costs, as well as certain period costs related to freight, packaging, stability, and quality testing. For the year ended December 31, 2021, no COGS was recognized as all inventory costs prior to regulatory approval on August 23, 2021 were expensed as incurred.
Research and Development (R&D) Expenses	Research and Development (R&D) Expenses R&D costs are charged to expense as incurred. Costs incurred under agreements with third parties are charged to expense as incurred in accordance with the specific contractual performance terms of such agreements. R&D expenses include, among other costs, compensation and other personnel-related costs, including consultant costs, and costs to conduct clinical trials using clinical research organizations, or CROs, which include upfront, milestone and monthly expenses as well as reimbursement for pass through costs. The amount of clinical trial expense recognized in any period varies depending on the duration and progress of each clinical trial, including the required level of patient enrollment, the rate at which patients actually enroll in and drop-out of the clinical trial, and the number of sites involved in the trial as well as the activities to be performed by the sites each period. R&D costs also include costs to manufacture product candidates and clinical supplies, laboratory supplies costs, facility-related costs and stock-based compensation for R&D personnel. Non-refundable R&D advance payments are deferred and capitalized as prepaid R&D expense. The capitalized amounts are expensed as the related goods are delivered or services are performed. As of December 31, 2021 and 2020, the Company recorded $1,481 and $11,286 as prepaid R&D expense, respectively.
General and Administrative (G&A) Expenses	General and Administrative (G&A) Expenses G&A costs are charged to expense as incurred. G&A expenses consist primarily of salaries and other related costs, including stock-based compensation, for personnel in executive, finance, accounting, legal, business development, information technology and human resources functions. Other costs include facility costs not otherwise included in R&D expenses, legal fees, insurance costs, investor relations costs, patent costs and fees for accounting and consulting services. As noted in Note 12, Collaboration and Licensing Agreements
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Deferred income tax assets are reduced, as necessary, by a valuation allowance when management determines it is more likely than not that some or all of the tax benefits will not be realized. There were no material uncertain tax positions taken as of December 31, 2021 and 2020. The Company does not have any interest or penalties accrued related to tax positions as it does not have any unrecognized tax benefits. In the event the Company determines that accrual of interest or penalties are necessary in the future, the amount will be presented as a component of interest expense.
Stock-Based Compensation	Stock-Based Compensation The Company grants stock options to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as compensation for services performed. All share-based payments, including grants of stock options, are recognized based on their grant date fair values. The grant date fair value of stock options is estimated using the Black-Scholes option valuation model. Using this model, fair value is calculated based on assumptions with respect to (i) the fair value or market price of the Company’s common stock on the grant date; (ii) expected volatility of the Company’s common stock price, (iii) the periods of time over which employees and members of the Company’s Board of Directors or non-employee consultants are expected to hold their options prior to exercise (expected term), (iv) expected dividend yield on the Company’s common stock, and (v) risk-free interest rates. The Company’s common stock has been traded on a public exchange only since January 31, 2014. Since that time, exercises of stock options have been limited due to various factors, including fluctuations in the Company’s stock price to below the exercise prices of awards and blackout periods during which exercises are not allowed, among others. Therefore, the Company believes that as of December 31, 2021, it does not have sufficient company-specific information available to determine the expected term based on its historical data. As a result, the expected term of stock options granted is determined using the average of the vesting period and term (6.25 years), an accepted method for the Company’s option grants under the SEC’s Staff Accounting Bulletin No. 110, Share-Based Payment. The Company calculates the expected volatility using company-specific trading activity of its common stock over the option’s expected term. The expected dividend yield is zero as the Company has never paid dividends and does not currently anticipate paying any in the foreseeable future. Risk-free interest rates are based on quoted U.S. Treasury rates for securities with maturities approximating the option’s expected term. The Company’s policy is to account for forfeitures of share-based payments as they occur. Compensation cost for all share-based payments granted with service-based graded vesting schedules is recognized using the straight-line method over the requisite service period.
(Loss) Income Per Share	(Loss) Income Per Share The Company computes basic net (loss) income per share by dividing net (loss) income by the weighted average number of shares of common stock outstanding. Diluted net (loss) income per share includes the potential dilutive effect of common stock equivalents as if such securities were converted or exercised during the period, when the effect is dilutive. Common stock equivalents may include outstanding stock options and restricted stock units, which are included under the treasury stock method when dilutive. For each of the years ended December 31, 2021 and 2019, the Company excluded the effects of potentially dilutive shares that were outstanding during those respective periods from the denominator as their inclusion would have been anti-dilutive due to the Company’s net losses for those periods. For the year ended December 31, 2020, the Company included the effects of dilutive shares that were outstanding in the denominator as their inclusion was dilutive due to the Company’s net income for the period.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business as one operating segment, which includes all activities related to the discovery, development, and commercialization of novel therapeutics to treat serious medical conditions, including pruritus and pain.
Leases	Leases The Company has two leases, a lease agreement for office space in Stamford, Connecticut, or the Stamford Lease, and an amendment to the Stamford Lease to add additional office space, or the Lease Amendment, which is included in operating lease right-of-use assets, or ROU assets, operating lease liabilities – current and operating lease liabilities – non-current as of December 31, 2021 and 2020 (see Note 18, Commitments and Contingencies: Leases In general, the Company determines if a contract, at its inception, is a lease or contains a lease based on whether the contract conveys the right to control the use of identified property, plant, or equipment (an identified asset) for a period of time in exchange for consideration. To determine whether a contract conveys the right to control the use of an identified asset for a period of time, the Company assesses whether, throughout the period of use, it has both the right to obtain substantially all of the economic benefits from use of the identified asset, and the right to direct the use of the identified asset. Both of these criteria are met by the Stamford Lease. Under ASC 842, the Company determines the amount of the operating lease liability based on the present value of the future minimum lease payments over the remaining lease term. The amount of the operating lease ROU asset is equal to the amount of the lease liability, less accrued rent and lease incentives received from the landlord. Initial direct costs were deemed to be immaterial. Since the Stamford Lease does not provide an implicit interest rate, the Company used an annual incremental borrowing rate of 7% based on the information available at the date of adoption for the purpose of determining the lease liability during the term of the lease. As noted above, upon adoption of ASC 842, the Company used hindsight in determining the term of the Stamford Lease. Although the Stamford Lease is renewable for one five-year term, upon inception of the lease the renewal term was not included in the lease term since it was not reasonably certain that the Company will exercise that option. Accordingly, the lease term of the Stamford Lease was not adjusted upon adoption of ASC 842 to determine the operating lease ROU asset and operating lease liability. The Stamford Lease contains both a lease and non-lease component which are accounted for separately. The Company allocates the consideration to the lease and the non-lease component on a relative standalone price basis. Lease expense under ASC 842 is recognized on a straight-line basis over the lease term in the Statements of Comprehensive (Loss) Income. There was no cumulative effect adjustment as a result of the adoption of ASC 842 on January 1, 2019. In June 2020, the Company entered into the Lease Amendment. The term of the Lease Amendment began when renovation of the additional space was completed and the Company took possession of the additional space in October 2020, or the Amendment Commencement Date, and ends on December 31, 2023. The Lease Amendment is also renewable for one five-year term, although this renewable period is not included as part of the lease term as defined in ASC 842 since it is not reasonably certain that the Company will exercise that option. The Lease Amendment contains both a lease and non-lease component which are accounted for separately. The Company allocates the consideration to the lease and non-lease component on a relative standalone price basis. The rent for the Lease Amendment is at market rate as of the signing of the Lease Amendment. The Lease Amendment requires monthly lease payments, including rent escalations, during the lease term. The Company began paying rent for the Lease Amendment on the Amendment Commencement Date. The Company accounted for the terms and conditions of the Lease Amendment as a lease modification, as defined in ASC 842, because it grants an additional right-of-use to an underlying asset (the new additional space). Under ASC 842, a lease modification can result in either a new lease that is accounted for separately from the original lease or as a single modified lease. The Lease Amendment is accounted for separately from the original Stamford Lease because the Lease Amendment grants the right-of-use to additional space and the price of the additional right-of-use is commensurate with its standalone price as no discounts were provided to the Company. Furthermore, there were no material changes to the original Stamford Lease. As of the Amendment Commencement Date, the Company recorded the lease liability for the Lease Amendment as the sum of the present value of the future minimum lease payments over the term for the new lease. Since the Lease Amendment does not provide an implicit interest rate, the Company used an incremental borrowing rate of 7%, which is based on the rate that the Company could obtain in the market for a fully collateralized loan equal to the term of the Lease Amendment. The Company also recorded a ROU asset equal to the amount of the lease liability, as no lease incentives were identified in the Lease Amendment. During the term of the Lease Amendment, interest expense will be calculated using the effective interest method and the ROU asset will be amortized on a straight-line basis over the lease term, and both will be recorded as lease expense.
Litigation Reserves	Litigation Reserves From time to time, the Company may become subject to arbitration, litigation or claims arising in the ordinary course of its business. Accruals are recorded when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. The Company reviews these reserves at least quarterly and adjusts these reserves to reflect current law, progress of each case, opinions and views of legal counsel and other advisers, the Company’s experience in similar matters and intended response to the litigation. The Company expenses amounts for administering or litigating claims as incurred. Accruals for legal proceedings, if any, are included in accounts payable and accrued expenses.
Accounting Pronouncements Recently Adopted and Recent Accounting Pronouncements Not Yet Adopted	Accounting Pronouncements Recently Adopted In December 2019, the Financial Accounting Standards Board, or FASB, issued ASU No. 2019-12, Income Taxes (Topic 740) and it did not have a material effect on its results of operations, financial position, and cash flows due to the full valuation allowance recorded. ​ ​ ​ Recent Accounting Pronouncements Not Yet Adopted In October 2021, the FASB issued ASU 2021-08, Business Combinations (Topic 805)

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Schedule of percentages of revenue recognized from license agreement partners	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Revenue ​ ​ Year Ended December 31, ​ 2021 2020 2019 License Agreement Partner: ​ VFMCRP 67 % 17 % 99 % Vifor 25 % 83 % — %
Summary of Useful Lives of Property and Equipment	Property and equipment (consisting of computer and office equipment, furniture and fixtures and leasehold improvements) are stated at cost, net of accumulated depreciation and amortization of leasehold improvements. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets. Leasehold improvements are amortized over the lesser of their useful lives or the life of the lease. ​ ​ ​ Asset Category Useful Lives Computer and office equipment 5 years Furniture and fixtures 7 years Leasehold improvements lesser of useful life of asset or life of lease (Stamford - 7 years) ​

Available-for-Sale Marketable_2

Available-for-Sale Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2021
Investments Debt And Equity Securities [Abstract]
Summary of Available-for-Sale Marketable Securities by Major Type of Security	The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2021 and 2020: As of December 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 11,573 ​ $ — ​ $ (3) ​ $ 11,570 U.S. government agency obligations ​ 17,020 ​ — ​ (45) ​ 16,975 Corporate bonds ​ 66,495 ​ — ​ (171) ​ 66,324 Commercial paper ​ ​ 106,914 ​ ​ 5 ​ ​ (31) ​ ​ 106,888 Municipal bonds ​ 21,692 ​ — ​ (113) ​ 21,579 Total available-for-sale marketable securities ​ $ 223,694 ​ $ 5 ​ $ (363) ​ $ 223,336 ​ As of December 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 20,710 ​ $ 41 ​ $ (1) ​ $ 20,750 U.S. government agency obligations ​ 22,125 ​ 4 ​ (1) ​ 22,128 Corporate bonds ​ 49,080 ​ 61 ​ (23) ​ 49,118 Commercial paper ​ 116,139 ​ 5 ​ (17) ​ 116,127 Municipal bonds ​ ​ 11,680 ​ ​ 12 ​ ​ (8) ​ ​ 11,684 Total available-for-sale marketable securities ​ $ 219,734 ​ $ 123 ​ $ (50) ​ $ 219,807
Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities	The following tables summarize the fair value and gross unrealized losses of the Company’s available-for-sale marketable securities by investment category and disaggregated by the length of time that individual debt securities have been in a continuous unrealized loss position as of December 31, 2021 and 2020: As of December 31, 2021 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Less than 12 Months ​ 12 Months or Greater ​ Total ​ ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Value Losses Value Losses Value Losses U.S. Treasury securities ​ $ 11,570 ​ $ (3) ​ $ — ​ $ — ​ $ 11,570 ​ $ (3) U.S. government agency obligations ​ 9,456 ​ (45) ​ — ​ — ​ 9,456 ​ (45) Corporate bonds ​ ​ 62,704 ​ ​ (170) ​ ​ 2,020 ​ ​ (1) ​ ​ 64,724 ​ ​ (171) Commercial paper ​ ​ 52,163 ​ ​ (31) ​ ​ — ​ ​ — ​ ​ 52,163 ​ ​ (31) Municipal bonds ​ 20,562 ​ (105) ​ 1,017 ​ (8) ​ 21,579 ​ (113) Total ​ $ 156,455 ​ $ (354) ​ $ 3,037 ​ $ (9) ​ $ 159,492 ​ $ (363) ​ As of December 31, 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Less than 12 Months ​ 12 Months or Greater ​ Total ​ ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ ​ ​ Gross ​ ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Fair ​ Unrealized ​ Value Losses Value Losses Value Losses U.S. Treasury securities ​ $ 12,682 ​ $ (1) ​ $ — ​ $ — ​ $ 12,682 ​ $ (1) U.S. government agency obligations ​ ​ 2,500 ​ ​ (1) ​ ​ — ​ ​ — ​ ​ 2,500 ​ ​ (1) Corporate bonds ​ 23,553 ​ (23) ​ — ​ — ​ 23,553 ​ (23) Commercial paper ​ 68,897 ​ (17) ​ — ​ — ​ 68,897 ​ (17) Municipal bonds ​ 6,259 ​ (8) ​ — ​ — ​ 6,259 ​ (8) Total ​ $ 113,891 ​ $ (50) ​ $ — ​ $ — ​ $ 113,891 ​ $ (50)
Schedule of Amortized Cost and Fair Values of Marketable Debt Securities by Contractual Maturity	The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2021, the Company’s marketable debt securities mature at various dates through November 2024. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2021 ​ As of December 31, 2020 Contractual maturity Amortized Cost Fair Value Amortized Cost Fair Value Less than one year ​ $ 153,631 ​ $ 153,582 ​ $ 149,164 ​ $ 149,242 One year to three years ​ 70,063 ​ 69,754 ​ 70,570 ​ 70,565 Total ​ $ 223,694 ​ $ 223,336 ​ $ 219,734 ​ $ 219,807

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive (Loss) Income (Tables)	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Summary of Changes in Accumulated Other Comprehensive (Loss) Income, Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities	The following table summarizes the changes in accumulated other comprehensive (loss) income, net of tax, from unrealized gains (losses) on available-for-sale marketable securities, the Company’s only component of accumulated other comprehensive (loss) income, for the years ended December 31, 2021, 2020 and 2019. ​ ​ ​ ​ ​ Total Accumulated ​ ​ Other Comprehensive ​ ​ (Loss) Income Balance, December 31, 2018 ​ $ (114) Other comprehensive income before reclassifications ​ 284 Amount reclassified from accumulated other comprehensive loss ​ — Net current period other comprehensive income ​ 284 Balance, December 31, 2019 ​ 170 Other comprehensive income before reclassifications ​ 175 Amount reclassified from accumulated other comprehensive income ​ (272) Net current period other comprehensive loss ​ (97) Balance, December 31, 2020 ​ $ 73 Other comprehensive loss before reclassifications ​ (392) Amount reclassified from accumulated other comprehensive income ​ (39) Net current period other comprehensive loss ​ (431) Balance, December 31, 2021 ​ $ (358)
Schedule of Reclassification Out of Accumulated Other Comprehensive (Loss) Income	Amounts reclassified out of accumulated other comprehensive (loss) income into net (loss) income are determined by specific identification. The reclassifications out of accumulated other comprehensive (loss) income and into net (loss) income were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Affected Line Item in the Component of Accumulated Other ​ Year Ended December 31, ​ Statements of Comprehensive (Loss) Income 2021 2020 2019 Comprehensive (Loss) Income Unrealized gains (losses) on available-for-sale marketable securities: ​ ​ ​ Realized gains on sales of securities ​ $ 39 ​ $ 272 ​ $ — Other income, net Income tax effect ​ — ​ — ​ — Benefit from income taxes Realized gains on sales of securities, net of tax ​ $ 39 ​ $ 272 ​ $ — ​ ​

Inventory, net (Tables)

Inventory, net (Tables)	12 Months Ended
	Dec. 31, 2021
Inventory Disclosure [Abstract]
Schedule of Inventories	​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Raw materials ​ $ 927 ​ $ — Work-in-process ​ 1,657 ​ — Total ​ $ 2,584 ​ $ —

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, net	Property and equipment, net consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 Computer and office equipment ​ $ 239 ​ $ 211 Laboratory equipment ​ — ​ 628 Furniture and fixtures ​ 330 ​ 330 Leasehold improvements ​ 1,223 ​ 1,212 ​ ​ $ 1,792 ​ $ 2,381 Less accumulated depreciation and amortization ​ 1,161 ​ 1,541 Property and equipment, net ​ $ 631 ​ $ 840 ​

Restricted Cash (Tables)

Restricted Cash (Tables)	12 Months Ended
	Dec. 31, 2021
Restricted Cash [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Balance Sheets that sum to the total of the same such amounts shown in the Statements of Cash Flows. ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Cash and cash equivalents ​ $ 13,453 ​ $ 31,683 Restricted cash, long-term assets ​ 408 ​ 408 Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows ​ $ 13,861 ​ $ 32,091

Accounts Payable and Accrued _2

Accounts Payable and Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2021
Payables And Accruals [Abstract]
Schedule of Accounts Payable and Accrued Expenses	Accounts payable and accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 December 31, 2020 Accounts payable ​ $ 5,625 ​ $ 4,893 Accrued research projects ​ 4,648 ​ 6,194 Accrued compensation and benefits ​ 4,959 ​ 4,955 Accrued professional fees and other ​ 629 ​ 839 Total ​ $ 15,861 ​ $ 16,881

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis	The following tables summarize the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2021 and 2020 and by level within the fair value hierarchy: Fair value measurement as of December 31, 2021: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted prices in ​ Significant other ​ Significant Financial assets ​ ​ ​ ​ active markets for ​ observable ​ unobservable ​ ​ ​ ​ ​ identical assets ​ inputs ​ inputs Type of Instrument Total (Level 1) (Level 2) (Level 3) Cash and cash equivalents: ​ ​ ​ ​ Money market funds and checking accounts ​ $ 13,453 ​ $ 13,453 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ ​ ​ ​ U.S. Treasury securities ​ 11,570 ​ — ​ 11,570 ​ — U.S. government agency obligations ​ 16,975 ​ — ​ 16,975 ​ — Corporate bonds ​ 66,324 ​ — ​ 66,324 ​ — Commercial paper ​ 106,888 ​ — ​ 106,888 ​ — Municipal bonds ​ 21,579 ​ — ​ 21,579 ​ — Restricted cash: ​ ​ ​ ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 237,197 ​ $ 13,861 ​ $ 223,336 ​ $ — ​ Fair value measurement as of December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted prices in ​ Significant other ​ Significant Financial assets ​ ​ ​ ​ active markets for ​ observable ​ unobservable ​ ​ ​ ​ ​ identical assets ​ inputs ​ inputs Type of Instrument Total (Level 1) (Level 2) (Level 3) Cash and cash equivalents: ​ ​ ​ ​ Money market funds and checking accounts ​ $ 31,683 ​ $ 31,683 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ U.S. Treasury securities ​ ​ 20,750 ​ ​ — ​ ​ 20,750 ​ ​ — U.S. government agency obligations ​ 22,128 ​ — ​ 22,128 ​ — Corporate bonds ​ 49,118 ​ — ​ 49,118 ​ — Commercial paper ​ 116,127 ​ — ​ 116,127 ​ — Municipal bonds ​ ​ 11,684 ​ ​ — ​ ​ 11,684 ​ ​ — Restricted cash: ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 251,898 ​ $ 32,091 ​ $ 219,807 ​ $ — ​

Revenue Recognition (Tables)

Revenue Recognition (Tables)	12 Months Ended
	Dec. 31, 2021
Revenue Recognition [Abstract]
Schedule of disaggregation of revenue	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 License and milestone fees ​ ​ ​ ​ ​ ​ ​ ​ ​ VFMCRP ​ $ 15,000 ​ $ 22,262 ​ $ 19,746 Vifor ​ ​ 5,031 ​ ​ 111,551 ​ ​ — Maruishi ​ ​ 1,192 ​ ​ — ​ ​ — CKDP ​ ​ — ​ ​ 626 ​ ​ — Total license and milestone fees ​ $ 21,223 ​ $ 134,439 ​ $ 19,746 Collaborative revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ Maruishi ​ $ 706 ​ $ — ​ $ — Total Collaborative revenue ​ $ 706 ​ $ — ​ $ — Commercial supply revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ Vifor (KORSUVA injection) ​ $ 701 ​ $ — ​ $ — Total commercial supply revenue ​ $ 701 ​ $ — ​ $ — Clinical compound revenue ​ ​ ​ ​ ​ ​ ​ ​ ​ VFMCRP (difelikefalin injection) ​ $ 361 ​ $ 115 ​ $ — Maruishi ​ ​ 37 ​ ​ 528 ​ ​ 140 Total clinical compound revenue ​ $ 398 ​ $ 643 ​ $ 140

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2021
Summary of Stock Option Activity	A summary of the Company’s stock option activity related to employees, non-employee members of the Board of Directors and non-employee consultants for the 2019 Plan and the 2014 Plan as of and for the year ended December 31, 2021 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ Number of ​ Average Exercise ​ Intrinsic ​ Shares Price Value Outstanding, December 31, 2020 5,469,393 ​ $ 15.02 ​ Granted 1,422,750 ​ 17.82 ​ Exercised (136,787) ​ 11.98 ​ Forfeited (239,457) ​ 18.01 ​ Expired (3,619) ​ 24.93 ​ Outstanding, December 31, 2021 6,512,280 ​ $ 15.58 ​ $ 3,537 Weighted average remaining contractual life as of December 31, 2021 (in years) 6.88 ​ ​ Options exercisable, December 31, 2021 4,120,917 ​ $ 14.73 ​ $ 3,525 Weighted average remaining contractual life as of December 31, 2021 (in years) 5.79 ​ ​ Options vested and expected to vest as of December 31, 2021 6,449,787 ​ $ 15.39 ​ $ 3,537 Weighted average remaining contractual life as of December 31, 2021 (in years) 6.06 ​ ​
Summary of Assumptions Used in Black-Scholes Option Pricing Model	During the years ended December 31, 2021, 2020 and 2019, the Company granted 1,422,750, 1,377,850 and 1,324,000 stock options, respectively, to employees and non-employee members of the Board of Directors. There were no options granted to nonemployee consultants during the years ended December 31, 2021, 2020 and 2019. The fair values of the stock options granted to those groups were estimated using the Black-Scholes option valuation model with the following ranges of assumptions (see Note 2, Summary of Significant Accounting Policies – Stock-Based Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 2019 Risk-free interest rate 0.66% - 1.41 % 0.35% - 1.57 % 1.55% - 2.62 % Expected volatility 71.6% - 83.5 % 71.8% - 74.8 % 71.1% - 75.2 % Expected dividend yield 0 % 0 % 0 % Expected life of employee and Board options (in years) 6.25 6.25 6.25 ​
Summary of Compensation Expense Relating to Stock Options	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Research and development ​ $ 7,126 ​ $ 6,765 ​ $ 5,206 General and administrative ​ 9,569 ​ 4,943 ​ 5,094 Total stock option expense ​ $ 16,695 ​ $ 11,708 ​ $ 10,300
Employees And Non-Employee Members Of Board Of Directors [Member]
Summary of Restricted Stock Unit Activity	A summary of restricted stock unit activity related to employees and non-employee members of the Company’s Board of Directors as of and for the year ended December 31, 2021 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ Number of ​ Average Grant ​ Units Date Fair Value Outstanding, December 31, 2020 235,250 ​ $ 16.25 Awarded 530,715 ​ 17.95 Vested and released (189,421) ​ 17.20 Outstanding, December 31, 2021 576,544 ​ $ 17.50 Restricted stock units exercisable (vested and deferred), December 31, 2021 — ​ ​ ​

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Components of Income Taxes (Benefit)	The Company’s benefit from income taxes is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 2019 Current: ​ ​ ​ Federal ​ $ — ​ $ — ​ $ — State ​ — ​ (691) ​ (816) ​ ​ — ​ (691) ​ (816) Deferred: ​ ​ ​ Federal ​ — ​ — ​ — State ​ — ​ — ​ — ​ ​ — ​ — ​ — Benefit from income taxes ​ $ — ​ $ (691) ​ $ (816)
Schedule of Reconciliation of Income Taxes Computed Using U.S. Federal Statutory Rate to that Reflected in Operations	A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 2019 Income taxes using U.S. federal statutory rate 21.00 % 21.00 % 21.00 % State income taxes, net of federal benefit 7.84 % (58.68) % (1.99) % Tax Cuts and Jobs Act 0.00 % 0.00 % 0.00 % Impact of R&D tax credit on effective tax rate 2.87 % (52.06) % 4.34 % Stock option shortfalls and cancellations (4.74) % 0.35 % (0.17) % Permanent items and other (1.29) % (5.08) % 0.36 % Change in valuation allowance (25.69) % 84.61 % (22.76) % Provision to return 0.01 % 0.92 % (0.02) % Non-taxable revenue 0.00 % 0.00 % 0.00 % ​ 0.00 % (8.94) % 0.76 % ​
Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2021 2020 Valuation allowance $ (143,388) $ (120,666) ​ ​ ​ ​ ​ ​ ​ Net operating loss carryforwards ​ ​ 113,636 ​ ​ 93,507 Federal and state tax credits ​ 24,428 ​ 19,982 Deferred revenue ​ — ​ — Stock-based compensation expense ​ 5,023 ​ 6,732 Other ​ 1,268 ​ 2,414 Deferred tax assets ​ 144,355 ​ 122,635 ​ ​ ​ ​ ​ ​ ​ Other ​ (967) ​ (1,969) Deferred tax liabilities: ​ (967) ​ (1,969) ​ ​ ​ ​ ​ Net deferred tax asset: ​ $ — ​ $ — ​

Net (Loss) Income Per Share (Ta

Net (Loss) Income Per Share (Tables)	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Computation of Denominators Used in Net (Loss) Income per Share	The denominators used in the net (loss) income per share computations are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Basic: Weighted average common shares outstanding 50,718,765 47,413,250 42,669,333 Diluted: Weighted average common shares outstanding - Basic 50,718,765 47,413,250 42,669,333 Common stock equivalents* — 501,780 — Denominator for diluted net (loss) income per share 50,718,765 47,915,030 42,669,333 * For the year end December 31, 2020, common stock equivalents include dilutive stock options and restricted stock units. For the years ended December 31, 2021 and 2019, no amounts were considered as their effects would have been anti-dilutive due to net losses for those periods.
Computation of Basic and Diluted Net (Loss) Income per Share	Basic and diluted net (loss) income per share is computed as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 Net (loss) income - basic and diluted ​ $ (88,441) ​ $ 8,410 ​ $ (106,373) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ 50,718,765 ​ 47,413,250 ​ 42,669,333 Effect of dilutive securities: ​ ​ ​ ​ ​ ​ Stock options ​ ​ — ​ ​ 481,254 ​ ​ — Restricted stock units ​ ​ — ​ ​ 20,526 ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 50,718,765 ​ ​ 47,915,030 ​ ​ 42,669,333 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net (loss) income per share: ​ ​ ​ Basic ​ $ (1.74) ​ $ 0.18 ​ $ (2.49) Diluted ​ $ (1.74) ​ $ 0.18 ​ $ (2.49)

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Schedule of Other Information related to Stamford Lease and lease amendment	Other information related to the Stamford Lease and Lease Amendment was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 ​ Cash paid for amounts included in the measurement of lease liabilities: ​ ​ ​ ​ Operating cash outflows relating to operating leases ​ $ 1,921 ​ $ 1,403 ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ — ​ $ 1,934 ​ Remaining lease term - operating leases (years) ​ 2.0 ​ 3.0 ​ Discount rate - operating leases ​ ​ 7.0 % ​ 7.0 %
Schedule of Future Minimum Lease Payments under Non-cancellable Operating Leases, Reconciliation of Undiscounted Cash Flows to the Operating Lease Liabilities	Future minimum lease payments under the non-cancellable operating leases for the Stamford lease and the Lease Amendment, as well as a reconciliation of these undiscounted cash flows to the operating lease liabilities as of December 31, 2021, were as follows: ​ ​ ​ ​ Year Ending December 31, ​ ​ 2022 $ 1,957 2023 ​ 1,991 Total future minimum lease payments, undiscounted ​ 3,948 Less imputed interest ​ (275) Total ​ $ 3,673 ​ ​ ​ ​ Operating lease liabilities reported as of December 31, 2021: ​ Operating lease liabilities - current ​ $ 1,755 Operating lease liabilities - non-current ​ 1,918 Total ​ $ 3,673

Business - Additional Informati

Business - Additional Information (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	Oct. 31, 2021USD ($)shares	Oct. 31, 2020USD ($)shares	May 31, 2018USD ($)shares	Dec. 31, 2021USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)shares
Nature Of Business [Line Items]
Date of formation				Jul. 2, 2004
Proceeds from equity and debt financing				$ 519,600
Number of follow-on public offerings | item				4
Payments received in connection with license agreements				$ 224,900
Proceeds from sale of common stock in connection with license agreement				44,969	$ 38,449
Unrestricted cash and cash equivalents and marketable securities				236,789
Accumulated deficit				480,758	392,317
Net (loss) income				(88,441)	8,410	$ (106,373)
Net cash used in operating activities				$ (60,087)	$ (5,487)	$ (109,225)
Vifor Agreement [Member]
Nature Of Business [Line Items]
Percentage of net profit sharing		60.00%
Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued | shares						6,325,000
Vifor International Ltd. [Member]
Nature Of Business [Line Items]
Proceeds from sale of common stock in connection with license agreement		$ 38,449	$ 14,556
Proceeds from sale of common stock in connection with regulatory milestone	$ 44,969
Vifor International Ltd. [Member] | Vifor Agreement [Member]
Nature Of Business [Line Items]
Percentage of net profit sharing		40.00%
Common stock, shares issued | shares		2,939,552
Vifor International Ltd. [Member] | Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued | shares	3,282,391	2,939,552	1,174,827

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2020item	Dec. 31, 2021USD ($)itemsegmentInstitution	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Jan. 01, 2019USD ($)lease	Dec. 31, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of suppliers		3
Cost of goods sold | $		$ 0
Prepaid R&D expense | $		$ 1,481	$ 11,286
Expected dividend yield		0.00%	0.00%	0.00%
Stockholders' equity | $		$ 227,522	$ 249,001	$ 186,713		$ 133,630
Number of operating segments | segment		1
Commercial Supply Revenue
Summary Of Significant Accounting Policies [Line Items]
Number of parties in net profit sharing arrangement		2
Vifor, VFMCRP, and CKDP Agreements [Member] | License and Milestone Fees
Summary Of Significant Accounting Policies [Line Items]
Number of performance obligations for revenue recognized		1
Stamford Operating Lease [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of renewable terms		1
Operating Lease, renewable term		5 years
Stamford Operating Lease [Member] | ASC 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of operating lease | lease					2
Number of renewable terms		1
Operating Lease, renewable term		5 years
Annual incremental borrowing rate		7.00%
Stamford Operating Lease [Member] | ASC 842 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Stockholders' equity | $					$ 0
New Stamford Lease [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of renewable terms	1
Operating Lease, renewable term	5 years
Annual incremental borrowing rate		7.00%
New Stamford Lease [Member] | ASC 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of renewable terms		1
Operating Lease, renewable term		5 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Period that would require for locating and qualifying alternate sources if the suppliers fail to meet supply commitments		2 years
Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)		6 years 3 months	6 years 3 months	6 years 3 months
Expected dividend yield		0.00%	0.00%	0.00%
Cash And Cash Equivalents And Marketable Securities [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of financial institutions | Institution		3

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Concentration of Credit Risk (Detail) - Revenue from Contract with Customer Benchmark [Member] - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Concentration Risk [Line Items]
Concentration of revenue percentage	67.00%	17.00%	99.00%
Vifor International Ltd. [Member]
Concentration Risk [Line Items]
Concentration of revenue percentage	25.00%	83.00%

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment, net (Detail)	12 Months Ended
	Dec. 31, 2021
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, description	lesser of useful life of asset or life of lease (Stamford - 7 years)
Stamford Operating Lease [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	7 years

Available-for-Sale Marketable_3

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 223,694	$ 219,734
Gross Unrealized Gains	5	123
Gross Unrealized Losses	(363)	(50)
Estimated Fair Value	223,336	219,807
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,573	20,710
Gross Unrealized Gains		41
Gross Unrealized Losses	(3)	(1)
Estimated Fair Value	11,570	20,750
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,020	22,125
Gross Unrealized Gains		4
Gross Unrealized Losses	(45)	(1)
Estimated Fair Value	16,975	22,128
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,495	49,080
Gross Unrealized Gains		61
Gross Unrealized Losses	(171)	(23)
Estimated Fair Value	66,324	49,118
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,914	116,139
Gross Unrealized Gains	5	5
Gross Unrealized Losses	(31)	(17)
Estimated Fair Value	106,888	116,127
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,692	11,680
Gross Unrealized Gains		12
Gross Unrealized Losses	(113)	(8)
Estimated Fair Value	$ 21,579	$ 11,684

Available-for-Sale Marketable_4

Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 156,455	$ 113,891
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(354)	(50)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value	3,037
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses	(9)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	159,492	113,891
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(363)	(50)
U.S. Treasury Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	11,570	12,682
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(3)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	11,570	12,682
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(3)	(1)
U.S. Government Agency Obligations [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	9,456	2,500
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(45)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	9,456	2,500
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(45)	(1)
Corporate Bonds [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	62,704	23,553
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(170)	(23)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value	2,020
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	64,724	23,553
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(171)	(23)
Commercial Paper [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	52,163	68,897
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(31)	(17)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	52,163	68,897
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(31)	(17)
Municipal Bonds [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	20,562	6,259
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(105)	(8)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value	1,017
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses	(8)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	21,579	6,259
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (113)	$ (8)

Available-for-Sale Marketable_5

Available-for-Sale Marketable Securities - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)position	Dec. 31, 2020USD ($)position
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale allowance for credit loss | $	$ 0	$ 0
Available-for-sale unrealized credit losses | $	$ 0	$ 0
Number of available-for-sale marketable securities in unrealized loss positions	58	30
Total number of positions	76	59
Number of positions of available-for-sale in unrealized loss position for 12 months or greater	2
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses | $	$ 9
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	3
Total number of positions	3
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	3
Total number of positions	5
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	23
Total number of positions	25
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	15
Total number of positions	29
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	14
Total number of positions	14

Available-for-Sale Marketable_6

Available-for-Sale Marketable Securities - Schedule of Amortized Cost and Fair Values of Marketable Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities [Abstract]
Amortized Cost of marketable debt securities, contractual maturity, less than one year	$ 153,631	$ 149,164
Amortized Cost of marketable debt securities, contractual maturity, One year to three years	70,063	70,570
Amortized Cost	223,694	219,734
Fair value of marketable debt securities, contractual maturities, less than one year	153,582	149,242
Fair value of marketable debt securities, contractual maturity, One year to three years	69,754	70,565
Total fair Value of marketable debt securities, contractual maturity	223,336	219,807
Proceeds from sale of available-for-sale marketable securities	10,029	41,600	$ 0
Realized gain	39	272
Interest receivable	$ 455	$ 311

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive (Loss) Income - Summary of Changes in Accumulated Other Comprehensive (Loss) Income, Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 249,001	$ 186,713	$ 133,630
Amount reclassified from accumulated other comprehensive (Loss) income	(39)	(272)
Net current period other comprehensive (loss) income	(431)	(97)	284
Balance, Value	227,522	249,001	186,713
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	73	170	(114)
Other comprehensive (loss) income before reclassifications	(392)	175	284
Amount reclassified from accumulated other comprehensive (Loss) income	(39)	(272)
Net current period other comprehensive (loss) income	(431)	(97)	284
Balance, Value	$ (358)	$ 73	$ 170

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive (Loss) Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income, net	$ 642	$ 2,334	$ 4,490
Benefit from income taxes	0	(691)	(816)
Net of tax	39	272
Accumulated Net Unrealized Investment Gain (Loss) Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income, net	39	272
Benefit from income taxes	$ 0	$ 0	$ 0

Inventory, net - Schedule of In

Inventory, net - Schedule of Inventories (Detail) $ in Thousands	Dec. 31, 2021USD ($)
Inventory Disclosure [Abstract]
Raw materials	$ 927
Work-in-process	1,657
Total	$ 2,584

Inventory, net - Additional Inf

Inventory, net - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)
Inventory Disclosure [Abstract]
Cost of goods sold	$ 0
Inventory write-down	$ 0

Prepaid Expenses - Additional I

Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 2,519	$ 12,076
Prepaid R&D clinical costs	1,481	11,286
Prepaid insurance	369	223
Other prepaid costs	$ 669	$ 567

Property and Equipment, net - P

Property and Equipment, net - Property and Equipment, net (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,792	$ 2,381
Less accumulated depreciation and amortization	1,161	1,541
Property, Plant and Equipment, net, Total	631	840
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	239	211
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		628
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	330	330
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,223	$ 1,212

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 248	$ 209	$ 198
Realized gain on sale of property and equipment	70	$ 0	$ 0
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Realized gain on sale of property and equipment	$ 70

Restricted Cash - Additional In

Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, long-term assets	$ 408	$ 408
Stamford Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, long-term assets	$ 408	$ 408

Restricted Cash - Schedule of R

Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash [Abstract]
Cash and cash equivalents	$ 13,453	$ 31,683
Restricted cash, long-term assets	408	408
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	$ 13,861	$ 32,091	$ 18,713	$ 15,850

Accounts Payable and Accrued _3

Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accounts payable	$ 5,625	$ 4,893
Accrued research projects	4,648	6,194
Accrued compensation and benefits	4,959	4,955
Accrued professional fees and other	629	839
Total	$ 15,861	$ 16,881

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands

Aug. 31, 2019USD ($)$ / shares

Jul. 29, 2019USD ($)$ / sharesshares

Oct. 31, 2021shares

Aug. 31, 2021shares

Jun. 30, 2021shares

Mar. 31, 2021shares

Feb. 28, 2021shares

Dec. 31, 2020$ / sharesshares

Oct. 31, 2020shares

Jun. 30, 2020shares

Feb. 29, 2020

Dec. 31, 2019$ / sharesshares

Aug. 31, 2019USD ($)$ / sharesshares

May 31, 2019shares

Mar. 31, 2019shares

Mar. 31, 2021shares

Jun. 30, 2020shares

Dec. 31, 2021USD ($)item$ / sharesshares

Dec. 31, 2020USD ($)item$ / sharesshares

Dec. 31, 2019USD ($)item$ / sharesshares

Apr. 04, 2019USD ($)

Mar. 20, 2019$ / shares

Sale Of Stock [Line Items]

Common stock, shares authorized

100,000,000

100,000,000

100,000,000

Common stock, par value | $ / shares

$ 0.001

$ 0.001

$ 0.001

Preferred stock, shares authorized

5,000,000

5,000,000

5,000,000

Preferred stock, par value | $ / shares

$ 0.001

$ 0.001

$ 0.001

Common stock, shares outstanding

49,872,213

53,480,812

49,872,213

Common stock, shares issued

49,872,213

53,480,812

49,872,213

Preferred stock, shares issued

0

0

0

Preferred Stock Shares Outstanding

0

0

0

Number of votes for each share of common stock | item

1

1

1

Unregistered common stock exchanged for consulting services | $

$ 197

Shareholder Consulting Agreement [Member]

Sale Of Stock [Line Items]

Number of shares of unregistered common stock exchanged for consulting services

10,195

Price per share | $ / shares

$ 19.37

Consulting services performance period

6 months

General and Administrative [Member] | Shareholder Consulting Agreement [Member]

Sale Of Stock [Line Items]

Compensation expense | $

$ 197

Regulatory Milestones [Member] | Vifor International Ltd. [Member]

Sale Of Stock [Line Items]

Common stock, shares issued

3,282,391

Enteris License Agreement [Member]

Sale Of Stock [Line Items]

Upfront fee | $

$ 8,000

$ 8,000

Upfront fee payment, cash | $

4,000

Upfront fee payment, stock | $

$ 4,000

Price per share | $ / shares

$ 23.42

$ 23.42

Share price average, Determination period

30 days

Percentage of payment, stock

50.00%

Exclusive License Agreement [Member] | Vifor International Ltd. [Member]

Sale Of Stock [Line Items]

Common stock, shares issued

2,939,552

Maximum [Member]

Sale Of Stock [Line Items]

Aggregate offering price of securities | $

$ 300,000

Follow-on Public Offering

Sale Of Stock [Line Items]

Price per share | $ / shares

$ 23

$ 23

Common Stock

Sale Of Stock [Line Items]

Common stock, shares issued

6,325,000

Restricted stock units vested shares

189,421

156,584

110,832

Number of shares of unregistered common stock exchanged for consulting services

10,195

Common Stock | Executive Officers [Member]

Sale Of Stock [Line Items]

Restricted stock units vested shares

74,166

Common Stock | Enteris License Agreement [Member]

Sale Of Stock [Line Items]

Common stock, shares issued

170,793

Upfront fee payable, stock | $

$ 4,000

$ 4,000

Common Stock | Follow-on Public Offering

Sale Of Stock [Line Items]

Common stock, shares issued

6,325,000

Price per share | $ / shares

$ 23

Proceeds from issuance of common stock net of underwriting discounts, commissions and estimated offering expenses | $

$ 136,498

Common Stock | Underwriter's Over-Allotment [Member]

Sale Of Stock [Line Items]

Common stock, shares issued

825,000

Underwriting discounts, commission and offering expenses | $

$ 8,977

Restricted Stock Units [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member]

Sale Of Stock [Line Items]

Common stock, shares issued

36,666

74,166

36,666

Restricted Stock Units [Member] | General and Administrative [Member] | Director [Member]

Sale Of Stock [Line Items]

Compensation expense | $

$ 565

$ 528

$ 287

Restricted Stock Units [Member] | General and Administrative [Member] | Executive Officers [Member]

Sale Of Stock [Line Items]

Compensation expense | $

2,324

1,167

$ 1,180

Restricted Stock Units [Member] | General and Administrative [Member] | 2014 Equity Incentive Plan [Member] | Director [Member] | June 2020 [Member]

Sale Of Stock [Line Items]

Compensation expense | $

$ 239

323

Restricted Stock Units [Member] | Common Stock | Director [Member]

Sale Of Stock [Line Items]

Vesting period of awards granted

1 year

Performance Restricted Stock Units [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member] | December 2020 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

36,750

Performance Restricted Stock Units [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member] | February 2020 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

40,000

36,750

113,500

Compensation expense | $

$ 1,256

601

Performance Restricted Stock Units [Member] | General and Administrative [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member] | February 2020 [Member]

Sale Of Stock [Line Items]

Compensation expense | $

$ 732

405

Performance Restricted Stock Units [Member] | Common Stock | December 2019 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

36,666

Performance Restricted Stock Units [Member] | Common Stock | Executive Officers [Member] | August 2021 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

44,002

Performance Restricted Stock Units [Member] | Common Stock | Executive Officers [Member] | February and March 2021 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

76,750

Performance Restricted Stock Units [Member] | Common Stock | Executive Officers [Member] | April And June 2020 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

95,834

Time-based Restricted Stock Units [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member] | February 2020 [Member]

Sale Of Stock [Line Items]

Vesting period of awards granted

3 years

Restricted stock units, vested

32,669

32,669

Compensation expense | $

$ 607

455

Time-based Restricted Stock Units [Member] | General and Administrative [Member] | 2014 Equity Incentive Plan [Member] | Executive Officers [Member] | February 2020 [Member]

Sale Of Stock [Line Items]

Compensation expense | $

$ 433

$ 306

Time-based Restricted Stock Units [Member] | Common Stock | Director [Member] | June 2021 [Member]

Sale Of Stock [Line Items]

Vesting period of awards granted

1 year

Restricted stock units, vested

36,000

Time-based Restricted Stock Units [Member] | Common Stock | Director [Member] | June 2020 [Member]

Sale Of Stock [Line Items]

Restricted stock units, vested

24,000

Time-based Restricted Stock Units [Member] | Common Stock | Executive Officers [Member] | February 2021 [Member]

Sale Of Stock [Line Items]

Vesting period of awards granted

3 years

Restricted stock units, vested

32,669

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Financial assets
Available-for-sale marketable securities	$ 223,336	$ 219,807
Recurring [Member]
Financial assets
Total financial assets	237,197	251,898
U.S. Government Agency Obligations [Member]
Financial assets
Available-for-sale marketable securities	16,975	22,128
U.S. Government Agency Obligations [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	16,975	22,128
Corporate Bonds [Member]
Financial assets
Available-for-sale marketable securities	66,324	49,118
Corporate Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	66,324	49,118
Municipal Bonds [Member]
Financial assets
Available-for-sale marketable securities	21,579	11,684
Municipal Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	21,579	11,684
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring [Member]
Financial assets
Total financial assets	13,861	32,091
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Total financial assets	223,336	219,807
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency Obligations [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	16,975	22,128
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	66,324	49,118
Significant Other Observable Inputs (Level 2) [Member] | Municipal Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	21,579	11,684
Money Market Funds [Member] | Recurring [Member]
Financial assets
Cash and cash equivalents	13,453	31,683
Restricted cash	408	408
Money Market Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring [Member]
Financial assets
Cash and cash equivalents	13,453	31,683
Restricted cash	408	408
U.S. Treasury Securities [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	11,570	20,750
U.S. Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	11,570	20,750
Commercial Paper [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	106,888	116,127
Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	$ 106,888	$ 116,127

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfer of financial asset into level 3 of fair value	$ 0	$ 0
Fair value assets level 2 to level 1 transfers	0	0
Fair value assets level 1 to level 2 transfers	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases of financial assets	0	0	$ 0
Sales of financial assets	0	0	0
Maturities of financial assets	0	0	0
Unrealized gains	0	0	0
Unrealized losses	$ 0	$ 0	$ 0

Collaboration and Licensing A_2

Collaboration and Licensing Agreements - Additional Information (Detail) $ / shares in Units, $ in Thousands	Oct. 15, 2020USD ($)item	May 17, 2018USD ($)item$ / sharesshares	Oct. 31, 2021USD ($)$ / sharesshares	Oct. 31, 2020USD ($)$ / sharesshares	Apr. 30, 2013item	Dec. 31, 2021USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from the sale of common stock								$ 136,498
Purchase of common stock value								136,498
Revenue from contract with customer						$ 23,028	$ 135,082	19,886
Cost of goods sold						0
Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						706
Commercial Supply Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						701
Clinical Compound Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						$ 398	643	140
Vifor Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net profit sharing				60.00%
Number of days to terminate agreement						60 days
Cost of goods sold						$ 0
Vifor Agreement [Member] | License [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations for revenue recognized | item	1
Revenue from contract with customer							111,551
Vifor Agreement [Member] | Commercial Supply Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						701
VFMCRP Agreement [Member] | Clinical Compound Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						361	115
Cost of clinical compound related to R&D expense						343	108
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable, non-creditable upfront payment		$ 50,000
Common stock, shares issued | shares		1,174,827
Purchase of common stock value		$ 20,000
Purchase common stock per share amount | $ / shares		$ 17.024
Closing prices of company common stock description		over a pre-determined average closing price of the Company’s common stock.
Premium from sale of stock		$ 5,444
Deferred revenue		$ 55,444
Number of combined performance obligations for revenue recognized | item		1
Number of performance obligations for revenue recognized | item		2
Number of days to terminate agreement		60 days
Termination notice effective period		12 months
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Clinical Compound Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						361	115
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Regulatory Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from milestone payments			$ 15,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Maximum [Member] | Regulatory and Commercial Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments		$ 455,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Maximum [Member] | Regulatory Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments						15,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Maximum [Member] | Tiered Commercial Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments						440,000
Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations for revenue recognized | item					2
Cost of clinical compound related to R&D expense						33	476	126
Maruishi Pharmaceutical Co., Ltd. [Member] | Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						706
Maruishi Pharmaceutical Co., Ltd. [Member] | Clinical Compound Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						37	$ 528	$ 140
Vifor International Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations for revenue recognized | item	1
Vifor International Ltd. [Member] | Commercial Supply Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer						701
Vifor International Ltd. [Member] | Regulatory and Commercial Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from the sale of common stock	$ 50,000
Vifor International Ltd. [Member] | Vifor Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net profit sharing				40.00%
Common stock, shares issued | shares				2,939,552
Purchase common stock per share amount | $ / shares				$ 17.0094
Vifor International Ltd. [Member] | Vifor Agreement [Member] | License [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable, non-creditable upfront payment				$ 100,000
Premium from sale of stock				$ 11,551
Vifor International Ltd. [Member] | Regulatory Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock, shares issued | shares			3,282,391
Proceeds from the sale of common stock			$ 50,000
Purchase common stock per share amount | $ / shares			$ 15.23
Premium from sale of stock						$ 5,031
Proceeds from milestone payments	$ 50,000
Percentage of premium on common stock investment	20.00%		20.00%
Stock issuance price measurement period	30 days		30 days
Vifor International Ltd. [Member] | Maximum [Member] | Sales-based Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments	$ 240,000

Revenue Recognition - Disaggreg

Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenue	$ 23,028	$ 135,082	$ 19,886
License and Milestone Fees
Disaggregation of Revenue [Line Items]
Total revenue	21,223	134,439	19,746
License and Milestone Fees | Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	15,000	22,262	19,746
License and Milestone Fees | Vifor International Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	5,031	111,551
License and Milestone Fees | Maruishi Pharmaceutical Co., Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	1,192
License and Milestone Fees | Chong Kun Dang Pharmaceutical Corp. [Member]
Disaggregation of Revenue [Line Items]
Total revenue		626
Collaborative Revenue
Disaggregation of Revenue [Line Items]
Total revenue	706
Collaborative Revenue | Maruishi Pharmaceutical Co., Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	706
Commercial Supply Revenue
Disaggregation of Revenue [Line Items]
Total revenue	701
Commercial Supply Revenue | Vifor International Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	701
Clinical Compound Revenue
Disaggregation of Revenue [Line Items]
Total revenue	398	643	140
Clinical Compound Revenue | Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	361	115
Clinical Compound Revenue | Maruishi Pharmaceutical Co., Ltd. [Member]
Disaggregation of Revenue [Line Items]
Total revenue	$ 37	$ 528	$ 140

Revenue Recognition - Additiona

Revenue Recognition - Additional Information (Detail) $ / shares in Units, ¥ in Billions

Oct. 15, 2020USD ($)item

May 17, 2018USD ($)item$ / sharesshares

Oct. 31, 2021USD ($)$ / sharesshares

May 31, 2021USD ($)

Jan. 31, 2021USD ($)

Apr. 30, 2013USD ($)item

Apr. 30, 2012USD ($)

Dec. 31, 2021USD ($)itemcontractMilestone

Dec. 31, 2020USD ($)item

Dec. 31, 2019USD ($)Milestone

Dec. 31, 2021USD ($)

Apr. 30, 2013JPY (¥)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Proceeds from the sale of common stock

$ 136,498,000

Number of contracts | contract

4

Total revenue

$ 23,028,000

$ 135,082,000

19,886,000

License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

21,223,000

134,439,000

$ 19,746,000

Royalty [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer

0

Collaborative Revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

706,000

Vifor Agreement [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

$ 111,551,000

Number of performance obligations for revenue recognized | item

1

1

upfront payment

$ 100,000,000

Premium from sale of stock

11,551,000

Revenue from contract with customer

111,551,000

Remaining performance obligations

$ 0

$ 0

VFMCRP Agreement [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of combined performance obligations for revenue recognized | item

1

1

Number of performance obligations for revenue recognized | item

2

upfront payment

$ 50,000,000

Premium from sale of stock

5,444,000

VFMCRP Agreement [Member] | Regulatory Milestones [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Proceeds from milestone payments

$ 15,000,000

VFMCRP Agreement [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Deferred revenue

$ 55,444,000

Total revenue

55,444,000

Number of performance obligations for revenue recognized | item

1

Remaining performance obligations

$ 0

0

Vifor, VFMCRP, and CKDP Agreements [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of performance obligations for revenue recognized | item

1

Vifor, VFMCRP, Maruishi and CKDP Agreements [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Receivables

$ 0

0

0

Other assets

0

0

0

Deferred revenue

$ 0

0

0

Vifor, Maruishi and CKDP Agreements [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

License agreements with IP as separate performance obligation | item

3

Maruishi and CKDP Agreements [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Remaining performance obligations

$ 0

0

Maruishi Agreement [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of performance obligations for revenue recognized | item

2

Premium from sale of stock

$ 337,000

Upfront non-refundable, non-creditable license fee and premium from sale of stock

15,337,000

Maruishi Agreement [Member] | Clinical Development and Regulatory Milestones [Member] | Maximum [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Potential milestone payments

10,500,000

Maruishi Agreement [Member] | One-time Sales Milestone [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Potential milestone payments | ¥

¥ 1

Maruishi Agreement [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

License and milestone fees revenue

$ 1,192,000

Proceeds from milestone payments

$ 1,898,000

Maruishi Agreement [Member] | License [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated selling price

10,200,000

Upfront payments on obligations

9,637,000

Maruishi Agreement [Member] | R&D Services [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated selling price

6,200,000

Upfront payments on obligations

$ 5,700,000

Maruishi Agreement [Member] | Collaborative Revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

$ 706,000

CKDP Agreement [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of performance obligations for revenue recognized | item

1

Premium from sale of stock

$ 83,000

Number of milestone event probable of occurrence or achieved | Milestone

0

0

CKDP Agreement [Member] | Clinical Development and Regulatory Milestones [Member] | Maximum [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Potential milestone payments

3,750,000

CKDP Agreement [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

626,000

CKDP Agreement [Member] | License [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer

$ 646,000

Maruishi Pharmaceutical Co., Ltd. [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of performance obligations for revenue recognized | item

2

Maruishi Pharmaceutical Co., Ltd. [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

$ 1,192,000

Maruishi Pharmaceutical Co., Ltd. [Member] | Collaborative Revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

706,000

Vifor International Ltd. [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of performance obligations for revenue recognized | item

1

Vifor International Ltd. [Member] | Regulatory Milestones [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Premium on Common Stock Investment, Percentage

20.00%

20.00%

Proceeds from the sale of common stock

$ 50,000,000

Stock Issued During Period Shares New Issues | shares

3,282,391

Stock Issuance Price Measurement Period

30 days

30 days

Shares Issued Price Per Share | $ / shares

$ 15.23

Premium from sale of stock

5,031,000

Proceeds from milestone payments

$ 50,000,000

Vifor International Ltd. [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

5,031,000

111,551,000

Vifor International Ltd. [Member] | License and Milestone Fees | Regulatory Milestones [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Premium from sale of stock

5,031,000

Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Stock Issued During Period Shares New Issues | shares

1,174,827

Shares Issued Price Per Share | $ / shares

$ 17.024

Deferred revenue

$ 55,444,000

Number of combined performance obligations for revenue recognized | item

1

Number of performance obligations for revenue recognized | item

2

upfront payment

$ 50,000,000

Premium from sale of stock

$ 5,444,000

Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Regulatory Milestones [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Proceeds from milestone payments

$ 15,000,000

Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Regulatory Milestones [Member] | Maximum [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Potential milestone payments

15,000,000

$ 15,000,000

Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | License and Milestone Fees

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Total revenue

$ 15,000,000

$ 22,262,000

$ 19,746,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands

Dec. 17, 2021installment$ / sharesshares

Nov. 01, 2021USD ($)

Oct. 29, 2021tranche$ / sharesshares

Jun. 03, 2021$ / sharesshares

Mar. 30, 2021installment$ / sharesshares

Jun. 04, 2020$ / sharesshares

Jun. 04, 2019$ / sharesshares

Aug. 31, 2021shares

Mar. 31, 2021shares

Feb. 28, 2021shares

Jun. 30, 2020shares

Apr. 30, 2020shares

Feb. 29, 2020installment$ / sharesshares

Dec. 31, 2019shares

May 31, 2019shares

Mar. 31, 2019$ / sharesshares

Dec. 31, 2021USD ($)$ / sharesshares

Dec. 31, 2020USD ($)$ / sharesshares

Dec. 31, 2019USD ($)$ / sharesshares

Dec. 31, 2015shares

Jan. 01, 2022shares

Nov. 20, 2019$ / sharesshares

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock, par value | $ / shares

$ 0.001

$ 0.001

Common stock, shares outstanding

53,480,812

49,872,213

Common stock, shares issued

53,480,812

49,872,213

Number of options outstanding

4,450,517

6,512,280

5,469,393

4,450,517

Options granted

1,422,750

Director [Member] | Restricted Stock Units [Member] | General and Administrative [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 565

$ 528

$ 287

Director [Member] | Share-based Payment Arrangement, Nonemployee [Member] | Restricted Stock Units [Member] | General and Administrative [Member] | June 2019 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

205

287

Executive Officers [Member] | Restricted Stock Units [Member] | Research and Development [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

1,201

1,432

604

Executive Officers [Member] | Restricted Stock Units [Member] | General and Administrative [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 2,324

$ 1,167

$ 1,180

Consultants [Member] | Share-based Payment Arrangement, Nonemployee [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Options granted

0

0

0

Employees And Non-Employee Members Of Board Of Directors [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair value of options granted | $ / shares

$ 12

$ 10.33

$ 11.67

Options granted

1,422,750

1,377,850

1,324,000

2019 Inducement Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock, par value | $ / shares

$ 0.001

2019 Inducement Plan [Member] | Incentive Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Options granted

0

0

2019 Inducement Plan [Member] | Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share-based compensation arrangement by share-based payment award, number of shares authorized

300,000

2019 Inducement Plan [Member] | Share-based Payment Arrangement, Employee [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Options granted

47,500

2019 Inducement Plan [Member] | Share-based Payment Arrangement, Employee [Member] | Share-Based Compensation Award, Tranche One [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Percentage of vested shares on first anniversary of grant date

25.00%

2019 Inducement Plan [Member] | Share-based Payment Arrangement, Employee [Member] | Share-based Compensation Award, Tranche Two [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

36 months

2019 Inducement Plan [Member] | Share-based Payment Arrangement, Employee [Member] | Share Based Compensation Subsequent Awards [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

4 years

2014 Equity Incentive Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of stock options that may be granted after the tenth anniversary of the 2014 Plan

0

Annual increases in number of shares reserved for issuance as a percentage of shares of capital stock outstanding through January 1, 2024

3.00%

Share-based compensation arrangement by share-based payment award, number of shares authorized

8,984,679

2014 Equity Incentive Plan [Member] | Subsequent Event [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share-based compensation arrangement by share-based payment award, number of shares authorized

10,589,103

2014 Equity Incentive Plan [Member] | Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Term of awards granted

10 years

2014 Equity Incentive Plan [Member] | Maximum [Member] | Incentive Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share-based compensation arrangement by share-based payment award, number of shares authorized

30,000,000

2014 Equity Incentive Plan [Member] | Director [Member] | Incentive Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

3 years

2014 Equity Incentive Plan [Member] | Director [Member] | Restricted Stock Units [Member] | General and Administrative [Member] | June 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 239

$ 323

2014 Equity Incentive Plan [Member] | Director [Member] | Share-based Payment Arrangement, Nonemployee [Member] | Restricted Stock Units [Member] | June 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

1 year

Awarded

36,000

Restricted stock units, grant date fair value | $ / shares

$ 15.62

Restricted stock units vesting date

Jun. 3, 2021

2014 Equity Incentive Plan [Member] | Director [Member] | Share-based Payment Arrangement, Nonemployee [Member] | Restricted Stock Units [Member] | June 2019 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

1 year

Awarded

24,000

Restricted stock units, grant date fair value | $ / shares

$ 20.47

Restricted stock units, vested

24,000

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Restricted Stock Units [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock, shares issued

36,666

74,166

36,666

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Awarded

176,000

176,000

Restricted stock units, grant date fair value | $ / shares

$ 20.59

Compensation expense | $

$ 906

Restricted stock units, vested

44,002

44,002

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Awarded

138,000

138,000

Restricted stock units, grant date fair value | $ / shares

$ 16.36

Compensation expense | $

$ 1,256

601

Restricted stock units, vested

40,000

36,750

113,500

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | March 2019 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Awarded

215,000

Restricted stock units, grant date fair value | $ / shares

$ 16.10

Compensation expense | $

$ 1,543

1,784

Restricted stock units, vested

30,000

65,834

36,666

74,166

215,000

Restricted stock units, Forfeited

8,334

Common stock, shares issued

30,000

65,834

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | Research and Development [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 329

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | Research and Development [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

524

196

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | Research and Development [Member] | March 2019 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

1,087

604

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | General and Administrative [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

577

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | General and Administrative [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

732

405

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Performance Restricted Stock Units [Member] | General and Administrative [Member] | March 2019 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 456

1,180

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | Modified March 2021 Awards [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 75

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | Modified February 2020 Awards [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 73

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | December 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

18 months

Awarded

63,573

63,573

Restricted stock units, grant date fair value | $ / shares

$ 12.45

Number of equal installment | installment

2

Compensation expense | $

$ 20

Restricted stock units, vested

0

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

3 years

Awarded

100,000

100,000

Restricted stock units, grant date fair value | $ / shares

$ 20.59

Number of equal installment | installment

3

Compensation expense | $

$ 592

Restricted stock units, vested

0

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

3 years

Awarded

98,000

98,000

Restricted stock units, grant date fair value | $ / shares

$ 16.36

Number of equal installment | installment

3

Compensation expense | $

$ 607

455

Restricted stock units, vested

32,669

32,669

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | Research and Development [Member] | December 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 8

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | Research and Development [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

166

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | Research and Development [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

174

149

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | General and Administrative [Member] | December 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

12

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | General and Administrative [Member] | March 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

426

2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Time-based Restricted Stock Units [Member] | General and Administrative [Member] | February 2020 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 433

$ 306

2014 Equity Incentive Plan [Member] | Chief Executive Officer [Member] | Time-based Restricted Stock Units [Member] | October 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Awarded

147,942

147,942

Restricted stock units, grant date fair value | $ / shares

$ 16.83

Number of restricted stock unit tranches | tranche

2

Restricted stock units, vested

0

2014 Equity Incentive Plan [Member] | Chief Executive Officer [Member] | Time-based Restricted Stock Units [Member] | General and Administrative [Member] | October 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 144

2014 Equity Incentive Plan [Member] | Chief Executive Officer [Member] | Share-Based Compensation Award, Tranche One [Member] | Time-based Restricted Stock Units [Member] | October 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Percentage of vested shares on first anniversary of grant date

25.00%

Awarded

142,000

2014 Equity Incentive Plan [Member] | Chief Executive Officer [Member] | Share-based Compensation Award, Tranche Two [Member] | Time-based Restricted Stock Units [Member] | October 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

36 months

Awarded

5,942

2014 Equity Incentive Plan [Member] | Employee And Nonemployee Consultants [Member] | Share-Based Compensation Award, Tranche One [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Percentage of vested shares on first anniversary of grant date

25.00%

2014 Equity Incentive Plan [Member] | Employee And Nonemployee Consultants [Member] | Share-based Compensation Award, Tranche Two [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

36 months

2014 Equity Incentive Plan [Member] | Employee And Nonemployee Consultants [Member] | Share Based Compensation Subsequent Awards [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

4 years

2014 Equity Incentive Plan [Member] | Non-employee Directors [Member] | Restricted Stock Units [Member] | June 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period of awards granted

1 year

Awarded

43,200

43,200

Restricted stock units, grant date fair value | $ / shares

$ 13.06

Restricted stock units, vested

0

2014 Equity Incentive Plan [Member] | Non-employee Directors [Member] | Restricted Stock Units [Member] | General and Administrative [Member] | June 2021 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation expense | $

$ 326

2014 Equity Incentive Plan [Member] | Employees And Non-Employee Members Of Board Of Directors [Member] | Restricted Stock Units [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Awarded

530,715

Restricted stock units, grant date fair value | $ / shares

$ 17.95

Restricted stock units, vested

189,421

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Unit Activity (Details) - $ / shares	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, Beginning Balance		235,250
Outstanding, Ending Balance	119,834	576,544
2014 Equity Incentive Plan [Member] | Employees And Non-Employee Members Of Board Of Directors [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, Beginning Balance		235,250
Awarded		530,715
Vested and released		(189,421)
Outstanding, Ending Balance		576,544
Weighted Average Grant Date Fair Value
Weighted-average grant date fair value, outstanding, Beginning Balance		$ 16.25
Weighted-average grant date fair value, awarded		17.95
Weighted-average grant date fair value, vested and released		17.20
Weighted-average grant date fair value, outstanding, Ending Balance		$ 17.50

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding, Beginning Balance	5,469,393	4,450,517
Number of Options, Granted	1,422,750
Number of Options, Exercised	(136,787)
Number of Options, Forfeited	(239,457)
Number of Options, Expired	(3,619)
Number of Options, Outstanding, Ending Balance	6,512,280	5,469,393	4,450,517
Number of Options, Outstanding, Weighted average remaining contractual life as of December 31, 2021	6 years 10 months 17 days
Number of Options, Options exercisable	4,120,917
Number of Options, Exercisable, Weighted average remaining contractual life as of December 31, 2021	5 years 9 months 14 days
Number of Options, Options vested and expected to vest at December 31, 2021	6,449,787
Number of Options, Vested and expected to vest, Weighted average remaining contractual life as of December 31, 2021	6 years 21 days
Weighted-Average Exercise Price, Outstanding, Beginning Balance	$ 15.02
Weighted-Average Exercise Price, Granted	17.82
Weighted-Average Exercise Price, Exercised	11.98
Weighted-Average Exercise Price, Forfeited	18.01
Weighted-Average Exercise Price, Expired	24.93
Weighted-Average Exercise Price, Outstanding, Ending Balance	15.58	$ 15.02
Weighted-Average Exercise Price, Options exercisable, December 31, 2021	14.73
Weighted-Average Exercise Price, Options vested and expected to vest at December 31, 2021	$ 15.39
Aggregate Intrinsic Value, Outstanding at December 31, 2021	$ 3,537
Aggregate Intrinsic Value, Options exercisable at December 31, 2021	3,525
Aggregate Intrinsic Value, Options vested and expected to vest at December 31, 2021	3,537
Fair value of options vested	12,844	$ 12,819	$ 10,074
Intrinsic value of options exercised	$ 965	$ 152	$ 5,741

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	0.66%	0.35%	1.55%
Risk-free interest rate, maximum	1.41%	1.57%	2.62%
Expected volatility, minimum	71.60%	71.80%	71.10%
Expected volatility, maximum	83.50%	74.80%	75.20%
Expected dividend yield	0.00%	0.00%	0.00%
Employee and Board of Directors Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	6 years 3 months	6 years 3 months	6 years 3 months

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Compensation Expense Relating to Stock Options (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from operations	$ 0	$ 0	$ 0
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from financing activities	0	0	0
Employees And Non-Employee Members Of Board Of Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense not yet recognized	$ 24,418
Weighted average period of compensation expense not yet recognized	2 years 6 months 29 days
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	$ 16,695	11,708	10,300
Options [Member] | Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	7,126	6,765	5,206
Options [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	9,569	4,943	5,094
Restricted Stock Units [Member] | Director [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	565	528	287
Restricted Stock Units [Member] | Executive Officers [Member] | Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	1,201	1,432	604
Restricted Stock Units [Member] | Executive Officers [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	$ 2,324	$ 1,167	1,180
Shareholder Consulting Agreement [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense			$ 197

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Award Modifications (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense		$ 16,695	$ 11,708	$ 10,300
Options [Member] | General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense		9,569	$ 4,943	$ 5,094
Former President and C E O [Member] | Performance Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense		$ 0
Modified Awards [Member] | Former President and C E O [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of awards granted	12 months	12 months
Term of awards granted	18 months
Modified Awards [Member] | Former President and C E O [Member] | Share-based Payment Arrangement, Option and Time Based Restricted Stock Units [Member] | General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total incremental stock compensation expense relating to modifications of Stock Awards		$ 5,056
Modified Awards [Member] | Former President and C E O [Member] | Options [Member] | General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Incremental stock compensation expense relating to modification of stock options		$ 4,908

Income Taxes - Components of In

Income Taxes - Components of Income Taxes (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Current:
State		$ (691)	$ (816)
Total Current		(691)	(816)
Deferred:
Benefit from income taxes	$ 0	$ (691)	$ (816)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Taxes Computed Using U.S. Federal Statutory Rate to that Reflected in Operations (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income taxes using U.S. federal statutory rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	7.84%	(58.68%)	(1.99%)
Tax Cuts and Jobs Act	0	0	0
Impact of R&D tax credit on effective tax rate	2.87%	(52.06%)	4.34%
Stock option shortfalls and cancellations	(4.74%)	0.35%	(0.17%)
Permanent items and other	(1.29%)	(5.08%)	0.36%
Change in valuation allowance	(25.69%)	84.61%	(22.76%)
Provision to return	0.01%	0.92%	(0.02%)
Non-taxable revenue	0.00%	0.00%	0.00%
Total	0.00%	(8.94%)	0.76%

Income Taxes - Schedule of Sign

Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Valuation allowance	$ (143,388)	$ (120,666)
Deferred tax assets:
Net operating loss carryforwards	113,636	93,507
Federal and state tax credits	24,428	19,982
Stock-based compensation expense	5,023	6,732
Other	1,268	2,414
Deferred tax assets	144,355	122,635
Deferred tax liabilities:
Other	(967)	(1,969)
Deferred tax liabilities	$ (967)	$ (1,969)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Benefit from income taxes	$ 0	$ (691)	$ (816)
Valuation allowance on the deferred tax asset	100.00%	100.00%
Change in valuation allowance	$ 22,722	$ 6,530
Unrecognized tax benefits, related interest and penalties	$ 0	$ 0
Tax examinations, description	tax years 2006 through 2021 remain open to U.S. federal and state tax examinations.
Income taxes using U.S. federal statutory rate	21.00%	21.00%	21.00%
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 443,134
Federal [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards expiration year	2026
Federal [Member] | R&D Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 19,864
Federal [Member] | R&D Tax Credit Carryforwards [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Tax credit expiration year	2025
State and Local [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 347,313
State and Local [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards expiration year	2027
State and Local [Member] | R&D Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 5,516

Net (Loss) Income per Share - C

Net (Loss) Income per Share - Computation of Denominators Used in Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic:
Weighted average common shares outstanding - Basic	50,718,765	47,413,250	42,669,333
Diluted:
Weighted average common shares outstanding - Basic	50,718,765	47,413,250	42,669,333
Common stock equivalents	0	501,780	0
Denominator for diluted net (loss) income per share	50,718,765	47,915,030	42,669,333

Net (Loss) Income per Share -_2

Net (Loss) Income per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net (loss) income - basic and diluted	$ (88,441)	$ 8,410	$ (106,373)
Weighted-average common shares outstanding:
Basic	50,718,765	47,413,250	42,669,333
Diluted	50,718,765	47,915,030	42,669,333
Stock options		481,254
Restricted stock units		20,526
Net (loss) income per share:
Basic	$ (1.74)	$ 0.18	$ (2.49)
Diluted	$ (1.74)	$ 0.18	$ (2.49)

Net (Loss) Income per Share - A

Net (Loss) Income per Share - Additional Information (Detail) - shares	Oct. 15, 2020	Oct. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2019
Number of options outstanding			5,469,393	6,512,280	4,450,517
Number of restricted stock units outstanding			235,250	576,544	119,834
Dilutive stock options			481,254
Dilutive restricted stock units			20,526
Vifor International Ltd. [Member] | Regulatory Milestones [Member]
Stock Issued During Period Shares New Issues		3,282,391
Percentage of premium on common stock investment	20.00%	20.00%
Stock issuance price measurement period	30 days	30 days

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Defined contribution retirement plan, description	All employees over the age of 21 are eligible to participate in the plan at the beginning of the next calendar month after three consecutive months of service
Employer contributions to the plan	$ 460	$ 349	$ 279
Defined contribution retirement plan, employer contribution percentage	3.00%
Minimum [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution retirement plan, employee eligible age to participate	21 years
Defined contribution retirement plan, employee eligible service period	3 months

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	Oct. 31, 2020USD ($)	Aug. 31, 2019USD ($)	Jul. 31, 2019	Jun. 30, 2020item	Jul. 31, 2019agreement	Dec. 31, 2021USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2016	May 31, 2016USD ($)
Other Commitments [Line Items]
Research and development expense						$ 82,701	$ 107,851	$ 113,820
Operating lease liabilities						3,673
Operating lease right-of-use assets						$ 2,973	$ 4,279
New Stamford Lease [Member]
Other Commitments [Line Items]
Operating lease liabilities	$ 1,934
Operating lease right-of-use assets	1,934
Lease incentives	$ 0
Stamford Operating Lease [Member]
Other Commitments [Line Items]
Number of renewable terms | item						1
Lease term									7 years
Operating Lease, renewable term						5 years
Operating lease, expiration date						Dec. 31, 2023
Tenant improvement expenses										$ 1,094
Operating lease cost								937
Stamford Operating Lease [Member] | Research and Development [Member]
Other Commitments [Line Items]
Operating lease cost								656
Stamford Operating Lease [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Operating lease cost								281
Stamford Operating Lease [Member] | ASC 842 [Member]
Other Commitments [Line Items]
Number of renewable terms | item						1
Operating Lease, renewable term						5 years
Stamford Operating Lease [Member] | Standby Letter of Credit [Member]
Other Commitments [Line Items]
Letter of credit, renewal term						automatically renewed annually through November 2023.
New Stamford Lease [Member]
Other Commitments [Line Items]
Number of renewable terms | item				1
Operating Lease, renewable term				5 years
Operating lease, expiration date				Dec. 31, 2023
New Stamford Lease [Member] | ASC 842 [Member]
Other Commitments [Line Items]
Number of renewable terms | item						1
Operating Lease, renewable term						5 years
Stamford Operating Lease and New Stamford Lease [Member]
Other Commitments [Line Items]
Lease term						2 years	3 years
Operating lease cost						$ 1,624	$ 1,116
Stamford Operating Lease and New Stamford Lease [Member] | Research and Development [Member]
Other Commitments [Line Items]
Operating lease cost						1,137	781
Stamford Operating Lease and New Stamford Lease [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Operating lease cost						487	335
Non-Exclusive License Agreement
Other Commitments [Line Items]
Research and development expense						15,000	5,000	$ 8,000
Non-Exclusive License Agreement | Enteris Biopharma, Inc. [Member]
Other Commitments [Line Items]
Upfront fee		$ 8,000
Upfront fee payment, cash		4,000
Upfront fee payment, stock		$ 4,000
Payment of milestone payment						$ 15,000	$ 5,000
License agreement expiration period, first commercial sale		10 years
Agreement termination period for material breach		60 days
Agreement termination period for material breach of a payment obligation		30 days
Agreement termination period for challenged patent rights		30 days
Agreement termination period prior to first regulatory approval		30 days
Agreement termination period after first regulatory approval		60 days
MSA [Member]
Other Commitments [Line Items]
Agreement initial term, expiration date			Dec. 31, 2023
Agreement renewal term			2 years
MSA [Member] | Minimum [Member]
Other Commitments [Line Items]
Termination notice effective period			18 months
MSA [Member] | Patheon and Patheon Manufacturing Services LLC [Member]
Other Commitments [Line Items]
Number of related product agreements | agreement					2
MSA or Product Agreement [Member]
Other Commitments [Line Items]
Agreement Termination Notice Description			Either party may terminate the MSA or a Product Agreement upon written notice if the other party (1) has failed to remedy a material breach within a specified time or (2) is declared insolvent or bankrupt, voluntarily files a petition of bankruptcy or assigns such agreement for the benefit of creditors. The Company may terminate a Product Agreement (a) upon 90 days’ prior written notice if any governmental agency takes any action that prevents the Company from selling the relevant product in the relevant territory, (b) upon six months’ prior written notice if it does not intend to order manufacturing services due to a product’s discontinuance in the market, or (c) upon 90 days’ prior written notice if it determines that the manufacture or supply of a product likely infringes third-party rights. Patheon may terminate the MSA or a Product Agreement (i) upon six months’ prior written notice if the Company assigns such agreement to an assignee that is unacceptable to Patheon for certain reasons, or (ii) upon 30 days’ prior written notice if, after the first year of commercial sales, the Company forecasts zero volume for 12 months.
Termination notice effective period upon government agency restrictions			90 days
Termination notice effective period upon discontinuance in the market			6 months
Termination notice period if likely infringes on third-party rights			90 days
Termination notice period if assignee is unacceptable			6 months
Termination notice period if zero volume is forecasted after first year of commercial sales			30 days

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Other Information related to Stamford Lease (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2016
Stamford Operating Lease [Member]
Other Commitments [Line Items]
Remaining lease term - operating leases (years)			7 years
Stamford Operating Lease and New Stamford Lease [Member]
Other Commitments [Line Items]
Operating cash outflows relating to operating leases	$ 1,921	$ 1,403
ROU assets obtained in exchange for new operating lease liabilities		$ 1,934
Remaining lease term - operating leases (years)	2 years	3 years
Discount rate - operating leases	7.00%	7.00%

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Non-cancellable Operating Leases, Reconciliation of Undiscounted Cash Flows to the Operating Lease Liability (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Operating Lease Liabilities, Payments Due [Abstract]
2022	$ 1,957
2023	1,991
Total future minimum lease payments, undiscounted	3,948
Less imputed interest	(275)
Operating lease liabilities	3,673
Operating lease liabilities reported as of December 31, 2021:
Operating lease liabilities - current	1,755	$ 1,602
Operating lease liabilities - non-current	1,918	$ 3,673
Total	$ 3,673

Related Party Transactions (Det

Related Party Transactions (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)shares
Vifor International Ltd. [Member]
Related Party Transaction [Line Items]
Shares owned as a result of upfront and milestone payments | shares	7,396,770
Related party ownership percentage	13.80%
Vifor International Ltd. [Member] | Commercial Supply Revenue
Related Party Transaction [Line Items]
Revenue from related parties	$ 701
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Clinical Compound Revenue
Related Party Transaction [Line Items]
Revenue from related parties	$ 361

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Thousands	Mar. 01, 2022	Apr. 04, 2019
Maximum [Member]
Subsequent Event [Line Items]
Aggregate Offering Price Of Securities		$ 300,000
Subsequent Event [Member] | Shelf Registration Statement [Member] | Maximum [Member]
Subsequent Event [Line Items]
Aggregate Offering Price Of Securities	$ 300,000
Subsequent Event [Member] | Unsold Securities Under Shelf Registration Statement dated April 4, 2019 [Member]
Subsequent Event [Line Items]
Aggregate Offering Price Of Securities	154,525
Subsequent Event [Member] | Jeffries LLC [Member] | Open Market Sales Agreement [Member]
Subsequent Event [Line Items]
Aggregate Offering Price Of Securities	$ 80,000
Stock Sale Commission Percentage	3.00%