Cara Therapeutics (CARA) 10-K 25 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 22, 2021	Jun. 30, 2020
Document And Entity Information [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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	Yes
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			$ 726,180,228
Entity Common Stock, Shares Outstanding		49,876,763
Entity Central Index Key	0001346830
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 31,683	$ 18,305
Marketable securities	149,242	136,701
Income tax receivable	1,507	816
Other receivables	557	971
Prepaid expenses	12,076	8,863
Total current assets	195,065	165,656
Operating lease right-of-use assets	4,279	3,036
Marketable securities, non-current	70,565	63,159
Property and equipment, net	840	700
Restricted cash	408	408
Total assets	271,157	232,959
Current liabilities:
Accounts payable and accrued expenses	16,881	19,665
Operating lease liabilities, current	1,602	967
Current portion of deferred revenue		22,262
Total current liabilities	18,483	42,894
Operating lease liabilities, non-current	3,673	3,352
Commitments and contingencies (Note 17)
Stockholders' equity:
Preferred stock; $0.001 par value; 5,000,000 shares authorized at December 31, 2020 and December 31, 2019, zero shares issued and outstanding at December 31, 2020 and December 31, 2019
Common stock; $0.001 par value; 100,000,000 shares authorized at December 31, 2020 and December 31, 2019, 49,872,213 shares and 46,720,225 shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	50	47
Additional paid-in capital	641,195	587,223
Accumulated deficit	(392,317)	(400,727)
Accumulated other comprehensive income	73	170
Total stockholders' equity	249,001	186,713
Total liabilities and stockholders' equity	$ 271,157	$ 232,959

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
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	49,872,213	46,720,225
Common stock, shares outstanding	49,872,213	46,720,225

Statements of Comprehensive Inc

Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Total revenue	$ 135,082	$ 19,886	$ 13,469
Operating expenses:
Research and development	107,851	113,820	75,531
General and administrative	21,846	17,745	15,320
Total operating expenses	129,697	131,565	90,851
Operating income (loss)	5,385	(111,679)	(77,382)
Other income, net	2,334	4,490	2,980
Income (loss) before benefit from income taxes	7,719	(107,189)	(74,402)
Benefit from income taxes	691	816	389
Net income (loss)	$ 8,410	$ (106,373)	$ (74,013)
Net income (loss) per share:
Basic	$ 0.18	$ (2.49)	$ (2.06)
Diluted	$ 0.18	$ (2.49)	$ (2.06)
Weighted average shares:
Basic	47,413,250	42,669,333	35,892,786
Diluted	47,915,030	42,669,333	35,892,786
Other comprehensive income (loss), net of tax of $0:
Change in unrealized gains (losses) on available-for-sale marketable securities	$ (97)	$ 284	$ (44)
Total comprehensive income (loss)	8,313	(106,089)	(74,057)
License and Milestone Fees
Revenue:
Total revenue	134,439	19,746	13,436
Clinical Compound Revenue
Revenue:
Total revenue	$ 643	$ 140	$ 33

Statements of Comprehensive I_2

Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Other comprehensive income, tax	$ 0	$ 0	$ 0

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance, Value at Dec. 31, 2017	$ 33	$ 307,158	$ (220,341)	$ (70)	$ 86,780
Balance, Shares at Dec. 31, 2017	32,662,255
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, value	$ 5	92,058			92,063
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, Shares	5,175,000
Sale of common stock under license agreement, value	$ 1	14,555			14,556
Sale of common stock under license agreement, shares	1,174,827
Stock-based compensation expense		7,785			7,785
Shares issued upon exercise of stock options, value		4,194			4,194
Shares issued upon exercise of stock options, shares	451,685
Shares issued upon vesting of restricted stock units, value		1,693			1,693
Shares issued upon vesting of restricted stock units, shares	83,791
Modification of equity awards		616			616
Net income (loss)			(74,013)		(74,013)
Other comprehensive income (loss)				(44)	(44)
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
Sale of common stock under license agreement, value		4,000			4,000
Sale of common stock under license agreement, 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 income (loss)			(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, value	$ 3	38,446			38,449
Sale of common stock under license agreement, 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				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 income (loss)			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

Statements of Stockholders' E_2

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Follow-on Public Offering [Member]
Price per share	$ 23	$ 19
Underwriting discounts and commissions and offering expenses	$ 8,977	$ 6,262
License Agreements [Member]
Price per share	$ 23.42

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net income (loss)	$ 8,410	$ (106,373)	$ (74,013)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Stock-based compensation expense	14,835	12,568	9,478
Modification of equity awards			616
Depreciation and amortization	209	198	370
Amortization expense component of lease expense	806	601
Noncash expense related to oral formulation license agreement		4,000
Amortization/(accretion) of available-for-sale marketable securities, net	154	(1,381)	(1,820)
Realized loss (gain) on sale of available-for-sale marketable securities	(272)		5
Deferred rent costs			(156)
Deferred revenue	(22,262)	(19,747)	42,009
Changes in operating assets and liabilities:
Income tax receivable	(691)	(152)	67
Other receivables	414	(45)	(803)
Prepaid expenses	(3,213)	(4,058)	(3,170)
Accounts payable and accrued expenses	(2,784)	6,043	5,116
Operating lease liabilities	(1,093)	(879)
Net cash used in operating activities	(5,487)	(109,225)	(22,301)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	144,320	253,584	175,300
Proceeds from redemptions of available-for-sale marketable securities, at par	27,035	2,000
Proceeds from sale of available-for-sale marketable securities	41,600	0	79,808
Purchases of available-for-sale marketable securities	(232,881)	(286,082)	(337,854)
Purchases of property and equipment	(349)	(18)	(73)
Net cash used in investing activities	(20,275)	(30,516)	(82,819)
Financing activities
Proceeds from the sale of common stock under license agreement	38,449		14,556
Proceeds from the sale of common stock in a follow-on public offering, net of issuance costs		136,498	92,063
Proceeds from the exercise of stock options	691	6,106	4,194
Net cash provided by financing activities	39,140	142,604	110,813
Net increase in cash, cash equivalents and restricted cash	13,378	2,863	5,693
Cash, cash equivalents and restricted cash at beginning of period	18,713	15,850	10,157
Cash, cash equivalents and restricted cash at end of period	$ 32,091	18,713	$ 15,850
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, 2020
Basis of Presentation
Business	1. Business Cara Therapeutics, Inc., or the Company, is a clinical-stage biopharmaceutical corporation formed on July 2, 2004. The Company is focused on developing and commercializing new chemical entities designed to alleviate pruritus by selectively targeting peripheral kappa opioid receptors. The Company’s primary activities to date have been organizing and staffing the Company, developing its product candidates and raising capital. As of December 31, 2020, 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 $201,893 under its license agreements for CR845/difelikefalin, primarily with Vifor (International) Ltd., or 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. 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 Note 11, Collaboration and Licensing Agreements As of December 31, 2020, the Company had unrestricted cash and cash equivalents and marketable securities of $251,490 and an accumulated deficit of $392,317. 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 net income of $8,410 and had net cash used in operating activities of $5,487 for the year ended December 31, 2020. 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 Food and Drug Administration, or FDA, and other government regulations. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate recurring product revenue or achieve profitability.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
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 periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, 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 and the periods over which those costs are expensed, 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. Concentrations of Credit Risk Financial instruments, which potentially subject the Company to significant concentrations of credit risk consist primarily of cash equivalents and marketable securities. 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 four 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. 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, 2020. 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 income (loss). 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, 2020 or December 31, 2019. Property and Equipment, net Property and equipment (consisting of computer, office and laboratory 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 Short-term laboratory equipment 2 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 On January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Collaboration and Licensing Agreements Revenue Recognition) The Company has entered into agreements to license its intellectual property, or IP, related to CR845/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 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 CR845/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. 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 CRO’s, 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, 2020 and 2019, the Company recorded $11,286 and $8,498 as prepaid R&D expense, respectively. 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, 2020 and December 31, 2019. 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, 2020, 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. During the period from January 31, 2014 to December 31, 2018, the Company’s stock had not traded on a public exchange for a sufficient period of time to approximate its expected term (as noted above). Therefore, the Company calculated the volatility for stock options granted using an analysis of guideline companies for the year ended December 31, 2018. Volatility calculated in this manner was in the range of 83% - 93% for stock options granted during the year ended December 31, 2018. Beginning on January 1, 2019, the Company determined that it had sufficient company-specific trading activity of its common stock available to use that activity exclusively to calculate the volatility of the Company’s common stock. Volatility calculated in this manner was in the range of 72% - 75% for stock options granted during the year ended December 31, 2020, and 71% - 75% for stock options granted during the year ended December 31, 2019. Volatility of the Company’s common stock from January 31, 2014 to December 31, 2018 was 75%, which was determined to be comparable to the method using guideline companies during the year ended December 31, 2018. 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. Prior to January 1, 2019, the Company accounted for stock options granted to non-employee consultants under ASC 505-50, Equity-Based Payments to Non-Employees On January 1, 2019, the Company adopted ASU 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Non-employee Share-Based Payment Accounting 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. Income (Loss) Per Share The Company computes basic net income (loss) per share by dividing net income (loss) by the weighted average number of shares of common stock outstanding. Diluted net income (loss) 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 the quarter and 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. For each of the years ended December 31, 2019 and 2018, 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. 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 and development of novel therapeutics to treat serious medical conditions, including pruritus and pain. Leases Prior to January 1, 2019, the Company recognized rent expense for operating leases on a straight-line basis over the term of the lease, beginning on the date the Company took possession of the property. Rent expense included the base amounts stated in the lease agreement as well as the effect of reduced or free rent and rent escalations. At lease inception, the Company determined the lease term by assuming the exercise of those renewal options that are reasonably assured because of the significant economic penalty that exists for not exercising those options. The exercise of renewal options is at the Company’s sole discretion. The expected lease term was one of the factors used to determine whether a lease was classified as operating or capital and was used to calculate the straight-line rent expense. The difference between the cash paid to the landlord and the amount recognized as rent expense on a straight-line basis was included in deferred rent and classified within long-term liabilities. Lease incentives made by landlords to or on behalf of the Company for leasehold improvements were recorded as deferred rent and classified as long-term liabilities. Deferred rent related to landlord incentives was amortized using the straight-line method over the lease term as an offset to rent expense for the year ended December 31, 2018. On January 1, 2019, the Company adopted ASC 842, Leases ● No re-evaluation of whether a contract is or contains a lease (embedded lease); ● Lease classification is grandfathered ● No reassessment of initial direct costs Upon adoption of ASC 842, the Company had only one lease, the Stamford Lease (see Note 17, 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 Statement of Comprehensive Income (Loss). There was no cumulative effect adjustment as a result of the adoption of ASC 842 on January 1, 2019, which reflects the difference between the amount of lease expense under ASC 842 that would have been recognized from inception of the Stamford Lease through December 31, 2018 and

Available-for-Sale Marketable S

Available-for-Sale Marketable Securities	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Marketable Securities	3. Available-for-Sale Marketable Securities As of December 31, 2020 and 2019, 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, 2020 and 2019: 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 ​ As of December 31, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 16,052 ​ $ 31 ​ $ (2) ​ $ 16,081 U.S. government agency obligations ​ 25,803 ​ 14 ​ (1) ​ 25,816 Corporate bonds ​ 115,788 ​ 125 ​ (23) ​ 115,890 Commercial paper ​ 38,547 ​ 27 ​ (1) ​ 38,573 Municipal bonds ​ ​ 3,500 ​ ​ — ​ ​ — ​ ​ 3,500 Total available-for-sale marketable securities ​ $ 199,690 ​ $ 197 ​ $ (27) ​ $ 199,860 ​ 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, 2020 and 2019: 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, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ $ 3,185 ​ $ (2) ​ $ — ​ $ — ​ $ 3,185 ​ $ (2) U.S. government agency obligations ​ 2,400 ​ (1) ​ — ​ — ​ 2,400 ​ (1) Corporate bonds ​ 28,895 ​ (23) ​ — ​ — ​ 28,895 ​ (23) Commercial paper ​ 4,264 ​ (1) ​ — ​ — ​ 4,264 ​ (1) Total ​ $ 38,744 ​ $ (27) ​ $ — ​ $ — ​ $ 38,744 ​ $ (27) ​ As of December 31, 2020 and 2019, 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, 2020 and 2019, the Company held a total of 30 out of 59 positions and 16 out of 81 positions, respectively, that were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. Unrealized losses individually and in aggregate 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. positions for its U.S. government agency obligations, that were in unrealized loss positions as of December 31, 2020. Corporate bonds, commercial paper, and municipal bonds. The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2020, the Company’s marketable debt securities mature at various dates through December 2023. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2020 ​ As of December 31, 2019 Contractual maturity Amortized Cost Fair Value Amortized Cost Fair Value Less than one year ​ $ 149,164 ​ $ 149,242 ​ $ 136,565 ​ $ 136,701 One year to three years ​ 70,570 ​ 70,565 ​ 63,125 ​ 63,159 Total ​ $ 219,734 ​ $ 219,807 ​ $ 199,690 ​ $ 199,860 ​ 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, 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, 2020 and 2019, accrued interest receivables on our available-for-sale debt securities were $311 and $971, respectively.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	4. Accumulated Other Comprehensive Income (Loss) The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax, from unrealized gains (losses) on available-for-sale marketable securities, the Company’s only component of accumulated other comprehensive income (loss), for the years ended December 31, 2020, 2019 and 2018. ​ ​ ​ ​ ​ Total Accumulated ​ ​ Other Comprehensive ​ ​ Income (Loss) Balance, December 31, 2017 ​ $ (70) Other comprehensive loss before reclassifications ​ (49) Amount reclassified from accumulated other comprehensive loss ​ 5 Net current period other comprehensive loss ​ (44) 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 ​ Amounts reclassified out of accumulated other comprehensive income (loss) into net income (loss) are determined by specific identification. The reclassifications out of accumulated other comprehensive income (loss) and into net income (loss) were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Affected Line Item in the Component of Accumulated Other ​ Year Ended December 31, ​ Statements of Comprehensive Income (Loss) 2020 2019 2018 Comprehensive Income (Loss) Unrealized gains (losses) on available-for- sale marketable securities ​ ​ ​ Realized gains (losses) on sale of securities ​ $ 272 ​ $ — ​ $ (5) Other income, net ​ ​ — ​ — ​ — Benefit from income taxes ​ ​ $ 272 ​ $ — ​ $ (5) ​ ​ ​

Prepaid Expenses

Prepaid Expenses	12 Months Ended
	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses	5. Prepaid Expenses 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. As of December 31, 2019, the amount of prepaid expenses was $8,863, consisting of $8,498 of prepaid R&D clinical costs, $181 of prepaid insurance and $184 of other costs.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, net	6. Property and Equipment, net Property and equipment, net consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 Computer and office equipment ​ $ 211 ​ $ 211 Laboratory equipment ​ 628 ​ 628 Furniture and fixtures ​ 330 ​ 61 Leasehold improvements ​ 1,212 ​ 1,132 ​ ​ $ 2,381 ​ $ 2,032 Less accumulated depreciation and amortization ​ 1,541 ​ 1,332 Property and equipment, net ​ $ 840 ​ $ 700 ​ Depreciation and amortization expense included in R&D expense and general and administrative, or G&A, expense was $209, $198 and $370 for the years ended December 31, 2020, 2019 and 2018, respectively. There were no gains or losses on sales of property and equipment during the years ended December 31, 2020, 2019 and 2018.

Restricted Cash

Restricted Cash	12 Months Ended
	Dec. 31, 2020
Receivables [Abstract]
Restricted Cash	7. Restricted Cash 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 17, Commitments and Contingencies: Leases The letter of credit balance for the Stamford lease was required to remain at $769 through May 2019 December 2023 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, 2020 December 31, 2019 Cash and cash equivalents ​ $ 31,683 ​ $ 18,305 Restricted cash, long-term assets ​ 408 ​ 408 Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows ​ $ 32,091 ​ $ 18,713 ​

Accounts Payable and Accrued Ex

Accounts Payable and Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Accounts Payable and Accrued Expenses	8. Accounts Payable and Accrued Expenses Accounts payable and accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 Accounts payable ​ $ 4,893 ​ $ 9,100 Accrued research projects ​ 6,194 ​ 6,637 Accrued professional fees ​ 839 ​ 635 Accrued compensation and benefits ​ 4,955 ​ 3,293 Total ​ $ 16,881 ​ $ 19,665 ​

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders' Equity	9. 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, 2020, there were 49,872,213 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 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 13, 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 11, 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 the earlier of (a) October 15, 2022 and (b) the date that the Company publicly discloses the receipt of a complete response letter from the FDA with respect to the Company’s NDA for CR845/difelikefalin injection, 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 13, 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 13, 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 13, 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 17, Commitments and Contingencies 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 13, 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 Income (Loss), all of which related to G&A expense. In December 2018, as a result of the achievement of a clinical performance target, an aggregate of 83,791 restricted stock units of various executive officers vested and were settled in shares of the Company’s common stock (see Note 13, Stock-Based Compensation In July 2018, the Company entered into an underwriting agreement with Jefferies LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as representatives of the several underwriters named therein, relating to the issuance and sale by the Company of up to 5,175,000 shares of its common stock, including 675,000 shares of common stock the underwriters had the option to purchase, at a public offering price of $19.00 per share. This offering was made pursuant to the Company’s Registration Statement on Form S-3 (File No. 333-216657), filed with the SEC on March 13, 2017 and declared effective on March 24, 2017, and a related prospectus dated March 24, 2017 and prospectus supplement dated July 18, 2018, which was filed with the SEC on July 20, 2018. On July 23, 2018, the Company closed the offering, including the full exercise of the underwriters’ option to purchase 675,000 additional shares of common stock. The Company received net proceeds of $92,063, after deducting $6,262 relating to underwriting discounts and commissions and offering expenses. In May 2018, the Company issued 1,174,827 shares of its common stock to Vifor in connection with the license agreement entered into with VFMCRP (refer to Note 11, Collaboration and Licensing Agreements

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	10. Fair Value Measurements The following tables summarize the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2020 and 2019 and by level within the fair value hierarchy: 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 ​ $ — ​ Fair value measurement as of December 31, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ $ 18,305 ​ $ 18,305 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ U.S. Treasury securities ​ ​ 16,081 ​ ​ — ​ ​ 16,081 ​ ​ — U.S. government agency obligations ​ 25,816 ​ — ​ 25,816 ​ — Corporate bonds ​ 115,890 ​ — ​ 115,890 ​ — Commercial paper ​ 38,573 ​ — ​ 38,573 ​ — Municipal bonds ​ ​ 3,500 ​ ​ — ​ ​ 3,500 ​ ​ — Restricted cash: ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 218,573 ​ $ 18,713 ​ $ 199,860 ​ $ — ​ 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, 2020, 2019 and 2018. There were no transfers of financial assets between Levels 1, 2, or 3 classifications during the years ended December 31, 2020 and 2019.

Collaboration and Licensing Agr

Collaboration and Licensing Agreements	12 Months Ended
	Dec. 31, 2020
Basis of Presentation
Collaboration and Licensing Agreements	11. Collaboration and Licensing Agreements Vifor (International) Ltd. 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 CR845/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, CR845/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 CR845/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 CR845/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 12, 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. 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 ​ Upon U.S. regulatory approval of CR845/difelikefalin, the Company will also be eligible to receive an additional $50,000 common stock investment at a 20% premium to the 30-day trailing average price of the Company’s common stock as of such date. 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. The license also requires Vifor to promote and take orders for Licensed Products 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 in 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. The supply price will be the Company’s cost of goods sold, as calculated under U.S. GAAP, plus an agreed upon margin. The Vifor Supply Agreement will co-terminate with the Vifor Agreement. In regards to a supply agreement, the Vifor Agreement only includes a requirement for the Company to negotiate in good faith with Vifor. After the execution of the Vifor Agreement, a separate agreement to supply them with the Licensed Product would be entered into, although the Company has no obligation to execute a supply agreement. The Vifor Supply Agreement will be 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 cost of goods sold plus an agreed upon margin, which is commensurate with the “cost of goods sold 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 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 Income (Loss) as sales of the Licensed Product occur. As of December 31, 2020, no supply agreement has been entered into between the Company and Vifor. The Vifor Agreement will continue in effect until its expiration upon the cessation of commercial sale of CR845/difelikefalin injection in the United States by Vifor and its affiliates and sublicensees, or until the earlier termination of the Vifor Agreement. In addition, upon the earlier of: (1) the acceptance for filing of an NDA covering CR845/difelikefalin injection submitted to the FDA; or (2) October 15, 2023, the Vifor Agreement may be terminated by Vifor in its entirety, with such termination effective upon 12 months’ notice. ​ The Vifor Agreement may also 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. 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 CR845/difelikefalin injection, or the Licensed Product, for all therapeutic uses to prevent, inhibit or treat itch associated with pruritus in hemodialysis and peritoneal-dialysis patients, or 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. If third-party costs incurred by the Company with respect to its clinical development for the EMA registration exceed $20,000, such excess costs will be shared equally by the Company and VFMCRP. 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. The Company is eligible to receive from VFMCRP regulatory and commercial milestone payments in the aggregate of up to $470,000, consisting of up to $30,000 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 CR845/difelikefalin injection in the Licensed Territories. The Company retains full commercialization rights for CR845/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 CR845/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. 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 will be recognized proportionately as the R&D services are conducted (i.e., prior to submission of an NDA). 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 cost of goods sold, 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 cost of goods sold plus an agreed upon margin, which is commensurate with the “cost of goods sold 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 Income (Loss) as sales of the Licensed Product occur. During the year ended December 31, 2020, the Company recognized clinical compound revenue of $115 from the sale of clinical compound to VFMCRP and as a result, the Company incurred R&D expense of $108 during this period. 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. 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 CR845/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 CR845/difelikefalin in the United States and Japan, respectively. In addition, the Company provided Maruishi specific clinical development services for CR845/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, 2020, 2019 and 2018, the Company recognized clinical compound revenue of $528, $140 and $33, respectively, from the sale of clinical compound to Maruishi. The Company incurred R&D expense related to the Maruishi Agreement of $476, $126 and $30 (all related to the cost of clinical compound sold to Maruishi) during the years ended December 31, 2020, 2019 and 2018, respectively. Chong Kun Dang Pharmaceutical Corporation In April 2012, the Company entered into a license agreement, or the CKDP Agreement, with Chong Kun Dang Pharmaceutical Corporation, or CKDP, in South Korea, under which the Company granted CKDP an exclusive license to develop, manufacture and commercialize drug products containing CR845/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 CR845/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 CR845/difelikefalin in South Korea, if any, and share in any sub-license fees.

Revenue Recognition

Revenue Recognition	12 Months Ended
	Dec. 31, 2020
Revenue Recognition [Abstract]
Revenue Recognition	12. Revenue Recognition The Company currently recognizes revenue in accordance with ASC 606, as amended, for the Vifor, VFMCRP, Maruishi and CKDP agreements (see Note 11 , Collaboration and Licensing Agreements Contract balances As of December 31, 2020, there were no material balances of receivables, and no other assets or deferred revenue other assets deferred revenue other assets deferred revenue relating to the Maruishi and CKDP agreements as of December 31, 2019. Performance obligations Under the Vifor Agreement, the Company’s only performance obligation is granting a license to allow Vifor to commercialize CR845/difelikefalin in the United States, which occurred at inception of the contract in October 2020 (see Note 11, Collaboration and Licensing Agreements Under the VFMCRP Agreement, the Company’s performance obligations of granting a license to allow VFMCRP to commercialize CR845/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 CR845/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 11, 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 CR845/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 CR845/difelikefalin and to conduct Phase 1 and proof-of-concept Phase 2 clinical trials of an intravenous formulation of CR845/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. 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 CR845/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 CR845/difelikefalin, which occurred at inception of the contract in 2012. 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, 2020, 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. For the year ended December 31, 2020, $22,262 was recognized as license and milestone fees revenue based on the percentage of R&D services that were completed during the period. As of December 31, 2020, 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, 2020, 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 11, 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 CR845/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 CR845/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. 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, 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 and sales milestones (see Note 11, 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 and sales milestones and sales royalties (see Note 11, 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 CR845/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 CR845/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 CR845/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 CR845/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 CR845/difelikefalin, conduct clinical trials, or commercialize CR845/difelikefalin, and is considered to be functional IP. During the license periods, the Company is continuing to develop and advance CR845/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 CR845/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, 2020, there is no remaining amount of the transaction price to be recognized as license and milestone fees revenue. 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). 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, 2019 and 2018. 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, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	13. 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 year ended December 31, 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 first anniversary of the date of grant. 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, 2021, the aggregate number of shares of common stock that may be issued pursuant to Stock Awards under the 2014 Plan automatically increased from 7,488,513 to 8,984,679. 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 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 will vest on the earlier of (i) June 4, 2021 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 will recognize 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, 2020, $323 of stock compensation expense relating to these restricted stock units was recognized in G&A expense. None of the 36,000 restricted stock units vested and were settled in shares of the Company’s common stock as of December 31, 2020. 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 the results of ongoing clinical trials, NDA filing and FDA approval as well as the recipient’s continuous service through each performance target. At the date of grant, the Company concluded that the probability of achievement of the performance targets could not be determined until the various milestones were probable of being achieved. Recognition of compensation expense associated with these awards begins when, and to the extent, the performance criteria have been achieved and therefore, the restricted stock units are earned and vesting has occurred. 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. As of December 31, 2020, 36,750 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. For the year ended December 31, 2020, the Company recognized $455 of stock compensation expense, with $149 recorded in R&D expense and $306 in G&A expense. None of the 98,000 restricted stock units vested or were settled in shares of the Company’s common stock as of December 31, 2020. 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. The restricted stock units vested on the earlier of (i) June 4, 2020 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 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, 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, 2020, 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 the results of ongoing clinical trials, subject to the recipient’s continuous service through the vesting events. At the date of grant, the Company concluded that the probability of achievement of the performance targets could not be determined until the milestones were probable of being achieved, and accordingly, the Company would recognize compensation expense associated with these awards when, and to the extent, the restricted stock units vested in accordance with achievement of the performance targets. In December 2019 and May 2019, performance targets relating to 36,666 and 74,166 restricted stock units, respectively, had been achieved and thus such restricted stock units vested and the awards were settled in shares common thus such restricted stock units vested and the awards were settled in shares common In September 2018, the Company granted a total of 83,791 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $20.21 per share. Vesting of the restricted stock units was contingent on the achievement of certain performance targets through the first quarter of 2019, subject to the recipient’s continuous service through the vesting events. At the date of grant, the Company concluded that the probability of achievement of the performance targets could not be determined until they were achieved, and accordingly, the Company would recognize compensation expense associated with these awards when, and to the extent, the restricted stock units vested in accordance with achievement of the performance targets. As of December 31, 2018, all of the performance targets had been achieved and, consequently, all of the restricted stock units had vested. As a result, $1,693 of stock compensation expense relating to the vesting of restricted stock units was recognized for the year ended December 31, 2018, with $1,217 recorded in G&A expense and $476 in R&D expense. In addition, all of the 83,791 restricted stock units vested and were settled in shares of the Company’s common stock as of December 31, 2018. 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, 2020 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ Number of ​ Average Exercise ​ Intrinsic ​ Shares Price Value Outstanding, December 31, 2019 4,450,517 ​ $ 14.73 ​ Granted 1,377,850 ​ 15.88 ​ Exercised (55,852) ​ 12.37 ​ Expired (175,971) ​ 14.06 ​ Forfeited (127,151) ​ 16.55 ​ Outstanding, December 31, 2020 5,469,393 ​ $ 15.02 ​ $ 8,441 Weighted average remaining contractual life as of December 31, 2020 (in years) 7.26 ​ ​ Options exercisable, December 31, 2020 3,192,128 ​ $ 13.96 ​ $ 7,589 Weighted average remaining contractual life as of December 31, 2020 (in years) 6.23 ​ ​ Options vested and expected to vest as of December 31, 2020 5,469,393 ​ $ 15.02 ​ $ 8,441 Weighted average remaining contractual life as of December 31, 2020 (in years) 7.26 ​ ​ ​ The total fair value of options vested during the years ended December 31, 2020, 2019 and 2018 was $12,819, $10,074, and $9,023, respectively. The intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 was $152, $5,741, and $3,893, respectively. During the years ended December 31, 2020, 2019 and 2018, the Company granted 1,377,850, 1,324,000 and 1,197,500 stock options, respectively, to employees and non-employee members of the Board of Directors. There were Summary of Significant Accounting Policies – Stock-Based Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 2019 2018 Risk-free interest rate 0.35% - 1.57 % 1.55% - 2.62 % 2.51% - 3.09 % Expected volatility 71.8% - 74.8 % 71.1% - 75.2 % 82.6% - 92.8 % 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, 2020, 2019 and 2018 was $10.33, $11.67, and $11.99, respectively. On January 1, 2019, the Company used the Black Scholes option valuation model to remeasure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07. At the end of each fiscal quarter during the year ended December 31, 2018, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50. The range of assumptions used by the Company on January 1, 2019 and during the year ended December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ January 1, 2019 Year Ended Risk-free interest rate 2.59% - 2.62% ​ 1.82% - 3.02% Expected volatility 58.9% - 84.6% ​ 58.2% - 101.0% Expected dividend yield 0% ​ 0% Expected life of non-employee options (in years) 0.81 - 8.19 0.25 - 8.94 ​ The weighted average fair value of outstanding options that had been granted to nonemployee consultants, as re-measured during the vesting period of each tranche during the year ended December 31, 2018, was $8.74. During the years ended December 31, 2020, 2019 and 2018, the Company recognized compensation expense relating to stock options as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Research and development ​ $ 6,765 ​ $ 5,206 ​ $ 3,919 General and administrative ​ 4,943 ​ 5,094 ​ 4,482 Total stock option expense ​ $ 11,708 ​ $ 10,300 ​ $ 8,401 ​ 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 $1,432, $604 and $476 in R&D expense for the years ended December 31, 2020, 2019 and 2018, respectively, and $1,167, $1,180 and $1,217 in G&A expense for the years ended December 31, 2020, 2019 and 2018, respectively; ii) compensation expense relating to the Board of Directors’ restricted stock units for $528 and $287 in G&A expense for the years ended December 31, 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. During the year ended December 31, 2018, the Company recognized additional compensation expense of $616 relating to modifications of outstanding Stock Awards for the former Chief Medical Officer and former Chief Financial Officer. This included expense based on marking to market the fair value of the modified Stock Awards in accordance with ASC 505-50, which is included within R&D expense in the total compensation table above for the year ended December 31, 2018. The Company determined that the acceleration of vesting for Stock Awards that would have vested based on their original vesting terms through the term of the consulting services were Type 1 modifications pursuant to ASC 718, Compensation – Stock Compensation As of December 31, 2020, 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 $23,110, which is expected to be realized over a weighted average period of 2.54 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, 2020, 2019 and 2018.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The Company’s benefit from income taxes is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 2018 Current: ​ ​ ​ Federal ​ $ — ​ $ — ​ $ — State ​ (691) ​ (816) ​ (389) ​ ​ (691) ​ (816) ​ (389) Deferred: ​ ​ ​ Federal ​ — ​ — ​ — State ​ — ​ — ​ — ​ ​ — ​ — ​ — Benefit from income taxes ​ $ (691) ​ $ (816) ​ $ (389) ​ 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. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 2018 Income taxes using U.S. federal statutory rate 21.00 % 21.00 % 21.00 % State income taxes, net of federal benefit (58.68) % (1.99) % 6.82 % Tax Cuts and Jobs Act 0.00 % 0.00 % 0.00 % Impact of R&D tax credit on effective tax rate (52.06) % 4.34 % 3.48 % Stock option shortfalls and cancellations 0.35 % (0.17) % (0.43) % Permanent items and other (5.08) % 0.36 % (0.15) % Change in valuation allowance 84.61 % (22.76) % (31.76) % Provision to return 0.92 % (0.02) % 0.03 % Non-taxable revenue 0.00 % 0.00 % 1.54 % ​ (8.94) % 0.76 % 0.53 % ​ Significant components of the Company’s deferred tax assets and liabilities are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 Valuation allowance $ (120,666) $ (114,136) ​ ​ ​ ​ ​ ​ ​ Net operating loss carryforwards ​ ​ 93,507 ​ ​ 84,608 Federal and state tax credits ​ 19,982 ​ 16,624 Deferred revenue ​ — ​ 5,994 Stock-based compensation expense ​ 6,732 ​ 4,481 Other ​ 2,414 ​ 3,409 Deferred tax assets ​ 122,635 ​ 115,116 ​ ​ ​ ​ ​ ​ ​ Other ​ (1,969) ​ (980) Deferred tax liabilities: ​ (1,969) ​ (980) ​ ​ ​ ​ ​ Net deferred tax asset: ​ $ — ​ $ — ​ A 100% valuation allowance has been recorded on the deferred tax asset as of December 31, 2020 and 2019 because management believes it is more likely than not that the asset will not be realized. The change in the valuation allowance during 2020 and 2019 was an increase of $6,530 and $24,321, 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, 2020 and 2019, 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, 2020, the Company had federal and state net operating loss carryforwards of $368,237 and $273,041, respectively. The federal and state tax loss carryforwards will begin to expire in 2026 and 2027, respectively, unless previously utilized. The federal net operating losses arising in taxable years beginning in 2018 and forward have an unlimited carryforward period, and, in the case of such federal net operating losses arising in taxable years beginning before 2021, may be carried back five years due to the Coronavirus Aid, Relief, and Economic Security Act of 2020, or the CARES Act. The federal losses may also be subject to limitation pursuant to Internal Revenue Code section 382. The Company also had federal and state R&D tax credit carryforwards of $18,358 and $1,795, respectively. The federal credits will begin expiring in 2025 unless previously utilized. The Connecticut credit carryforwards have no expiration period. Because of the net operating loss and research credit carryforwards, tax years 2006 through 2020 remain open to U.S. federal and state tax examinations. On March 27, 2020, 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. On December 22, 2017, the United States enacted the Tax Cuts and Jobs Act, or the TCJA. The TCJA, which is also commonly referred to as “U.S. tax reform”, significantly changes U.S. corporate income tax laws by, among other provisions, reducing the maximum U.S. corporate income tax rate from 35% to 21% starting in 2018. As of December 31, 2020 and 2019, the Company did not have any foreign subsidiaries and the international aspects of the TCJA were not applicable. On December, 22, 2017, Staff Accounting Bulletin 118, or SAB 118, was issued by the SEC due to the complexities involved in accounting for the TCJA. SAB 118 requires the Company to include in its financial statements a reasonable estimate of the impact of the TCJA on earnings to the extent such estimate has been determined. The Company finalized its accounting for the TCJA as of December 31, 2018, which resulted in insignificant adjustments.

Net Income (Loss) Per Share

Net Income (Loss) Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	15. Net Income (loss) per Share The Company computes net income (loss) per share in accordance with ASC 260-10, Earnings per Share Significant Accounting Policies – Income (Loss) per Share The denominators used in the net income (loss) per share computations are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Basic: Weighted average common shares outstanding 47,413,250 42,669,333 35,892,786 Diluted: Weighted average common shares outstanding - Basic 47,413,250 42,669,333 35,892,786 Common stock equivalents* 501,780 — — Denominator for diluted net income (loss) per share 47,915,030 42,669,333 35,892,786 * For the year end December 31, 2020, common stock equivalents include dilutive stock options and restricted stock units. For the years ended December 31, 2019 and 2018, no amounts were considered as their effects would have been anti-dilutive due to net losses for those periods. Basic and diluted net income (loss) per share is computed as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Net income (loss) - basic and diluted ​ $ 8,410 ​ $ (106,373) ​ $ (74,013) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ 47,413,250 ​ 42,669,333 ​ 35,892,786 Effect of dilutive securities: ​ ​ ​ ​ ​ ​ Stock options ​ ​ 481,254 ​ ​ — ​ ​ — Restricted stock units ​ ​ 20,526 ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 47,915,030 ​ ​ 42,669,333 ​ ​ 35,892,786 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) per share: ​ ​ ​ Basic ​ $ 0.18 ​ $ (2.49) ​ $ (2.06) Diluted ​ $ 0.18 ​ $ (2.49) ​ $ (2.06) ​ 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. In addition, the Company entered into the Vifor Agreement in October 2020. The Company will be eligible to receive an additional $50,000 common stock investment upon U.S. regulatory approval of CR845/difelikefalin at a 20% premium to the 30-day trailing average price of the Company’s common stock as of such date, which could potentially dilute basic earnings per share in the future (see Note 11, Collaboration and Licensing Agreements 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. As of December 31, 2018, 4,004,422 stock options 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, 2020
Postemployment Benefits [Abstract]
Employee Benefit Plan	16. Employee Benefit Plan In February 2006, the Company adopted a defined contribution retirement plan that 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 calendar quarter 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, 2020, 2019 and 2018, employer contributions to the plan were $349 , $279 and $198 , respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	17. 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 9, 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 its obligation to pay any royalties under the Enteris License Agreement in exchange for a lump sum payment in cash, or the Royalty Buyout. Subject to certain conditions, the Company may elect to pay 50% of the lump sum due under the Royalty Buyout in shares of the Company’s common stock pursuant to the Purchase Agreement. During the year ended December 31, 2020, the Company paid $5,000 to Enteris for a milestone earned during the year ended December 31, 2020 in relation to the Enteris License Agreement. As a result, the Company recognized $5,000 of R&D expense related to the Enteris License Agreement during the year ended December 31, 2020. 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 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 CR845/difelikefalin injection, the Company’s lead product candidate. Pursuant to the Product Agreements, Patheon and Patheon Greenville will manufacture commercial supplies of CR845/difelikefalin injection at the Monza, Italy and Greenville, North Carolina manufacturing sites, respectively, from active pharmaceutical ingredient 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 In December 2015, the Company entered into a lease agreement, or the Stamford Lease, for office space in Stamford, Connecticut, or the Premises, for the purpose of relocating its headquarters. The initial term of the Stamford Lease commenced in May 2016, or the Commencement Date, and ends in December 2023 The Stamford Lease requires monthly lease payments, including rent escalations and rent holidays, during the initial lease term. The Company began to make rental payments from the Commencement Date. Prior to January 1, 2019, the Company recorded monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through December 31, 2018. Total rent expense under the Stamford Lease was $974 for the year ended December 31, 2018. As of the Commencement Date, the Stamford Lease landlord had made tenant improvements of $1,094 to the leased premises which was included in Property and equipment, net. 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 7, Restricted Cash On January 1, 2019, the Company adopted ASC 842 (see Note 2 – Summary of Significant Accounting Policies - Leases Upon adoption of ASC 842, the Company established an operating lease ROU asset and operating lease liability for the Stamford Lease. In establishing the ROU asset, the operating lease liability of $5,198 was reduced by lease incentives relating to tenant improvements of $698 and deferred lease obligation of $864, which were outstanding on December 31, 2018. 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, $1,116 and $937 of operating lease cost, or lease expense, was recognized for the years ended December 31, 2020 and 2019, respectively, consisting of $781 relating to R&D lease expense and $335 relating to G&A lease expense for the 2020 period, and $656 relating to R&D lease expense and $281 relating to G&A lease expense for the 2019 period. Other information related to the Stamford Lease and Lease Amendment was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ ​ Operating cash outflows relating to operating leases ​ $ 1,403 ​ $ 1,215 ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ 1,934 ​ $ 3,636 ​ Remaining lease term - operating leases (years) ​ 3.0 ​ 3.9 ​ 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 liability as of December 31, 2020, were as follows: ​ ​ ​ ​ Year Ending December 31, ​ ​ 2021 $ 1,921 2022 ​ 1,957 2023 ​ 1,992 Total future minimum lease payments, undiscounted ​ 5,870 Less imputed interest ​ (595) Total ​ $ 5,275 ​ ​ ​ ​ Operating lease liabilities reported as of December 31, 2020: ​ Operating lease liabilities - current ​ $ 1,602 Operating lease liabilities - non-current ​ 3,673 Total ​ $ 5,275 ​

Legal Matters

Legal Matters	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Legal Matters	18. 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.

Quarterly Results of Operations

Quarterly Results of Operations (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations (Unaudited)	19. Quarterly Results of Operations (Unaudited) The following tables contain selected financial data for each quarter of the years ended December 31, 2020 and 2019. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for each quarter of the years ended December 31, 2020 and 2019. The operating results for any period are not necessarily indicative of results for any future periods. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, 2020 ​ First Second Third Fourth ​ ​ Quarter ​ Quarter ​ Quarter ​ Quarter Revenues ​ $ 8,093 ​ $ 5,634 ​ $ 9,266 ​ $ 112,089 (a) Operating expenses ​ $ 38,094 ​ $ 31,518 ​ $ 26,286 ​ $ 33,799 ​ Net income (loss) - Basic and Diluted ​ $ (28,922) ​ $ (25,068) ​ $ (16,509) ​ $ 78,909 ​ Income (loss) per share - Basic ​ $ (0.62) ​ $ (0.54) ​ $ (0.35) ​ $ 1.60 (b) Income (loss) per share - Diluted ​ $ (0.62) ​ $ (0.54) ​ $ (0.35) ​ $ 1.59 (c) ​ (a) Revenues for the fourth quarter of 2020 include $111,551 relating to the Vifor Agreement (see Note 12, Revenue Recognition ). (b) The difference between the sum of net income (loss) per share – basic of $0.09, as calculated on a quarterly basis for 2020, and net income per share – basic of $0.18 for the year ended December 31, 2020, is due to the denominator used for the year ended December 31, 2020, which weights shares outstanding on a cumulative basis and reflects the issuance of 2,939,552 shares of the Company’s common stock during the year ended December 31, 2020 (see Note 9, Stockholders’ Equity ). (c) The difference between the sum of net income (loss) per share – diluted of $0.08, as calculated on a quarterly basis for 2020, and net income per share – diluted of $0.18 for the year ended December 31, 2020, is due to: i) the issuance of shares of the Company’s common stock as described in b) above; and ii) first, second and third quarter diluted per share amounts did not include stock options and restricted stock units that were dilutive in those periods as those periods were in a net loss position; however, those same stock options and restricted stock units were included in the annual diluted per share amounts since the Company had net income for the year ended December 31, 2020. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, 2019 ​ ​ First Second Third Fourth ​ ​ ​ Quarter ​ Quarter ​ Quarter ​ Quarter ​ Revenues ​ $ 4,382 ​ $ 5,208 ​ $ 5,785 ​ $ 4,511 ​ Operating expenses ​ $ 27,516 ​ $ 29,350 ​ $ 40,218 ​ $ 34,481 ​ Net loss - Basic and Diluted ​ $ (21,960) ​ $ (22,960) ​ $ (32,842) ​ $ (28,611) ​ Loss per share - Basic and Diluted ​ $ (0.56) ​ $ (0.58) ​ $ (0.74) ​ $ (0.61) ​ ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
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 periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, 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 and the periods over which those costs are expensed, 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.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments, which potentially subject the Company to significant concentrations of credit risk consist primarily of cash equivalents and marketable securities. 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 four 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.
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, 2020. 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 income (loss). 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, 2020 or December 31, 2019.
Property and Equipment, net	Property and Equipment, net Property and equipment (consisting of computer, office and laboratory 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 Short-term laboratory equipment 2 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 On January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Collaboration and Licensing Agreements Revenue Recognition) The Company has entered into agreements to license its intellectual property, or IP, related to CR845/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 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 CR845/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.
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 CRO’s, 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, 2020 and 2019, the Company recorded $11,286 and $8,498 as prepaid R&D expense, respectively.
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, 2020 and December 31, 2019. 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, 2020, 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. During the period from January 31, 2014 to December 31, 2018, the Company’s stock had not traded on a public exchange for a sufficient period of time to approximate its expected term (as noted above). Therefore, the Company calculated the volatility for stock options granted using an analysis of guideline companies for the year ended December 31, 2018. Volatility calculated in this manner was in the range of 83% - 93% for stock options granted during the year ended December 31, 2018. Beginning on January 1, 2019, the Company determined that it had sufficient company-specific trading activity of its common stock available to use that activity exclusively to calculate the volatility of the Company’s common stock. Volatility calculated in this manner was in the range of 72% - 75% for stock options granted during the year ended December 31, 2020, and 71% - 75% for stock options granted during the year ended December 31, 2019. Volatility of the Company’s common stock from January 31, 2014 to December 31, 2018 was 75%, which was determined to be comparable to the method using guideline companies during the year ended December 31, 2018. 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. Prior to January 1, 2019, the Company accounted for stock options granted to non-employee consultants under ASC 505-50, Equity-Based Payments to Non-Employees On January 1, 2019, the Company adopted ASU 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Non-employee Share-Based Payment Accounting 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.
Income (Loss) Per Share	Income (Loss) Per Share The Company computes basic net income (loss) per share by dividing net income (loss) by the weighted average number of shares of common stock outstanding. Diluted net income (loss) 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 the quarter and 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. For each of the years ended December 31, 2019 and 2018, 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.
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 and development of novel therapeutics to treat serious medical conditions, including pruritus and pain.
Leases	Leases Prior to January 1, 2019, the Company recognized rent expense for operating leases on a straight-line basis over the term of the lease, beginning on the date the Company took possession of the property. Rent expense included the base amounts stated in the lease agreement as well as the effect of reduced or free rent and rent escalations. At lease inception, the Company determined the lease term by assuming the exercise of those renewal options that are reasonably assured because of the significant economic penalty that exists for not exercising those options. The exercise of renewal options is at the Company’s sole discretion. The expected lease term was one of the factors used to determine whether a lease was classified as operating or capital and was used to calculate the straight-line rent expense. The difference between the cash paid to the landlord and the amount recognized as rent expense on a straight-line basis was included in deferred rent and classified within long-term liabilities. Lease incentives made by landlords to or on behalf of the Company for leasehold improvements were recorded as deferred rent and classified as long-term liabilities. Deferred rent related to landlord incentives was amortized using the straight-line method over the lease term as an offset to rent expense for the year ended December 31, 2018. On January 1, 2019, the Company adopted ASC 842, Leases ● No re-evaluation of whether a contract is or contains a lease (embedded lease); ● Lease classification is grandfathered ● No reassessment of initial direct costs Upon adoption of ASC 842, the Company had only one lease, the Stamford Lease (see Note 17, 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 Statement of Comprehensive Income (Loss). There was no cumulative effect adjustment as a result of the adoption of ASC 842 on January 1, 2019, which reflects the difference between the amount of lease expense under ASC 842 that would have been recognized from inception of the Stamford Lease through December 31, 2018 and the amount of rent expense actually recognized under ASC 840 during that same period. 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, 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.
Recent Accounting Pronouncements	Accounting Pronouncements Recently Adopted On January 1, 2020, the Company adopted ASU 2016-13 which replaces the incurred loss impairment methodology in prior GAAP that delays recognition of a credit loss until it is probable that such loss has been incurred, with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates (see Note 2, Summary of Significant Accounting Policies: Marketable Securities). On January 1, 2020, the Company adopted ASU 2019-08, Compensation – Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606) On January 1, 2020, the Company adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 Collaboration and Licensing Agreements Revenue Recognition . On January 1, 2020, the Company adopted ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement included in Level 3 fair value measurements and no retrospective treatment was applicable. As a result, the adoption of ASU 2018-13 did not have a material effect on its results of operations, financial position or cash flows. Recent Accounting Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740)

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Useful Lives of Property and Equipment	Property and equipment (consisting of computer, office and laboratory 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 Short-term laboratory equipment 2 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, 2020
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, 2020 and 2019: 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 ​ As of December 31, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ Estimated Fair Type of Security Amortized Cost Gains Losses Value U.S. Treasury securities ​ $ 16,052 ​ $ 31 ​ $ (2) ​ $ 16,081 U.S. government agency obligations ​ 25,803 ​ 14 ​ (1) ​ 25,816 Corporate bonds ​ 115,788 ​ 125 ​ (23) ​ 115,890 Commercial paper ​ 38,547 ​ 27 ​ (1) ​ 38,573 Municipal bonds ​ ​ 3,500 ​ ​ — ​ ​ — ​ ​ 3,500 Total available-for-sale marketable securities ​ $ 199,690 ​ $ 197 ​ $ (27) ​ $ 199,860
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, 2020 and 2019: 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, 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ $ 3,185 ​ $ (2) ​ $ — ​ $ — ​ $ 3,185 ​ $ (2) U.S. government agency obligations ​ 2,400 ​ (1) ​ — ​ — ​ 2,400 ​ (1) Corporate bonds ​ 28,895 ​ (23) ​ — ​ — ​ 28,895 ​ (23) Commercial paper ​ 4,264 ​ (1) ​ — ​ — ​ 4,264 ​ (1) Total ​ $ 38,744 ​ $ (27) ​ $ — ​ $ — ​ $ 38,744 ​ $ (27)
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, 2020, the Company’s marketable debt securities mature at various dates through December 2023. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2020 ​ As of December 31, 2019 Contractual maturity Amortized Cost Fair Value Amortized Cost Fair Value Less than one year ​ $ 149,164 ​ $ 149,242 ​ $ 136,565 ​ $ 136,701 One year to three years ​ 70,570 ​ 70,565 ​ 63,125 ​ 63,159 Total ​ $ 219,734 ​ $ 219,807 ​ $ 199,690 ​ $ 199,860

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Summary of Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax, from unrealized gains (losses) on available-for-sale marketable securities, the Company’s only component of accumulated other comprehensive income (loss), for the years ended December 31, 2020, 2019 and 2018. ​ ​ ​ ​ ​ Total Accumulated ​ ​ Other Comprehensive ​ ​ Income (Loss) Balance, December 31, 2017 ​ $ (70) Other comprehensive loss before reclassifications ​ (49) Amount reclassified from accumulated other comprehensive loss ​ 5 Net current period other comprehensive loss ​ (44) 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
Schedule of Reclassification Out of Accumulated Other Comprehensive Income (Loss)	Amounts reclassified out of accumulated other comprehensive income (loss) into net income (loss) are determined by specific identification. The reclassifications out of accumulated other comprehensive income (loss) and into net income (loss) were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Affected Line Item in the Component of Accumulated Other ​ Year Ended December 31, ​ Statements of Comprehensive Income (Loss) 2020 2019 2018 Comprehensive Income (Loss) Unrealized gains (losses) on available-for- sale marketable securities ​ ​ ​ Realized gains (losses) on sale of securities ​ $ 272 ​ $ — ​ $ (5) Other income, net ​ ​ — ​ — ​ — Benefit from income taxes ​ ​ $ 272 ​ $ — ​ $ (5) ​ ​

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, net	Property and equipment, net consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 Computer and office equipment ​ $ 211 ​ $ 211 Laboratory equipment ​ 628 ​ 628 Furniture and fixtures ​ 330 ​ 61 Leasehold improvements ​ 1,212 ​ 1,132 ​ ​ $ 2,381 ​ $ 2,032 Less accumulated depreciation and amortization ​ 1,541 ​ 1,332 Property and equipment, net ​ $ 840 ​ $ 700 ​

Restricted Cash (Tables)

Restricted Cash (Tables)	12 Months Ended
	Dec. 31, 2020
Receivables [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, 2020 December 31, 2019 Cash and cash equivalents ​ $ 31,683 ​ $ 18,305 Restricted cash, long-term assets ​ 408 ​ 408 Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows ​ $ 32,091 ​ $ 18,713

Accounts Payable and Accrued _2

Accounts Payable and Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Schedule of Accounts Payable and Accrued Expenses	Accounts payable and accrued expenses consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 Accounts payable ​ $ 4,893 ​ $ 9,100 Accrued research projects ​ 6,194 ​ 6,637 Accrued professional fees ​ 839 ​ 635 Accrued compensation and benefits ​ 4,955 ​ 3,293 Total ​ $ 16,881 ​ $ 19,665

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 and 2019 and by level within the fair value hierarchy: 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 ​ $ — ​ Fair value measurement as of December 31, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ $ 18,305 ​ $ 18,305 ​ $ — ​ $ — Available-for-sale marketable securities: ​ ​ ​ ​ U.S. Treasury securities ​ ​ 16,081 ​ ​ — ​ ​ 16,081 ​ ​ — U.S. government agency obligations ​ 25,816 ​ — ​ 25,816 ​ — Corporate bonds ​ 115,890 ​ — ​ 115,890 ​ — Commercial paper ​ 38,573 ​ — ​ 38,573 ​ — Municipal bonds ​ ​ 3,500 ​ ​ — ​ ​ 3,500 ​ ​ — Restricted cash: ​ ​ ​ ​ Commercial money market account ​ 408 ​ 408 ​ — ​ — Total financial assets ​ $ 218,573 ​ $ 18,713 ​ $ 199,860 ​ $ — ​

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ Aggregate ​ ​ Number of ​ Average Exercise ​ Intrinsic ​ Shares Price Value Outstanding, December 31, 2019 4,450,517 ​ $ 14.73 ​ Granted 1,377,850 ​ 15.88 ​ Exercised (55,852) ​ 12.37 ​ Expired (175,971) ​ 14.06 ​ Forfeited (127,151) ​ 16.55 ​ Outstanding, December 31, 2020 5,469,393 ​ $ 15.02 ​ $ 8,441 Weighted average remaining contractual life as of December 31, 2020 (in years) 7.26 ​ ​ Options exercisable, December 31, 2020 3,192,128 ​ $ 13.96 ​ $ 7,589 Weighted average remaining contractual life as of December 31, 2020 (in years) 6.23 ​ ​ Options vested and expected to vest as of December 31, 2020 5,469,393 ​ $ 15.02 ​ $ 8,441 Weighted average remaining contractual life as of December 31, 2020 (in years) 7.26 ​ ​
Summary of Assumptions Used in Black-Scholes Option Pricing Model	There were Summary of Significant Accounting Policies – Stock-Based Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 2019 2018 Risk-free interest rate 0.35% - 1.57 % 1.55% - 2.62 % 2.51% - 3.09 % Expected volatility 71.8% - 74.8 % 71.1% - 75.2 % 82.6% - 92.8 % 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	During the years ended December 31, 2020, 2019 and 2018, the Company recognized compensation expense relating to stock options as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Research and development ​ $ 6,765 ​ $ 5,206 ​ $ 3,919 General and administrative ​ 4,943 ​ 5,094 ​ 4,482 Total stock option expense ​ $ 11,708 ​ $ 10,300 ​ $ 8,401
Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | Re-Measurement [Member]
Summary of Assumptions Used in Black-Scholes Option Pricing Model	On January 1, 2019, the Company used the Black Scholes option valuation model to remeasure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07. At the end of each fiscal quarter during the year ended December 31, 2018, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50. The range of assumptions used by the Company on January 1, 2019 and during the year ended December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ January 1, 2019 Year Ended Risk-free interest rate 2.59% - 2.62% ​ 1.82% - 3.02% Expected volatility 58.9% - 84.6% ​ 58.2% - 101.0% Expected dividend yield 0% ​ 0% Expected life of non-employee options (in years) 0.81 - 8.19 0.25 - 8.94 ​

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Components of Income Taxes (Benefit)	The Company’s benefit from income taxes is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 2018 Current: ​ ​ ​ Federal ​ $ — ​ $ — ​ $ — State ​ (691) ​ (816) ​ (389) ​ ​ (691) ​ (816) ​ (389) Deferred: ​ ​ ​ Federal ​ — ​ — ​ — State ​ — ​ — ​ — ​ ​ — ​ — ​ — Benefit from income taxes ​ $ (691) ​ $ (816) ​ $ (389)
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, ​ 2020 2019 2018 Income taxes using U.S. federal statutory rate 21.00 % 21.00 % 21.00 % State income taxes, net of federal benefit (58.68) % (1.99) % 6.82 % Tax Cuts and Jobs Act 0.00 % 0.00 % 0.00 % Impact of R&D tax credit on effective tax rate (52.06) % 4.34 % 3.48 % Stock option shortfalls and cancellations 0.35 % (0.17) % (0.43) % Permanent items and other (5.08) % 0.36 % (0.15) % Change in valuation allowance 84.61 % (22.76) % (31.76) % Provision to return 0.92 % (0.02) % 0.03 % Non-taxable revenue 0.00 % 0.00 % 1.54 % ​ (8.94) % 0.76 % 0.53 % ​
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, ​ 2020 2019 Valuation allowance $ (120,666) $ (114,136) ​ ​ ​ ​ ​ ​ ​ Net operating loss carryforwards ​ ​ 93,507 ​ ​ 84,608 Federal and state tax credits ​ 19,982 ​ 16,624 Deferred revenue ​ — ​ 5,994 Stock-based compensation expense ​ 6,732 ​ 4,481 Other ​ 2,414 ​ 3,409 Deferred tax assets ​ 122,635 ​ 115,116 ​ ​ ​ ​ ​ ​ ​ Other ​ (1,969) ​ (980) Deferred tax liabilities: ​ (1,969) ​ (980) ​ ​ ​ ​ ​ Net deferred tax asset: ​ $ — ​ $ — ​

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computation of Denominators Used in Net Income (Loss) per Share	The denominators used in the net income (loss) per share computations are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Basic: Weighted average common shares outstanding 47,413,250 42,669,333 35,892,786 Diluted: Weighted average common shares outstanding - Basic 47,413,250 42,669,333 35,892,786 Common stock equivalents* 501,780 — — Denominator for diluted net income (loss) per share 47,915,030 42,669,333 35,892,786 * For the year end December 31, 2020, common stock equivalents include dilutive stock options and restricted stock units. For the years ended December 31, 2019 and 2018, 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 Income (Loss) per Share	Basic and diluted net income (loss) per share is computed as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2020 2019 2018 Net income (loss) - basic and diluted ​ $ 8,410 ​ $ (106,373) ​ $ (74,013) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ 47,413,250 ​ 42,669,333 ​ 35,892,786 Effect of dilutive securities: ​ ​ ​ ​ ​ ​ Stock options ​ ​ 481,254 ​ ​ — ​ ​ — Restricted stock units ​ ​ 20,526 ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 47,915,030 ​ ​ 42,669,333 ​ ​ 35,892,786 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) per share: ​ ​ ​ Basic ​ $ 0.18 ​ $ (2.49) ​ $ (2.06) Diluted ​ $ 0.18 ​ $ (2.49) ​ $ (2.06)

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
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, ​ 2020 2019 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ ​ Operating cash outflows relating to operating leases ​ $ 1,403 ​ $ 1,215 ​ ROU assets obtained in exchange for new operating lease liabilities ​ $ 1,934 ​ $ 3,636 ​ Remaining lease term - operating leases (years) ​ 3.0 ​ 3.9 ​ 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 Liability	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 liability as of December 31, 2020, were as follows: ​ ​ ​ ​ Year Ending December 31, ​ ​ 2021 $ 1,921 2022 ​ 1,957 2023 ​ 1,992 Total future minimum lease payments, undiscounted ​ 5,870 Less imputed interest ​ (595) Total ​ $ 5,275 ​ ​ ​ ​ Operating lease liabilities reported as of December 31, 2020: ​ Operating lease liabilities - current ​ $ 1,602 Operating lease liabilities - non-current ​ 3,673 Total ​ $ 5,275

Quarterly Results of Operatio_2

Quarterly Results of Operations (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Schedule of Selected Financial Data for Each Quarter	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, 2020 ​ First Second Third Fourth ​ ​ Quarter ​ Quarter ​ Quarter ​ Quarter Revenues ​ $ 8,093 ​ $ 5,634 ​ $ 9,266 ​ $ 112,089 (a) Operating expenses ​ $ 38,094 ​ $ 31,518 ​ $ 26,286 ​ $ 33,799 ​ Net income (loss) - Basic and Diluted ​ $ (28,922) ​ $ (25,068) ​ $ (16,509) ​ $ 78,909 ​ Income (loss) per share - Basic ​ $ (0.62) ​ $ (0.54) ​ $ (0.35) ​ $ 1.60 (b) Income (loss) per share - Diluted ​ $ (0.62) ​ $ (0.54) ​ $ (0.35) ​ $ 1.59 (c) ​ (a) Revenues for the fourth quarter of 2020 include $111,551 relating to the Vifor Agreement (see Note 12, Revenue Recognition ). (b) The difference between the sum of net income (loss) per share – basic of $0.09, as calculated on a quarterly basis for 2020, and net income per share – basic of $0.18 for the year ended December 31, 2020, is due to the denominator used for the year ended December 31, 2020, which weights shares outstanding on a cumulative basis and reflects the issuance of 2,939,552 shares of the Company’s common stock during the year ended December 31, 2020 (see Note 9, Stockholders’ Equity ). (c) The difference between the sum of net income (loss) per share – diluted of $0.08, as calculated on a quarterly basis for 2020, and net income per share – diluted of $0.18 for the year ended December 31, 2020, is due to: i) the issuance of shares of the Company’s common stock as described in b) above; and ii) first, second and third quarter diluted per share amounts did not include stock options and restricted stock units that were dilutive in those periods as those periods were in a net loss position; however, those same stock options and restricted stock units were included in the annual diluted per share amounts since the Company had net income for the year ended December 31, 2020. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, 2019 ​ ​ First Second Third Fourth ​ ​ ​ Quarter ​ Quarter ​ Quarter ​ Quarter ​ Revenues ​ $ 4,382 ​ $ 5,208 ​ $ 5,785 ​ $ 4,511 ​ Operating expenses ​ $ 27,516 ​ $ 29,350 ​ $ 40,218 ​ $ 34,481 ​ Net loss - Basic and Diluted ​ $ (21,960) ​ $ (22,960) ​ $ (32,842) ​ $ (28,611) ​ Loss per share - Basic and Diluted ​ $ (0.56) ​ $ (0.58) ​ $ (0.74) ​ $ (0.61) ​

Business - Additional Informati

Business - Additional Information (Detail) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Oct. 31, 2020USD ($)shares	May 31, 2018USD ($)shares	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)shares
Nature Of Business [Line Items]
Date of formation											Jul. 2, 2004
Proceeds from equity and debt financing			$ 519,600								$ 519,600
Number of follow-on public offerings | item											4
Upfront and milestone payments received in connection with license agreements			201,893								$ 201,893
Proceeds from sale of common stock in connection with license agreement											38,449		$ 14,556
Unrestricted cash and cash equivalents and marketable securities			251,490								251,490
Accumulated deficit			392,317				$ 400,727				392,317	$ 400,727
Net income (loss)			$ 78,909	$ (16,509)	$ (25,068)	$ (28,922)	$ (28,611)	$ (32,842)	$ (22,960)	$ (21,960)	8,410	(106,373)	(74,013)
Net cash used in operating activities											$ (5,487)	$ (109,225)	$ (22,301)
Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued | shares												6,325,000	5,175,000
Vifor International Ltd. [Member]
Nature Of Business [Line Items]
Proceeds from sale of common stock in connection with license agreement	$ 38,449	$ 14,556
Vifor International Ltd. [Member] | Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued | shares	2,939,552	1,174,827

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	Jan. 01, 2020agreement	Jan. 01, 2018contract	Dec. 31, 2020USD ($)Institutionitemsegment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2020	Jan. 01, 2019USD ($)lease	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of revenue related contracts | contract		2
Prepaid R&D expense | $			$ 11,286	$ 8,498
Expected volatility, minimum			71.80%	71.10%	82.60%
Expected volatility, maximum			74.80%	75.20%	92.80%
Expected dividend yield			0.00%	0.00%	0.00%
Cumulative effect adjustment | $			$ 249,001	$ 186,713	$ 133,630	$ 133,630			$ 86,780
Number of renewable terms			1
Lease, Practical Expedient, Use of Hindsight [true false]			true
Lease, Practical Expedients, Package [true false]			true
Number of operating segments | segment			1
Number of collaboration and licensing agreements | agreement	3
Stamford Operating Lease [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, renewable term			5 years
Annual incremental borrowing rate			7.00%	7.00%
Stamford Operating Lease [Member] | ASC 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of operating lease | lease								1
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 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment | $								$ 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
Employee Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)			6 years 3 months
Expected volatility, minimum			72.00%	71.00%	83.00%
Expected volatility, maximum			75.00%	75.00%	93.00%
Expected volatility						75.00%
Expected dividend yield			0.00%	0.00%	0.00%
Non Employee Stock Option [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)			10 years
Cash And Cash Equivalents And Marketable Securities [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of financial institutions | Institution			4

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment, net (Detail)	12 Months Ended
	Dec. 31, 2020
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Short-term Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	2 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, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 219,734	$ 199,690
Gross Unrealized Gains	123	197
Gross Unrealized Losses	(50)	(27)
Estimated Fair Value	219,807	199,860
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,710	16,052
Gross Unrealized Gains	41	31
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	20,750	16,081
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,125	25,803
Gross Unrealized Gains	4	14
Gross Unrealized Losses	(1)	(1)
Estimated Fair Value	22,128	25,816
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,080	115,788
Gross Unrealized Gains	61	125
Gross Unrealized Losses	(23)	(23)
Estimated Fair Value	49,118	115,890
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,139	38,547
Gross Unrealized Gains	5	27
Gross Unrealized Losses	(17)	(1)
Estimated Fair Value	116,127	38,573
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,680	3,500
Gross Unrealized Gains	12
Gross Unrealized Losses	(8)
Estimated Fair Value	$ 11,684	$ 3,500

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, 2020	Dec. 31, 2019
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	$ 113,891	$ 38,744
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(50)	(27)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses	0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	113,891	38,744
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(50)	(27)
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	12,682	3,185
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(1)	(2)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	12,682	3,185
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(1)	(2)
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	2,500	2,400
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(1)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	2,500	2,400
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(1)	(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	23,553	28,895
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(23)	(23)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	23,553	28,895
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(23)	(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	68,897	4,264
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(17)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	68,897	4,264
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(17)	$ (1)
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	6,259
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(8)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	6,259
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (8)

Available-for-Sale Marketable_5

Available-for-Sale Marketable Securities - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)position	Dec. 31, 2019USD ($)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	30	16
Total number of positions	59	81
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses | $	$ 0
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	6
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	1
Total number of positions	6
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale marketable securities in unrealized loss positions	8
Total number of positions	18
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	22
Municipal Bonds [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	7

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Amortized Cost of marketable debt securities, contractual maturity, less than one year	$ 149,164	$ 136,565
Amortized Cost of marketable debt securities, contractual maturity, One year to three years	70,570	63,125
Amortized Cost	219,734	199,690
Fair value of marketable debt securities, contractual maturities, less than one year	149,242	136,701
Fair value of marketable debt securities, contractual maturity, One year to three years	70,565	63,159
Total fair Value of marketable debt securities, contractual maturity	219,807	199,860
Proceeds from sale of available-for-sale marketable securities	41,600	0	$ 79,808
Realized gain (loss)	272		$ (5)
Interest receivable	$ 311	$ 971

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 186,713	$ 133,630	$ 86,780
Amount reclassified from accumulated other comprehensive income	(272)		5
Net current period other comprehensive income (loss)	(97)	284	(44)
Balance, Value	249,001	186,713	133,630
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	170	(114)	(70)
Other comprehensive income (loss) before reclassifications	175	284	(49)
Amount reclassified from accumulated other comprehensive income	(272)		5
Net current period other comprehensive income (loss)	(97)	284	(44)
Balance, Value	$ 73	$ 170	$ (114)

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income, net	$ 2,334	$ 4,490	$ 2,980
Benefit from income taxes	(691)	(816)	(389)
Net of tax	272		(5)
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	272		(5)
Benefit from income taxes	$ 0	$ 0	$ 0

Prepaid Expenses - Additional I

Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 12,076	$ 8,863
Prepaid R&D clinical costs	11,286	8,498
Prepaid insurance	223	181
Other prepaid costs	$ 567	$ 184

Property and Equipment, net - P

Property and Equipment, net - Property and Equipment, net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,381	$ 2,032
Less accumulated depreciation and amortization	1,541	1,332
Property, Plant and Equipment, net, Total	840	700
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	211	211
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	628	628
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	330	61
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,212	$ 1,132

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 209	$ 198	$ 370
Net proceeds from sale of property and equipment	$ 0	$ 0	$ 0

Restricted Cash - Additional In

Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	May 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash balance	$ 408	$ 408
Stamford Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Letter of credit, remaining amount	$ 408		$ 769
Letter of credit, remaining amount date	2019-05
Operating lease, expiration date	Dec. 31, 2023
Restricted cash balance	$ 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, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 31,683	$ 18,305
Restricted cash balance	408	408
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	$ 32,091	$ 18,713	$ 15,850	$ 10,157

Accounts Payable and Accrued _3

Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accounts payable	$ 4,893	$ 9,100
Accrued research projects	6,194	6,637
Accrued professional fees	839	635
Accrued compensation and benefits	4,955	3,293
Total	$ 16,881	$ 19,665

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands	Aug. 31, 2019USD ($)$ / shares	Jul. 29, 2019USD ($)$ / sharesshares	Jul. 23, 2018USD ($)shares	Dec. 31, 2020$ / sharesshares	Oct. 31, 2020shares	Jun. 30, 2020shares	Dec. 31, 2019$ / sharesshares	Aug. 31, 2019USD ($)$ / sharesshares	May 31, 2019shares	Mar. 31, 2019shares	Dec. 31, 2018$ / sharesshares	Jul. 31, 2018$ / sharesshares	May 31, 2018shares	Jun. 30, 2020shares	Dec. 31, 2020USD ($)item$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / 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	100,000,000
Common stock, par value | $ / shares				$ 0.001			$ 0.001								$ 0.001	$ 0.001
Preferred stock, shares authorized				5,000,000			5,000,000								5,000,000	5,000,000
Preferred stock, par value | $ / shares				$ 0.001			$ 0.001								$ 0.001	$ 0.001
Common stock, shares outstanding				49,872,213			46,720,225								49,872,213	46,720,225
Common stock, shares issued				49,872,213			46,720,225								49,872,213	46,720,225
Preferred stock, shares issued				0			0								0	0
Preferred Stock Shares Outstanding				0			0								0	0
Number of votes for each share of common stock | item															1
Restricted stock units vested shares											83,791
Unregistered common stock exchanged for consulting services | $																$ 197
2014 Equity Incentive Plan [Member] | Director [Member]
Sale Of Stock [Line Items]
Vesting period of awards granted															3 years
2014 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | Executive Officers [Member]
Sale Of Stock [Line Items]
Common stock, shares issued							36,666		74,166							36,666
2014 Equity Incentive Plan [Member] | Restricted Stock Units [Member] | Executive Officers [Member] | February 2020 [Member]
Sale Of Stock [Line Items]
Restricted stock units, vested				36,750
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
Vifor International Ltd. [Member]
Sale Of Stock [Line Items]
Common stock, shares issued													1,174,827
General and Administrative [Member] | Restricted Stock Units [Member] | Director [Member]
Sale Of Stock [Line Items]
Compensation expense | $															$ 528	$ 287
General and Administrative [Member] | Restricted Stock Units [Member] | Executive Officers [Member]
Sale Of Stock [Line Items]
Compensation expense | $															$ 1,167	1,180	$ 1,217
General and Administrative [Member] | Shareholder Consulting Agreement [Member]
Sale Of Stock [Line Items]
Compensation expense | $																$ 197
Non-Exclusive License Agreement
Sale Of Stock [Line Items]
Upfront fee | $	$ 8,000							$ 8,000
Upfront fee payable, cash | $	4,000							4,000
Upfront fee payable, stock | $	$ 4,000							$ 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										2,939,552
Maximum [Member]
Sale Of Stock [Line Items]
Aggregate offering price of securities | $																		$ 300,000
Follow-on Public Offering [Member]
Sale Of Stock [Line Items]
Price per share | $ / shares							$ 23				$ 19					$ 23	$ 19
Common Stock [Member]
Sale Of Stock [Line Items]
Common stock, shares issued																6,325,000	5,175,000
Restricted stock units vested shares							36,666								156,584	110,832	83,791
Number of shares of unregistered common stock exchanged for consulting services																10,195
Common Stock [Member] | Director [Member]
Sale Of Stock [Line Items]
Vesting period of awards granted						1 year
Restricted stock units vested shares						24,000
Common Stock [Member] | Executive Officers [Member]
Sale Of Stock [Line Items]
Restricted stock units vested shares									74,166					95,834
Common Stock [Member] | Non-Exclusive License Agreement
Sale Of Stock [Line Items]
Common stock, shares issued								170,793
Upfront fee payable, stock | $	$ 4,000							$ 4,000
Common Stock [Member] | Follow-on Public Offering [Member]
Sale Of Stock [Line Items]
Common stock, shares issued		6,325,000
Common stock, shares issued												5,175,000
Price per share | $ / shares		$ 23										$ 19
Proceeds from issuance of common stock net of underwriting discounts, commissions and estimated offering expenses | $		$ 136,498
Proceeds from issuance of common stock net of underwriting discounts and offering costs | $			$ 92,063
Common Stock [Member] | Underwriter's Over-Allotment [Member]
Sale Of Stock [Line Items]
Common stock, shares issued		825,000	675,000
Underwriting discounts, commission and offering expenses | $		$ 8,977	$ 6,262

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial assets
Available-for-sale marketable securities	$ 219,807	$ 199,860
Recurring [Member]
Financial assets
Total financial assets	251,898	218,573
U.S. Government Agency Obligations [Member]
Financial assets
Available-for-sale marketable securities	22,128	25,816
U.S. Government Agency Obligations [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	22,128	25,816
Corporate Bonds [Member]
Financial assets
Available-for-sale marketable securities	49,118	115,890
Corporate Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	49,118	115,890
Municipal Bonds [Member]
Financial assets
Available-for-sale marketable securities	11,684	3,500
Municipal Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	11,684	3,500
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring [Member]
Financial assets
Total financial assets	32,091	18,713
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Total financial assets	219,807	199,860
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency Obligations [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	22,128	25,816
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	49,118	115,890
Significant Other Observable Inputs (Level 2) [Member] | Municipal Bonds [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	11,684	3,500
Money Market Funds [Member] | Recurring [Member]
Financial assets
Cash and cash equivalents	31,683	18,305
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	31,683	18,305
Restricted cash	408	408
U.S. Treasury Securities [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	20,750	16,081
U.S. Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	20,750	16,081
Commercial Paper [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	116,127	38,573
Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Financial assets
Available-for-sale marketable securities	$ 116,127	$ 38,573

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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
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]
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, 2020USD ($)$ / sharesshares

Apr. 30, 2013item

Dec. 31, 2020USD ($)

Sep. 30, 2020USD ($)

Jun. 30, 2020USD ($)

Mar. 31, 2020USD ($)

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Purchase of common stock value

$ 136,498

$ 92,063

Revenue from contract with customer

$ 112,089

$ 9,266

$ 5,634

$ 8,093

$ 4,511

$ 5,785

$ 5,208

$ 4,382

$ 135,082

19,886

13,469

Clinical Compound Revenue

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue from contract with customer

$ 643

140

33

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

Vifor Agreement [Member] | License [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential milestone payments

$ 50,000

Percentage of premium on common stock investment

20.00%

Stock issuance price measurement period

30 days

Number of performance obligations for revenue recognized | item

1

Revenue from contract with customer

$ 111,551

VFMCRP Agreement [Member] | Clinical Compound Revenue

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue from contract with customer

115

Cost of clinical compound related to R&D expense

$ 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

Contract liabilities

$ 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] | Maximum [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Third-party cost for clinical development

$ 20,000

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

470,000

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

30,000

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

$ 476

126

30

Maruishi Pharmaceutical Co., Ltd. [Member] | Clinical Compound Revenue

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue from contract with customer

$ 528

$ 140

$ 33

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] | Regulatory and Commercial Milestones [Member] | License [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential milestone payments

$ 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%

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

Common stock, shares issued | shares

2,939,552

Purchase common stock per share amount | $ / shares

$ 17.0094

Premium from sale of stock

$ 11,551

Vifor International Ltd. [Member] | Maximum [Member] | Tiered Commercial Milestones [Member] | License [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential milestone payments

$ 240,000

Revenue Recognition - Additiona

Revenue Recognition - Additional Information (Detail) ¥ in Billions	Oct. 15, 2020item	May 17, 2018USD ($)item	Apr. 30, 2013USD ($)item	Apr. 30, 2012USD ($)item	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2020USD ($)contractitem	Dec. 31, 2019USD ($)Milestone	Dec. 31, 2018USD ($)Milestone	Dec. 31, 2020USD ($)	Apr. 30, 2013JPY (¥)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue, current									$ 22,262,000					$ 22,262,000
Number of contracts | contract													4
Total revenue					$ 112,089,000	$ 9,266,000	$ 5,634,000	$ 8,093,000	4,511,000	$ 5,785,000	$ 5,208,000	$ 4,382,000	$ 135,082,000	19,886,000	$ 13,469,000
License and Milestone Fees
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total revenue													134,439,000	19,746,000	$ 13,436,000
Royalty [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer													0
Vifor Agreement [Member] | License and Milestone Fees
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total revenue					111,551,000								111,551,000
Number of performance obligations for revenue recognized | item	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			$ 0
VFMCRP Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue, current									22,262,000					22,262,000
Receivables									0					0
Other assets									0					0
Deferred revenue									0					0
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
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													22,262,000			55,444,000
Number of performance obligations for revenue recognized | item		1
Remaining performance obligations					0								0			0
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]
Receivables									0					0
Other assets									0					0
Deferred revenue									$ 0					$ 0
Remaining performance obligations					$ 0								$ 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 [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
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

Stock-Based Compensation - Addi

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

Jun. 04, 2020

Jun. 04, 2019

Dec. 31, 2020

Jun. 30, 2020

Apr. 30, 2020

Feb. 29, 2020

Dec. 31, 2019

May 31, 2019

Mar. 31, 2019

Sep. 30, 2018

Dec. 31, 2020

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2015

Jan. 01, 2021

Nov. 20, 2019

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

Common stock, par value

$ 0.001

$ 0.001

$ 0.001

$ 0.001

$ 0.001

Common stock, shares outstanding

49,872,213

46,720,225

49,872,213

49,872,213

46,720,225

Common stock, shares issued

49,872,213

46,720,225

49,872,213

49,872,213

46,720,225

Number of options outstanding

5,469,393

4,450,517

5,469,393

5,469,393

4,450,517

4,004,422

Options granted

1,377,850

Incentive Stock Options [Member]

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

Compensation expense

$ 11,708

$ 10,300

$ 8,401

Incentive Stock Options [Member] | Research and Development [Member]

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

Compensation expense

6,765

5,206

3,919

Incentive Stock Options [Member] | General and Administrative [Member]

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

Compensation expense

4,943

5,094

4,482

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

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

Compensation expense

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

1,432

604

476

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

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

Compensation expense

$ 1,167

$ 1,180

$ 1,217

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

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

Options granted

0

0

0

Weighted average fair values of outstanding options

$ 8.74

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

$ 10.33

$ 11.67

$ 11.99

Options granted

1,377,850

1,324,000

1,197,500

2019 Inducement Plan [Member]

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

Common stock, par value

$ 0.001

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

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

Options granted

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

7,488,513

7,488,513

7,488,513

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

8,984,679

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]

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] | 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

Restricted stock units, approved and granted

36,000

Restricted stock units, grant date fair value

$ 15.62

Restricted stock units vesting date

Jun. 4, 2021

Restricted stock units, vested

0

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

Restricted stock units, approved and granted

24,000

Restricted stock units, grant date fair value

$ 20.47

Restricted stock units grant date

Jun. 4, 2019

Restricted stock units, vested

24,000

2014 Equity Incentive Plan [Member] | Director [Member] | Share-based Payment Arrangement, Nonemployee [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

$ 323

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

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

Restricted stock units, approved and granted

83,791

Common stock, shares issued

36,666

74,166

36,666

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

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

Restricted stock units, vested

36,750

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]

Restricted stock units, approved and granted

138,000

138,000

Restricted stock units, grant date fair value

$ 16.36

Compensation expense

$ 601

Restricted stock units, vested

36,750

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]

Restricted stock units, approved and granted

215,000

Restricted stock units, grant date fair value

$ 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] | September 2018 [Member]

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

Restricted stock units, approved and granted

83,791

Restricted stock units, grant date fair value

$ 20.21

Compensation expense

$ 1,693

Common stock, shares outstanding

83,791

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

$ 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] | Research and Development [Member] | September 2018 [Member]

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

Compensation expense

$ 476

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

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] | Performance Restricted Stock Units [Member] | General and Administrative [Member] | September 2018 [Member]

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

Compensation expense

$ 1,217

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

Restricted stock units, approved and granted

98,000

98,000

Restricted stock units, grant date fair value

$ 16.36

Compensation expense

$ 455

Restricted stock units, vested

0

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

$ 149

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

$ 306

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

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding	4,450,517	4,004,422
Number of Options, Granted	1,377,850
Number of Options, Exercised	(55,852)
Number of Options, Expired	(175,971)
Number of Options, Forfeited	(127,151)
Number of Options, Outstanding	5,469,393	4,450,517	4,004,422
Number of Options, Outstanding, Weighted average remaining contractual life as of December 31, 2020	7 years 3 months 3 days
Number of Options, Options exercisable	3,192,128
Number of Options, Exercisable, Weighted average remaining contractual life as of December 31, 2020	6 years 2 months 23 days
Number of Options, Options vested and expected to vest at December 31, 2020	5,469,393
Number of Options, Vested and expected to vest, Weighted average remaining contractual life as of December 31, 2020	7 years 3 months 3 days
Weighted-Average Exercise Price, Outstanding	$ 14.73
Weighted-Average Exercise Price, Granted	15.88
Weighted-Average Exercise Price, Exercised	12.37
Weighted-Average Exercise Price, Expired	14.06
Weighted-Average Exercise Price, Forfeited	16.55
Weighted-Average Exercise Price, Outstanding	15.02	$ 14.73
Weighted-Average Exercise Price, Options exercisable, December 31, 2020	13.96
Weighted-Average Exercise Price, Options vested and expected to vest at December 31, 2020	$ 15.02
Aggregate Intrinsic Value, Outstanding at December 31, 2020	$ 8,441
Aggregate Intrinsic Value, Options exercisable at December 31, 2020	7,589
Aggregate Intrinsic Value, Options vested and expected to vest at December 31, 2020	8,441
Fair value of options vested	12,819	$ 10,074	$ 9,023
Intrinsic value of options exercised	$ 152	$ 5,741	$ 3,893

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum		0.35%	1.55%	2.51%
Risk-free interest rate, maximum		1.57%	2.62%	3.09%
Expected volatility, minimum		71.80%	71.10%	82.60%
Expected volatility, maximum		74.80%	75.20%	92.80%
Expected dividend yield		0.00%	0.00%	0.00%
Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | Re-Measurement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	2.59%			1.82%
Risk-free interest rate, maximum	2.62%			3.02%
Expected volatility, minimum	58.90%			58.20%
Expected volatility, maximum	84.60%			101.00%
Expected dividend yield	0.00%			0.00%
Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | Re-Measurement [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	9 months 21 days			3 months
Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | Re-Measurement [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	8 years 2 months 8 days			8 years 11 months 8 days
Employee Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility					75.00%
Expected volatility, minimum		72.00%	71.00%	83.00%
Expected volatility, maximum		75.00%	75.00%	93.00%
Expected dividend yield		0.00%	0.00%	0.00%
Expected life of options (in years)		6 years 3 months
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 ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	$ 23,110,000
Weighted average period of compensation expense not yet recognized	2 years 6 months 14 days
Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	$ 11,708,000	10,300,000	8,401,000
Incentive Stock Options [Member] | Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	6,765,000	5,206,000	3,919,000
Incentive Stock Options [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense	4,943,000	5,094,000	4,482,000
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	528,000	287,000
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	1,432,000	604,000	476,000
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	$ 1,167,000	1,180,000	1,217,000
Modified Awards [Member] | Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense			$ 616,000
Shareholder Consulting Agreement [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Compensation expense		$ 197,000

Income Taxes - Components of In

Income Taxes - Components of Income Taxes (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
State	$ (691)	$ (816)	$ (389)
Total Current	(691)	(816)	(389)
Deferred:
Benefit from income taxes	$ (691)	$ (816)	$ (389)

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, 2020	Dec. 31, 2019	Dec. 31, 2018
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	(58.68%)	(1.99%)	6.82%
Tax Cuts and Jobs Act	0	0	0
Impact of R&D tax credit on effective tax rate	(52.06%)	4.34%	3.48%
Stock option shortfalls and cancellations	0.35%	(0.17%)	(0.43%)
Permanent items and other	(5.08%)	0.36%	(0.15%)
Change in valuation allowance	84.61%	(22.76%)	(31.76%)
Provision to return	0.92%	(0.02%)	0.03%
Non-taxable revenue	0.00%	0.00%	1.54%
Total	(8.94%)	0.76%	0.53%

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, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Valuation allowance	$ (120,666)	$ (114,136)
Deferred tax assets:
Net operating loss carryforwards	93,507	84,608
Federal and state tax credits	19,982	16,624
Deferred revenue		5,994
Stock-based compensation expense	6,732	4,481
Other	2,414	3,409
Deferred tax assets	122,635	115,116
Deferred tax liabilities:
Other	(1,969)	(980)
Deferred tax liabilities	$ (1,969)	$ (980)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Percentage of annual research and development credit for cash	65.00%	65.00%	65.00%
Valuation allowance on the deferred tax asset	100.00%	100.00%
Change in valuation allowance	$ 6,530,000	$ 24,321,000
Unrecognized tax benefits, related interest and penalties	$ 0	$ 0
Tax examinations, description	tax years 2006 through 2020 remain open to U.S. federal and state tax examinations.
Income taxes using U.S. federal statutory rate	21.00%	21.00%	21.00%
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Income taxes using U.S. federal statutory rate				35.00%
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 368,237,000
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	$ 18,358,000
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	$ 273,041,000
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	$ 1,795,000

Net Income (Loss) per Share - C

Net Income (Loss) per Share - Computation of Denominators Used in Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic:
Weighted average common shares outstanding - Basic	47,413,250	42,669,333	35,892,786
Diluted:
Weighted average common shares outstanding - Basic	47,413,250	42,669,333	35,892,786
Common stock equivalents	501,780	0	0
Denominator for diluted net income (loss) per share	47,915,030	42,669,333	35,892,786

Net Income (Loss) per Share -_2

Net Income (Loss) per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income (loss) - Basic and Diluted	$ 78,909	$ (16,509)	$ (25,068)	$ (28,922)	$ (28,611)	$ (32,842)	$ (22,960)	$ (21,960)	$ 8,410	$ (106,373)	$ (74,013)
Weighted-average common shares outstanding:
Weighted average common shares outstanding - Basic									47,413,250	42,669,333	35,892,786
Stock options									481,254
Restricted stock units									20,526
Weighted-average common shares outstanding - diluted									47,915,030	42,669,333	35,892,786
Net income (loss) per share:
Basic	$ 1.60	$ (0.35)	$ (0.54)	$ (0.62)					$ 0.18	$ (2.49)	$ (2.06)
Diluted	$ 1.59	$ (0.35)	$ (0.54)	$ (0.62)					$ 0.18	$ (2.49)	$ (2.06)

Net Income (Loss) per Share - A

Net Income (Loss) per Share - Additional Information (Detail) - USD ($) $ in Thousands	Oct. 15, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of options outstanding		5,469,393	4,450,517	4,004,422
Number of restricted stock units outstanding		235,250	119,834
Dilutive stock options		481,254
Dilutive restricted stock units		20,526
Vifor Agreement [Member] | License [Member]
Potential milestone payment	$ 50,000
Percentage of premium on common stock investment	20.00%
Stock issuance price measurement period	30 days

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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 calendar quarter after three consecutive months of service
Employer contributions to the plan	$ 349	$ 279	$ 198
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	Jul. 31, 2019agreement	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2020	Jan. 01, 2019USD ($)	May 31, 2016USD ($)
Other Commitments [Line Items]
Research and development expense					$ 107,851	$ 113,820	$ 75,531
Number of renewable terms | item					1
Operating lease right-of-use assets					$ 4,279	3,036
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]
Operating Lease, renewable term					5 years
Operating lease, expiration date					Dec. 31, 2023
Operating Lease, description					The Stamford Lease requires monthly lease payments, including rent escalations and rent holidays, during the initial lease term. The Company began to make rental payments from the Commencement Date. Prior to January 1, 2019, the Company recorded monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through December 31, 2018.
Tenant improvement expenses										$ 1,094
Deferred lease obligation							864
Lease, rent expense							974
Operating lease liabilities					$ 5,275
Tenant improvement incentives							$ 698
Operating lease cost						937
Stamford Operating Lease [Member] | Research and Development [Member]
Other Commitments [Line Items]
Operating lease cost					781	656
Stamford Operating Lease [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Operating lease cost					$ 335	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] | ASC 842 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Other Commitments [Line Items]
Operating lease liabilities									$ 5,198
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]
Operating lease cost					$ 1,116
Non-Exclusive License Agreement
Other Commitments [Line Items]
Upfront fee		$ 8,000
Upfront fee payable, cash		4,000
Upfront fee payable, stock		$ 4,000
Research and development expense					5,000	$ 8,000
Payment of royalty buyout		50.00%
Payment of milestone payment					$ 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
Other Commitments [Line Items]
Agreement initial term, expiration date			Dec. 31, 2023
Agreement renewal term			2 years
Number of related product agreements | agreement				2
MSA | Minimum [Member]
Other Commitments [Line Items]
Termination notice effective period			18 months
MSA or Product Agreement
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) - Stamford Operating Lease [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Other Commitments [Line Items]
Operating cash outflows relating to operating leases	$ 1,403	$ 1,215
ROU assets obtained in exchange for new operating lease liabilities	$ 1,934	$ 3,636
Remaining lease term - operating leases (years)	3 years	3 years 10 months 24 days
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, 2020	Dec. 31, 2019
Operating lease liabilities reported as of December 31, 2020:
Operating lease liabilities - current	$ 1,602	$ 967
Operating lease liabilities - non-current	3,673	$ 3,352
Stamford Operating Lease [Member]
Operating Lease Liabilities, Payments Due [Abstract]
2021	1,921
2022	1,957
2023	1,992
Total future minimum lease payments, undiscounted	5,870
Less imputed interest	(595)
Operating lease liabilities	5,275
Operating lease liabilities reported as of December 31, 2020:
Operating lease liabilities - current	1,602
Operating lease liabilities - non-current	3,673
Total	$ 5,275

Quarterly Results of Operatio_3

Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Oct. 31, 2020	May 31, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues			$ 112,089	$ 9,266	$ 5,634	$ 8,093	$ 4,511	$ 5,785	$ 5,208	$ 4,382	$ 135,082	$ 19,886	$ 13,469
Operating expenses			33,799	26,286	31,518	38,094	34,481	40,218	29,350	27,516	129,697	131,565	90,851
Net income (loss) - Basic and Diluted			$ 78,909	$ (16,509)	$ (25,068)	$ (28,922)	$ (28,611)	$ (32,842)	$ (22,960)	$ (21,960)	$ 8,410	$ (106,373)	$ (74,013)
Basic			$ 1.60	$ (0.35)	$ (0.54)	$ (0.62)					$ 0.18	$ (2.49)	$ (2.06)
Diluted			$ 1.59	$ (0.35)	$ (0.54)	$ (0.62)					0.18	$ (2.49)	$ (2.06)
Basic - sum measured on a quarterly basis											0.09
Diluted- sum measured on a quarterly basis											$ 0.08
Loss per share - Basic and Diluted							$ (0.61)	$ (0.74)	$ (0.58)	$ (0.56)
Vifor International Ltd. [Member]
Stock Issued During Period Shares New Issues		1,174,827
Exclusive License Agreement [Member] | Vifor International Ltd. [Member]
Stock Issued During Period Shares New Issues	2,939,552										2,939,552
License and Milestone Fees
Revenues											$ 134,439	$ 19,746	$ 13,436
License and Milestone Fees | Vifor Agreement [Member]
Revenues			$ 111,551								$ 111,551