Ardelyx (ARDX) 10-Q 6 Aug 20 2020 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Aug. 03, 2020
Document And Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	001-36485
Entity Registrant Name	ARDELYX, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1303944
Entity Address, Address Line One	34175 Ardenwood Boulevard
Entity Address, City or Town	Fremont
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94555
City Area Code	510
Local Phone Number	745-1700
Title of 12(b) Security	Common Stock, par value $0.0001
Trading Symbol	ARDX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		89,140,563
Entity Central Index Key	0001437402
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Amendment Flag	false

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 100,494	$ 181,133
Short-term investments	104,347	66,379
Unbilled revenue	750	750
Prepaid expenses and other current assets	6,035	3,800
Total current assets	211,626	252,062
Property and equipment, net	2,501	3,436
Right-of-use assets	2,945	3,970
Other assets	271	314
Total assets	217,343	259,782
Current liabilities:
Accounts payable	4,212	2,187
Accrued compensation and benefits	3,081	4,453
Current portion of operating lease liability	2,826	2,608
Loan payable, current portion	13,716	1,183
Deferred revenue	2,241	4,541
Accrued expenses and other current liabilities	7,574	7,248
Total current liabilities	33,650	22,220
Operating lease liability, net of current portion	608	2,076
Loan payable, net of current portion	36,735	48,831
Total liabilities	70,993	73,127
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; no shares issued and outstanding as of June 30, 2020 and December 31, 2019, respectively.
Common stock, $0.0001 par value; 300,000,000 shares authorized; 89,140,563 and 88,817,741 shares issued and outstanding as of June 30, 2020 and December 31, 2019, respectively.	9	9
Additional paid-in capital	653,805	647,078
Accumulated deficit	(507,781)	(460,452)
Accumulated other comprehensive income	317	20
Total stockholders' equity	146,350	186,655
Total liabilities and stockholders' equity	$ 217,343	$ 259,782

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Jun. 30, 2020	Dec. 31, 2019
CONDENSED BALANCE SHEETS (Parenthetical)
Preferred stock, par value	$ 0.0001	$ 0.0001
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	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	89,140,563	88,817,741
Common stock, shares outstanding	89,140,563	88,817,741

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Total revenues	$ 1,836	$ 18	$ 3,049	$ 18
Operating expenses:
Cost of revenue	141		141
Research and development	18,864	19,475	34,708	39,856
General and administrative	7,038	5,371	14,176	10,488
Total operating expenses	26,043	24,846	49,025	50,344
Loss from operations	(24,207)	(24,828)	(45,976)	(50,326)
Interest expense	(1,226)	(1,451)	(2,583)	(2,885)
Other income, net	477	812	1,230	1,602
Loss before provision for income taxes	(24,956)	(25,467)	(47,329)	(51,609)
Provision for income taxes				2
Net loss	$ (24,956)	$ (25,467)	$ (47,329)	$ (51,611)
Net loss per common share, basic and diluted	$ (0.28)	$ (0.41)	$ (0.53)	$ (0.82)
Shares used in computing net loss per share - basic and diluted	89,080,046	62,651,863	88,980,353	62,599,371
Comprehensive loss:
Net loss	$ (24,956)	$ (25,467)	$ (47,329)	$ (51,611)
Unrealized gains on available-for-sale securities	361	4	297	54
Comprehensive loss	(24,595)	(25,463)	(47,032)	(51,557)
Licensing
Revenues:
Total revenues	706		706
Collaborative development
Revenues:
Total revenues	1,125		2,300
Other
Revenues:
Total revenues	$ 5	$ 18	$ 43	$ 18

CONDENSED STATEMENTS OF CHANGES

CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income	Total
Balance, amount at Dec. 31, 2018	$ 6	$ 481,357	$ (365,512)	$ (38)	$ 115,813
Balance, shares at Dec. 31, 2018	62,516,627
Issuance of common stock under employee stock purchase plan, amount		198			198
Issuance of common stock under employee stock purchase plan, shares	83,046
Issuance of common stock for services, amount		311			311
Issuance of common stock for services, shares	113,136
Issuance of common stock upon exercise of options, amount		7			7
Issuance of common stock upon exercise of options, shares	2,451
Issuance of common stock upon vesting of restricted stock units, shares	85,609
Stock-based compensation		3,845			3,845
Unrealized gains on available-for-sale securities				54	54
Net loss			(51,611)		(51,611)
Balance, amount at Jun. 30, 2019	$ 6	485,718	(417,123)	16	68,617
Balance, shares at Jun. 30, 2019	62,800,869
Balance, amount at Mar. 31, 2019	$ 6	483,479	(391,656)	12	91,841
Balance, shares at Mar. 31, 2019	62,600,443
Issuance of common stock for services, amount		311			311
Issuance of common stock for services, shares	113,136
Issuance of common stock upon exercise of options, amount		5			5
Issuance of common stock upon exercise of options, shares	1,681
Issuance of common stock upon vesting of restricted stock units, shares	85,609
Stock-based compensation		1,923			1,923
Unrealized gains on available-for-sale securities				4	4
Net loss			(25,467)		(25,467)
Balance, amount at Jun. 30, 2019	$ 6	485,718	(417,123)	16	68,617
Balance, shares at Jun. 30, 2019	62,800,869
Balance, amount at Dec. 31, 2019	$ 9	647,078	(460,452)	20	186,655
Balance, shares at Dec. 31, 2019	88,817,741
Issuance of common stock under employee stock purchase plan, amount		375			375
Issuance of common stock under employee stock purchase plan, shares	75,804
Issuance of common stock for services, amount		310			310
Issuance of common stock for services, shares	42,403
Issuance of common stock upon exercise of options, amount		420			420
Issuance of common stock upon exercise of options, shares	204,615
Stock-based compensation		5,622			5,622
Unrealized gains on available-for-sale securities				297	297
Net loss			(47,329)		(47,329)
Balance, amount at Jun. 30, 2020	$ 9	653,805	(507,781)	317	146,350
Balance, shares at Jun. 30, 2020	89,140,563
Balance, amount at Mar. 31, 2020	$ 9	650,617	(482,825)	(44)	167,757
Balance, shares at Mar. 31, 2020	89,035,096
Issuance of common stock for services, amount		310			310
Issuance of common stock for services, shares	42,403
Issuance of common stock upon exercise of options, amount		204			204
Issuance of common stock upon exercise of options, shares	63,064
Stock-based compensation		2,674			2,674
Unrealized gains on available-for-sale securities				361	361
Net loss			(24,956)		(24,956)
Balance, amount at Jun. 30, 2020	$ 9	$ 653,805	$ (507,781)	$ 317	$ 146,350
Balance, shares at Jun. 30, 2020	89,140,563

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (47,329)	$ (51,611)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	960	1,353
Amortization of deferred financing costs	282	282
Amortization of deferred compensation for services	158	154
Amortization of premium on investment securities	(227)	(537)
Non-cash lease expense	1,025	891
Stock-based compensation	5,622	3,845
Change in derivative liabilities	152	61
Non-cash interest associated with debt discount accretion	257	231
Changes in operating assets and liabilities:
Unbilled revenue		5,000
Prepaid expenses and other assets	(2,144)	12
Accounts payable	2,025	(887)
Accrued compensation and benefits	(1,372)	(372)
Lease liabilities	(1,251)	(825)
Accrued and other liabilities	174	(2,330)
Deferred revenue	(2,300)
Net cash used in operating activities	(43,968)	(44,733)
Investing activities
Proceeds from maturities of investments	25,519	86,454
Purchases of investments	(62,960)	(30,857)
Purchases of property and equipment	(25)	(211)
Net cash (used in) provided by investing activities	(37,466)	55,386
Financing activities
Proceeds from issuance of common stock under stock plans	375	198
Issuance of common stock upon exercise of options	420	7
Net cash provided by financing activities	795	205
Net (decrease) increase in cash and cash equivalents	(80,639)	10,858
Cash and cash equivalents at beginning of period	181,133	78,768
Cash and cash equivalents at end of period	$ 100,494	89,626
Supplementary disclosure of cash flow information:
Income taxes paid		2
Supplementary disclosure of non-cash activities:
Right-of-use assets obtained in exchange for lease obligations		$ 5,810

Organization and Basis of Prese

Organization and Basis of Presentation	6 Months Ended
	Jun. 30, 2020
Organization and Basis of Presentation
Organization and Basis of Presentation	NOTE 1. ORGANIZATION AND BASIS OF PRESENTATION Ardelyx, Inc. (the “Company,” “we,” “us” or “our”) is a specialized biopharmaceutical company focused on developing innovative first-in-class medicines to improve treatment for people with kidney and cardiovascular diseases. The Company operates in one business segment, which is the research and development of biopharmaceutical products. Basis of Presentation These condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the requirements of the Securities and Exchange Commission (“SEC”) for interim reporting. As permitted under those rules and regulations, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted. These condensed financial statements have been prepared on the same basis as the Company’s most recent annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary to present fairly the Company’s financial position at June 30, 2020 and results of operations, changes in stockholders’ equity, and cash flows for the interim periods ended June 30, 2020 and 2019. The accompanying condensed financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The results for the three and six months ended June 30, 2020 are not necessarily indicative of results to be expected for the entire year ending December 31, 2020, or for any other interim period or future year. ​ Prior Period Errors ​ In connection with our review of our financial statements as of and for the six months ended June 30, 2019, we corrected errors related to the accounting for clinical trial accruals that had resulted in an overstatement of research and development expenses during the three months ended March 31, 2019 and during the year ended December 31, 2018. Specifically, management concluded that the Company’s research and development expenses recorded during the three months ended March 31, 2019 and during year ended December 31, 2018 had been overstated by $0.5 million and $3.6 million, respectively, and that the Company’s accrued expenses other current liabilities ​ Management analyzed the potential impact of these errors in accordance with the SEC’s Staff Accounting Bulletin No. 108, Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements ​ Liquidity As of June 30, 2020, the Company had cash, cash equivalents and short-term investments of approximately $204.8 million. The Company believes its current available cash, cash equivalents and short-term investments will be sufficient to fund the Company’s planned expenditures and meet its obligations for at least 12 months following August 6, 2020, which is the date that these condensed financial statements are being issued. ​ Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the amounts reported in the financial statements and accompanying notes thereto. On an ongoing basis, management evaluates its estimates, including those related to recognition of revenue, clinical trial accruals, contract manufacturing accruals, the fair value of assets and liabilities, income taxes and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could materially differ from those estimates. ​ ​ Summary of Significant Accounting Policies There have been no changes to the significant accounting policies disclosed in the Company’s most recent Annual Report on Form 10-K. Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In December 2019, as part of its initiative to reduce complexity in the accounting standards, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 Revenue from Contracts with Customers Fair Value Measurement (Topic 820): Disclosure Framework— Changes to the Disclosure Requirements for Fair Value Measurement Recent Accounting Pronouncements Not Yet Adopted In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments

Cash, Cash Equivalents and Shor

Cash, Cash Equivalents and Short-Term Investments	6 Months Ended
	Jun. 30, 2020
Cash, Cash Equivalents and Short-Term Investments
Cash, Cash Equivalents and Short-Term Investments	NOTE 2. CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS Securities classified as cash, cash equivalents and short-term investments as of June 30, 2020 and December 31, 2019 are summarized below. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value Cash and cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 99,410 ​ $ — ​ $ — ​ $ 99,410 Cash ​ ​ 1,084 ​ ​ — ​ ​ — ​ ​ 1,084 Total cash and cash equivalents ​ ​ 100,494 ​ ​ — ​ ​ — ​ ​ 100,494 Short-term investments ​ ​ ​ ​ ​ Commercial paper ​ $ 46,328 ​ $ 115 ​ $ — ​ $ 46,443 Corporate bonds ​ ​ 35,119 ​ ​ 170 ​ ​ — ​ ​ 35,289 Asset-backed securities ​ 10,507 ​ 25 ​ — ​ 10,532 U.S. government-sponsored agency bonds ​ ​ 8,077 ​ ​ — ​ ​ (2) ​ ​ 8,075 U.S. treasury notes ​ 3,999 ​ 9 ​ — ​ 4,008 Total short-term investments ​ ​ 104,030 ​ ​ 319 ​ ​ (2) ​ ​ 104,347 Total cash equivalents and short-term investments ​ $ 204,524 ​ $ 319 ​ $ (2) ​ $ 204,841 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value Cash and cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 147,208 ​ $ — ​ $ — ​ $ 147,208 Commercial paper ​ 19,357 ​ 3 ​ — ​ 19,360 Corporate bonds ​ 11,441 ​ — ​ — ​ 11,441 Cash ​ 3,124 ​ — ​ — ​ 3,124 Total cash and cash equivalents ​ ​ 181,130 ​ ​ 3 ​ ​ — ​ ​ 181,133 Short-term investments ​ ​ ​ ​ ​ Commercial paper ​ $ 36,667 ​ $ 14 ​ $ — ​ $ 36,681 Corporate bonds ​ 21,690 ​ 6 ​ (3) ​ 21,693 Asset-backed securities ​ 8,005 ​ — ​ — ​ 8,005 Total short-term investments ​ ​ 66,362 ​ ​ 20 ​ ​ (3) ​ ​ 66,379 Total cash equivalents and short-term investments ​ $ 247,492 ​ $ 23 ​ $ (3) ​ $ 247,512 ​ All available-for-sale securities held as of June 30, 2020 had contractual maturities of less than one year. The Company’s available-for-sale securities are subject to a periodic impairment review. The Company considers a debt security to be impaired when the fair value of that security is less than its carrying cost, in which case the Company would further evaluate the investment to determine whether the security is other-than-temporarily impaired. When the Company evaluates an investment for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below cost basis, the financial condition or creditworthiness of the issuer and any changes thereto, intent to sell, and whether it is more likely than not the Company will be required to sell the investment before the recovery of its cost basis. If an investment is other-than-temporarily impaired, the Company writes the investment down through the statement of operations to its fair value and establishes that value as the new cost basis for the investment. Management has determined that none of the Company’s available-for-sale securities were other-than-temporarily impaired in any of the periods presented, and as of June 30, 2020, no investment was in a continuous unrealized loss position for more than one year. As such, the Company believes that it is more likely than not that the investments will be held until maturity or a forecasted recovery of fair value. While our investment policy requires that we only invest in highly-rated securities and limit our exposure to any single issuer, the COVID-19 pandemic may materially affect the financial conditions of issuers, which could result in a default by one or more issuers or result in downgrades below our minimum credit rating requirements.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2020
Fair Value Measurements
Fair Value Measurements	NOTE 3. FAIR VALUE MEASUREMENTS The Company’s financial instruments consist of cash and cash equivalents, short-term investments, prepaid expenses, other current assets, accounts payable, accrued expenses, and the Term Loan, as defined and discussed in Note 5. Fair value estimates of these instruments are made at a specific point in time based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment. The carrying amounts of financial instruments such as cash and cash equivalents, prepaid expenses, other current assets, accounts payable and accrued expenses approximate the related fair values due to the short maturities of these instruments. Based on prevailing borrowing rates available to the Company for loans with similar terms, the Company believes the fair value of the Term Loan, considering level 2 inputs, approximates this instrument’s carrying value. ​ Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. ​ The three-level hierarchy for the inputs to valuation techniques is briefly summarized as follows: Level 1 – Valuations are based on quoted prices in active markets for identical assets or liabilities and readily accessible by the Company at the reporting date. Examples of assets and liabilities utilizing Level 1 inputs are certain money market funds, U.S. treasuries and trading securities with quoted prices on active markets. Level 2 – Valuations based on inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Examples of assets and liabilities utilizing Level 2 inputs are corporate bonds, commercial paper, certificates of deposit and over-the-counter derivatives. Level 3 – Valuations based on unobservable inputs in which there is little or no market data, which require the Company to develop its own assumptions. The following table sets forth the fair value of the Company’s financial assets and liabilities that are measured or disclosed on a recurring basis: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ Total Level 1 Level 2 Level 3 Assets: ​ ​ ​ ​ Money market funds ​ $ 99,410 ​ $ 99,410 ​ $ — ​ $ — Commercial paper ​ 46,443 ​ — ​ 46,443 ​ — Corporate bonds ​ 35,289 ​ — ​ 35,289 ​ — Asset-backed securities ​ 10,532 ​ — ​ 10,532 ​ — U.S. government-sponsored agency bonds ​ ​ 8,075 ​ ​ 8,075 ​ ​ ​ ​ ​ ​ U.S. treasury notes ​ 4,008 ​ 4,008 ​ — ​ — Total ​ $ 203,757 ​ $ 111,493 ​ $ 92,264 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative liability for Exit Fee ​ $ 1,121 ​ $ — ​ $ — ​ $ 1,121 Total ​ $ 1,121 ​ $ — ​ $ — ​ $ 1,121 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ Total Level 1 Level 2 Level 3 Assets: ​ ​ ​ ​ Money market funds ​ $ 147,208 ​ $ 147,208 ​ $ — ​ $ — Commercial paper ​ 56,041 ​ — ​ 56,041 ​ — Corporate bonds ​ 33,134 ​ — ​ 33,134 ​ — Asset-backed securities ​ 8,005 ​ — ​ 8,005 ​ — Total ​ $ 244,388 ​ $ 147,208 ​ $ 97,180 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative liability for Exit Fee ​ $ 969 ​ $ — ​ $ — ​ $ 969 Total ​ $ 969 ​ $ — ​ $ — ​ $ 969 ​ Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds, U.S. treasury securities and U.S. treasury notes as Level 1. When quoted market prices are not available for the specific security, the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes and issuer spreads. The Company classifies corporate bonds, commercial paper, asset-backed securities and foreign currency derivative contracts as Level 2. In certain cases, where there is limited activity or less transparency around inputs to valuation, securities or derivative liabilities such as the Exit Fee, as defined and discussed in Note 4, are classified as Level 3. There were no transfers

Derivative Liability

Derivative Liability	6 Months Ended
	Jun. 30, 2020
Derivative Liability
Derivative Liability	NOTE 4. DERIVATIVE LIABILITY ​ In May 2018, in connection with entering into the Loan Agreement, as defined and discussed in Note 5, the Company entered into an agreement pursuant to which the Company agreed to pay $1.5 million in cash (the “Exit Fee”) upon any change of control transaction in respect of the Company or if the Company obtains both (i) U.S. Food and Drug Administration (“FDA”) approval of tenapanor for the treatment of hyperphosphatemia in chronic kidney disease patients on dialysis and (ii) FDA approval of tenapanor for the treatment of patients with irritable bowel syndrome with constipation (“IBS-C”), which was obtained on September 12, 2019 when the FDA approved IBSRELA® (tenapanor), a 50 milligram, twice daily oral pill for the treatment of IBS-C in adults (the “Exit Fee Agreement”). Notwithstanding the prepayment or termination of the Term Loan, as defined and discussed in Note 5, the Company’s obligation to pay the Exit Fee will expire on May 16, 2028. The Company concluded that the Exit Fee is a freestanding derivative which should be accounted for at fair value on a recurring basis. The estimated fair value of the Exit Fee is recorded as a derivative liability and included in accrued expenses and other current liabilities on the accompanying condensed balance sheets. The fair value of the derivative liability was determined using a discounted cash flow analysis, and the key assumptions included in the calculation of the estimated fair value of the derivative liability include: (i) the Company’s estimates of both the probability and timing of payment of the Exit Fee to Solar Capital Ltd. and Western Alliance Bank as a result of the FDA approvals and (ii) a variable discount rate. Generally, increases or decreases in the probability of occurrence would result in a directionally similar impact in the fair value measurement of the derivative liability, and it is estimated that a 10% increase or decrease fluctuation Changes in fair value, which are presented as other income, net, in the Company's condensed statements of operations, were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2020 ​ ​ 2019 ​ Fair value of Exit Fee derivative liability at January 1 ​ $ 969 ​ $ 533 ​ Change in estimated fair value of derivative liability ​ ​ 152 ​ ​ 61 ​ Fair value of Exit Fee derivative liability at June 30 ​ $ 1,121 ​ $ 594 ​ ​ ​

Borrowing

Borrowing	6 Months Ended
	Jun. 30, 2020
Borrowing
Borrowing	NOTE 5. BORROWING ​ Solar Capital and Western Alliance Bank Loan Agreement ​ On May 16, 2018, the Company entered into a loan and security agreement (the “Loan Agreement”), with Solar Capital Ltd. and Western Alliance Bank (collectively the “Lenders”). The Loan Agreement provides for a $50.0 million term loan facility with a maturity date of November 1, 2022 (the “Term Loan”). ​ Borrowings under the Term Loan bear interest at a floating per annum rate equal to 7.45% plus the one ​ The Company paid a closing fee of 1% of the Term Loan, or $0.5 million, upon the closing of the Term Loan, and the Company is obligated to pay a final fee equal to 3.95% of the Term Loan upon the earliest to occur of the maturity date, the acceleration of the Term Loan, the prepayment or repayment of the Term Loan or the termination of the Loan Agreement. The Company may voluntarily prepay the outstanding Term Loan, subject to a prepayment premium of (i) 3% of the principal amount of the Term Loan if prepaid prior to or on the first anniversary of the Closing Date, (ii) 2% of the principal amount of the Term Loan if prepaid after the first anniversary of the Closing Date through and including the second anniversary of the Closing Date, or (iii) 1% of the principal amount of the Term Loan if prepaid after the second anniversary of the Closing Date and prior to the maturity date. The Term Loan is secured by substantially all the Company’s assets, except for the Company’s intellectual property and certain other customary exclusions. Additionally, in connection with the Term Loan, the Company entered into the Exit Fee Agreement, as discussed in Note 4. The Loan Agreement contains customary representations and warranties and customary affirmative and negative covenants. As of June 30, 2020, the Company was in compliance with all of the covenants set forth in the Loan Agreement. The Loan Agreement also contains customary events of default that entitle the Lender to cause the Company’s indebtedness under the Loan Agreement to become immediately due and payable, and to exercise remedies against the Company and the collateral securing the Term Loan, including the Company’s cash. Upon the occurrence and for the duration of an event of default, an additional default interest rate equal to 4.0% per annum will apply to all amounts owed under the Loan Agreement. As of June 30, 2020, to the Company’s knowledge, there were no facts or circumstances in existence that would give rise to an event of default. ​ As of June 30, 2020, the Company’s future payment obligations related to the Term Loan, excluding interest payments and the Exit Fee, are as follows: ​ ​ ​ ​ ​ ​ Remainder of 2020 ​ $ 2,083 2021 ​ 25,000 2022 ​ 24,892 Total principal and final fee payments ​ ​ 51,975 Less: Unamortized discount and debt issuance costs ​ ​ (561) Less: Unaccreted value of final fee ​ ​ (963) Loan payable ​ ​ 50,451 Less: Loan payable, current portion ​ ​ 13,716 Loan payable, net of current portion ​ $ 36,735 ​

Leases

Leases	6 Months Ended
	Jun. 30, 2020
Leases
Leases	​ NOTE 6. LEASES ​ All of the Company’s leases, which primarily include the right to use office and laboratory space, are operating leases, and certain of the leases have both lease and non-lease components. The Company has elected to account for each separate lease component and the non-lease components associated with that lease component as a single lease component for all classes of underlying assets. The following table provides additional details related to our facility leases, as presented in the Company’s condensed balance sheet as of June 30, 2020: ​ ​ ​ ​ ​ ​ Facilities ​ ​ ​ ​ Right of use assets ​ $ 2,945 ​ ​ ​ ​ ​ ​ Current portion of lease liabilities ​ ​ 2,826 ​ Operating lease liability, net of current portion ​ ​ 608 ​ Total ​ $ 3,434 ​ ​ ​ ​ ​ ​ Weighted-average remaining life (years) ​ ​ 1.25 ​ Weighted-average discount rate ​ ​ 12.99 % ​ Other information related to the Company’s facilities lease is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ 2020 2019 ​ 2020 2019 Operating lease expense $ 648 ​ $ 648 ​ $ 1,296 ​ $ 1,296 Cash paid for operating lease $ 761 ​ $ 616 ​ $ 1,521 ​ $ 1,230 ​ The following table summarizes the Company’s undiscounted cash payment obligations for its operating lease liabilities as of June 30, 2020: ​ ​ ​ ​ ​ Remainder of 2020 ​ $ 1,544 2021 ​ ​ 2,183 Total undiscounted operating lease payments ​ ​ 3,727 Imputed interest expenses ​ ​ (293) Total operating lease liabilities ​ ​ 3,434 Less: Current portion of operating lease liability ​ ​ 2,826 Operating lease liability, net of current portion ​ $ 608 ​

Equity Incentive Plans

Equity Incentive Plans	6 Months Ended
	Jun. 30, 2020
Equity Incentive Plans
Equity Incentive Plans	​ NOTE 7. EQUITY INCENTIVE PLANS ​ Stock Option Plan During the three and six months ended June 30, 2020, options were granted to employees and members of the board of directors to purchase 729,727 and 2,649,516 shares, respectively, of the Company's common stock. The weighted-average grant-date estimated fair value of options granted during the three and six months ended June 30, 2020 was $5.14 and $5.19, respectively. The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following weighted-average assumptions: ​ ​ ​ ​ ​ ​ ​ Six Months Ended June 30, ​ 2020 Expected term (years) 6.00 Expected volatility 83.55 % Risk-free interest rate 1.33 % Dividend yield — % ​ During the three and six months ended June 30, 2020, options were exercised to purchase 63,064 and 204,615 shares, respectively, of the Company's common stock, resulting in corresponding approximately $0.2 million and $0.4 million net proceeds, respectively, to the Company. During the three and six months ended June 30, 2019, options were exercised to purchase 1,681 and 2,451 shares, respectively, of the Company's common stock, resulting in corresponding insignificant net proceeds to the Company. ​ Restricted Stock Units (“RSUs”) In July 2018, the Company granted 903,374 performance-based restricted stock units (“PRSUs”) to its employees that vest upon the achievement of certain performance conditions, subject to the employees’ continued service relationship with the Company through the achievement date. As of June 30, 2020, 849,328 of these PRSUs were outstanding and none of these PRSUs were vested. Additionally, during the three months ended June 30, 2020, the Company granted 30,000 PRSUs with the same performance criteria to its recently hired chief commercial officer and chief financial officer, of which none vested and those 30,000 PRSU’s are also outstanding at June 30, 2020. Based on the evaluation of the performance conditions, the Company recorded stock-based compensation expense of $0.6 million and $1.4 million for the three and six months ended June 30, 2020, respectively. The Company had not recorded stock-based compensation expense for the three and six months ended June 30, 2019 related to these PRSUs. The related compensation cost is recognized as an expense ratably over the estimated vesting period. The expense recognized for these awards is based on the grant date fair value of the Company’s common stock multiplied by the number of units granted. For each of the three and six months ended June 30, 2020 and 2019, the Company issued zero and 85,609 shares of its common stock upon vesting of RSUs to its employees. Employee Stock Purchase Plan In February 2020, the Company sold 75,804 shares of its common stock under the Company’s employee stock purchase program (“ESPP”). The shares were purchased by employees at a purchase price of $4.95 per share with proceeds to the Company of approximately $0.4 million. In February 2019, the Company sold 83,046 shares of its common stock under the ESPP. The shares were purchased by employees at a purchase price of $2.39 per share with proceeds to the Company of approximately $0.2 million. Issuance of Common Stock for Services For the three and six months ended June 30, 2020, the Company issued 42,403 shares of its common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Company’s Amended and Restated Non-Employee Director Compensation Program. The shares issued were valued at $0.3 million based on the fair value of the common stock on the date of grant. For the three and six months ended June 30, 2019, the Company issued 113,136 shares of its common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Company’s Amended and Restated Non-Employee Director Compensation Program. The shares issued were valued at $0.3 million based on the fair value of the common stock on the date of grant. Stock-Based Compensation Stock-based compensation expense recognized for stock options, RSUs, PRSUs and the Company’s ESPP are recorded as operating expenses in the Company’s condensed statements of operations and comprehensive loss, as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ ​ 2020 2019 2020 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Research and development $ 1,144 ​ $ 767 ​ $ 2,202 ​ $ 1,527 ​ General and administrative ​ 1,530 ​ ​ 1,156 ​ ​ 3,420 ​ 2,318 ​ Total ​ $ 2,674 ​ $ 1,923 ​ $ 5,622 ​ $ 3,845 ​ ​ As of June 30, 2020, the Company’s total unrecognized stock-based compensation expense, net of estimated forfeitures, and average remaining vesting period, included the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ Unrecognized Compensation Expense ​ Average Remaining Vesting Period (Years) Stock options $ 15,202 ​ ​ 2.8 PRSUs ​ $ 396 ​ ​ 0.2 ESPP $ 78 ​ ​ 0.2 ​ Warrants ​ In June 2015, the Company had sold and issued warrants to purchase 2,172,899 shares of common stock. The purchase price for the warrants was $0.125 per warrant. The warrants were exercisable for an exercise price of $13.91 per share at any time prior to the earlier of (i) 5 years from the date of issuance or (ii) certain changes in control of the Company. The Company had determined that the warrants should be classified as equity. In June 2020, the warrants expired with none of the warrants exercised. ​

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2020
Net Loss Per Share
Net Loss Per Share	NOTE 8. NET LOSS PER SHARE Basic net loss per share is calculated by dividing net loss by the weighted-average number of common shares outstanding during the period, less shares subject to repurchase, and excludes any dilutive effects of stock-based awards and warrants. Diluted net loss per common share is computed giving effect to all potential dilutive common shares, including common stock issuable upon exercise of stock options, and unvested restricted common stock and stock units. As the Company had net losses for the three and six months ended June 30, 2020 and 2019, all potential common shares were determined to be anti-dilutive. The following table sets forth the computation of net loss per common share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended June 30, ​ ​ 2020 2019 2020 2019 Numerator: ​ ​ ​ ​ Net loss ​ $ (24,956) ​ $ (25,467) ​ $ (47,329) ​ $ (51,611) ​ Denominator: ​ ​ ​ ​ ​ Weighted average common shares outstanding - basic and diluted ​ 89,080,046 ​ 62,651,863 ​ 88,980,353 ​ 62,599,371 ​ Net loss per share - basic and diluted ​ $ (0.28) ​ $ (0.41) ​ $ (0.53) ​ $ (0.82) ​ ​ For the three and six months ended June 30, 2020, the total number of securities that could potentially dilute basic net loss per share in the future that were not included in the computation of diluted net loss per share because the effect would have been antidilutive was 11.5 million and 11.7 million, respectively. For the three and six months ended June 30, 2019, the total number of securities that could potentially dilute basic net loss per share in the future that were not included in the computation of diluted net loss per share because the effect would have been antidilutive was 10.2 million and 10.0 million, respectively.

Collaboration and Licensing Agr

Collaboration and Licensing Agreements	6 Months Ended
	Jun. 30, 2020
Collaboration and Licensing Agreements
Collaboration and Licensing Agreements	NOTE 9. COLLABORATION AND LICENSING AGREEMENTS Kyowa Kirin Co., Ltd. (“KKC”) 2019 KKC Agreement In November 2019, the Company entered into a research collaboration and option agreement with KKC (the “2019 KKC Agreement”) for research associated with identifying two pre-clinical compounds that are ready for designation as development compounds, with one compound inhibiting the first undisclosed target (“Program 1”), and a second inhibiting the second undisclosed target (“Program 2”). Pursuant to the 2019 KKC Agreement, upon completion of the research and designation by the research steering committee of one or more development candidates (“DCs”), KKC has the right to execute one or more separate collaborative agreements relating to the development and commercialization of one or both DCs in certain specified territories. Under the terms of the 2019 KKC Agreement, KKC agreed to pay the Company a non-refundable, non-creditable upfront fee of $10.0 million, payable as follows: the first installment of $5.0 million within 30 days of November 11, 2019 (the “Effective Date”), and the second installment of $5.0 million on the first anniversary of the Effective Date, unless the 2019 KKC Agreement is earlier terminated by KKC due to material breach by the Company. The term of the 2019 KKC Agreement commenced on the Effective Date and ends on the earliest of: (i) two years following the Effective Date, or (ii) the nomination of a program DC for both programs, (iii) or the nomination of one program DC and the decision by the parties to cease research for the other program, or (iv) the decision by the parties to cease research for both programs. The Company assessed the 2019 KKC Agreement in accordance with ASC 606 and concluded that the contract’s counterparty, KKC, is a customer. Management also considered the modification guidance prescribed in ASC 606 and concluded that the 2019 KKC Agreement should be accounted for as a separate contract from the 2017 KKC Agreement, as defined and discussed below. The Company identified various promises in the 2019 KKC Agreement, including: the grant of an initial research license; the Program 1 research; the Program 2 research; the right to obtain certain development and commercialization rights to a Program 1 DC in certain territories; the right to obtain development and commercialization rights to a Program 2 DC in certain territories; and participation in a joint steering committee (“JSC”). The Company determined that KKC could not benefit from either of the research programs without the research license and participation in the JSC. As such, the combined license, research programs and participation in the JSC were deemed to be the highest level of goods and services that can be deemed distinct for each of the Program 1 research and Program 2 research. The Company concluded that the options to obtain additional development and commercialization rights that are exercisable by KKC under certain circumstances are not performance obligations of the contract at inception because the option fees reflect the standalone selling price of the options, and therefore, the options are not considered to be material rights. At the outset of the 2019 KKC Agreement, the Company determined that the initial transaction price amounted to $10.0 million and that revenue associated with the combined performance obligations will be recognized as services are provided using an input method. Since transfer of control occurs over time, in management’s judgment this input method is the best measure of progress towards satisfying the performance obligations and reflects a faithful depiction of the transfer of goods and services. Revenue will be recognized over the Program 1 and Program 2 research periods, which are currently expected to extend through the end of 2021. Management will re-evaluate the estimates related to the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur and adjust the timing of revenue recognition as necessary. During the three and six months ended June 30, 2020, the Company recognized $1.1 million and $2.3 million, respectively, as revenue under the 2019 KKC Agreement in the accompanying statement of operations and comprehensive loss. The aggregate amount of the transaction price allocated to the Company’s partially unsatisfied performance obligations as of June 30, 2020 was $7.2 million, of which $2.2 million is presented in the accompanying condensed balance sheet as deferred revenue. As of June 30, 2020, the Company expects to recognize the remaining transaction price allocated to the Company’s partially unsatisfied performance obligations over the remaining research terms, which, as noted above, are currently expected to extend through the end of 2021. There were no changes in estimates associated with the 2019 KKC Agreement during the three and six months ended June 30, 2020. 2017 KKC Agreement In November 2017, the Company entered into an exclusive license agreement with KKC (the “2017 KKC Agreement”), for the development, commercialization and distribution of tenapanor in Japan for cardiorenal indications. The Company granted KKC an exclusive license to develop and commercialize certain sodium hydrogen exchanger 3 (“NHE3”) inhibitors including tenapanor in Japan for the treatment of cardiorenal diseases and conditions, excluding cancer. The Company retained the rights to tenapanor outside of Japan, and also retained the rights to tenapanor in Japan for indications other than those stated above. Pursuant to the 2017 KKC Agreement, KKC is responsible for all costs and expenses incurred in the development and commercialization of tenapanor for the treatment of cardiorenal diseases and conditions, excluding cancer in Japan. Under the 2017 KKC Agreement, the Company is responsible for supplying the tenapanor drug product for KKC’s use in development and during commercialization until KKC has assumed such responsibility. Additionally, the Company is responsible for supplying the tenapanor drug substance for KKC’s use in development and commercialization throughout the term of the 2017 KKC Agreement, provided that KKC may exercise an option to manufacture the tenapanor drug substance under certain conditions. ​ The Company assessed these arrangements in accordance with ASC 606 and concluded that the contract counterparty, KKC, is a customer. Under the terms of the 2017 KKC Agreement, the Company received $30.0 million in upfront license fees, which was recognized as revenue when the agreement was executed. Based on the Company’s assessment, management determined that the license and the manufacturing supply services were its material performance obligations at the inception of the 2017 KKC Agreement, and as such, each of the performance obligations is distinct. Additionally, the Company recorded unbilled revenue of $5.0 million and an increase in uncharged license fees of $1.0 million related to the first milestone under the 2017 KKC Agreement that KKC; this first milestone was achieved in February 2019 . In addition to the $30.0 million upfront license fee, the Company may be entitled to receive up to $55.0 million in total development milestones, of which $5.0 million has been received to date, and approximately $78.9 million in commercialization milestones, as well as reimbursement of costs plus a reasonable overhead for the supply of product and high-teen royalties on net sales throughout the term of the agreement. The variable consideration related to the remaining development milestone payments has not been included in the transaction price as these were fully constrained at June 30, 2020. During the three and six months ended June 30, 2020, the Company recognized $5,000 and $43,000 , respectively, as other revenue related to the manufacturing supply of tenapanor and other materials to KKC pursuant to the 2017 KKC Agreement. Similarly, for each of the three and six months ended June 30, 2019, $18,000 was recognized as other revenue.. Xuanzhu (HK) Biopharmaceutical Limited (“XuanZhu”) In November 2019, the Company entered into a license agreement with XuanZhu (the “XuanZhu Agreement”), pursuant to which the Company granted XuanZhu a license to certain specific patent and patent applications. The Company assessed the XuanZhu Agreement in accordance with ASC 606 and concluded that the contract counterparty, XuanZhu, is a customer. Under the terms of the XuanZhu Agreement, the Company recognized $1.5 million in license fees, which constituted the initial transaction price, when the agreement was executed, of which $750,000 was received upfront in November 2019, and achievement for the second $750,000 payment, related to the issuance and grant of a specific patent, was determined to be not materially at risk and probable of achievement. Based on management’s assessment, the Company determined that it has one combined performance obligation, which is the license and the specific patent grant. ​ No revenue related to the XuanZhu Agreement was recorded during the three and six months ended June 30, 2020. Shanghai Fosun Pharmaceutical Industrial Development Co. Ltd. (“Fosun Pharma”) In December 2017, the Company entered into an exclusive license agreement with Fosun Pharma ( the “ Fosun Agreement ”), for the development, commercialization and distribution of tenapanor in China for both hyperphosphatemia and IBS-C. The Company assessed these arrangements in accordance with ASC 606 and concluded that the contract counterparty, Fosun Pharma, is a customer. Under the terms of the Fosun Agreement, the Company received $12.0 million in upfront license fees which was recognized as revenue when the agreement was executed. Based on management’s assessment, the Company determined that the license and the manufacturing supply services represented the material performance obligations at the inception of the agreement, and as such , each of the performance obligations is distinct . The Company may be entitled to additional development and commercialization milestones of up to $110.0 million, as well as reimbursement of cost plus a reasonable overhead for the supply of product and tiered royalties on net sales ranging from the mid-teens to 20% . The variable consideration related to the remaining development milestone payments has not been included in the transaction price as these were fully constrained at June 30, 2020. The Company has recorded no revenue during the three and six months ended June 30, 2020 or 2019 related to the Fosun Agreement. Knight Therapeutics, Inc. (“Knight“) In March 2018, the Company entered into an exclusive license agreement with Knight (the “Knight Agreement”), for the development, commercialization and distribution of tenapanor in Canada for hyperphosphatemia and IBS-C. The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Knight, is a customer. Based on management’s assessment, the Company determined that the license and the manufacturing supply services represented the material performance obligations at the inception of the agreement, and as such, each of the performance obligations is distinct. Under the terms of the agreement, the Company is eligible to receive up to $18.3 million, including an upfront payment, development and sales milestones, reimbursement of supply costs on a schedule specifying cost per tablet, with a reasonable mark up for overhead, as well as tiered royalty rates on net sales ranging from the mid-single digits to the low twenties. The variable consideration related to the remaining development milestone payments has not been included in the transaction price as these were fully constrained at June 30, 2020. In April 2020, Knight announced that it had received approval from Health Canada for IBSRELA ® (tenapanor) for the treatment of IBS-C, and in May 2020, the Company received a development milestone payment from Knight related to the achievement of the aforementioned milestone. For each of the three and six months ended June 30, 2020 the Company recognized $0.7 million as licensing revenue and for the three and six months ended June 30, 2019 no revenue was recognized related to the Knight Agreement. AstraZeneca AB (“AstraZeneca”) In June 2015, the Company entered into a termination agreement with AstraZeneca (the “AstraZeneca Termination Agreement”) pursuant to which the Company has agreed to pay AstraZeneca fees for (i) future royalties at a royalty rate of 10% of net sales of tenapanor or other NHE3 products by the Company or its licensees, and (ii) 20% of non-royalty revenue received from a new collaboration partner should the Company elect to license, or otherwise provide rights to develop and commercialize tenapanor or another NHE3 inhibitor, up to a maximum of $75.0 million in aggregate for (i) and (ii). As of June 30, 2020, to date in aggregate, the Company has recognized $10.6 million of the $75.0 million, recorded as cost of revenue, and has paid AstraZeneca $10.6 million. For each of the three and six months ended June 30, 2020 and 2019 the Company has recognized and recorded as cost of revenue $0.1 million and zero, respectively, related to the AstraZeneca Termination Agreement. The following table presents changes in the Company’s deferred revenue balance, which is attributable entirely to the 2019 KKC Agreement discussed above, during the reporting period: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue Balance at December 31, 2019 $ 4,541 Increases due to cash received, excluding amounts recognized as revenue during the period ​ ​ — ​ Decreases due to revenue recognized in the period for which cash has been received ​ ​ (1,150) ​ Decreases due to revenue recognized in the period for which cash has not been received ​ ​ (1,150) ​ Balance at June 30, 2020 ​ $ 2,241 ​ ​

Contingencies

Contingencies	6 Months Ended
	Jun. 30, 2020
Contingencies
Contingencies	NOTE 10. CONTINGENCIES From time to time the Company may be involved in claims arising in connection with its business. Based on information currently available, management believes that the amount, or range, of reasonably possible losses in connection with any pending actions against the Company will not be material to the Company’s financial condition or cash flows, and no contingent liabilities were accrued as of June 30, 2020 or 2019. ​

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2020
Income Taxes
Income Taxes	​ NOTE 11. INCOME TAXES On March 27, 2020, the Coronavirus Aid, Relief and Economic Security Act (the “CARES Act”) was signed into law. The CARES Act includes, among other features, provisions relating to refundable payroll tax credits, deferment of employer side social security payments, net operating loss carryback periods, alternative minimum tax credit refunds, modifications to the net interest deduction limitations and technical corrections to tax depreciation methods for qualified improvement property. While the Company continues to evaluate the impact of the CARES Act, it does not expect the provisions of the legislation to have a significant impact on the effective tax rate or on the income tax provision or deferred income tax positions of the Company. ​ ​ ​ ​ ​

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2020
Subsequent Events
Subsequent Events	​ ​ NOTE 12. SUBSEQUENT EVENT ​ On July 9, 2020 the Company filed a Form S-3 registration statement containing (i) a base prospectus for the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $250,000,000 of the Company’s common stock, preferred stock, debt securities, warrants and/or units, from time to time in one or more offerings; and (ii) a prospectus supplement for the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $100,000,000 of the Company’s common stock that may be issued and sold, from time to time, under a sales agreement with Jefferies LLC, deemed to be “at the market offerings”. This prospectus supplement supersedes prior “at the market offerings” agreement. Subsequently, on July 24, 2020 the Company filed an amendment to the said Form S-3, which became effective on August 3, 2020. ​ ​

Organization and Basis of Pre_2

Organization and Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2020
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation These condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the requirements of the Securities and Exchange Commission (“SEC”) for interim reporting. As permitted under those rules and regulations, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted. These condensed financial statements have been prepared on the same basis as the Company’s most recent annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary to present fairly the Company’s financial position at June 30, 2020 and results of operations, changes in stockholders’ equity, and cash flows for the interim periods ended June 30, 2020 and 2019. The accompanying condensed financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The results for the three and six months ended June 30, 2020 are not necessarily indicative of results to be expected for the entire year ending December 31, 2020, or for any other interim period or future year.
Prior Period Errors	Prior Period Errors ​ In connection with our review of our financial statements as of and for the six months ended June 30, 2019, we corrected errors related to the accounting for clinical trial accruals that had resulted in an overstatement of research and development expenses during the three months ended March 31, 2019 and during the year ended December 31, 2018. Specifically, management concluded that the Company’s research and development expenses recorded during the three months ended March 31, 2019 and during year ended December 31, 2018 had been overstated by $0.5 million and $3.6 million, respectively, and that the Company’s accrued expenses other current liabilities ​ Management analyzed the potential impact of these errors in accordance with the SEC’s Staff Accounting Bulletin No. 108, Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements
Liquidity	Liquidity As of June 30, 2020, the Company had cash, cash equivalents and short-term investments of approximately $204.8 million. The Company believes its current available cash, cash equivalents and short-term investments will be sufficient to fund the Company’s planned expenditures and meet its obligations for at least 12 months following August 6, 2020, which is the date that these condensed financial statements are being issued.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the amounts reported in the financial statements and accompanying notes thereto. On an ongoing basis, management evaluates its estimates, including those related to recognition of revenue, clinical trial accruals, contract manufacturing accruals, the fair value of assets and liabilities, income taxes and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could materially differ from those estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In December 2019, as part of its initiative to reduce complexity in the accounting standards, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 Revenue from Contracts with Customers Fair Value Measurement (Topic 820): Disclosure Framework— Changes to the Disclosure Requirements for Fair Value Measurement Recent Accounting Pronouncements Not Yet Adopted In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments
Fair Value Measurements	The Company’s financial instruments consist of cash and cash equivalents, short-term investments, prepaid expenses, other current assets, accounts payable, accrued expenses, and the Term Loan, as defined and discussed in Note 5. Fair value estimates of these instruments are made at a specific point in time based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment. The carrying amounts of financial instruments such as cash and cash equivalents, prepaid expenses, other current assets, accounts payable and accrued expenses approximate the related fair values due to the short maturities of these instruments. Based on prevailing borrowing rates available to the Company for loans with similar terms, the Company believes the fair value of the Term Loan, considering level 2 inputs, approximates this instrument’s carrying value. ​ Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. ​ The three-level hierarchy for the inputs to valuation techniques is briefly summarized as follows: Level 1 – Valuations are based on quoted prices in active markets for identical assets or liabilities and readily accessible by the Company at the reporting date. Examples of assets and liabilities utilizing Level 1 inputs are certain money market funds, U.S. treasuries and trading securities with quoted prices on active markets. Level 2 – Valuations based on inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Examples of assets and liabilities utilizing Level 2 inputs are corporate bonds, commercial paper, certificates of deposit and over-the-counter derivatives. Level 3 – Valuations based on unobservable inputs in which there is little or no market data, which require the Company to develop its own assumptions.

Cash, Cash Equivalents and Sh_2

Cash, Cash Equivalents and Short-Term Investments (Tables)	6 Months Ended
	Jun. 30, 2020
Cash, Cash Equivalents and Short-Term Investments
Schedule of Securities Classified as Cash, Cash Equivalents and Short-Term Investments	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value Cash and cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 99,410 ​ $ — ​ $ — ​ $ 99,410 Cash ​ ​ 1,084 ​ ​ — ​ ​ — ​ ​ 1,084 Total cash and cash equivalents ​ ​ 100,494 ​ ​ — ​ ​ — ​ ​ 100,494 Short-term investments ​ ​ ​ ​ ​ Commercial paper ​ $ 46,328 ​ $ 115 ​ $ — ​ $ 46,443 Corporate bonds ​ ​ 35,119 ​ ​ 170 ​ ​ — ​ ​ 35,289 Asset-backed securities ​ 10,507 ​ 25 ​ — ​ 10,532 U.S. government-sponsored agency bonds ​ ​ 8,077 ​ ​ — ​ ​ (2) ​ ​ 8,075 U.S. treasury notes ​ 3,999 ​ 9 ​ — ​ 4,008 Total short-term investments ​ ​ 104,030 ​ ​ 319 ​ ​ (2) ​ ​ 104,347 Total cash equivalents and short-term investments ​ $ 204,524 ​ $ 319 ​ $ (2) ​ $ 204,841 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value Cash and cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 147,208 ​ $ — ​ $ — ​ $ 147,208 Commercial paper ​ 19,357 ​ 3 ​ — ​ 19,360 Corporate bonds ​ 11,441 ​ — ​ — ​ 11,441 Cash ​ 3,124 ​ — ​ — ​ 3,124 Total cash and cash equivalents ​ ​ 181,130 ​ ​ 3 ​ ​ — ​ ​ 181,133 Short-term investments ​ ​ ​ ​ ​ Commercial paper ​ $ 36,667 ​ $ 14 ​ $ — ​ $ 36,681 Corporate bonds ​ 21,690 ​ 6 ​ (3) ​ 21,693 Asset-backed securities ​ 8,005 ​ — ​ — ​ 8,005 Total short-term investments ​ ​ 66,362 ​ ​ 20 ​ ​ (3) ​ ​ 66,379 Total cash equivalents and short-term investments ​ $ 247,492 ​ $ 23 ​ $ (3) ​ $ 247,512

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2020
Fair Value Measurements
Summary of Fair Value Measurements of Company's Financial Assets and Liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ Total Level 1 Level 2 Level 3 Assets: ​ ​ ​ ​ Money market funds ​ $ 99,410 ​ $ 99,410 ​ $ — ​ $ — Commercial paper ​ 46,443 ​ — ​ 46,443 ​ — Corporate bonds ​ 35,289 ​ — ​ 35,289 ​ — Asset-backed securities ​ 10,532 ​ — ​ 10,532 ​ — U.S. government-sponsored agency bonds ​ ​ 8,075 ​ ​ 8,075 ​ ​ ​ ​ ​ ​ U.S. treasury notes ​ 4,008 ​ 4,008 ​ — ​ — Total ​ $ 203,757 ​ $ 111,493 ​ $ 92,264 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative liability for Exit Fee ​ $ 1,121 ​ $ — ​ $ — ​ $ 1,121 Total ​ $ 1,121 ​ $ — ​ $ — ​ $ 1,121 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ Total Level 1 Level 2 Level 3 Assets: ​ ​ ​ ​ Money market funds ​ $ 147,208 ​ $ 147,208 ​ $ — ​ $ — Commercial paper ​ 56,041 ​ — ​ 56,041 ​ — Corporate bonds ​ 33,134 ​ — ​ 33,134 ​ — Asset-backed securities ​ 8,005 ​ — ​ 8,005 ​ — Total ​ $ 244,388 ​ $ 147,208 ​ $ 97,180 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Derivative liability for Exit Fee ​ $ 969 ​ $ — ​ $ — ​ $ 969 Total ​ $ 969 ​ $ — ​ $ — ​ $ 969

Derivative Liability (Tables)

Derivative Liability (Tables)	6 Months Ended
	Jun. 30, 2020
Derivative Liability
Schedule of Changes in Fair Value	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2020 ​ ​ 2019 ​ Fair value of Exit Fee derivative liability at January 1 ​ $ 969 ​ $ 533 ​ Change in estimated fair value of derivative liability ​ ​ 152 ​ ​ 61 ​ Fair value of Exit Fee derivative liability at June 30 ​ $ 1,121 ​ $ 594 ​

Borrowing (Tables)

Borrowing (Tables)	6 Months Ended
	Jun. 30, 2020
Borrowing
Schedule of Future Debt Payment Obligations	​ ​ ​ ​ ​ Remainder of 2020 ​ $ 2,083 2021 ​ 25,000 2022 ​ 24,892 Total principal and final fee payments ​ ​ 51,975 Less: Unamortized discount and debt issuance costs ​ ​ (561) Less: Unaccreted value of final fee ​ ​ (963) Loan payable ​ ​ 50,451 Less: Loan payable, current portion ​ ​ 13,716 Loan payable, net of current portion ​ $ 36,735

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2020
Leases
Summary of Additional Details of the Leases	​ ​ ​ ​ ​ ​ Facilities ​ ​ ​ ​ Right of use assets ​ $ 2,945 ​ ​ ​ ​ ​ ​ Current portion of lease liabilities ​ ​ 2,826 ​ Operating lease liability, net of current portion ​ ​ 608 ​ Total ​ $ 3,434 ​ ​ ​ ​ ​ ​ Weighted-average remaining life (years) ​ ​ 1.25 ​ Weighted-average discount rate ​ ​ 12.99 %
Summary of Lease Costs	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ 2020 2019 ​ 2020 2019 Operating lease expense $ 648 ​ $ 648 ​ $ 1,296 ​ $ 1,296 Cash paid for operating lease $ 761 ​ $ 616 ​ $ 1,521 ​ $ 1,230
Summary of Undiscounted Cash Payment Obligations for Operating Lease Liabilities	​ ​ ​ ​ ​ Remainder of 2020 ​ $ 1,544 2021 ​ ​ 2,183 Total undiscounted operating lease payments ​ ​ 3,727 Imputed interest expenses ​ ​ (293) Total operating lease liabilities ​ ​ 3,434 Less: Current portion of operating lease liability ​ ​ 2,826 Operating lease liability, net of current portion ​ $ 608

Equity Incentive Plans (Tables)

Equity Incentive Plans (Tables)	6 Months Ended
	Jun. 30, 2020
Equity Incentive Plans
Summary of Weighted-Average Assumptions to Estimate Fair Value of Employee Stock Options	​ ​ ​ ​ ​ ​ ​ Six Months Ended June 30, ​ 2020 Expected term (years) 6.00 Expected volatility 83.55 % Risk-free interest rate 1.33 % Dividend yield — %
Summary of Stock-Based Compensation Expense Recognized	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ ​ 2020 2019 2020 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Research and development $ 1,144 ​ $ 767 ​ $ 2,202 ​ $ 1,527 ​ General and administrative ​ 1,530 ​ ​ 1,156 ​ ​ 3,420 ​ 2,318 ​ Total ​ $ 2,674 ​ $ 1,923 ​ $ 5,622 ​ $ 3,845 ​
Summary of Total Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures	​ ​ ​ ​ ​ ​ ​ ​ ​ Unrecognized Compensation Expense ​ Average Remaining Vesting Period (Years) Stock options $ 15,202 ​ ​ 2.8 PRSUs ​ $ 396 ​ ​ 0.2 ESPP $ 78 ​ ​ 0.2

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2020
Net Loss Per Share
Computation of Basic and Diluted Loss Per Share Attributable to Common Stockholders	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended June 30, ​ ​ 2020 2019 2020 2019 Numerator: ​ ​ ​ ​ Net loss ​ $ (24,956) ​ $ (25,467) ​ $ (47,329) ​ $ (51,611) ​ Denominator: ​ ​ ​ ​ ​ Weighted average common shares outstanding - basic and diluted ​ 89,080,046 ​ 62,651,863 ​ 88,980,353 ​ 62,599,371 ​ Net loss per share - basic and diluted ​ $ (0.28) ​ $ (0.41) ​ $ (0.53) ​ $ (0.82) ​

Collaboration and Licensing A_2

Collaboration and Licensing Agreements (Tables)	6 Months Ended
	Jun. 30, 2020
Collaboration and Licensing Agreements
Schedule of Changes in Deferred Revenue	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue Balance at December 31, 2019 $ 4,541 Increases due to cash received, excluding amounts recognized as revenue during the period ​ ​ — ​ Decreases due to revenue recognized in the period for which cash has been received ​ ​ (1,150) ​ Decreases due to revenue recognized in the period for which cash has not been received ​ ​ (1,150) ​ Balance at June 30, 2020 ​ $ 2,241 ​

Organization and Basis of Pre_3

Organization and Basis of Presentation - Additional Information (Details)	6 Months Ended
	Jun. 30, 2020segment
Organization and Basis of Presentation
Number of operating segments	1

Organization and Basis of Pre_4

Organization and Basis of Presentation - Prior Period Errors (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Research and Development Expense	$ 18,864	$ 19,475		$ 34,708	$ 39,856
Accrued Liabilities and Other Liabilities	$ 7,574			$ 7,574			$ 7,248
Overstatement due to clinical trial accrual errors | Effect of Change
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Research and Development Expense			$ 500			$ 3,600
Accrued Liabilities and Other Liabilities			$ 500			$ 3,600

Organization and Basis of Pre_5

Organization and Basis of Presentation - Cumulative adjustments (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)
Reduction in accrued and other liabilities
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Correction of cumulative errors	$ 4.1
Reduction in research and development expenses
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Correction of cumulative errors	$ 4.1

Organization and Basis of Pre_6

Organization and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Organization and Basis of Presentation
Cash equivalents and short-term investments, Fair Value	$ 204,841	$ 247,512

Cash, Cash Equivalents and Sh_3

Cash, Cash Equivalents and Short-Term Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 100,494	$ 181,133	$ 89,626	$ 78,768
Cash and cash equivalents, Amortized Cost		181,130
Cash and cash equivalents, Gross Unrealized Gains		3
Cash and cash equivalents, Fair Value	100,494	181,133
Short-term investments, Amortized Cost	104,030	66,362
Short-term investments, Gross Unrealized Gains	319	20
Short-term investments, Gross Unrealized Losses	(2)	(3)
Short-term investments, Fair Value	104,347	66,379
Cash equivalents and short-term investments, Amortized Cost	204,524	247,492
Cash equivalents and short-term investments, Gross Unrealized Gains	319	23
Cash equivalents and short-term investments, Gross Unrealized Losses	(2)	(3)
Cash equivalents and short-term investments, Fair Value	204,841	247,512
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	99,410	147,208
Cash and cash equivalents, Fair Value	99,410	147,208
Commercial Paper (Cash Equivalents) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost		19,357
Cash and cash equivalents, Gross Unrealized Gains		3
Cash and cash equivalents, Fair Value		19,360
US Government Agencies Short-term Debt Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	8,077
Short-term investments, Gross Unrealized Losses	(2)
Short-term investments, Fair Value	8,075
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost		11,441
Cash and cash equivalents, Fair Value		11,441
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	1,084	3,124
Cash and cash equivalents, Fair Value	1,084	3,124
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	46,328	36,667
Short-term investments, Gross Unrealized Gains	115	14
Short-term investments, Fair Value	46,443	36,681
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	35,119	21,690
Short-term investments, Gross Unrealized Gains	170	6
Short-term investments, Gross Unrealized Losses		(3)
Short-term investments, Fair Value	35,289	21,693
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	10,507	8,005
Short-term investments, Gross Unrealized Gains	25
Short-term investments, Fair Value	10,532	$ 8,005
U.S. Treasury Notes [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	3,999
Short-term investments, Gross Unrealized Gains	9
Short-term investments, Fair Value	$ 4,008

Cash, Cash Equivalents and Sh_4

Cash, Cash Equivalents and Short-Term Investments - Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Cash Cash Equivalents And Short Term Investments [Line Items]
Investment in continuous unrealized loss position for more than one year	$ 0
Available-for-sale securities that were other-than-temporarily impaired	$ 0	$ 0
Maximum
Cash Cash Equivalents And Short Term Investments [Line Items]
Available for sale securities contractual maturity period	1 year

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets and Liabilities (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Assets at fair value	$ 203,757	$ 244,388
Liabilities:
Derivative liability	1,121	969
Level 1 [Member]
Assets:
Assets at fair value	111,493	147,208
Level 2 [Member]
Assets:
Assets at fair value	92,264	97,180
Level 3 [Member]
Liabilities:
Derivative liability	1,121	969
Derivative Liability Exit Fee [Member]
Liabilities:
Derivative liability	1,121	969
Derivative Liability Exit Fee [Member] | Level 3 [Member]
Liabilities:
Derivative liability	1,121	969
Money Market Funds [Member]
Assets:
Assets at fair value	99,410	147,208
Money Market Funds [Member] | Level 1 [Member]
Assets:
Assets at fair value	99,410	147,208
Commercial Paper [Member]
Assets:
Assets at fair value	46,443	56,041
Commercial Paper [Member] | Level 2 [Member]
Assets:
Assets at fair value	46,443	56,041
Corporate Bonds [Member]
Assets:
Assets at fair value	35,289	33,134
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Assets at fair value	35,289	33,134
Asset-Backed Securities [Member]
Assets:
Assets at fair value	10,532	8,005
Asset-Backed Securities [Member] | Level 2 [Member]
Assets:
Assets at fair value	10,532	$ 8,005
U.S. Treasury Notes [Member]
Assets:
Assets at fair value	4,008
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Assets at fair value	4,008
US Government Agencies Short-term Debt Securities [Member]
Assets:
Assets at fair value	8,075
US Government Agencies Short-term Debt Securities [Member] | Level 1 [Member]
Assets:
Assets at fair value	$ 8,075

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) $ in Thousands	Jun. 30, 2020USD ($)
Fair Value Measurements
Transfer of assets from Level 1 to Level 2	$ 0
Transfer of assets from Level 2 to Level 1	$ 0

Derivative Liability - Addition

Derivative Liability - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended
	May 31, 2018	Jun. 30, 2020
Derivative [Line Items]
Exit Fee	$ 1.5
Solar Capital and Western Alliance Bank Loan Agreement [Member] | Level 3 [Member]
Derivative [Line Items]
Percentage of increase in risk component		10.00%
Percentage decrease in risk component		10.00%
Fair value fluctuation due to increase in risk component		$ 0.1
Fair value fluctuation due to decrease in risk component		$ 0.1

Derivative Liability - Schedule

Derivative Liability - Schedule of Changes in Fair Value (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Derivative Liability
Fair value of Exit Fee derivative liability at beginning	$ 969	$ 533
Change in estimated fair value of derivative liability	152	61
Fair value of Exit Fee derivative liability at ending	$ 1,121	$ 594

Borrowing (Details)

Borrowing (Details) - USD ($) $ in Thousands	May 16, 2018	Jun. 30, 2020	Dec. 31, 2019
Term Loans
Amount of term loan facility	$ 50,000	$ 51,975
Base interest rate	7.45%
Equal monthly installment	$ 2,100
Percentage of premium payable on redemption of the term loan	1.00%
Amount of fee payable upon closing of the term loan	$ 500
Additional default interest rate	4.00%
Future debt payment obligations
Remainder of 2020		2,083
2021		25,000
2022		24,892
Total principal and final fee payments	$ 50,000	51,975
Less: Unamortized discount and debt issuance costs		(561)
Less: Unaccreted value of final fee		(963)
Loan payable		50,451
Loan payable, current portion		13,716	$ 1,183
Loan payable, net of current portion		$ 36,735	$ 48,831
At Maturity
Term Loans
Prepayment fee (as a percent)	3.95%
Prior to first anniversary of closing date
Term Loans
Prepayment fee (as a percent)	3.00%
After first anniversary of closing date
Term Loans
Prepayment fee (as a percent)	2.00%
After second anniversary to maturity date
Term Loans
Prepayment fee (as a percent)	1.00%
LIBOR
Term Loans
Floating interest rate	one-month LIBOR

Leases - Balance Sheet and Stat

Leases - Balance Sheet and Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Additional details of the leases
Right-of-use assets	$ 2,945		$ 2,945		$ 3,970
Current portion of lease liabilities	2,826		2,826		2,608
Operating lease liability, net of current portion	608		608		$ 2,076
Total operating lease liabilities	$ 3,434		$ 3,434
Weighted-average remaining life	1 year 3 months		1 year 3 months
Weighted-average discount rate	12.99%		12.99%
Facilities
Operating lease expense	$ 648	$ 648	$ 1,296	$ 1,296
Cash paid for operating lease	$ 761	$ 616	$ 1,521	$ 1,230

Leases - Undiscounted Cash Paym

Leases - Undiscounted Cash Payment Obligations (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Undiscounted cash payment obligations for its operating lease liabilities
Remainder of 2020	$ 1,544
2021	2,183
Total undiscounted operating lease payments	3,727
Imputed interest expenses	(293)
Total operating lease liabilities	3,434
Current portion of operating lease liability	2,826	$ 2,608
Operating lease liability, net of current portion	$ 608	$ 2,076

Equity Incentive Plans - Additi

Equity Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended		6 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Jul. 31, 2018	Jun. 30, 2015	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date estimated fair value of options granted					$ 5.14		$ 5.19
Net proceeds from stock option exercised							$ 420	$ 7
Stock-based compensation expenses					$ 2,674	$ 1,923	5,622	3,845
Proceeds							375	198
Issuance of common stock for services, amount					$ 310	$ 311	$ 310	$ 311
Warrants issued to purchase common stock				2,172,899
Common stock to be issued if warrants are exercised (in shares)				2,172,899
Purchase price of common shares				$ 0.125
Exercise price for warrants				$ 13.91
Warrant exercise period				5 years
Warrants exercised							0
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued under ESPP	75,804	83,046
Purchase price	$ 4.95	$ 2.39
Proceeds	$ 400	$ 200
Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted					729,727		2,649,516
Number of options exercised					63,064	1,681	204,615	2,451
Net proceeds from stock option exercised					$ 200		$ 400
Issuance of common stock for services, shares					42,403		42,403	113,136
Issuance of common stock for services, amount					$ 300	$ 300	$ 300	$ 300
Performance-Based Restricted Stock Units (PRSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted			903,374		30,000
Number of shares, outstanding					849,328		849,328
Number of vested shares							0
Stock-based compensation expenses					$ 600		$ 1,400
Performance-Based Restricted Stock Units (PRSUs) [Member] | Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, outstanding					30,000		30,000
Time Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted					0	0	0	0
Number of shares issued upon vesting of restricted stock units					0	85,609	0	85,609

Equity Incentive Plans - Summar

Equity Incentive Plans - Summary of Weighted-Average Assumptions to Estimate Fair value of Employee Stock Options (Details)	6 Months Ended
	Jun. 30, 2020
Equity Incentive Plans
Expected term (years)	6 years
Volatility	83.55%
Risk-free interest rate	1.33%

Equity Incentive Plans - Summ_2

Equity Incentive Plans - Summary of Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses	$ 2,674	$ 1,923	$ 5,622	$ 3,845
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses	1,144	767	2,202	1,527
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses	$ 1,530	$ 1,156	$ 3,420	$ 2,318

Equity Incentive Plans - Unreco

Equity Incentive Plans - Unrecognized Stock-Based Compensation (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020USD ($)
Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Expense	$ 15,202
Average Vesting Period	2 years 9 months 18 days
Performance-Based Restricted Stock Units (PRSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Expense	$ 396
Average Vesting Period	2 months 12 days
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Expense	$ 78
Average Vesting Period	2 months 12 days

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net loss	$ (24,956)	$ (25,467)	$ (47,329)	$ (51,611)
Denominator:
Weighted average common shares outstanding - basic and diluted	89,080,046	62,651,863	88,980,353	62,599,371
Net loss per share - basic and diluted	$ (0.28)	$ (0.41)	$ (0.53)	$ (0.82)

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Details) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Loss Per Share
Potential common shares that would have been included in diluted income per share were not anti-dilutive effect caused by the net loss, computed by converting these securities using the treasury stock method	11.5	10.2	11.7	10

Collaboration and Licensing A_3

Collaboration and Licensing Agreements - Additional Information (Details)	1 Months Ended					3 Months Ended		6 Months Ended		12 Months Ended	32 Months Ended
	Nov. 30, 2019USD ($)item	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Nov. 30, 2017USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2020USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2020USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue						$ 1,836,000	$ 18,000	$ 3,049,000	$ 18,000
Unbilled revenue						750,000		750,000			$ 750,000	$ 750,000
Cost of revenue						141,000		141,000
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue						5,000	18,000	43,000	18,000
Astra Zeneca
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost of revenue								100,000	0
Cost of revenue, aggregate amount recognized						10,600,000		10,600,000			10,600,000
Cost of revenue, Amount Paid								10,600,000
Astra Zeneca | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment per termination agreement					$ 75,000,000			75,000,000
2019 KKC Agreement | Kyowa Kirin Co. Ltd
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of pre-clinical study-ready compounds | item	2
Number of compounds inhibiting the first undisclosed target | item	1
Upfront payment recognized	$ 10,000,000
License fee receivable, first installment	$ 5,000,000
Term of payment of license fee, first installment	30 days
License fee receivable, Second instalment	$ 5,000,000
Term of agreement	2 years
Initial transaction price	$ 10,000,000
Revenue						1,100,000		2,300,000
Change in estimates								0
Transaction price allocated to partially unsatisfied performance obligations						7,200,000		7,200,000			7,200,000
Uncharged license fees						2,200,000		2,200,000			2,200,000
2019 KKC Agreement | Kyowa Kirin Co. Ltd | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of development candidates | item	1
Number of separate collaborative agreements | item	1
2017 KKC Agreement | Kyowa Kirin Co. Ltd
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment recognized				$ 30,000,000
Uncharged license fees												1,000,000
Upfront payment received				30,000,000
Unbilled revenue												$ 5,000,000
Development milestones Received											$ 5,000,000
Commercialization milestones				78,900,000
2017 KKC Agreement | Kyowa Kirin Co. Ltd | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestones				$ 55,000,000
2017 KKC Agreement | Kyowa Kirin Co. Ltd | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue						5,000		43,000	18,000
XuanZhu Agreement | XuanZhu
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								0
Upfront payment received	$ 750,000
License fee recognized	1,500,000
Second milestone payment	$ 750,000
Fosun Agreement | Fosun Pharma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment recognized			$ 12,000,000							$ 12,000,000
Revenue						0	0	0	0
Future development milestones										$ 110,000,000
Fosun Agreement | Fosun Pharma | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Threshold percentage of net sales for tiered royalties			20.00%
Knight Agreement | Knight
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue						$ 700,000	$ 0	$ 700,000	$ 0
Total payments, including an up-front payment and development and sales milestones to be received		$ 18,300,000
Development and Commercialization | Astra Zeneca
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of royalty revenue (as a percent)					10.00%
Percentage of non royalty revenue (as a percent)					20.00%

Collaboration and Licensing A_4

Collaboration and Licensing Agreements - Schedule of Changes in Deferred Revenue (Details) - 2019 KKC Agreement $ in Thousands	6 Months Ended
	Jun. 30, 2020USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Balance at December 31, 2019	$ 4,541
Decreases due to revenue recognized in the period for which cash has been received	(1,150)
Decreases due to revenue recognized in the period for which cash has not been received	$ (1,150)

Contingencies - Additional Info

Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Contingencies
Contingent liabilities	$ 0	$ 0

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event [Member]	Jul. 09, 2020USD ($)
Subsequent Event [Line Items]
Maximum Aggregate Offering Price	$ 250,000,000
AT The Market Offering [Member] | Common Stock
Subsequent Event [Line Items]
Maximum Aggregate Offering Price	$ 100,000,000