Mirum Pharmaceuticals (MIRM) 10-Q 15 Nov 21 2021 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 10, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity File Number	001-38981
Entity Registrant Name	Mirum Pharmaceuticals, Inc.
Entity Central Index Key	0001759425
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	83-1281555
Entity Address, Address Line One	950 Tower Lane, Suite 1050,
Entity Address, City or Town	Foster City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94404
City Area Code	650
Local Phone Number	667-4085
Trading Symbol	MIRM
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		30,638,194
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common stock, par value $0.0001 per share
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 55,844	$ 142,086
Other Receivable	400	0
Short-term investments	134,190	89,734
Prepaid expenses and other current assets	6,371	4,530
Total current assets	196,805	236,350
Investments	14,997	0
Property and equipment, net	1,060	1,293
Operating lease right-of-use assets	1,664	1,949
Intangible assets, net	19,000	0
Other assets	1,641	1,272
Total assets	235,167	240,864
Current liabilities:
Accounts payable	5,003	3,151
Accrued expenses	43,847	13,411
Operating lease liabilities	690	636
Derivative liability	847	1,264
Total current liabilities	50,387	18,462
Revenue interest liability, net	126,277	47,651
Operating lease liabilities, noncurrent	2,088	2,627
Other liabilities	19	29
Total liabilities	178,771	68,769
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized as of September 30, 2021 and December 31, 2020, respectively; and zero shares issued and outstanding as of September 30, 2021 and December 31, 2020, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized as of September 30, 2021 and December 31, 2020, respectively; 30,562,167 shares issued and 30,317,173 shares outstanding, excluding 155,864 shares subject to repurchase as of September 30, 2021; and 30,032,600 shares issued and 29,776,544 shares outstanding, excluding 256,056 shares subject to repurchase as of December 31, 2020	3	3
Additional paid-in capital	371,104	345,180
Accumulated deficit	(314,705)	(173,171)
Accumulated other comprehensive (loss) income	(6)	83
Total stockholders’ equity	56,396	172,095
Total liabilities and stockholders’ equity	$ 235,167	$ 240,864

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares, issued	0	0
Preferred stock, shares, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares, issued	30,562,167	30,032,600
Common stock, shares, outstanding	30,406,303	29,776,544
Common stock, subject to repurchase	155,864	256,056

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue	$ 5,000	$ 0	$ 16,000	$ 0
Revenue, Product and Service [Extensible List]	License [Member]	License [Member]	License [Member]	License [Member]
Operating expenses:
Research and development	$ 30,471	$ 15,984	$ 103,653	$ 51,879
General and administrative	17,353	5,732	40,185	15,466
Total operating expenses	47,824	21,716	143,838	67,345
Loss from operations	(42,824)	(21,716)	(127,838)	(67,345)
Other income (expense):
Interest income	72	237	301	1,391
Interest expense	(5,667)	0	(13,824)	0
Change in fair value of derivative liability	1,355	0	417	0
Other income (expense), net	(35)	(30)	(565)	(109)
Net loss before provision for income taxes	(47,099)	(21,509)	(141,509)	(66,063)
Provision for income taxes	9	(3)	25	4
Net loss	$ (47,108)	$ (21,506)	$ (141,534)	$ (66,067)
Net loss per share, basic and diluted	$ (1.55)	$ (0.86)	$ (4.68)	$ (2.65)
Weighted-average shares of common stock outstanding, basic and diluted	30,367,727	25,132,916	30,250,127	24,965,178

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (47,108)	$ (21,506)	$ (141,534)	$ (66,067)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale investments	7	(151)	(83)	39
Cumulative translation adjustments	0	5	(6)	(4)
Comprehensive loss	$ (47,101)	$ (21,652)	$ (141,623)	$ (66,024)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Follow-on Public Offering	At The Market Offering	Public Offering	Common Stock	Common StockFollow-on Public Offering	Common StockAt The Market Offering	Common StockPublic Offering	Additional Paid-In Capital	Additional Paid-In CapitalFollow-on Public Offering	Additional Paid-In CapitalAt The Market Offering	Additional Paid-In CapitalPublic Offering	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2019	$ 130,349				$ 2				$ 200,119				$ (69,901)	$ 129
Balance, Shares at Dec. 31, 2019					22,600,338
Issuance of common stock		$ 44,659				$ 1				$ 44,658
Issuance of common stock, shares						2,400,000
Restricted common stock vested in the period, Shares					33,398
Stock-based compensation	2,573								2,573
Net loss	(21,310)												(21,310)
Other comprehensive income (loss)	(153)													(153)
Balance at Mar. 31, 2020	156,118				$ 3				247,350				(91,211)	(24)
Balance, Shares at Mar. 31, 2020					25,033,736
Balance at Dec. 31, 2019	130,349				$ 2				200,119				(69,901)	129
Balance, Shares at Dec. 31, 2019					22,600,338
Net loss	(66,067)
Balance at Sep. 30, 2020	120,255				$ 3				256,048				(135,968)	172
Balance, Shares at Sep. 30, 2020					25,210,885
Balance at Dec. 31, 2019	130,349				$ 2				200,119				(69,901)	129
Balance, Shares at Dec. 31, 2019					22,600,338
Balance at Dec. 31, 2020	172,095				$ 3				345,180				(173,171)	83
Balance, Shares at Dec. 31, 2020					29,776,544
Balance at Mar. 31, 2020	156,118				$ 3				247,350				(91,211)	(24)
Balance, Shares at Mar. 31, 2020					25,033,736
Issuance of common stock in connection with common stock option exercises	25								25
Issuance of common stock in connection with common stock option exercises, Shares					4,203
Restricted common stock vested in the period, Shares					33,398
Stock-based compensation	2,974								2,974
Net loss	(23,251)												(23,251)
Other comprehensive income (loss)	342													342
Balance at Jun. 30, 2020	136,208				$ 3				250,349				(114,462)	318
Balance, Shares at Jun. 30, 2020					25,071,337
Issuance of common stock in connection with common stock option exercises	27								27
Issuance of common stock in connection with common stock option exercises, Shares					7,442
Issuance of common stock			$ 2,244								$ 2,244
Issuance of common stock, shares							98,708
Restricted common stock vested in the period, Shares					33,398
Stock-based compensation	3,428								3,428
Net loss	(21,506)												(21,506)
Other comprehensive income (loss)	(146)													(146)
Balance at Sep. 30, 2020	120,255				$ 3				256,048				(135,968)	172
Balance, Shares at Sep. 30, 2020					25,210,885
Balance at Dec. 31, 2020	172,095				$ 3				345,180				(173,171)	83
Balance, Shares at Dec. 31, 2020					29,776,544
Issuance of common stock in connection with common stock option exercises	80								80
Issuance of common stock in connection with common stock option exercises, Shares					12,535
Issuance of common stock				$ 7,038								$ 7,038
Issuance of common stock, shares								375,654
Restricted common stock vested in the period, Shares					33,396
Stock-based compensation	5,285								5,285
Net loss	(50,532)												(50,532)
Other comprehensive income (loss)	(83)													(83)
Balance at Mar. 31, 2021	133,883				$ 3				357,583				(223,703)
Balance, Shares at Mar. 31, 2021					30,198,129
Balance at Dec. 31, 2020	172,095				$ 3				345,180				(173,171)	83
Balance, Shares at Dec. 31, 2020					29,776,544
Net loss	(141,534)
Balance at Sep. 30, 2021	56,396				$ 3				371,104				(314,705)	(6)
Balance, Shares at Sep. 30, 2021					30,406,303
Balance at Mar. 31, 2021	133,883				$ 3				357,583				(223,703)
Balance, Shares at Mar. 31, 2021					30,198,129
Issuance of common stock in connection with common stock option exercises	295								295
Issuance of common stock in connection with common stock option exercises, Shares					41,668
Restricted common stock vested in the period, Shares					33,400
Issuance of shares in connection with ESPP	643								643
Issuance of shares in connection with ESPP, Shares					43,976
Stock-based compensation	4,823								4,823
Net loss	(43,894)												(43,894)
Other comprehensive income (loss)	(13)													(13)
Balance at Jun. 30, 2021	95,737				$ 3				363,344				(267,597)	(13)
Balance, Shares at Jun. 30, 2021					30,317,173
Issuance of common stock in connection with common stock option exercises	345								345
Issuance of common stock in connection with common stock option exercises, Shares					55,734
Restricted common stock vested in the period, Shares					33,396
Stock-based compensation	7,415								7,415
Net loss	(47,108)												(47,108)
Other comprehensive income (loss)	7													7
Balance at Sep. 30, 2021	$ 56,396				$ 3				$ 371,104				$ (314,705)	$ (6)
Balance, Shares at Sep. 30, 2021					30,406,303

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - Common Stock - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Follow-on Public Offering
Issuance of common stock upon public offering, issuance costs			$ 3,342
At The Market Offering
Issuance of common stock upon public offering, issuance costs		$ 138
Public Offering
Issuance of common stock upon public offering, issuance costs	$ 476

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Operating activities
Net loss	$ (141,534)	$ (66,067)
Reconciliation of net loss to net cash used in operating activities:
Stock-based compensation	17,523	8,975
Depreciation and amortization	253	220
Amortization of operating lease right-of-use assets	285	238
Net (accretion) amortization of discounts on investments	(80)	32
Non-cash interest expense on revenue interest purchase agreement	13,824
Change in fair value of derivative liability	(417)
Change in operating assets and liabilities:
Prepaid expenses and other current assets	(2,241)	(1,699)
Operating lease right-of-use assets		94
Other assets	(374)
Accounts payable, accrued expenses and other liabilities	13,635	5,655
Operating lease liabilities	(485)	(244)
Net cash used in operating activities	(99,611)	(52,796)
Investing activities
Proceeds from maturities of investments	117,600	71,400
Proceeds from paydown of investments	2,000	23,608
Purchase of investments	(179,055)	(51,991)
Purchase of property and equipment	(21)	(197)
Net cash provided by (used in) investing activities	(59,476)	42,820
Financing activities
Proceeds from issuance of common stock in public offerings, net of issuance costs	6,914	46,903
Proceeds from revenue interest liability, net of issuance costs	64,574
Proceeds from issuance of common stock pursuant to equity award plans	1,363	52
Payment of deferred offering costs in connection with the shelf registration and sales agreement		(175)
Net cash provided by financing activities	72,851	46,780
Effect of exchange rate on cash and cash equivalents	(6)	4
Net increase (decrease) in cash and cash equivalents	(86,242)	36,808
Cash and cash equivalents at beginning of period	142,086	11,970
Cash and cash equivalents at end of period	55,844	48,778
Supplemental disclosure of cash flow information:
Operating cash flows paid for operating lease	645	362
Accrued milestone payments classified as intangible assets, net	$ 19,000

Organization and Description of

Organization and Description of Business	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Description of Business	1. Organization and Description of Business Mirum Pharmaceuticals, Inc. (the “Company”) was incorporated in the State of Delaware on May 2, 2018 , and is headquartered in Foster City, California. The Company is a biopharmaceutical company focused on the identification, acquisition, development and commercialization of novel therapies for debilitating rare liver diseases. The Company received U.S. Food and Drug Administration (“FDA”) approval for LIVMARLI (maralixibat) oral solution (“Livmarli”), the first and only FDA-approved medication for the treatment of cholestatic pruritus in patients with Alagille syndrome (“ALGS”) one year of age and older, on September 29, 2021. The Company’s development pipeline consists of two clinical-stage product candidates, maralixibat and volixibat, with mechanisms of action that have potential utility across a wide range of orphan liver diseases. The Company commenced significant operations in November 2018. The unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions among the consolidated entities have been eliminated in consolidation. The Company views its operations and manages its business as one operating segment. Liquidity The Company has a limited operating history, has incurred significant operating losses since its inception, and the revenue and income potential of the Company’s business and market are unproven. As of September 30, 2021, the Company had an accumulated deficit of $ 314.7 million and cash, cash equivalents and investments of $ 205.0 million, which are available to fund future operations. The Company believes that its cash, cash equivalents and investments as of September 30, 2021 provide sufficient capital resources to continue its operations for at least twelve months from the issuance date of the accompanying unaudited condensed consolidated financial statements. The unaudited condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business. The unaudited condensed consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. Management expects to continue to incur additional substantial losses in the foreseeable future as a result of the Company’s research and development activities.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (“SEC”). Accordingly, the accompanying unaudited condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The unaudited interim financial statements reflect all adjustments which, in the opinion of management, are necessary for a fair statement of the results for the periods presented. All such adjustments are of a normal and recurring nature. The consolidated balance sheet as of December 31, 2020 has been derived from the audited consolidated financial statements at that date but does not include all information and footnotes required by GAAP for complete financial statements. The operating results presented in these unaudited condensed consolidated financial statements are not necessarily indicative of the results that may be expected for any future periods. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto in the Company’s Annual Report on Form 10-K (“Annual Report”) for the fiscal year ended December 31, 2020, as filed with the SEC on March 9, 2021. Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in the financial statements and accompanying notes. The most significant estimates in the Company’s unaudited condensed consolidated financial statements relate to the revenue interest liability, accrued research and development expenses, the valuation of derivative liabilities, equity award compensation and the valuation allowance of deferred tax assets resulting from net operating losses. These estimates and assumptions are based upon historical experience, knowledge of current events and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results could differ materially from those estimates. Significant Accounting Policies There have been no significant changes to the accounting policies during the nine months ended September 30, 2021 , as compared to the significant accounting policies described in Note 2 of the “Notes to Consolidated Financial Statements” in the Company’s audited consolidated financial statements included in the Annual Report, except as discussed below. Revenue Recognition The Company accounts for contracts with customers under Accounting Standards Codification (“ASC”), Revenue from Contracts with Customers (Topic 606) (“ASC 606”). Under ASC 606, the Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration the Company expects to receive in exchange for those goods or services. License and Collaboration Arrangements The Company enters into collaborative arrangements with partners that are within the scope of ASC, Collaborative Arrangements (Topic 808) (“ASC 808”). The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808 to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. The accounting for some of the activities under collaboration arrangements may be subject to ASC 606 for distinct units of account that are reflective of a vendor-customer relationship. For other elements of collaboration arrangements, such as assistance with development of the drug products, the Company applies the illustrative examples in ASC 808 and generally records reimbursements received as a reduction of research and development expenses. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the contracts with customers; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) determination and measurement of the transaction price, including any constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations in the contract; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The terms of the Company’s license and collaborative research and development agreements include upfront license fees, research, development and other funding or reimbursements, milestone and other contingent payments for the achievement of defined collaboration objectives and certain development, regulatory and sales-based events, as well as royalties on sales of commercialized products. Arrangements that include upfront payments may require deferral of revenue recognition to a future period until we perform obligations under these arrangements. A performance obligation is a promise in a contract to transfer a distinct good or service and is the unit of accounting in Topic 606. A contract’s transaction price is allocated among each distinct performance obligation based on relative standalone selling price and recognized as revenue when, or as, the applicable performance obligation is satisfied. If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues attributed to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price as variable consideration using the most likely amount method or expected value method, depending on the nature of the contingency and the variable payments. If it is probable that a significant reversal of cumulative revenue recognized for the contract would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not generally considered probable of being achieved until those approvals are received. Given the high degree of uncertainty around the occurrence of these events, the Company generally determines the milestone and other contingent amounts to be fully constrained until the uncertainty associated with these payments is resolved. At the end of each reporting period, the Company re-evaluates the probability of achievement of any development milestones, and if necessary, adjusts its estimate of the transaction price. Any such adjustments would be recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, the Company recognizes revenue at the later of (i) when or as the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). Accounting for these arrangements requires the Company to develop assumptions that require judgment to determine the stand-alone selling price of each performance obligation identified in the contract. The Company has never sold the performance obligations in its collaborative arrangements separately; therefore, an observable stand-alone selling price does not exist. Accordingly, the Company estimates a stand-alone selling price through maximizing the use of observable inputs such as market data, project cost estimates, and targeted margins. Stock-Based Compensation The Company recognizes stock-based compensation for all stock-based awards based on the grant date fair value of the award. For stock-based awards with service conditions, the fair value of the awards is amortized on a straight-line basis over the requisite service period in which the awards are expected to vest. For stock-based awards with performance vesting conditions, stock-based compensation is recognized when it is considered probable that the performance conditions will be satisfied. At each reporting period, the Company reassesses the probability of the achievement of the performance vesting conditions. Any change in stock-based compensation resulting from an adjustment in the vesting is treated as a cumulative catch-up in the period of adjustment. For stock-based awards with market conditions, stock-based compensation is recognized over the appropriate requisite service period. The Company accounts for forfeitures as they occur. Intangibles Assets, Net Upon FDA approval of Livmarli in September 2021, contractual milestone payments for which the Company was then obligated to pay to licensors were evaluated as intangible assets for the completed regulatory approval and right to commercialize the product. The evaluation of intangible assets includes assessing the amortization period for which the asset is expected to contribute to the future cash flows of the Company. The Company determined the pattern of this intangible asset’s future cash flows could not be readily determined with a high level of precision. As a result, the intangible asset is being amortized on a straight-line basis over the estimated useful life of 11 year s. The Company tests its definite lived intangible assets for impairment annually if events or changes in circumstances indicate that it is more likely than not that the asset is impaired. If it is determined that the asset becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in the consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments to its intangible assets. Net Loss Per Share Basic net loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding for the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury-stock and if-converted methods. Diluted net loss per share excludes the potential impact of the Company’s common stock subject to repurchase, common stock options, contingently issuable employee stock purchase plan shares, contingently issuable performance stock units (“PSUs”) and contingently issuable underwriter option shares because their effect would be anti-dilutive due to the Company’s net loss. Since the Company incurred a net loss in each of the periods presented, basic and diluted net loss per share were the same. The following outstanding potentially dilutive shares of common stock have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, As of December 31, 2021 2020 Options to purchase common stock 7,006,115 5,071,740 Common stock subject to repurchase 155,864 256,056 Employee stock purchase plan contingently issuable 40,429 12,931 PSUs contingently issuable 151,753 — Underwriter option shares contingently issuable — 562,500 Total 7,354,161 5,903,227 Recently Adopted Accounting Pronouncements In November 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No.2018-18, Collaborative Arrangements (Topic 818): Clarifying the Interaction Between Topic 808 and Topic 606, which clarifies when transactions between participants in a collaborative arrangement are within the scope of the FASB’s revenue standard, Topic 606. The standard is effective for fiscal years beginning after December 15, 2019 and interim periods within those fiscal years, with early adoption permitted. The Company adopted this standard on January 1, 2021 , as it had no collaboration arrangements prior to that date. There was no impact on the accompanying consolidated financial statements as of the adoption date. Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). ASU 2016-13 requires an entity to utilize a new impairment model that requires measurement and recognition of expected credit losses for most financial assets and certain other instruments, including but not limited to available-for-sale debt securities. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses rather than as a direct write-down to the security. The new guidance requires the use of forward-looking expected credit loss models based on historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount, which may result in earlier recognition of credit losses under the new guidance. The new guidance also modifies the impairment models for available-for-sale debt securities and for purchased financial assets with credit deterioration since their origination. Subsequent to the issuance of ASU 2016-13, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments - Credit Losses . This ASU does not change the core principle of the guidance in ASU 2016-13, instead these amendments are intended to clarify and improve operability of certain topics included within the credit losses guidance. The FASB also subsequently issued ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 842), which did not change the core principle of the guidance in ASU 2016-13 but clarified that expected recoveries of amounts previously written off and expected to be written off should be included in the valuation account and should not exceed amounts previously written off and expected to be written off. In March 2020, the FASB issued ASU No. 2020-3, Codification Improvements to Financial Instruments which makes narrow-scope improvements to various financial instruments topics, including the new credit losses standard and clarifies the following areas (i) the contractual term of a net investment in a lease should be the contractual term used to measure expected credit losses; (ii) when an entity regains control of financial assets sold, an allowance for credit losses should be recorded. The guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies. For smaller reporting companies, the guidance will be effective during the first quarter of 2023. The Company is in the process of assessing the impact adoption will have on its consolidated financial statements. In August 2020, the FASB issued ASU No. 2020-06, Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging-Contracts in Entity’s Own Equity (Subtopic 815-40) (“ASU 2020-06”), which simplifies the accounting for convertible instruments. The guidance removes certain accounting models that separate the embedded conversion features from the host contract for convertible instruments. ASU 2020-06 allows for a modified or full retrospective method of transition. This update is effective for fiscal years beginning after December 15, 2021, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact this change will have on its consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type are presented in the following table (in thousands): September 30, 2021 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 52,509 $ — $ — $ 52,509 Commercial paper — 134,190 — 134,190 U.S. government bonds — 14,997 — 14,997 Total financial assets $ 52,509 $ 149,187 $ — $ 201,696 Financial liabilities: Derivative liability $ — $ — $ 847 $ 847 Total financial liabilities $ — $ — $ 847 $ 847 December 31, 2020 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 127,783 $ — $ — $ 127,783 U.S. treasury bills 29,997 — — 29,997 Corporate debt securities — 23,201 — 23,201 Commercial paper — 41,460 — 41,460 U.S. government bonds — 5,066 — 5,066 Asset-backed securities — 2,006 — 2,006 Total financial assets $ 157,780 $ 71,733 $ — $ 229,513 Financial liabilities: Derivative liability $ — $ — $ 1,264 $ 1,264 Total financial liabilities $ — $ — $ 1,264 $ 1,264 The carrying amounts of certain financial instruments such as cash and cash equivalents, prepaid expenses, other current assets, accounts payable, accrued expenses, and other current liabilities as of September 30, 2021 and December 31, 2020 approximate their related fair values due to the short-term maturities of these instruments. Certain financial instruments classified within Level 2 of the fair value hierarchy include the types of instruments that trade in markets that are not considered to be active, but are valued based on quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. The carrying amount of the revenue interest liability as of September 30, 2021 and December 31, 2020 approximates its fair value and is based on the Company’s contractual repayment obligation to the Purchasers (as defined below), based on the current estimates of future revenues, over the life of the Revenue Interest Purchase Agreement (“RIPA”). The derivative liability is considered a Level 3 input based on the three-level hierarchy. Refer to Note 6 “Revenue Interest Purchase Agreement” for further information. Derivative Liability The debt pursuant to the RIPA contains embedded derivatives requiring bifurcation as a single compound derivative instrument. The Company estimated the fair value of the derivative liability using a “with-and-without” method. The “with-and-without” methodology involves valuing the whole instrument on an as-is basis and then valuing the instrument without the individual embedded derivative. The difference between the entire instrument with the embedded derivative compared to the instrument without the embedded derivative was the fair value of the derivative liability at September 30, 2021 and December 31, 2020. The estimated probability and timing of underlying events triggering the exercisability of the put options contained within the RIPA, forecasted cash flows and the discount rate are significant unobservable inputs used to determine the estimated fair value of the entire instrument with the embedded derivative. As of September 30, 2021 and December 31, 2020, the discount rate used for valuation of the derivative liability is 15.7 % and 15.9 %, respectively. The following table provides a summary of the change in the estimated fair value of the Company’s derivative liability, classified as Level 3 in the fair value hierarchy: Balance at December 31, 2020 $ 1,264 Change in fair value of derivative liability ( 334 ) Balance at March 31, 2021 930 Change in fair value of derivative liability 1,272 Balance at June 30, 2021 2,202 Change in fair value of derivative liability ( 1,355 ) Balance at September 30, 2021 $ 847

Financial Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2021
Financial Instruments Owned At Fair Value [Abstract]
Financial Instruments	4. Financial Instruments The fair value and amortized cost of cash equivalents and available-for-sale investments by major security type are presented in the following table (in thousands): September 30, 2021 Amortized Unrealized Unrealized Estimated Cash equivalents and investments: Money market fund $ 52,509 $ — $ — $ 52,509 Commercial paper 134,190 — — 134,190 U.S. government bonds 14,999 — ( 2 ) 14,997 Total cash equivalents and investments $ 201,698 $ — $ ( 2 ) $ 201,696 Classified as: Cash equivalents $ 52,509 Short-term investments 134,190 Investments 14,997 Total cash equivalents and investments $ 201,696 December 31, 2020 Amortized Unrealized Unrealized Estimated Cash equivalents and investments: Money market fund $ 127,783 $ — $ — $ 127,783 U.S. treasury bills 29,995 2 — 29,997 Corporate debt securities 23,126 75 — 23,201 Commercial paper 41,460 — — 41,460 U.S. government bonds 5,067 — ( 1 ) 5,066 Asset-backed securities 2,001 5 — 2,006 Total cash equivalents and investments $ 229,432 $ 82 $ ( 1 ) $ 229,513 Classified as: Cash equivalents $ 139,779 Short-term investments 89,734 Total cash equivalents and investments $ 229,513 As of September 30, 2021, the remaining contractual maturities of available-for-sale debt securities were as follows: (in thousands): Estimated Due within one year $ 134,190 One to two years 14,997 Total $ 149,187 During the three and nine months ended September 30, 2021 and 2020 , there have been no significant realized gains or losses on available-for-sale investments, no investments had been in a continuous unrealized loss position for more than 12 months, and the Company did no t recognize any other-than-temporary impairment losses on these securities.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Accrued clinical trials $ 6,026 $ 3,673 Accrued milestone payments 19,000 — Accrued professional service fees 4,462 2,157 Accrued contract manufacturing and non-clinical costs 7,644 2,780 Accrued compensation and related benefits 6,715 4,801 Total accrued expenses $ 43,847 $ 13,411

Revenue Interest Purchase Agree

Revenue Interest Purchase Agreement	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Revenue Interest Purchase Agreement	6. Revenue Interest Purchase Agreement In December 2020, the Company entered into the RIPA, as amended in September 2021, with Mulholland SA LLC, an affiliate of Oberland Capital LLC, as agent for the purchasers party thereto (the “Purchasers”), and the Purchasers to obtain financing for the commercialization and further development of maralixibat and other working capital needs. Pursuant to the RIPA, the Company has received $ 115.0 million consisting of an upfront payment of $ 50.0 million in December 2020 and $ 65.0 million in April 2021 associated with the acceptance for filing by the FDA of a New Drug Application (“NDA”) for maralixibat for the treatment of cholestatic pruritus in patients with ALGS, less certain transaction expenses. The Company may be entitled to receive up to approximately $ 50.0 million at the option of the Purchasers to finance in-licenses or other acquisitions on or prior to December 31, 2022. The Company was entitled to receive an additional $ 35.0 million upon FDA approval of Livmarli, which it elected to forgo. As consideration for such payments, the Purchasers have the right to receive certain revenue interests (the “Revenue Interests”) from the Company based on annual net sales of maralixibat, which will be tiered payments (the “Revenue Interest Payments”) based on whether such annual net sales are (i) less than or equal to $ 350.0 million (“Tier 1”), (ii) exceeding $ 350.0 million and less than or equal to $ 1.1 billion (“Tier 2”), or (iii) exceeding $ 1.1 billion (“Tier 3”). The Revenue Interest Payments will initially be 9.75 % (at Tier 1) and 2.0 % (at Tier 2 and Tier 3) of such annual net sales. If the Purchasers have received Revenue Interest Payments in an amount equal to or greater than 110.0 % of the total payments actually made by the Purchasers to the Company, exclusive of transaction expenses (the “Cumulative Purchaser Payments”), on or prior to December 31, 2026, the Revenue Interests shall be reduced to 2.0 % at Tier 1 and 0.0 % at Tier 3 for all subsequent calendar years beginning on January 1, 2027. If the Purchasers have not received Revenue Interest Payments in an amount equal to or greater than 110.0% of the Cumulative Purchaser Payments on or prior to December 31, 2026, the Revenue Interests shall be increased for all subsequent calendar years beginning on January 1, 2027 to a single defined rate (with no separate tiers) that would have provided the Purchasers with an amount equal to 110.0% of the Cumulative Purchaser Payments on or prior to December 31, 2026 had such rate applied to Tier 1 of initial Revenue Interest Payments. The Purchasers’ rights to receive the Revenue Interest Payments shall terminate on the date on which the Purchasers have received Revenue Interest Payments of 195.0 % of the Cumulative Purchaser Payments, unless the RIPA is terminated earlier. Under the RIPA, the Company has an option (the “Call Option”) to terminate the RIPA and repurchase future Revenue Interests at any time upon advance written notice. Additionally, the Purchasers have an option (the “Put Option”) to terminate the RIPA and to require the Company to repurchase future Revenue Interests upon enumerated events such as a bankruptcy event, an uncured material breach, a material adverse effect or a change of control, or upon the 12 th anniversary of the first payment made by Purchasers. If the Put Option is exercised prior to the first anniversary of the closing date by the Purchasers (except pursuant to a change of control), the required repurchase price will be 120.0 % of the Cumulative Purchaser Payments (minus all payments Company has made to the Purchasers in connection with the Revenue Interests). In all other cases, if the Put Option or the Call Option are exercised, the required repurchase price will be 175.0 % of the Cumulative Purchaser Payments (minus all payments Company has made to the Purchasers in connection with the Revenue Interests), if such option is exercised prior to the third anniversary of the closing date, and 195.0 % of the Cumulative Purchaser Payments (minus all payments Company has made to the Purchasers in connection with the Revenue Interests) if such option is exercised thereafter. In addition, the RIPA contains various representations and warranties, information rights, non-financial and financial covenants, indemnification obligations and other provisions that are customary for a transaction of this nature. The Purchaser’s obligations to fund the scheduled installments are subject to certain customary conditions as set forth in the RIPA. Concurrently with the RIPA, the Company entered into a Common Stock Purchase Agreement (“CSPA”) with certain affiliates of Oberland, pursuant to which the Company sold an aggregate of 509,164 shares of its common stock for an aggregate purchase price of $ 10.0 million. The $ 50.0 million upfront payment received pursuant to the RIPA and $ 10.0 million received pursuant to the CSPA was allocated between the resulting financial instruments on a relative fair value basis, with $ 49.2 million allocated to the debt under the RIPA and $ 10.8 million allocated to the common stock issued under the CSPA. The Put Options under the RIPA that are exercisable by Purchasers upon certain contingent events were determined to be embedded derivatives requiring bifurcation and separately accounted for as a single compound derivative instrument. The Company recorded the initial fair value of the derivative liability of $ 1.3 million as a debt discount, which will be amortized to interest expense over the expected term of the debt using the effective interest method. In connection with the RIPA, as of September 30, 2021 and December 31, 2020, $ 126.3 million and $ 47.7 million, respectively, was recorded as a revenue interest liability on the accompanying unaudited condensed consolidated balance sheets. The Company imputes interest expense associated with this liability using the effective interest rate method. The effective interest rate is calculated based on the rate that would enable the debt to be repaid in full over the anticipated life of the arrangement. The interest rate on this liability may vary during the term of the agreement depending on a number of factors, including the level of forecasted net sales. The Company evaluates the interest rate quarterly based on its current net sales forecasts utilizing the prospective method. A significant increase or decrease in net sales will materially impact the revenue interest liability, interest expense and the time period for repayment. The Company recorded $ 5.7 million and $ 13.8 million in interest expense related to this arrangement for the three and nine months ended September 30, 2021. As of September 30, 2021, the Company had incurred $ 0.9 million of issuance costs in connection with the RIPA, which are amortized to interest expense over the estimated term of the debt. Revenue Interest Payments made as a result of the Company’s net product sales will reduce the revenue interest liability. During the three and nine months ended September 30, 2021, the Company did no t record any product sales revenue. The following table summarizes the revenue interest liability activity during the three and nine months ended September 30, 2021 (in thousands): Revenue interest liability at December 31, 2020 $ 47,651 Interest expense recognized 3,381 Capitalized issuance costs ( 172 ) Revenue interest liability at March 31, 2021 $ 50,860 Proceeds from purchaser payments 65,000 Interest expense recognized 4,776 Capitalized issuance costs ( 26 ) Revenue interest liability at June 30, 2021 $ 120,610 Interest expense recognized 5,667 Revenue interest liability at September 30, 2021 $ 126,277

Asset Acquisitions

Asset Acquisitions	9 Months Ended
	Sep. 30, 2021
Asset Acquisitions [Abstract]
Asset Acquisitions	7. Asset Acquisitions Assignment and License Agreement with Shire International GmbH (Takeda) In November 2018, the Company entered into an Assignment and License Agreement (the “Shire Agreement”) with Shire International GmbH (“Shire”), which was subsequently acquired by Takeda Pharmaceutical Company Limited, and made an upfront payment to Shire of $ 7.5 million and issued Shire 1,859,151 shares of redeemable common stock with an estimated fair value of $ 7.0 million, or $ 3.76 per share. Under the terms of the Shire Agreement, Shire granted the Company an exclusive, royalty bearing worldwide license to develop and commercialize its two product candidates, maralixibat and volixibat. As part of the Shire Agreement, the Company was assigned license agreements held by Shire with Satiogen Pharmaceuticals, Inc. (“Satiogen”), Pfizer Inc. (“Pfizer”) and Sanofi-Aventis Deutschland GmbH (“Sanofi”). The Company has the right to sublicense under the Shire Agreement and additionally has the right to sublicense under the Satiogen, Pfizer and Sanofi licenses subject to the terms of those license agreements. The Company is obligated to pay Shire up to an aggregate of $ 109.5 million upon the achievement of certain clinical development and regulatory milestones for maralixibat in certain indications and an additional $ 25.0 million upon regulatory approval of maralixibat for each and every other indication. In addition, the Company is required to pay up to an aggregate of $ 30.0 million upon the achievement of certain clinical development and regulatory milestones for volixibat solely for the first indication sought. Upon commercialization, the Company is obligated to pay Shire product sales milestones on total licensed products up to an aggregate of $ 30.0 million. The Company is also obligated to pay tiered royalties with rates ranging from low double-digits to mid-teens based upon annual worldwide net sales for all licensed products; however, these royalties are reduced in part by royalties due under the Satiogen and Sanofi licenses, as discussed below, related to maralixibat and volixibat, as applicable. The Company’s royalty obligations will continue on a licensed product-by-licensed product and country-by-country basis until the later to occur of the expiration of the last valid claim in a licensed patent covering the applicable licensed product in such country, expiration of any regulatory exclusivity for the licensed product in a country and ten years after the first commercial sale of a licensed product in such country. For the periods presented, the Company paid development milestones of $ 10.0 million in December 2020 upon acceptance of a marketing authorization application filing to the European Medicines Agency for maralixibat for the treatment of PFIC2, $ 2.0 million in January 2021 associated with the initiation of the VISTAS Phase 2b clinical trial of volixibat in primary sclerosing cholangitis, $ 15.0 million in April 2021 consisting of (1) $ 5.0 million which would have been due had the Company initiated a Phase 3 clinical trial in ALGS, and (2) $ 10.0 million associated with the acceptance of the Company’s NDA filing by the FDA for maralixibat for the treatment of cholestatic pruritus in patients with ALGS based upon the results of the Company’s Phase 2b ICONIC clinical trial, and $ 2.0 million in July 2021 associated with the initiation of the EMBARK Phase 2b clinical trial of maralixibat in biliary atresia. Development milestones achieved prior to regulatory approval of the product were expensed by the Company in the period incurred as research and development. As of September 30, 2021, the Company accrued a $ 15 .0 million regulatory milestone associated with the FDA approval of Livmarli, which was subsequently paid in October 2021. Satiogen License Through the Shire Agreement, the Company was assigned a license agreement with Satiogen pursuant to which the Company obtained an exclusive, worldwide license to certain patents and know-how, with the right to sublicense to a third party subject to certain financial considerations. The Company is obligated to pay to Satiogen up to an aggregate of $ 10.5 million upon the achievement of certain milestones, of which $ 0.5 million was for initiation of certain development activities, $ 5.0 million for the completion of regulatory approvals and $ 5.0 million for commercialization activities. Additionally, the Company will be required to pay a low single-digit royalty on net sales. The Company’s royalty obligations continue on a licensed product-by-licensed product and country-by-country basis until the expiration of the last valid claim in a licensed patent covering the applicable licensed product in such country. Royalty obligations under the Satiogen license are creditable against the royalty obligations to Shire under the Shire Agreement. As of September 30, 2021 , the Company accrued $ 4.0 million associated with milestones for the FDA approval and for the first commercial sale of Livmarli which was considered probable as of September 30, 2021, both of which were subsequently paid in October 2021. Pfizer License Through the Shire Agreement, the Company was assigned a license agreement with Pfizer pursuant to which the Company obtained an exclusive, worldwide license to certain Pfizer know-how with a right to sublicense. Upon commercialization of any product utilizing the licensed product, the Company will be required to pay to Pfizer a low single-digit royalty on net sales of product sold by the Company, its affiliates or sublicensees. The Company’s royalty obligations continue on a licensed product-by-licensed product basis until the eighth anniversary of the first commercial sale of such licensed product anywhere in the world. Sanofi License Through the Shire Agreement, the Company was assigned a license agreement with Sanofi pursuant to which the Company obtained an exclusive, worldwide license to certain patents and know-how with the right to sublicense to a third party subject to certain financial considerations. The Company is obligated to pay up to an aggregate of $ 36.0 million upon the achievement of certain regulatory, commercialization and product sales milestones. Additionally, upon commercialization, the Company is required to pay tiered royalties in the mid to high single-digit range based upon net sales of licensed products sold by the Company and sublicensees in a calendar year, subject to adjustments in certain circumstances. The Company’s royalty obligations continue on a licensed product-by-licensed product and country-by-country basis until the later to occur of the expiration of the last valid claim in a licensed patent covering the applicable licensed product in such country and ten years after the first commercial sale of a licensed product in such country. Royalty obligations under the Sanofi license are creditable against the royalty obligations to Shire under the Shire Agreement. The Company has not paid milestone payments pursuant to this agreement for the periods presented. As of September 30, 2021 , no milestones had been accrued as there were no potential milestones considered probable.

Collaboration and License Agree

Collaboration and License Agreements	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration and License Agreements	8. Collaboration and License Agreements Option, License and Collaboration Agreement with Vivet In April 2021, the Company entered into an Option, License and Collaboration Agreement (“Vivet Collaboration Agreement”) with Vivet Therapeutics SAS (“Vivet”). Pursuant to the Vivet Collaboration Agreement, Vivet granted the Company the exclusive option, at the Company's discretion, to develop and subsequently commercialize Vivet’s two proprietary AAV gene therapy programs for PFIC, subtypes 3 and 2. Under the terms of the Vivet Collaboration Agreement, the Company paid an upfront fee of $ 4.2 million and agreed to provide funding to support certain research and development costs associated with the two gene therapy programs through July 2023. For the three and nine months ended September 30, 2021, pursuant to the terms of the Vivet Collaboration Agreement, the Company recorded research and development expense of $ 6.3 and $ 16.7 million, respectively, on the accompanying condensed consolidated statements of operations. License and Collaboration Agreement with CANbridge In April 2021, the Company entered into an exclusive license and collaboration agreement with CANbridge Pharmaceuticals, Inc. (“CANbridge”). Under the terms of the agreement, CANbridge has obtained the exclusive right to develop and commercialize maralixibat within the Greater China regions (China, Hong Kong, Macau and Taiwan). In connection with the agreement, the Company received an upfront payment of $ 11.0 million, which, upon satisfaction of the performance obligation and receipt by CANbridge of the right to use and benefit from the license, was recorded as license revenue in the accompanying condensed consolidated statements of operations. Additionally, the Company is eligible to receive up to $ 5.0 million in research and development funding, and up to $ 109.0 million for the achievement of future regulatory and commercial milestones, with double-digit tiered royalties based on product net sales. The Company concluded at inception of the agreement that the transaction price should not include the variable consideration related to unachieved developmental and regulatory milestones as this consideration was considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal in cumulative revenue. The Company will recognize any consideration related to sales-based payments (including milestones and royalties) when the related sales occur, as the Company has determined that these amounts relate predominantly to the license granted and therefore will be recognized at the later of (i) when or as the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). The Company re-evaluates the transaction price at each reporting period as uncertain events are resolved and other changes in circumstances occur. For the three and nine months ended September 30, 2021 , no adjustments were made to the transaction price. For the three and nine months ended September 30, 2021 , the Company recorded research and development funding of $ 0.4 million and $ 1.4 million, respectively, payable by CANbridge to the Company which is reflected as a reduction of research and development expense on the accompanying condensed consolidated statements of operations. As of September 30, 2021 , such research and development funding of $ 0.3 million was recorded as a receivable which was included in prepaids and other current assets on the accompanying condensed consolidated balance sheets. License and Collaboration Agreement with GC Pharma In July 2021, the Company entered into an exclusive license and collaboration agreement with GC Pharma. Under the terms of the agreement, GC Pharma has obtained the exclusive right to develop and commercialize maralixibat within South Korea for ALGS, PFIC, and biliary atresia. In connection with the agreement, the Company received a $ 5.0 million upfront payment, which, upon satisfaction of the performance obligation and receipt by GC Pharma of the right to use and benefit from the license, was recorded as license revenue in the accompanying condensed consolidated financial statements of operations. Additionally, the Company is entitled to certain research and development funding and up to $ 23.0 million for the achievement of future regulatory and commercial milestones, with double-digit tiered royalties based on product net sales. At inception of the agreement, the Company concluded that the transaction price should not include the variable consideration related to unachieved developmental and regulatory milestones as this consideration was considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal in cumulative revenue for this contract when the uncertainty is resolved in the future. The Company will recognize any consideration related to sales-based payments (including milestones and royalties) when the related sales occur, as the Company has determined that these amounts relate predominantly to the license granted and therefore will be recognized on the later to occur of satisfaction of the performance obligation or the occurrence of the related sales. The Company will re-evaluate the transaction price at each reporting period as uncertain events are resolved and other changes in circumstances occur. For the three and nine months ended September 30, 2021, no adjustments were made to the transaction price. For the three and nine months ended September 30, 2021, the Company recorded research and development funding of $ 0.1 million payable by GC Pharma to the Company which is reflected as a reduction of research and development expense on the accompanying condensed consolidated statements of operations. As of September 30, 2021, such research and development funding of $ 0.1 million was recorded as a receivable which was included in prepaids and other current assets on the accompanying condensed consolidated balance sheets.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	9. Leases In January 2019, the Company entered into an operating lease agreement for office space which consisted of approximately 5,600 square feet (the “initial lease”). The lease term is approximately four years with an option to extend the term for one five-year term, which at the time was not reasonably assured of exercise and therefore, not included in the lease term. The lease contained a tenant improvement allowance of $ 0.4 million, which has been recorded as leasehold improvements in the accompanying unaudited condensed consolidated balance sheets with a corresponding reduction of the right-of-use (“ROU”) asset at inception of the lease. Rent payments commenced in August 2019. In November 2019, the Company amended the operating lease agreement (the “amended agreement”) to extend the term of the initial lease through March 2025. This extension was accounted for as a lease modification and the Company recorded an increase to the ROU asset and lease liability of $ 0.6 million at the time of the amendment. Additionally, pursuant to the amended agreement, the Company expanded the office space by 5,555 square feet for a five-year term expiring in March 2025 (the “expanded space”). The Company accounted for the expanded space as a separate contract as there were material additional rights of use that were not included in the initial lease. The amended lease contained a tenant improvement allowance of $ 0.8 million in connection with the expanded space, which has been recorded as leasehold improvements within property and equipment, net on the accompanying unaudited condensed consolidated balance sheets with a corresponding reduction of the ROU asset at inception of the lease for the expanded space. The ROU and corresponding lease liabilities were estimated using a weighted-average incremental borrowing rate of 8.0 %. As of September 30, 2021 , the Company recorded an aggregate ROU asset of $ 1.7 million and an aggregate lease liability of $ 2.8 million in the accompanying unaudited condensed consolidated balance sheets. The weighted-average remaining lease term is 3.4 years. As of September 30, 2021, undiscounted future minimum payments under the Company’s operating leases are as follows (in thousands): Year Ending December 31, Undiscounted 2021 (remaining three months) 219 2022 888 2023 916 2024 925 2025 230 Total undiscounted lease payments 3,178 Less: imputed interest ( 400 ) Total lease liability $ 2,778 Rent expense was $ 0.2 million and $ 0.2 million for the three months ended September 30, 2021 and 2020 , respectively, and $ 0.5 million and $ 0.5 million for the nine months ended September 30, 2021 and 2020, respectively. Variable lease payments for operating expenses were immaterial for the three and nine months ended September 30, 2021 and 2020 .

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' Equity	10. Stockholders’ Equity Common Stock In January 2020, the Company completed a follow-on public offering of its common stock, pursuant to which the Company sold 2,400,000 shares of common stock at a public offering price of $ 20.00 per share, resulting in net proceeds of $ 44.7 million after deducting underwriting discounts, commissions and offering expenses. In August 2020, the SEC declared effective a registration statement on Form S-3 (“Shelf Registration”) covering the sale of up to $ 300.0 million of the Company’s securities. Also, in August 2020, the Company entered into a sales agreement (“Sales Agreement”) with SVB Leerink LLC (“SVB Leerink”) pursuant to which the Company may elect to issue and sell, from time to time, shares of common stock having an aggregate offering price of up to $ 75.0 million under the Shelf Registration through SVB Leerink acting as the sales agent and/or principal. During the nine months ended September 30, 2021 , the Company did no t sell any common stock pursuant to the Sales Agreement. As of December 31, 2020, the Company had sold 305,969 shares of common stock in an at-the-market offering pursuant to the Sales Agreement at a weighted-average price of $ 21.55 per share, resulting in gross proceeds of $ 6.6 million. The net proceeds after deducting sales commissions to SVB Leerink and other issuance expenses were approximately $ 6.3 million. The remaining capacity under the Sales Agreement is approximately $ 68.4 million as of September 30, 2021. In December 2020, the Company completed an underwritten public offering of its common stock pursuant to the Shelf Registration. The Company sold 3,750,000 shares of common stock at a public offering price of $ 20.00 per share, resulting in net proceeds of $ 70.0 million after deducting underwriting discounts, commissions and offering expenses. In addition, the Company granted the underwriters an option, exercisable for 30 days, to purchase up to 562,500 additional shares of its common stock at the public offering price, less the underwriting discounts and commissions. In January 2021, the underwriters partially exercised their option and purchased 375,654 shares of the Company’s common stock at a public offering price of $ 20.00 per share, resulting in net proceeds of $ 7.1 million after deducting underwriting discounts. The remaining options expired unexercised. As of September 30, 2021 and December 31, 2020, 155,864 shares and 256,056 shares of common stock, respectively, were subject to repurchase by the Company. The unvested stock liability related to these shares is immaterial to all periods presented. Common Stock Reserved for Issuance Common stock reserved for issuance is as follows: As of September 30, As of December 31, 2021 2020 Stock options issued and outstanding 7,006,115 5,071,740 Reserved for future stock awards or option grants 1,238,948 2,009,410 Reserved for employee stock purchase plan 956,373 700,023 Reserved for PSUs 227,780 — Reserved for underwriter option shares — 562,500 9,429,216 8,343,673

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation Equity Incentive Plans In November 2018, the Company adopted the 2018 Equity Incentive Plan (the “2018 Plan”), which permits the granting of stock awards and incentive and nonstatutory stock options to employees, directors and consultants of the Company. In July 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Equity Incentive Plan (the “2019 Plan”). The 2019 Plan became effective on July 17, 2019. Under the 2019 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units (“RSUs”) and other stock or cash-based awards to individuals who are then employees, officers, directors or consultants of the Company. Shares subject to outstanding awards under the 2018 Plan as of the effective date of the 2019 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2019 Plan. In addition, the number of shares of common stock available for issuance under the 2019 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2019 Plan, beginning with January 1, 2020 and ending with January 1, 2029 , by an amount equal to 5 % of the outstanding number of shares of the Company’s common stock on December 31st of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of September 30, 2021 , 1,225,948 shares of common stock were available for issuance under the 2019 Plan. In March 2020, the compensation committee of the Company’s board of directors approved and adopted the 2020 Inducement Plan (the “2020 Inducement Plan”). Under the 2020 Inducement Plan, the Company may grant nonstatutory stock options, stock appreciation rights, restricted stock and RSUs to new employees entering into employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). At adoption, the 2020 Inducement Plan authorized 750,000 shares of the Company’s common stock for future issuance. In December 2020, the Company’s board of directors authorized an additional 750,000 shares of the Company’s common stock for future issuance. As of September 30, 2021 , 13,000 shares of common stock were available for issuance under the 2020 Inducement Plan. Stock Options The following table summarizes stock option activity during the nine months ended September 30, 2021 (in thousands, except share and per share data): Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2020 5,071,740 $ 9.99 8.6 $ 40,631 Granted 2,263,200 $ 18.17 Exercised ( 109,937 ) $ 6.55 Canceled and forfeited ( 218,888 ) $ 13.28 Outstanding as of September 30, 2021 7,006,115 $ 12.59 8.3 $ 52,040 Vested and exercisable as of September 30, 2021 2,685,172 $ 8.46 7.6 $ 31,028 Intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that had exercise prices that were lower than the per share fair value of the common stock on the date of exercise. The weighted-average grant date fair value per share of stock options granted during the nine months ended September 30, 2021 and 2020 was $ 13.47 and $ 12.56 per share, respectively. The total intrinsic value of options exercised during the nine months ended September 30, 2021 and 2020 was $ 1.2 million and $ 0.2 million, re spectively. As of September 30, 2021, the total unrecognized stock-based compensation related to unvested stock option awards granted was $ 47.3 million, which the Company expects to recognize over a weighted-average period of approximate ly 2.7 years. In connection with a separation agreement with an employee, the Company accelerated the vesting of certain unvested stock options and extended the exercise period related to the vested awards. The modification resulted in incremental stock-based compensat ion during the nine months ended September 30, 2021 of $ 0.6 million, which was recognized immediately. The fair value of each employee and non-employee stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Due to the Company’s limited operating history and a lack of company specific historical and implied volatility data, the Company estimated expected volatility based on the historical volatility of a group of similar companies that are publicly traded. The historical volatility data was computed using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. Due to the lack of historical exercise history, the expected term of the Company’s stock options for employees has been determined utilizing the “simplified” method for awards. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following assumptions were used to estimate the fair value of stock option awards granted during the following periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Exercise price $ 14.05 -$ 18.15 $ 19.03 -$ 26.59 $ 14.05 -$ 21.22 $ 10.40 -$ 26.59 Expected term (in years) 5.5 - 6.1 5.5 - 6.1 5.5 - 6.1 5.5 - 6.1 Expected volatility 82.76 %- 83.88 % 92.45 %- 94.82 % 82.76 %- 94.06 % 77.07 %- 95.85 % Risk-free interest rate 0.75 %- 0.97 % 0.33 %- 0.40 % 0.62 %- 1.12 % 0.33 %- 1.73 % Expected dividend yield — — — — Performance Stock Units In March 2021, the Company granted PSUs to employees which are subject to a performance condition of FDA marketing approval of Livmarli (“NDA Approval”) by a certain date. Upon achievement, 50 % will vest immediately and 50 % will vest on June 30, 2023, subject to the employees’ continuous service through each vesting date. As of September 29, 2021, the Company determined achievement of the NDA Approval performance condition was met. As a result, 76,027 of the PSUs vested on that date. Additionally, in March 2021, the Company granted an aggregate of 75,688 PSUs to certain executive participants (“Executive PSUs”). The Executive PSUs are subject to performance and market conditions: (1) NDA Approval by a certain date and (2) achievement of a specified stock price. The Executive PSU’s will vest as follows:  25 % of the granted Executive PSUs will vest upon NDA Approval and achievement of a specified stock price as measured by the volume-weighted average price of the Company’s common stock for the 30 trading days ended December 31, 2021 (“VWAP”). Up to an additional 25 % of the granted Executive PSUs will vest as determined by the incremental performance of the common stock over a specified stock price measured by the VWAP (altogether the “Base Units”).  An equal number of units to the Base Units will vest on June 30, 2023, subject to the executive participants’ continued service with the Company. As the Executive PSU’s contained a market condition, the grant date fair value was determined using a Monte Carlo simulation model and the weighted-average grant date fair value of these Executive PSU's was $ 7.09 per share. As of September 30, 2021, the Company determined achievement of the NDA Approval performance condition was met as applied under the Accounting Standards Codification 718, Compensation — Stock Compensation (Topic 718) . The total unrecognized stock-based compensation related to the PSUs and the Executive PSUs w as $ 1.4 million a s of September 30, 2021. The following table summarizes the activity related to PSUs for the nine months ended September 30, 2021: Number of Weighted-Average Grant Date Fair Value Unvested and Outstanding as of December 31, 2020 — $ — Granted 229,856 $ 14.78 Vested ( 76,027 ) $ 18.56 Cancelled/Forfeited ( 2,076 ) $ 18.50 Unvested and Outstanding as of September 30, 2021 151,753 12.84 2019 Employee Stock Purchase Plan In July 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Employee Stock Purchase Plan (“ESPP”). The ESPP became effective on July 17, 2019. A total of 500,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029 , by an amount equal to the lesser of (i) 1 % of the outstanding number of shares of common stock on December 31st of the preceding calendar year, (ii) 1,500,000 shares of common stock or (iii) such lesser amount as determined by the Company’s board of directors. During the three and nine months ended September 30, 2021 , 43,976 shares were issued under the ESPP. As of September 30, 2021 , the Company had 956,373 shares available for future issuance under the ESPP. The stock-based compensation related to the ESPP for the three and nine months ended September 30, 2021 was $ 0.2 million and $ 0.5 million, respectively. Restricted Stock In November 2018, in connection with the issuance of Series A Preferred Stock, the Company’s founders agreed to modify their outstanding shares of common stock to include vesting provisions that require continued service to the Company in order to vest in those shares. As such, the 562,500 modified shares of common stock became compensatory upon such modification. The modified shares have a four-year vesting period and a measurement date fair value of $ 2.936 per share. For the three months ended September 30, 2021 and 2020 , 33,396 shares and 33,398 sha res vested, respectively. For the nine months ended September 30, 2021 and 2020 , 100,192 shares and 100,194 sha res vested, respectively. As of September 30, 2021 , the total unrecognized compensation expense related to unvested restricted stock was $ 0.5 million expected to be recognized over a weighted-average period of approximately 1.2 years. Total stock-based compensation is reflected in the unaudited condensed consolidated statements of operations as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 General and administrative $ 4,380 $ 2,067 $ 9,731 $ 5,313 Research and development 3,035 1,361 7,792 3,662 Total $ 7,415 $ 3,428 $ 17,523 $ 8,975

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Contingencies	12. Contingencies The Company is subject to potential liabilities under government regulations and various claims and legal actions that are pending or may be asserted from time-to-time. These matters arise in the ordinary course and conduct of the Company’s business and may include, for example, commercial, intellectual property, and employment matters. The Company intends to defend itself vigorously in such matters and when warranted, take legal action against others. Furthermore, the Company regularly assesses contingencies to determine the degree of probability and range of possible loss for potential accrual in its financial statements. An estimated loss contingency is accrued in the Company’s financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. The Company does not accrue amounts for liabilities that it does not believe are probable. Litigation is inherently unpredictable, and unfavorable resolutions could occur. As a result, assessing contingencies is highly subjective and requires judgment about future events. During the periods presented, the Company has not recorded any accrual for loss contingencies associated with such government regulations, claims or legal actions, determined that an unfavorable outcome is probable or reasonably possible, or determined that the amount or range of any possible loss is reasonably estimable.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (“SEC”). Accordingly, the accompanying unaudited condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The unaudited interim financial statements reflect all adjustments which, in the opinion of management, are necessary for a fair statement of the results for the periods presented. All such adjustments are of a normal and recurring nature. The consolidated balance sheet as of December 31, 2020 has been derived from the audited consolidated financial statements at that date but does not include all information and footnotes required by GAAP for complete financial statements. The operating results presented in these unaudited condensed consolidated financial statements are not necessarily indicative of the results that may be expected for any future periods. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto in the Company’s Annual Report on Form 10-K (“Annual Report”) for the fiscal year ended December 31, 2020, as filed with the SEC on March 9, 2021.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in the financial statements and accompanying notes. The most significant estimates in the Company’s unaudited condensed consolidated financial statements relate to the revenue interest liability, accrued research and development expenses, the valuation of derivative liabilities, equity award compensation and the valuation allowance of deferred tax assets resulting from net operating losses. These estimates and assumptions are based upon historical experience, knowledge of current events and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results could differ materially from those estimates.
Significant Accounting Policies	Significant Accounting Policies There have been no significant changes to the accounting policies during the nine months ended September 30, 2021 , as compared to the significant accounting policies described in Note 2 of the “Notes to Consolidated Financial Statements” in the Company’s audited consolidated financial statements included in the Annual Report, except as discussed below.
Revenue Recognition	Revenue Recognition The Company accounts for contracts with customers under Accounting Standards Codification (“ASC”), Revenue from Contracts with Customers (Topic 606) (“ASC 606”). Under ASC 606, the Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration the Company expects to receive in exchange for those goods or services.
License and Collaboration Arrangements	License and Collaboration Arrangements The Company enters into collaborative arrangements with partners that are within the scope of ASC, Collaborative Arrangements (Topic 808) (“ASC 808”). The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808 to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. The accounting for some of the activities under collaboration arrangements may be subject to ASC 606 for distinct units of account that are reflective of a vendor-customer relationship. For other elements of collaboration arrangements, such as assistance with development of the drug products, the Company applies the illustrative examples in ASC 808 and generally records reimbursements received as a reduction of research and development expenses. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the contracts with customers; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) determination and measurement of the transaction price, including any constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations in the contract; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The terms of the Company’s license and collaborative research and development agreements include upfront license fees, research, development and other funding or reimbursements, milestone and other contingent payments for the achievement of defined collaboration objectives and certain development, regulatory and sales-based events, as well as royalties on sales of commercialized products. Arrangements that include upfront payments may require deferral of revenue recognition to a future period until we perform obligations under these arrangements. A performance obligation is a promise in a contract to transfer a distinct good or service and is the unit of accounting in Topic 606. A contract’s transaction price is allocated among each distinct performance obligation based on relative standalone selling price and recognized as revenue when, or as, the applicable performance obligation is satisfied. If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues attributed to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price as variable consideration using the most likely amount method or expected value method, depending on the nature of the contingency and the variable payments. If it is probable that a significant reversal of cumulative revenue recognized for the contract would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not generally considered probable of being achieved until those approvals are received. Given the high degree of uncertainty around the occurrence of these events, the Company generally determines the milestone and other contingent amounts to be fully constrained until the uncertainty associated with these payments is resolved. At the end of each reporting period, the Company re-evaluates the probability of achievement of any development milestones, and if necessary, adjusts its estimate of the transaction price. Any such adjustments would be recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, the Company recognizes revenue at the later of (i) when or as the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). Accounting for these arrangements requires the Company to develop assumptions that require judgment to determine the stand-alone selling price of each performance obligation identified in the contract. The Company has never sold the performance obligations in its collaborative arrangements separately; therefore, an observable stand-alone selling price does not exist. Accordingly, the Company estimates a stand-alone selling price through maximizing the use of observable inputs such as market data, project cost estimates, and targeted margins.
Stock-Based Compensation	Stock-Based Compensation The Company recognizes stock-based compensation for all stock-based awards based on the grant date fair value of the award. For stock-based awards with service conditions, the fair value of the awards is amortized on a straight-line basis over the requisite service period in which the awards are expected to vest. For stock-based awards with performance vesting conditions, stock-based compensation is recognized when it is considered probable that the performance conditions will be satisfied. At each reporting period, the Company reassesses the probability of the achievement of the performance vesting conditions. Any change in stock-based compensation resulting from an adjustment in the vesting is treated as a cumulative catch-up in the period of adjustment. For stock-based awards with market conditions, stock-based compensation is recognized over the appropriate requisite service period. The Company accounts for forfeitures as they occur.
Intangibles Assets, Net	Intangibles Assets, Net Upon FDA approval of Livmarli in September 2021, contractual milestone payments for which the Company was then obligated to pay to licensors were evaluated as intangible assets for the completed regulatory approval and right to commercialize the product. The evaluation of intangible assets includes assessing the amortization period for which the asset is expected to contribute to the future cash flows of the Company. The Company determined the pattern of this intangible asset’s future cash flows could not be readily determined with a high level of precision. As a result, the intangible asset is being amortized on a straight-line basis over the estimated useful life of 11 year s. The Company tests its definite lived intangible assets for impairment annually if events or changes in circumstances indicate that it is more likely than not that the asset is impaired. If it is determined that the asset becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in the consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments to its intangible assets.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding for the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury-stock and if-converted methods. Diluted net loss per share excludes the potential impact of the Company’s common stock subject to repurchase, common stock options, contingently issuable employee stock purchase plan shares, contingently issuable performance stock units (“PSUs”) and contingently issuable underwriter option shares because their effect would be anti-dilutive due to the Company’s net loss. Since the Company incurred a net loss in each of the periods presented, basic and diluted net loss per share were the same. The following outstanding potentially dilutive shares of common stock have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, As of December 31, 2021 2020 Options to purchase common stock 7,006,115 5,071,740 Common stock subject to repurchase 155,864 256,056 Employee stock purchase plan contingently issuable 40,429 12,931 PSUs contingently issuable 151,753 — Underwriter option shares contingently issuable — 562,500 Total 7,354,161 5,903,227
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In November 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No.2018-18, Collaborative Arrangements (Topic 818): Clarifying the Interaction Between Topic 808 and Topic 606, which clarifies when transactions between participants in a collaborative arrangement are within the scope of the FASB’s revenue standard, Topic 606. The standard is effective for fiscal years beginning after December 15, 2019 and interim periods within those fiscal years, with early adoption permitted. The Company adopted this standard on January 1, 2021 , as it had no collaboration arrangements prior to that date. There was no impact on the accompanying consolidated financial statements as of the adoption date.
Recent Accounting Pronouncements Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). ASU 2016-13 requires an entity to utilize a new impairment model that requires measurement and recognition of expected credit losses for most financial assets and certain other instruments, including but not limited to available-for-sale debt securities. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses rather than as a direct write-down to the security. The new guidance requires the use of forward-looking expected credit loss models based on historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount, which may result in earlier recognition of credit losses under the new guidance. The new guidance also modifies the impairment models for available-for-sale debt securities and for purchased financial assets with credit deterioration since their origination. Subsequent to the issuance of ASU 2016-13, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments - Credit Losses . This ASU does not change the core principle of the guidance in ASU 2016-13, instead these amendments are intended to clarify and improve operability of certain topics included within the credit losses guidance. The FASB also subsequently issued ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 842), which did not change the core principle of the guidance in ASU 2016-13 but clarified that expected recoveries of amounts previously written off and expected to be written off should be included in the valuation account and should not exceed amounts previously written off and expected to be written off. In March 2020, the FASB issued ASU No. 2020-3, Codification Improvements to Financial Instruments which makes narrow-scope improvements to various financial instruments topics, including the new credit losses standard and clarifies the following areas (i) the contractual term of a net investment in a lease should be the contractual term used to measure expected credit losses; (ii) when an entity regains control of financial assets sold, an allowance for credit losses should be recorded. The guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies. For smaller reporting companies, the guidance will be effective during the first quarter of 2023. The Company is in the process of assessing the impact adoption will have on its consolidated financial statements. In August 2020, the FASB issued ASU No. 2020-06, Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging-Contracts in Entity’s Own Equity (Subtopic 815-40) (“ASU 2020-06”), which simplifies the accounting for convertible instruments. The guidance removes certain accounting models that separate the embedded conversion features from the host contract for convertible instruments. ASU 2020-06 allows for a modified or full retrospective method of transition. This update is effective for fiscal years beginning after December 15, 2021, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact this change will have on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Outstanding Potentially Dilutive Shares of Common Stock Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares of common stock have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, As of December 31, 2021 2020 Options to purchase common stock 7,006,115 5,071,740 Common stock subject to repurchase 155,864 256,056 Employee stock purchase plan contingently issuable 40,429 12,931 PSUs contingently issuable 151,753 — Underwriter option shares contingently issuable — 562,500 Total 7,354,161 5,903,227

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities to Fair Value Measurements On Recurring Basis and Level of Input Measurements	Financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type are presented in the following table (in thousands): September 30, 2021 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 52,509 $ — $ — $ 52,509 Commercial paper — 134,190 — 134,190 U.S. government bonds — 14,997 — 14,997 Total financial assets $ 52,509 $ 149,187 $ — $ 201,696 Financial liabilities: Derivative liability $ — $ — $ 847 $ 847 Total financial liabilities $ — $ — $ 847 $ 847 December 31, 2020 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 127,783 $ — $ — $ 127,783 U.S. treasury bills 29,997 — — 29,997 Corporate debt securities — 23,201 — 23,201 Commercial paper — 41,460 — 41,460 U.S. government bonds — 5,066 — 5,066 Asset-backed securities — 2,006 — 2,006 Total financial assets $ 157,780 $ 71,733 $ — $ 229,513 Financial liabilities: Derivative liability $ — $ — $ 1,264 $ 1,264 Total financial liabilities $ — $ — $ 1,264 $ 1,264
Summary of Change in Estimated Fair Value of Company's Derivative Liability Classified as Level 3	The following table provides a summary of the change in the estimated fair value of the Company’s derivative liability, classified as Level 3 in the fair value hierarchy: Balance at December 31, 2020 $ 1,264 Change in fair value of derivative liability ( 334 ) Balance at March 31, 2021 930 Change in fair value of derivative liability 1,272 Balance at June 30, 2021 2,202 Change in fair value of derivative liability ( 1,355 ) Balance at September 30, 2021 $ 847

Financial Instruments (Tables)

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Fair Value and Amortized Cost of Cash Equivalents and Available-for-sale Investments by Major Security Type	The fair value and amortized cost of cash equivalents and available-for-sale investments by major security type are presented in the following table (in thousands): September 30, 2021 Amortized Unrealized Unrealized Estimated Cash equivalents and investments: Money market fund $ 52,509 $ — $ — $ 52,509 Commercial paper 134,190 — — 134,190 U.S. government bonds 14,999 — ( 2 ) 14,997 Total cash equivalents and investments $ 201,698 $ — $ ( 2 ) $ 201,696 Classified as: Cash equivalents $ 52,509 Short-term investments 134,190 Investments 14,997 Total cash equivalents and investments $ 201,696 December 31, 2020 Amortized Unrealized Unrealized Estimated Cash equivalents and investments: Money market fund $ 127,783 $ — $ — $ 127,783 U.S. treasury bills 29,995 2 — 29,997 Corporate debt securities 23,126 75 — 23,201 Commercial paper 41,460 — — 41,460 U.S. government bonds 5,067 — ( 1 ) 5,066 Asset-backed securities 2,001 5 — 2,006 Total cash equivalents and investments $ 229,432 $ 82 $ ( 1 ) $ 229,513 Classified as: Cash equivalents $ 139,779 Short-term investments 89,734 Total cash equivalents and investments $ 229,513
Remaining contractual maturities of available-for-sale debt securities	As of September 30, 2021, the remaining contractual maturities of available-for-sale debt securities were as follows: (in thousands): Estimated Due within one year $ 134,190 One to two years 14,997 Total $ 149,187

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Accrued clinical trials $ 6,026 $ 3,673 Accrued milestone payments 19,000 — Accrued professional service fees 4,462 2,157 Accrued contract manufacturing and non-clinical costs 7,644 2,780 Accrued compensation and related benefits 6,715 4,801 Total accrued expenses $ 43,847 $ 13,411

Revenue Interest Purchase Agr_2

Revenue Interest Purchase Agreement (Tables)	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Revenue Interest Liability	The following table summarizes the revenue interest liability activity during the three and nine months ended September 30, 2021 (in thousands): Revenue interest liability at December 31, 2020 $ 47,651 Interest expense recognized 3,381 Capitalized issuance costs ( 172 ) Revenue interest liability at March 31, 2021 $ 50,860 Proceeds from purchaser payments 65,000 Interest expense recognized 4,776 Capitalized issuance costs ( 26 ) Revenue interest liability at June 30, 2021 $ 120,610 Interest expense recognized 5,667 Revenue interest liability at September 30, 2021 $ 126,277

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Schedule of Undiscounted Future Minimum Payments under Operating Leases	As of September 30, 2021, undiscounted future minimum payments under the Company’s operating leases are as follows (in thousands): Year Ending December 31, Undiscounted 2021 (remaining three months) 219 2022 888 2023 916 2024 925 2025 230 Total undiscounted lease payments 3,178 Less: imputed interest ( 400 ) Total lease liability $ 2,778

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	Common stock reserved for issuance is as follows: As of September 30, As of December 31, 2021 2020 Stock options issued and outstanding 7,006,115 5,071,740 Reserved for future stock awards or option grants 1,238,948 2,009,410 Reserved for employee stock purchase plan 956,373 700,023 Reserved for PSUs 227,780 — Reserved for underwriter option shares — 562,500 9,429,216 8,343,673

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Summary of Stock Option Activity	The following table summarizes stock option activity during the nine months ended September 30, 2021 (in thousands, except share and per share data): Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2020 5,071,740 $ 9.99 8.6 $ 40,631 Granted 2,263,200 $ 18.17 Exercised ( 109,937 ) $ 6.55 Canceled and forfeited ( 218,888 ) $ 13.28 Outstanding as of September 30, 2021 7,006,115 $ 12.59 8.3 $ 52,040 Vested and exercisable as of September 30, 2021 2,685,172 $ 8.46 7.6 $ 31,028
Schedule of Assumptions Used to Estimate Fair Value of Stock Option Awards Granted	The following assumptions were used to estimate the fair value of stock option awards granted during the following periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Exercise price $ 14.05 -$ 18.15 $ 19.03 -$ 26.59 $ 14.05 -$ 21.22 $ 10.40 -$ 26.59 Expected term (in years) 5.5 - 6.1 5.5 - 6.1 5.5 - 6.1 5.5 - 6.1 Expected volatility 82.76 %- 83.88 % 92.45 %- 94.82 % 82.76 %- 94.06 % 77.07 %- 95.85 % Risk-free interest rate 0.75 %- 0.97 % 0.33 %- 0.40 % 0.62 %- 1.12 % 0.33 %- 1.73 % Expected dividend yield — — — —
Summary of PSU Activity	The following table summarizes the activity related to PSUs for the nine months ended September 30, 2021: Number of Weighted-Average Grant Date Fair Value Unvested and Outstanding as of December 31, 2020 — $ — Granted 229,856 $ 14.78 Vested ( 76,027 ) $ 18.56 Cancelled/Forfeited ( 2,076 ) $ 18.50 Unvested and Outstanding as of September 30, 2021 151,753 12.84
Summary of Stock-based Compensation Reflected in Unaudited Condensed Consolidated Statements of Operations	Total stock-based compensation is reflected in the unaudited condensed consolidated statements of operations as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 General and administrative $ 4,380 $ 2,067 $ 9,731 $ 5,313 Research and development 3,035 1,361 7,792 3,662 Total $ 7,415 $ 3,428 $ 17,523 $ 8,975

Organization and Description _2

Organization and Description of Business - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)Segment	Dec. 31, 2020USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	May 2, 2018
Number of operating segments | Segment	1
Accumulated deficit	$ 314,705	$ 173,171
Cash, cash equivalents and investments	$ 205,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Outstanding Potentially Dilutive Shares of Common Stock Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	7,354,161	5,903,227
Underwriter Option Shares Contingently Issuable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	0	562,500
Options to Purchase Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	7,006,115	5,071,740
Common Stock Subject to Repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	155,864	256,056
Employee Stock Purchase Plan Contingently Issuable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	40,429	12,931
PSUs Contingently Issuable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	151,753	0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2021
Summary Of Significant Accounting Policies [Line Items]
Intangible asset useful lives	11 years
Accounting Standards Update 2018-18
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Change in accounting principle, accounting standards update, immaterial effect [true false]	true

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities to Fair Value Measurement On Recurring Basis and Level of Input Measurement (Details) - Fair Value, Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Fair value measurements	$ 201,696	$ 229,513
Financial liabilities:
Derivative liability	847	1,264
Total financial liabilities	847	1,264
Level 1
Financial assets:
Fair value measurements	52,509	157,780
Financial liabilities:
Derivative liability	0	0
Total financial liabilities	0	0
Level 2
Financial assets:
Fair value measurements	149,187	71,733
Financial liabilities:
Derivative liability	0	0
Total financial liabilities	0	0
Level 3
Financial assets:
Fair value measurements	0	0
Financial liabilities:
Derivative liability	847	1,264
Total financial liabilities	847	1,264
Money Market Fund
Financial assets:
Fair value measurements	52,509	127,783
Money Market Fund | Level 1
Financial assets:
Fair value measurements	52,509	127,783
Money Market Fund | Level 2
Financial assets:
Fair value measurements	0	0
Money Market Fund | Level 3
Financial assets:
Fair value measurements	0	0
U.S. treasury bills
Financial assets:
Fair value measurements		29,997
U.S. treasury bills | Level 1
Financial assets:
Fair value measurements		29,997
U.S. treasury bills | Level 2
Financial assets:
Fair value measurements		0
U.S. treasury bills | Level 3
Financial assets:
Fair value measurements		0
Corporate Debt Securities
Financial assets:
Fair value measurements		23,201
Corporate Debt Securities | Level 1
Financial assets:
Fair value measurements		0
Corporate Debt Securities | Level 2
Financial assets:
Fair value measurements		23,201
Corporate Debt Securities | Level 3
Financial assets:
Fair value measurements		0
Commercial Paper
Financial assets:
Fair value measurements	134,190	41,460
Commercial Paper | Level 1
Financial assets:
Fair value measurements		0
Commercial Paper | Level 2
Financial assets:
Fair value measurements	134,190	41,460
Commercial Paper | Level 3
Financial assets:
Fair value measurements	0	0
U.S. Government Bonds
Financial assets:
Fair value measurements	14,997	5,066
U.S. Government Bonds | Level 1
Financial assets:
Fair value measurements		0
U.S. Government Bonds | Level 2
Financial assets:
Fair value measurements	14,997	5,066
U.S. Government Bonds | Level 3
Financial assets:
Fair value measurements	$ 0	0
Asset Backed Securities
Financial assets:
Fair value measurements		2,006
Asset Backed Securities | Level 1
Financial assets:
Fair value measurements		0
Asset Backed Securities | Level 2
Financial assets:
Fair value measurements		2,006
Asset Backed Securities | Level 3
Financial assets:
Fair value measurements		$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details)	Sep. 30, 2021	Dec. 31, 2020
Discount Rate
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Discount rate used for valuation to derivative liability	0.157	0.159

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Change in Estimated Fair Value of Company's Derivative Liability Classified as Level 3 (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value of derivative liability				$ 417
Level 3
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 2,202	$ 930	$ 1,264	1,264
Change in fair value of derivative liability	(1,355)	1,272	(334)
Ending balance	$ 847	$ 2,202	$ 930	$ 847

Financial Instruments - Summary

Financial Instruments - Summary of Fair Value and Amortized Cost of Cash Equivalents and Available-for-sale Investments by Major Security Type (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents and investments, Amortized cost	$ 201,698	$ 229,432
Cash equivalents and investments, Unrealized gain	0	82
Cash equivalents and investments, Unrealized loss	(2)	(1)
Cash equivalents and investments, Estimated Fair Value	201,696	229,513
Cash equivalents, Estimated Fair Value	52,509	139,779
Short-term investments	134,190	89,734
Investments	14,997
Total cash equivalents and investments	201,696	229,513
Money Market Fund
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents, Amortized Cost	52,509	127,783
Cash equivalents, Estimated Fair Value	52,509	127,783
U.S. treasury bills
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost		29,995
Investments, Unrealized Gain		2
Investments, Unrealized Loss		0
Investments, Estimated Fair Value		29,997
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost		23,126
Investments, Unrealized Gain		75
Investments, Unrealized Loss		0
Investments, Estimated Fair Value		23,201
U.S. Government Bonds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	14,999	5,067
Investments, Unrealized Gain	0	0
Investments, Unrealized Loss	(2)	(1)
Investments, Estimated Fair Value	14,997	5,066
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	134,190	41,460
Investments, Unrealized Gain	0	0
Investments, Unrealized Loss	0	0
Investments, Estimated Fair Value	$ 134,190	41,460
Asset Backed Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost		2,001
Investments, Unrealized Gain		5
Investments, Unrealized Loss		0
Investments, Estimated Fair Value		$ 2,006

Financial Instruments - Additio

Financial Instruments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financial Instruments Owned At Fair Value [Abstract]
Realized gains or losses on available-for-sale investments	$ 0	$ 0	$ 0	$ 0
Investments in continuous unrealized loss position for more than 12 months	$ 0	$ 0	0	0
Other-than-temporary impairment losses			$ 0	$ 0

Financial Instruments - Remaini

Financial Instruments - Remaining contractual maturities of available-for-sale debt securities (Details) $ in Thousands	Sep. 30, 2021USD ($)
Financial Instruments Owned At Fair Value [Abstract]
Due within one year	$ 134,190
One to two years	14,997
Debt Securities, Held-to-maturity, Fair Value, Total	$ 149,187

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued clinical trials	$ 6,026	$ 3,673
Accrued milestone payments	19,000	0
Accrued professional service fees	4,462	2,157
Accrued contract manufacturing and non-clinical costs	7,644	2,780
Accrued compensation and related benefits	6,715	4,801
Total accrued expenses	$ 43,847	$ 13,411

Revenue Interest Purchase Agr_3

Revenue Interest Purchase Agreement - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2021	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue interest liability		$ 47,651,000	$ 126,277,000	$ 126,277,000	$ 47,651,000
Sales
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues			0	$ 0
Revenue Interest Purchase Agreement | Mulholland SA LLC,
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of revenue interest payments on annual net sales at Tier 1				9.75%
Percentage of revenue interest payments on annual net sales at Tier 2 and Tier 3				2.00%
Revenue interest rate percentage shall be reduced, if revenue interest payment percentage greater than or equal to 110% at Tier				2.00%
Revenue interest rate percentage shall be reduced, if revenue interest payment percentage greater than or equal to 110% at Tier 3				0.00%
Required revenue interest payment percentage of cumulative purchaser payments for termination				195.00%
Required repurchase price percentage of cumulative purchaser prior to first anniversary of closing date				120.00%
Required repurchase price percentage of cumulative purchaser payments after first anniversary and prior to third anniversary of closing date				175.00%
Required repurchase price percentage of cumulative purchaser after third anniversary of closing date				195.00%
Purchase agreement amount allocated to debt					49,200,000
Initial fair value of derivative liability			1,300,000	$ 1,300,000
Revenue interest liability		47,700,000	126,300,000	126,300,000	$ 47,700,000
Interest expense			5,700,000	13,800,000
Debt issuance costs			$ 900,000	$ 900,000
Revenue Interest Purchase Agreement | Mulholland SA LLC, | On or Prior to December 31, 2026
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Minimum required revenue interest payment percentage to cumulative purchaser payments to reduce interest rate				110.00%
Revenue Interest Purchase Agreement | Mulholland SA LLC, | Maximum | On or Prior to December 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payment to be received at the option of purchasers				$ 50,000,000
Revenue Interest Purchase Agreement | Mulholland SA LLC, | Maralixibat
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone and upfront payments received				115,000,000
Upfront payment received		$ 50,000,000
Milestone payment received	$ 65,000,000
Revenue Interest Purchase Agreement | Mulholland SA LLC, | Livmarli
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payment received upon regulatory approval				35,000,000
Tier I | Mulholland SA LLC, | Maralixibat | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual net sales				350,000,000
Tier 2 | Mulholland SA LLC, | Maralixibat | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual net sales				350,000,000
Tier 2 | Mulholland SA LLC, | Maralixibat | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual net sales				1,100,000,000
Tier 3 | Mulholland SA LLC, | Maralixibat | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual net sales				$ 1,100,000
CSPA | Mulholland SA LLC,
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares sold					509,164
Aggregate purchase price					$ 10,000,000
Net proceeds from transaction					10,000,000
Purchase agreement amount allocated to common stock issued					$ 10,800,000

Revenue Interest Purchase Agr_4

Revenue Interest Purchase Agreement - Summary of Revenue Interest Liability (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Revenue interest liability	$ 120,610	$ 50,860	$ 47,651
Proceeds from purchaser payments		65,000
Interest expense recognized	5,667	4,776	3,381
Capitalized issuance costs		(26)	(172)
Revenue interest liability	$ 126,277	$ 120,610	$ 50,860

Asset Acquisitions - Additional

Asset Acquisitions - Additional Information (Details)	1 Months Ended			9 Months Ended
	Apr. 30, 2021USD ($)	Dec. 31, 2020USD ($)	Nov. 30, 2018USD ($)ProductCandidate$ / sharesshares	Sep. 30, 2021USD ($)	Jul. 30, 2021USD ($)
Asset Acquisitions [Line Items]
Accrued milestones		$ 0		$ 19,000,000
Shire
Asset Acquisitions [Line Items]
Accrued milestones					$ 2,000,000
Shire Agreement | Livmarli | Satiogen Pharmaceuticals, Inc.
Asset Acquisitions [Line Items]
Accrued milestones				4,000,000
Shire Agreement | Shire
Asset Acquisitions [Line Items]
Number of product candidates | ProductCandidate			2
Upfront payment			$ 7,500,000
Product sales milestone payments, payable			30,000,000
Milestone payments, upon clinical trial treatment				2,000,000
Accrued milestones				15,000,000
Shire Agreement | Shire | Maralixibat
Asset Acquisitions [Line Items]
Milestone payments, payable			109,500,000
Milestone payments, payable upon approval			25,000,000
Milestone payment		$ 10,000,000
Accrued milestones				10,000,000
Shire Agreement | Shire | Volixibat
Asset Acquisitions [Line Items]
Milestone payments, payable upon commercialization			$ 30,000,000
Shire Agreement | Shire | Clinical Trial in ALGS
Asset Acquisitions [Line Items]
Accrued milestones				5,000,000
Shire Agreement | Shire | Livmarli
Asset Acquisitions [Line Items]
Accrued milestones				15,000,000
Shire Agreement | Shire | Redeemable Common Stock
Asset Acquisitions [Line Items]
Temporary equity, shares issued | shares			1,859,151
Issuance of Series A redeemable convertible preferred stock, net of issuance costs			$ 7,000,000
Shares issued, price per share | $ / shares			$ 3.76
Assigned License Agreement | Satiogen Pharmaceuticals, Inc.
Asset Acquisitions [Line Items]
Milestone payments, payable			$ 10,500,000
Milestone payments, payable upon approval			5,000,000
Milestone payments, payable upon initiation			500,000
Milestone payments, payable upon commercialization			5,000,000
Assigned License Agreement | Sanofi-Aventis Deutschland GmbH
Asset Acquisitions [Line Items]
Milestone payments, payable			$ 36,000,000
Royalty obligations payment period			10 years
Milestones accrued				$ 0
Exclusive Licensing Agreement | Maralixibat | CANbridge | Maximum
Asset Acquisitions [Line Items]
Potential regulatory and commercial milestone payment to be received	$ 109,000,000

Collaboration and License Agr_2

Collaboration and License Agreements (Additional Information) (Details) - USD ($)	1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 31, 2021	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2021
Vivet Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License and collaboration agreement date		Apr. 30, 2021
Upfront fee		$ 4,200,000
Research and development expense			$ 6,300,000	$ 16,700,000
Exclusive Licensing Agreement | Maralixibat | CANbridge
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		11,000,000
Transaction price, adjustments			0	0
Research and development funding received			400,000	1,400,000
Exclusive Licensing Agreement | Maralixibat | CANbridge | Prepaid and Other Current Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Other research and development receivable			300,000	300,000
Exclusive Licensing Agreement | Maralixibat | CANbridge | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential regulatory and commercial milestone payment to be received		109,000,000
Research and development funding received		$ 5,000,000
Exclusive Licensing Agreement | Maralixibat | GC Pharma
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 5,000,000
Transaction price, adjustments			0	0
Research and development funding received			100,000	100,000
Exclusive Licensing Agreement | Maralixibat | GC Pharma | Prepaid and Other Current Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Other research and development receivable			$ 100,000	$ 100,000
Exclusive Licensing Agreement | Maralixibat | GC Pharma | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Achievement of future regulatory and commercial milestones payment	$ 23,000,000

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 30, 2019USD ($)ft²	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Jan. 31, 2019USD ($)	Jan. 22, 2019ft²
Lessee Lease Description [Line Items]
Area of office space | ft²								5,600
Term of lease								4 years
Operating lease, option to extend, description				The lease term is approximately four years with an option to extend the term for one five-year term, which at the time was not reasonably assured of exercise and therefore, not included in the lease term.
Existence of option to extend				true
Term of extension of lease							5 years
Operating lease right-of-use assets		$ 1,664		$ 1,664		$ 1,949
Lease liability		$ 2,778		$ 2,778
Tenant improvement allowance							$ 400
Weighted-average incremental borrowing rate		8.00%		8.00%
Weighted-average remaining lease term		3 years 4 months 24 days		3 years 4 months 24 days
Rent expense		$ 200	$ 200	$ 500	$ 500
Amended Operating Lease Agreement
Lessee Lease Description [Line Items]
Term of lease	5 years
Additional area of office space | ft²	5,555
Lease expiration, month and year	2025-03
Amended Operating Lease Agreement | Property, Plant and Equipment
Lessee Lease Description [Line Items]
Tenant improvement allowance	$ 800
Amended Operating Lease Agreement | Restatement Adjustment
Lessee Lease Description [Line Items]
Operating lease right-of-use assets	600
Lease liability	$ 600

Leases - Schedule of Undiscount

Leases - Schedule of Undiscounted Future Minimum Payments under Operating Leases (Detail) $ in Thousands	Sep. 30, 2021USD ($)
Leases [Abstract]
2021 (remaining three months)	$ 219
2022	888
2023	916
2024	925
2025	230
Total undiscounted lease payments	3,178
Less: imputed interest	(400)
Total lease liability	$ 2,778

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	1 Months Ended			3 Months Ended			9 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2020	Jan. 31, 2020	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Dec. 31, 2020	Aug. 31, 2020
Class Of Stock [Line Items]
Maximum amount of sale covered in shelf registration statement									$ 300,000,000
Common stock aggregate offering price, remaining							$ 68,400,000
SVB Leerink LLC
Class Of Stock [Line Items]
Maximum amount of offering issuance and sale covered in sales agreement									$ 75,000,000
Common Stock
Class Of Stock [Line Items]
Number of shares subject to repurchase		256,056					155,864	256,056
Common Stock | Sales Agreement
Class Of Stock [Line Items]
Proceeds from issuance of shares								$ 6,300,000
Gross proceeds from issuance of common stock								$ 6,600,000
Follow-on Public Offering
Class Of Stock [Line Items]
Proceeds from issuance of shares			$ 44,700,000
Follow-on Public Offering | Common Stock
Class Of Stock [Line Items]
Number of shares issued			2,400,000			2,400,000
Shares issued, public offering price per share			$ 20
At The Market Offering | Common Stock
Class Of Stock [Line Items]
Number of shares issued					98,708
At The Market Offering | Common Stock | Sales Agreement
Class Of Stock [Line Items]
Number of shares issued		3,750,000					0	305,969
Shares issued, public offering price per share		$ 20						$ 20
Proceeds from issuance of shares		$ 70,000,000
At The Market Offering | Common Stock | Sales Agreement | Weighted Average
Class Of Stock [Line Items]
Shares issued, public offering price per share		$ 21.55						$ 21.55
Underwritten Public Offerings | Common Stock | Sales Agreement | Maximum
Class Of Stock [Line Items]
Number of shares issued		562,500
Public Offering
Class Of Stock [Line Items]
Proceeds from issuance of shares	$ 7,100,000
Public Offering | Common Stock
Class Of Stock [Line Items]
Number of shares issued				375,654
Shares issued, public offering price per share	$ 20
Number of shares purchased	375,654

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock reserved for issuance	9,429,216	8,343,673
Stock Options
Class Of Stock [Line Items]
Common stock reserved for issuance	7,006,115	5,071,740
Reserved for Future Stock Awards or Option Grants
Class Of Stock [Line Items]
Common stock reserved for issuance	1,238,948	2,009,410
Reserved for Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock reserved for issuance	956,373	700,023
Reserved for PSUs
Class Of Stock [Line Items]
Common stock reserved for issuance	227,780
Reserved for Underwriter Option Shares
Class Of Stock [Line Items]
Common stock reserved for issuance	0	562,500

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	Jun. 30, 2023	Sep. 29, 2021shares	Nov. 05, 2018$ / sharesshares	Mar. 31, 2021dshares	Dec. 31, 2020shares	Jul. 31, 2019shares	Sep. 30, 2021USD ($)shares	Sep. 30, 2020USD ($)shares	Sep. 30, 2021USD ($)shares	Sep. 30, 2020USD ($)shares	Mar. 31, 2020shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock approved and reserved for issuance					8,343,673		9,429,216		9,429,216
Incremental stock-based compensation | $									$ 600
Stock-based compensation expense | $							$ 7,415	$ 3,428	$ 17,523	$ 8,975
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock approved and reserved for issuance					5,071,740		7,006,115		7,006,115
Stock-based compensation, Intrinsic value of options exercised | $									$ 1,200	$ 200
Total unrecognized stock-based compensation related to unvested stock option awards granted | $							$ 47,300		$ 47,300
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period									2 years 8 months 12 days
Expected dividend yield							0.00%	0.00%	0.00%	0.00%
Weighted-average grant-date fair value									13.47%	12.56%
PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock approved and reserved for issuance							227,780		227,780
Granted									7.09
Weighted-average grant-date fair value				50.00%
Total unrecognized stock-based compensation | $							$ 1,400		$ 1,400
PSUs | NDA Approval and Achievement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted		76,027
PSUs | Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value	50.00%
Executive Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted				75,688
Common stock trading days | d				30
Executive Performance Stock Units | NDA Approval and Achievement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value				25.00%
Executive Performance Stock Units | Incremental Performance of Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value				25.00%
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period									1 year 2 months 12 days
Vested							33,396	33,398	100,192	100,194
Total unrecognized compensation expense related to unvested restricted stock | $							$ 500		$ 500
Restricted Stock | Founder
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted | $ / shares			$ 2.936
Restricted Stock | Common Stock | Founder
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued for services			562,500
Vesting period			4 years
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plans, description									Shares subject to outstanding awards under the 2018 Plan as of the effective date of the 2019 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2019 Plan. In addition, the number of shares of common stock available for issuance under the 2019 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2019 Plan, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31st of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of September 30, 2021, 1,225,948 shares of common stock were available for issuance under the 2019 Plan.
Shares of common stock expiration term						10 years
Shares of common stock beginning date						Jan. 1, 2020
Shares of common stock ending date						Jan. 1, 2029
Percentage of annual increase in common stock available for issuance						5.00%
Number of common stock for future issuance							1,225,948		1,225,948
2020 Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock for future issuance							13,000		13,000		750,000
Number of additional common stock for future issuance					750,000
2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plans, description									A total of 500,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to the lesser of (i) 1% of the outstanding number of shares of common stock on December 31st of the preceding calendar year, (ii) 1,500,000 shares of common stock or (iii) such lesser amount as determined by the Company’s board of directors. During the three and nine months ended September 30, 2021, 43,976 shares were issued under the ESPP. As of September 30, 2021, the Company had 956,373 shares available for future issuance under the ESPP.
Shares of common stock expiration term						10 years
Shares of common stock beginning date						Jan. 1, 2020
Shares of common stock ending date						Jan. 1, 2029
Percentage of annual increase in common stock available for issuance						1.00%
Number of common stock approved and reserved for issuance							956,373		956,373
Stock issued for services						500,000	43,976		43,976
Annual increase in common stock available for issuance, shares						1,500,000
2019 Employee Stock Purchase Plan | Executive Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense | $							$ 200		$ 500

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - Stock Options $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Options, Outstanding
Number of shares, Outstanding, Beginning balance | shares	5,071,740
Number of shares, Granted | shares	2,263,200
Number of shares, Exercised | shares	(109,937)
Cancelled/Forfeited | shares	(218,888)
Number of shares, Outstanding, Ending balance | shares	7,006,115	5,071,740
Number of shares, Vested and exercisable | shares	2,685,172
Weighted-average exercise price, Outstanding
Weighted-average exercise price, Outstanding, Beginning balance | $ / shares	$ 9.99
Weighted-average exercise price, Granted | $ / shares	18.17
Weighted-average exercise price, Exercised | $ / shares	6.55
Weighted-average exercise price, Canceled and forfeited | $ / shares	13.28
Weighted-average exercise price, Outstanding, Ending balance | $ / shares	12.59	$ 9.99
Weighted-average exercise price, Vested and exercisable | $ / shares	$ 8.46
Share-based Payment Award, Options, Additional Disclosures
Weighted-average remaining contractual life, Outstanding	8 years 3 months 18 days	8 years 7 months 6 days
Weighted-average remaining contractual life, Vested and exercisable	7 years 7 months 6 days
Aggregate intrinsic value, Outstanding | $	$ 52,040	$ 40,631
Aggregate intrinsic value, Vested and exercisable | $	$ 31,028

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions Used to Estimate Fair Value of Stock Option Awards Granted (Details) - Stock Options - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, minimum	$ 14.05	$ 19.03	$ 14.05	$ 10.40
Exercise price, maximum	$ 18.15	$ 26.59	$ 21.22	$ 26.59
Expected volatility, minimum	82.76%	92.45%	82.76%	77.07%
Expected volatility, maximum	83.88%	94.82%	94.06%	95.85%
Risk-free interest rate, minimum	0.75%	0.33%	0.62%	0.33%
Risk-free interest rate, maximum	0.97%	0.40%	1.12%	1.73%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 6 months	5 years 6 months	5 years 6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of PSU Activity (Details) - Performance Shares [Member] - $ / shares	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested and Outstanding as of December 31, 2020	151,753	0
Granted	229,856
Vested	(76,027)
Cancelled/Forfeited	(2,076)
Unvested and Outstanding as of September 30, 2021	151,753	0
Weighted-average exercise price, Granted	$ 14.78
Weighted-average exercise price, Vested	18.56
Weighted-average exercise price, Canceled and forfeited	18.50
Weighted-average exercise price, Vested and exercisable	$ 12.84

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-based Compensation Reflected in Unaudited Condensed Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 7,415	$ 3,428	$ 17,523	$ 8,975
General and Administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	4,380	2,067	9,731	5,313
Research and Development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 3,035	$ 1,361	$ 7,792	$ 3,662