Mirum Pharmaceuticals (MIRM) 10-Q 6 Nov 19 2019 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 01, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
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		22,989,987
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common stock
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 30,814	$ 51,963
Short-term investments	95,445
Prepaid expenses and other current assets	3,058	12
Total current assets	129,317	51,975
Long-term investments	27,300
Property and equipment, net	631
Operating lease right-of-use assets	941
Other assets	84
Total assets	158,273	51,975
Current liabilities:
Accounts payable	2,665	269
Accrued expenses	7,912	2,180
Operating lease liabilities	332
Total current liabilities	10,909	2,449
Operating lease liabilities, noncurrent	1,124
Other liabilities	37
Total liabilities	12,070	2,449
Commitments and contingencies	0
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 and zero shares authorized as of September 30, 2019 (unaudited) and December 31, 2018, respectively; zero shares issued and outstanding as of September 30, 2019 (unaudited) and December 31, 2018, respectively; and liquidation value of $0 as of September 30, 2019 (unaudited) and December 31, 2018, respectively	0
Common stock, $0.0001 par value; 200,000,000 and 180,000,000 shares authorized as of September 30, 2019 (unaudited) and December 31, 2018, respectively; 22,989,987 shares issued and 22,566,940 outstanding, excluding 423,047 shares subject to repurchase as of September 30, 2019 (unaudited); and 1,187,500 shares issued and 636,719 outstanding, excluding 550,781 shares subject to repurchase as of December 31, 2018	2	1
Additional paid-in capital	198,052	34
Accumulated deficit	(51,949)	(17,348)
Accumulated other comprehensive income	98
Total stockholders’ equity (deficit)	146,203	(17,313)
Total liabilities, redeemable convertible preferred stock, redeemable common stock and stockholders’ equity (deficit)	158,273	51,975
Series A Redeemable Convertible Preferred Stock
Temporary equity:
Redeemable stock	0	59,849
Redeemable Common Stock
Temporary equity:
Redeemable stock	$ 0	$ 6,990

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares, issued	0	0
Preferred stock, shares, outstanding	0	0
Preferred stock, liquidation preference	$ 0	$ 0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	180,000,000
Common stock, shares, issued	22,989,987	1,187,500
Common stock, shares, outstanding	22,566,940	636,719
Common stock, subject to repurchase	423,047	550,781
Series A Redeemable Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	0	120,000,000
Temporary equity, shares issued	0	59,908,284
Temporary equity, shares outstanding	0	59,908,284
Temporary equity, liquidation preference	$ 0	$ 60,064
Redeemable Common Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	1,859,151
Temporary equity, shares outstanding	0	1,859,151

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019
Operating expenses:
Research and development	$ 12,159	$ 3	$ 3	$ 28,611
General and administrative	3,708	62	72	7,474
Total operating expenses	15,867	65	75	36,085
Loss from operations	(15,867)	(65)	(75)	(36,085)
Interest income (expense)	785	(2)	(2)	1,485
Other income (expense), net	(5)			(1)
Net loss	$ (15,087)	$ (67)	$ (77)	$ (34,601)
Net loss per common share, basic and diluted	$ (0.84)	$ (0.14)	$ (0.26)	$ (4.47)
Weighted-average common shares outstanding, basic and diluted	17,996,065	476,905	290,570	7,745,241

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (15,087)	$ (67)	$ (77)	$ (34,601)
Other comprehensive loss:
Unrealized gain on available-for-sale investments	77			123
Cumulative translation adjustments	(7)			(25)
Comprehensive loss	$ (15,017)	$ (67)	$ (77)	$ (34,503)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Series A Redeemable Convertible Preferred Stock	Redeemable Common Stock
Balance at May. 02, 2018	$ 0	$ 0	$ 0	$ 0	$ 0
Balance, Shares at May. 02, 2018		0
Issuance of common stock, Shares		26
Net loss	(10)			(10)
Balance at Jun. 30, 2018	(10)			(10)
Balance, Shares at Jun. 30, 2018		26
Issuance of common stock	1	$ 1
Issuance of common stock, Shares		812,474
Net loss	(67)			(67)
Balance at Sep. 30, 2018	(76)	$ 1		(77)
Balance, Shares at Sep. 30, 2018		812,500
Balance at Dec. 31, 2018						$ 59,849	$ 6,990
Balance, Shares at Dec. 31, 2018						59,908,284	1,859,151
Balance at Dec. 31, 2018	(17,313)	$ 1	34	(17,348)
Balance, Shares at Dec. 31, 2018		636,719
Restricted common stock vested in the period, Shares		35,156
Stock-based compensation	218		218
Net loss	(5,957)			(5,957)
Balance at Mar. 31, 2019						$ 59,849	$ 6,990
Balance, Shares at Mar. 31, 2019						59,908,284	1,859,151
Balance at Mar. 31, 2019	(23,052)	$ 1	252	(23,305)
Balance, Shares at Mar. 31, 2019		671,875
Balance at Dec. 31, 2018						$ 59,849	$ 6,990
Balance, Shares at Dec. 31, 2018						59,908,284	1,859,151
Balance at Dec. 31, 2018	(17,313)	$ 1	34	(17,348)
Balance, Shares at Dec. 31, 2018		636,719
Net loss	(34,601)
Balance at Sep. 30, 2019						$ 0	$ 0
Balance, Shares at Sep. 30, 2019						0	0
Balance at Sep. 30, 2019	146,203	$ 2	198,052	(51,949)	98
Balance, Shares at Sep. 30, 2019		22,566,940
Balance at Mar. 31, 2019						$ 59,849	$ 6,990
Balance, Shares at Mar. 31, 2019						59,908,284	1,859,151
Balance at Mar. 31, 2019	(23,052)	$ 1	252	(23,305)
Balance, Shares at Mar. 31, 2019		671,875
Restricted common stock vested in the period, Shares		33,398
Issuance of Series A convertible preferred stock at $1.00259507 per share, net of issuance costs of $23						$ 59,977
Issuance of Series A convertible preferred stock at $1.00259507 per share, net of issuance costs of $23, Shares						59,844,699
Stock-based compensation	1,641		1,641
Net loss	(13,557)			(13,557)
Other comprehensive income	28				28
Balance at Jun. 30, 2019						$ 119,826	$ 6,990
Balance, Shares at Jun. 30, 2019						119,752,983	1,859,151
Balance at Jun. 30, 2019	(34,940)	$ 1	1,893	(36,862)	28
Balance, Shares at Jun. 30, 2019		705,273
Restricted common stock vested in the period, Shares		33,398
Stock-based compensation	2,144		2,144
Conversion of Series A redeemable convertible preferred stock into common stock on initial public offering	119,826	$ 1	119,825			$ (119,826)
Conversion of Series A redeemable convertible preferred stock into common stock on initial public offering, Shares		14,969,118				(119,752,983)
Reclassification of redeemable common stock into common stock on initial public offering	6,990		6,990				$ (6,990)
Reclassification of redeemable common stock into common stock on initial public offering, Shares		1,859,151					(1,859,151)
Issuance of common stock	67,200		67,200
Issuance of common stock, Shares		5,000,000
Net loss	(15,087)			(15,087)
Other comprehensive income	70				70
Balance at Sep. 30, 2019						$ 0	$ 0
Balance, Shares at Sep. 30, 2019						0	0
Balance at Sep. 30, 2019	$ 146,203	$ 2	$ 198,052	$ (51,949)	$ 98
Balance, Shares at Sep. 30, 2019		22,566,940

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019
Common Stock
Issuance of common stock upon initial public offering, issuance costs	$ 7,800
Series A Redeemable Convertible Preferred Stock
Issuance of stock at per share		$ 1.00259507
Issuance of Series A redeemable convertible preferred stock, issuance costs		$ 23

Condensed Consolidated Statem_5

Condensed Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	5 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019
Operating activities
Net loss	$ (77)	$ (34,601)
Reconciliation of net loss to net cash used in operating activities:
Stock-based compensation		4,003
Depreciation and amortization		80
Amortization of operating lease right-of-use assets		124
Amortization of premium on investments		(199)
Interest expense on convertible note	2
Change in operating assets and liabilities:
Prepaid and other current assets		(3,046)
Operating lease right-of-use assets		(33)
Other assets		(84)
Accounts payable, accrued expenses and other liabilities	56	8,079
Operating lease liabilities		13
Net cash used in operating activities	(19)	(25,664)
Investing activities
Purchase of investments		(130,923)
Proceeds from maturities of investments		8,500
Purchase of property and equipment		(214)
Net cash used in investing activities		(122,637)
Financing activities
Proceeds from convertible note, related party	50
Proceeds from the sale of common stock	1
Proceeds from the issuance of Series A redeemable convertible preferred stock, net		59,977
Proceeds from issuance of common stock in initial public offering, net of issuance costs		67,200
Net cash provided by financing activities	51	127,177
Effect of exchange rate on cash and cash equivalents		(25)
Net (decrease) increase in cash and cash equivalents	32	(21,149)
Cash and cash equivalents at beginning of period		51,963
Cash and cash equivalents at end of period	32	30,814
Supplemental disclosure of cash flow information:
Operating lease right-of-use asset obtained in exchange for operating lease liability		1,444
Landlord paid tenant improvements		455
Deferred Series A redeemable convertible preferred stock issuance costs in accounts payable	$ 55
Conversion of Series A redeemable convertible preferred stock into common stock on initial public offering		119,826
Reclassification of redeemable common stock into common stock on initial public offering		6,990
Property and equipment purchases included in accounts payable		$ 43

Description of Business

Description of Business	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. 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 development and commercialization of a late-stage pipeline of novel therapies for debilitating liver diseases. The Company’s 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 subsidiary, Mirum Pharmaceuticals AG. All intercompany balances and transactions among the consolidated entities have been eliminated in consolidation. Reverse Stock Split On July 3, 2019, the Company effected a 1-for-8 reverse stock split of its common stock. The par value and the authorized number of shares of the common stock were not adjusted as a result of the reverse stock split. The reverse stock split resulted in an adjustment to the conversion price of the Company’s Series A redeemable convertible preferred stock (the “Series A Preferred Stock”) to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying unaudited condensed consolidated financial statements and notes to the unaudited condensed consolidated financial statements give retroactive effect to the reverse stock split for all periods presented. Initial Public Offering On July 22, 2019, the Company completed its initial public offering (“IPO”) of its common stock. In connection with its IPO, the Company issued and sold 5,000,000 shares of its common stock at a price of $15.00 per share. As a result of the IPO, the Company received $67.2 million in net proceeds, after deducting underwriting discounts, commissions and offering expenses. At the closing of the IPO, 119,752,983 shares of outstanding Series A Preferred Stock were automatically converted into 14,969,118 shares of common stock , and 1,859,151 shares of redeemable common stock were reclassified into permanent equity due to the expiration of the deemed redemption feature 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, 2019, the Company had an accumulated deficit of $51.9 million and cash, cash equivalents and investments of $153.6 million, which is available to fund future operations. The Company believes that its cash, cash equivalents and investments as of September 30, 2019 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. As such, 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [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 unaudited condensed consolidated balance sheet as of December 31, 2018 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 for the period from May 2, 2018 to December 31, 2018 included in the Company’s prospectus dated July 17, 2019 that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-232251), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended, on July 18, 2019. Use of Estimates The preparation of condensed 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 accrued research and development expenses, the valuation of common stock, equity awards 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. Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. The carrying amounts reported in the unaudited condensed consolidated balance sheet for cash and cash equivalents are valued at cost, which approximate their fair value. Cash equivalents may consist of money market accounts, money market funds, U.S. treasury bills and repurchase agreements. The Company invests in certain reverse repurchase agreements, which are collateralized by deposits in the form of U.S. Treasury Securities for an amount no less than 102% of their value. The Company does not record an asset or liability for the collateral as the Company does not intend to sell or re-pledge the collateral. The collateral has the prevailing credit rating of at least the U.S. Government Treasuries and Agencies. The Company utilizes a third-party custodian to manage the exchange of funds and ensure that collateral received is maintained at 102% of the value of the reverse repurchase agreements on a daily basis. Investments The Company classifies all investments as available-for-sale, as the sale of such securities may be required prior to maturity . Management determines the appropriate classification of its investments in debt securities at the time of purchase. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from the balance sheet date, are classified as a current asset. Investments are carried at fair value, with unrealized gains and losses reported as accumulated other comprehensive income (loss) until realized . The Company periodically evaluates whether declines in fair values of its available-for-sale securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the available-for-sale security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any available-for-sale securities before recovery of its amortized cost basis. Fair Value of Financial Instruments The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1: Observable inputs such as quoted prices in active markets; Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. The carrying amounts of all cash equivalents, investments, accounts payable and accrued liabilities are reasonable estimates of their fair value. There were no transfers between Levels 1, 2 or 3 for the periods presented. Property and Equipment Property and equipment are recorded at cost less accumulated depreciation, ranging from three to five years. Leasehold improvements are amortized over the shorter of their useful lives or the related lease term. As of September 30, 2019, and December 31, 2018, property and equipment consisted primarily of leasehold improvements of $0.5 million and zero, respectively. Accumulated depreciation as of September 30, 2019 and December 31, 2018 was $0.1 million and zero, respectively. Accrued Research and Development Expenses The Company accrues and expenses clinical trial activities performed by third parties based upon estimates of the proportion of work completed over the life of the individual study and patient enrollment rates in accordance with agreements established with clinical research organizations and clinical trial sites. The Company determines the estimates by reviewing contracts, vendor agreements and purchase orders and through discussions with internal clinical personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services. The Company makes estimates of accrued expenses as of each balance sheet date based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception. Nonrefundable advance payments for goods and services are deferred and recognized as expense in the period that the related goods are consumed or services are performed. Research and Development Expenses Research and development expenses consist primarily of fees paid to contract research organizations and other vendors for clinical, non-clinical and manufacturing services, salaries and benefits, including stock-based compensation expense, consultant expenses, costs related to acquiring manufacturing materials, costs related to compliance with regulatory requirements and license payments related to acquiring intellectual property rights for the Company’s product candidates. Research and development expenses are expensed as incurred. Leases In accordance with Accounting Standards Update (“ASU”) No. 2016-02, as adopted on January 1, 2019, the Company determines if an arrangement is a lease at inception. Operating lease right-of-use (“ROU”) assets represent the Company’s right to use an underlying asset during the lease term, and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating leases are included in ROU assets, current operating lease liabilities, and long-term operating lease liabilities on our unaudited condensed consolidated balance sheet. Operating lease ROU assets and lease liabilities are initially recognized based on the present value of the future minimum lease payments over the lease term at commencement date calculated using the Company’s incremental borrowing rate applicable to the lease asset, unless the implicit rate is readily determinable. Operating lease ROU assets also include any lease payments made at or before lease commencement and exclude any lease incentives received. The Company determines the lease term as the noncancelable period of the lease and may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Leases with a term of 12 months or less are not recognized on the unaudited condensed consolidated balance sheet. The Company’s leases do not contain any residual value guarantees. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Net Loss Per Share Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted-average number of 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 number of 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 and common stock options because their effect would be anti-dilutive due to the Company’s net loss. Since the Company had a net loss in each of the periods presented, basic and diluted net loss per common share were the same. The following outstanding potentially dilutive shares 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, 2019 2018 Options to purchase common stock 3,206,812 — Common stock subject to repurchase 423,047 — Total 3,629,859 — Recently Adopted Accounting Pronouncements On January 1, 2019, the Company adopted ASU No. 2016-02 , Leases (Topic 842) Other recent accounting pronouncements issued by the Financial Accounting Standards Board (“FASB”) (including its Emerging Issues Task Force), the American Institute of Certified Public Accountants, and the SEC did not, or are not believed by management to, have a material impact on the Company’s present or future consolidated financial position, results of operations or cash flows. Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, Financial Instruments – Credit Losses (Topic 326), In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The guidance eliminates the requirements to disclose the amount and reasons for transfers between Level 1 and Level 2 assets, the policy for timing and transfers between levels and the valuation process for Level 3 fair value measurements. The guidance modifies disclosure requirements for investments in certain entities that calculate net asset value and clarifies the purpose of the measurement uncertainty disclosure. The guidance adds requirements to disclose changes in unrealized gains or losses included in other comprehensive income for recurring Level 3 fair value measurements and to disclose the range and weighted average used to develop significant unobservable inputs for Level 3 fair value measurements. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company is currently in the process of evaluating the impact the standard will have on its unaudited condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Money market funds and U.S. treasury bills are measured at fair value on a recurring basis using quoted prices and are classified as Level 1. I nvestments measured at fair value based on inputs other than quoted prices that are derived from observable market data are classified as Level 2. 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, 2019 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 28,716 $ — $ — $ 28,716 U.S. treasury bills 13,973 — — 13,973 Corporate debt securities — 36,570 — 36,570 Commercial paper — 24,777 — 24,777 U.S. government bonds — 22,482 — 22,482 Asset backed securities — 24,943 — 24,943 Total $ 42,689 $ 108,772 $ — $ 151,461 There were no financial assets and liabilities subject to fair value measurements on a recurring basis as of December 31, 2018.

Financial Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2019
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, 2019 Amortized Cost Unrealized Gain Unrealized Loss Estimated Fair Value Cash equivalents and investments: Money market fund $ 28,716 $ — $ — $ 28,716 U.S. treasury bills 13,966 7 — 13,973 Corporate debt securities 36,471 99 — 36,570 Commercial paper 24,777 — — 24,777 U.S. government bonds 22,482 1 (1 ) 22,482 Asset back securities 24,926 20 (3 ) 24,943 Total cash equivalents and investments $ 151,338 $ 127 $ (4 ) $ 151,461 Classified as: Cash equivalents $ 28,716 Short-term investments 95,445 Long-term investments 27,300 Total cash equivalents and investments $ 151,461 The Company did not hold investments as of December 31, 2018. As of September 30, 2019, the remaining contractual maturities of available-for-sale debt securities were as follows (in thousands): Estimated Fair Value Due within one year $ 95,445 One to two years 23,289 Three years or more 4,011 Total $ 122,745 During the nine months ended September 30, 2019, there have been no significant realized gains or losses on available-for-sale investments. During the nine months ended September 30, 2019, the Company did not recognize any other-than-temporary impairment losses

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consists of the following (in thousands): September 30, December 31, 2019 2018 Accrued costs for Shire $ — $ 1,310 Accrued clinical trials 3,484 785 Accrued professional service fees 1,099 70 Accrued contract manufacturing and non-clinical costs 2,035 — Accrued compensation and related benefits 1,294 15 Total accrued expenses $ 7,912 $ 2,180

Asset Acquisitions

Asset Acquisitions	9 Months Ended
	Sep. 30, 2019
Asset Acquisitions [Abstract]
Asset Acquisitions	6. Asset Acquisitions Assignment and License Agreement with Shire International GmbH On November 5, 2018, the Company entered into an Assignment and License Agreement (the “Shire Agreement”) with Shire International GmbH (“Shire”). 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. In consideration for the rights granted to the Company under the Shire Agreement, the Company made an upfront payment to Shire on November 5, 2018 of $7.5 million and issued Shire 1,859,151 shares of its redeemable common stock with an estimated fair value of $7.0 million, or $3.76 per share. The fair value of the shares was determined using an option pricing model with key assumptions as of the date of issuance including the probabilities of liquidity scenarios, enterprise value, time to liquidity, risk-free interest rates, volatility and discount for lack of marketability. The Company accounted for the transaction as an asset acquisition as substantially all of the fair value of the gross assets acquired were concentrated in a group of similar identifiable assets thus satisfying the requirements of the screen test in ASU 2017-01. The assets acquired in the transaction were measured based on the upfront payment to Shire and the fair value of the common stock shares issued to Shire, as the fair value of the consideration given was more readily determinable than the fair value of the assets received. Because the assets had not yet received regulatory approval and have no alternative future use, the fair value attributable to these assets were initially recorded as in process research and development expenses. The Company is also 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. In July 2019, the Company achieved a development milestone related to the initiation of the Phase 3 MARCH-PFIC clinical trial and made a $2.5 million payment to Shire. As of September 30, 2019, no additional milestones had been accrued as there were no other potential milestones yet considered probable. Concurrent with the Shire Agreement, the Company also entered into a Transition Services Agreement (“TSA”) with Shire, which covers services provided by Shire to transfer the research and development activities and the related know-how from Shire to the Company, including continuation of work on any existing clinical trials and manufacturing activities until fully transferred. As of June 30, 2019, all transition services were completed as well as settlement of any pass-through costs. The Company recorded $0.4 million for services provided by Shire under the TSA for the nine months ended September 30, 2019. Additionally, the Company recorded a reduction of estimated expenses of $0.1 million for pass-through costs related to continuation of work on existing clinical trials and manufacturing activities for the nine months ended September 30, 2019. The reduction of estimated expenses for the nine months ended September 30, 2019 was related to a final reconciliation of expenses and agreement on final amounts due to Shire. There were no expenses for Shire provided services or pass-through costs recorded for the three months ended September 30, 2019, September 30, 2018 and for the period from May 2, 2018 to September 30, 2018. As of September 30, 2019, there was no accrued expenses for these services. As of December 31, 2018, $2.1 million was recorded in accrued expenses on the consolidated balance sheets for these services. 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. In July 2019, the Company achieved a development milestone related to the initiation of the Phase 3 MARCH-PFIC clinical trial and made a $0.5 million payment to Satiogen. As of September 30, 2019, no additional milestones had been accrued as there were no other potential milestones yet considered probable. 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. As of September 30, 2019, no milestones had been accrued as there were no potential milestones yet considered probable.

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Stockholders’ Equity (Deficit)	7. Stockholders’ Equity (Deficit) In connection with the Company’s IPO, all of the outstanding shares of the Company’s Series A Preferred Stock automatically converted into 14,969,118 shares of common stock and the 1,859,151 shares of the Company’s redeemable common stock classified in mezzanine equity were reclassified to permanent equity due to the expiration of the deemed redemption feature associated with the stock. Common Stock In August and October 2018, the Company issued 1,187,500 shares of common stock as founder shares for services rendered to the Company, valued at $0.0001 per share for consideration of approximately $950. On November 5, 2018, in connection with the issuance of the Series A Preferred Stock, vesting conditions were placed on 562,500 previously issued founder shares. These shares vest over 4 years and are subject to repurchase by the Company in the event of termination of services. Shares subject to repurchase are not deemed, for accounting purposes, to be outstanding until those shares vest. In April 2019, the Company repurchased 25,782 shares of the common stock from a former employee in connection with termination of employment. As of September 30, 2019 and December 31, 2018, 423,047 and 550,781 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. Each share of common stock is entitled to one voting right. Common stockholders are entitled to dividends when funds are legally available and declared by the Board. Common Stock Reserved for Issuance Common stock reserved for issuance is as follows: As of September 30, As of December 31, 2019 2018 Conversion of Series A Preferred Stock — 7,488,530 Stock options issued and outstanding 3,206,812 — Reserved for future stock awards or option grants 1,272,443 1,859,151 Reserved for employee stock purchase plan 500,000 — 4,979,255 9,347,681

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Equity Incentive Plans On November 5, 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 and other stock or cash-based awards to individuals who are then employees, officers, directors or consultants of the Company. A total of 1,401,443 shares of common stock were approved to be initially reserved for issuance under the 2019 Plan, including 101,443 shares that remained available for issuance under the 2018 Plan as of July 17, 2019. 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, 2019, 1,272,443 shares of common stock were available for issuance under the 2019 Plan. Stock Options 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. Prior to its IPO, the Company was a private company and lacked company-specific historical and implied volatility information. Therefore, it estimated its expected volatility based on the historical volatility of a publicly traded set of peer companies. 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. No stock options were granted for the period from May 2, 2018 to September 30, 2018. 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, 2019 2019 Exercise price $9.86-$15.00 $2.94-$15.00 Expected term (in years) 6.0-6.3 6.0-6.3 Expected volatility 75.31%-75.98% 73.88%-75.98% Risk-free interest rate 1.46%-1.89% 1.46%-2.46% Expected dividend yield - - Grant date fair value of options granted $6.62-$10.01 $6.62-$10.46 The following table summarizes stock option activity during the nine months ended September 30, 2019 (in thousands, except share and per share data): Number of Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2018 — $ — — $ — Granted 3,206,812 $ 4.98 9.6 Exercised — $ — — Canceled and forfeited — $ — — Outstanding as of September 30, 2019 3,206,812 $ 4.98 9.6 $ 16,292 Vested and exercisable as of September 30, 2019 1,465 $ 2.94 9.5 $ 10 As of September 30, 2019, the total unrecognized stock-based compensation related to unvested stock option awards granted was $23.2 million, which the Company expects to recognize over a weighted-average period of approximately 3.4 years. Restricted Stock On November 5, 2018, in connection with the issuance of the 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 total compensation cost resulting from the modification was $1.7 million. The modified shares have a four-year vesting period and a measurement date fair value of $2.936 per share. For the three and nine months ended September 30, 2019, 33,398 and 101,952 shares vested, respectively. As of September 30, 2019, the total unrecognized compensation expense related to unvested restricted stock was $1.2 million expected to be recognized over a weighted-average period of approximately 3.2 years. Stock-based compensation expense is reflected in the unaudited condensed consolidated statements of operations (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2019 General and administrative $ 1,314 $ 2,464 Research and development 830 1,539 Total $ 2,144 $ 4,003 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 (the “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 lessor 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. The Company had 500,000 shares available for future issuance under the 2019 ESPP as of September 30, 2019.

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases	9. Leases In January 2019, the Company entered into an operating lease agreement for office space which consists of approximately 5,600 square feet. The lease term is approximately four years with an option to extend the term for one five-year term, which is not reasonably assured of exercise and therefore, not included in the lease term. The lease additionally contains a tenant improvement allowance of $0.4 million, which has been recorded in the accompanying unaudited condensed consolidated balance sheet as of September 30, 2019 as leasehold improvements with a corresponding reduction of the ROU asset at inception of the lease. In connection with this lease, the Company recognized an operating lease ROU asset of $0.9 million as of September 30, 2019 and an aggregate lease liability of $1.5 million in the accompanying balance sheet. The remaining lease term is 3.7 years, and the estimated incremental borrowing rate is 8.0%. Rent payments began in August 2019. Year Ending December 31, Undiscounted Rent Payments 2019 (remaining three months) $ 108 2020 436 2021 448 2022 461 2023 215 2024 15 Total undiscounted lease payments 1,683 Less: imputed interest (227 ) Total lease liability $ 1,456 Rent expense was $0.1 million and $0.2 million for the three and nine months ended September 30, 2019, respectively. Variable lease payments for operating expenses were immaterial for the three and nine months ended September 30, 2019. There was a total commitment of $0.4 million at September 30, 2019 related to the short-term portion of the lease.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company did not record tax expense for the periods of the three months ended September 30, 2019 and 2018, the nine months ended September 30, 2019, and for the period from May 2, 2018 to September 30, 2018, due to the Company’s loss position and full valuation allowance.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [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 unaudited condensed consolidated balance sheet as of December 31, 2018 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 for the period from May 2, 2018 to December 31, 2018 included in the Company’s prospectus dated July 17, 2019 that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-232251), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended, on July 18, 2019.
Use of Estimates	Use of Estimates The preparation of condensed 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 accrued research and development expenses, the valuation of common stock, equity awards 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.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. The carrying amounts reported in the unaudited condensed consolidated balance sheet for cash and cash equivalents are valued at cost, which approximate their fair value. Cash equivalents may consist of money market accounts, money market funds, U.S. treasury bills and repurchase agreements. The Company invests in certain reverse repurchase agreements, which are collateralized by deposits in the form of U.S. Treasury Securities for an amount no less than 102% of their value. The Company does not record an asset or liability for the collateral as the Company does not intend to sell or re-pledge the collateral. The collateral has the prevailing credit rating of at least the U.S. Government Treasuries and Agencies. The Company utilizes a third-party custodian to manage the exchange of funds and ensure that collateral received is maintained at 102% of the value of the reverse repurchase agreements on a daily basis.
Investments	Investments The Company classifies all investments as available-for-sale, as the sale of such securities may be required prior to maturity . Management determines the appropriate classification of its investments in debt securities at the time of purchase. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from the balance sheet date, are classified as a current asset. Investments are carried at fair value, with unrealized gains and losses reported as accumulated other comprehensive income (loss) until realized . The Company periodically evaluates whether declines in fair values of its available-for-sale securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the available-for-sale security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any available-for-sale securities before recovery of its amortized cost basis.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1: Observable inputs such as quoted prices in active markets; Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. The carrying amounts of all cash equivalents, investments, accounts payable and accrued liabilities are reasonable estimates of their fair value. There were no transfers between Levels 1, 2 or 3 for the periods presented.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost less accumulated depreciation, ranging from three to five years. Leasehold improvements are amortized over the shorter of their useful lives or the related lease term. As of September 30, 2019, and December 31, 2018, property and equipment consisted primarily of leasehold improvements of $0.5 million and zero, respectively. Accumulated depreciation as of September 30, 2019 and December 31, 2018 was $0.1 million and zero, respectively.
Accrued Research and Development Expenses	Accrued Research and Development Expenses The Company accrues and expenses clinical trial activities performed by third parties based upon estimates of the proportion of work completed over the life of the individual study and patient enrollment rates in accordance with agreements established with clinical research organizations and clinical trial sites. The Company determines the estimates by reviewing contracts, vendor agreements and purchase orders and through discussions with internal clinical personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services. The Company makes estimates of accrued expenses as of each balance sheet date based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception. Nonrefundable advance payments for goods and services are deferred and recognized as expense in the period that the related goods are consumed or services are performed.
Research and Development Expenses	Research and Development Expenses Research and development expenses consist primarily of fees paid to contract research organizations and other vendors for clinical, non-clinical and manufacturing services, salaries and benefits, including stock-based compensation expense, consultant expenses, costs related to acquiring manufacturing materials, costs related to compliance with regulatory requirements and license payments related to acquiring intellectual property rights for the Company’s product candidates. Research and development expenses are expensed as incurred.
Leases	Leases In accordance with Accounting Standards Update (“ASU”) No. 2016-02, as adopted on January 1, 2019, the Company determines if an arrangement is a lease at inception. Operating lease right-of-use (“ROU”) assets represent the Company’s right to use an underlying asset during the lease term, and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating leases are included in ROU assets, current operating lease liabilities, and long-term operating lease liabilities on our unaudited condensed consolidated balance sheet. Operating lease ROU assets and lease liabilities are initially recognized based on the present value of the future minimum lease payments over the lease term at commencement date calculated using the Company’s incremental borrowing rate applicable to the lease asset, unless the implicit rate is readily determinable. Operating lease ROU assets also include any lease payments made at or before lease commencement and exclude any lease incentives received. The Company determines the lease term as the noncancelable period of the lease and may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Leases with a term of 12 months or less are not recognized on the unaudited condensed consolidated balance sheet. The Company’s leases do not contain any residual value guarantees. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term.
Net Loss Per Share	Net Loss Per Share Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted-average number of 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 number of 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 and common stock options because their effect would be anti-dilutive due to the Company’s net loss. Since the Company had a net loss in each of the periods presented, basic and diluted net loss per common share were the same. The following outstanding potentially dilutive shares 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, 2019 2018 Options to purchase common stock 3,206,812 — Common stock subject to repurchase 423,047 — Total 3,629,859 —
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements On January 1, 2019, the Company adopted ASU No. 2016-02 , Leases (Topic 842) Other recent accounting pronouncements issued by the Financial Accounting Standards Board (“FASB”) (including its Emerging Issues Task Force), the American Institute of Certified Public Accountants, and the SEC did not, or are not believed by management to, have a material impact on the Company’s present or future consolidated financial position, results of operations or cash flows.
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) Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, Financial Instruments – Credit Losses (Topic 326), In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The guidance eliminates the requirements to disclose the amount and reasons for transfers between Level 1 and Level 2 assets, the policy for timing and transfers between levels and the valuation process for Level 3 fair value measurements. The guidance modifies disclosure requirements for investments in certain entities that calculate net asset value and clarifies the purpose of the measurement uncertainty disclosure. The guidance adds requirements to disclose changes in unrealized gains or losses included in other comprehensive income for recurring Level 3 fair value measurements and to disclose the range and weighted average used to develop significant unobservable inputs for Level 3 fair value measurements. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company is currently in the process of evaluating the impact the standard will have on its unaudited condensed consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares 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, 2019 2018 Options to purchase common stock 3,206,812 — Common stock subject to repurchase 423,047 — Total 3,629,859 —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 Level 1 Level 2 Level 3 Total Financial assets: Money market fund $ 28,716 $ — $ — $ 28,716 U.S. treasury bills 13,973 — — 13,973 Corporate debt securities — 36,570 — 36,570 Commercial paper — 24,777 — 24,777 U.S. government bonds — 22,482 — 22,482 Asset backed securities — 24,943 — 24,943 Total $ 42,689 $ 108,772 $ — $ 151,461

Financial Instruments (Tables)

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 Amortized Cost Unrealized Gain Unrealized Loss Estimated Fair Value Cash equivalents and investments: Money market fund $ 28,716 $ — $ — $ 28,716 U.S. treasury bills 13,966 7 — 13,973 Corporate debt securities 36,471 99 — 36,570 Commercial paper 24,777 — — 24,777 U.S. government bonds 22,482 1 (1 ) 22,482 Asset back securities 24,926 20 (3 ) 24,943 Total cash equivalents and investments $ 151,338 $ 127 $ (4 ) $ 151,461 Classified as: Cash equivalents $ 28,716 Short-term investments 95,445 Long-term investments 27,300 Total cash equivalents and investments $ 151,461
Summary of Remaining Contractual Maturities of Available-for-sale Debt Securities	As of September 30, 2019, the remaining contractual maturities of available-for-sale debt securities were as follows (in thousands): Estimated Fair Value Due within one year $ 95,445 One to two years 23,289 Three years or more 4,011 Total $ 122,745

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consists of the following (in thousands): September 30, December 31, 2019 2018 Accrued costs for Shire $ — $ 1,310 Accrued clinical trials 3,484 785 Accrued professional service fees 1,099 70 Accrued contract manufacturing and non-clinical costs 2,035 — Accrued compensation and related benefits 1,294 15 Total accrued expenses $ 7,912 $ 2,180

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit) (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 2018 Conversion of Series A Preferred Stock — 7,488,530 Stock options issued and outstanding 3,206,812 — Reserved for future stock awards or option grants 1,272,443 1,859,151 Reserved for employee stock purchase plan 500,000 — 4,979,255 9,347,681

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
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, 2019 2019 Exercise price $9.86-$15.00 $2.94-$15.00 Expected term (in years) 6.0-6.3 6.0-6.3 Expected volatility 75.31%-75.98% 73.88%-75.98% Risk-free interest rate 1.46%-1.89% 1.46%-2.46% Expected dividend yield - - Grant date fair value of options granted $6.62-$10.01 $6.62-$10.46
Summary of Stock Option Activity	The following table summarizes stock option activity during the nine months ended September 30, 2019 (in thousands, except share and per share data): Number of Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2018 — $ — — $ — Granted 3,206,812 $ 4.98 9.6 Exercised — $ — — Canceled and forfeited — $ — — Outstanding as of September 30, 2019 3,206,812 $ 4.98 9.6 $ 16,292 Vested and exercisable as of September 30, 2019 1,465 $ 2.94 9.5 $ 10
Summary of Stock-based Compensation Expense Reflected in Unaudited Condensed Consolidated Statements of Operations and Comprehensive Loss	Stock-based compensation expense is reflected in the unaudited condensed consolidated statements of operations (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2019 General and administrative $ 1,314 $ 2,464 Research and development 830 1,539 Total $ 2,144 $ 4,003

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Schedule of Lease Liability	Year Ending December 31, Undiscounted Rent Payments 2019 (remaining three months) $ 108 2020 436 2021 448 2022 461 2023 215 2024 15 Total undiscounted lease payments 1,683 Less: imputed interest (227 ) Total lease liability $ 1,456

Description of Business - Addit

Description of Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Jul. 22, 2019USD ($)$ / sharesshares	Jul. 03, 2019	Jun. 30, 2018$ / sharesshares	Sep. 30, 2019USD ($)shares	Sep. 30, 2018$ / sharesshares	Sep. 30, 2018USD ($)$ / shares	Sep. 30, 2019USD ($)shares	Jul. 23, 2019shares	Jun. 30, 2019shares	Mar. 31, 2019shares	Dec. 31, 2018USD ($)shares
Organization And Basis Of Presentation [Line Items]
Date of incorporation							May 2, 2018
Reverse stock split ratio		0.125
Proceeds from issuance of shares | $						$ 1
Accumulated deficit | $				$ (51,949)			$ (51,949)				$ (17,348)
Cash, cash equivalents and investments | $				$ 153,600			$ 153,600
Common Stock
Organization And Basis Of Presentation [Line Items]
Number of shares issued			26	5,000,000	812,474
Shares issued, price per share | $ / shares			$ 0.0001		$ 0.0001	$ 0.0001
Redeemable common stock reclassified to equity	1,859,151
Series A Redeemable Convertible Preferred Stock
Organization And Basis Of Presentation [Line Items]
Redeemable stock, outstanding	119,752,983			0			0	0	119,752,983	59,908,284	59,908,284
Redeemable Common Stock
Organization And Basis Of Presentation [Line Items]
Redeemable stock, outstanding				0			0	0	1,859,151	1,859,151	1,859,151
IPO
Organization And Basis Of Presentation [Line Items]
Proceeds from issuance of shares | $	$ 67,200
IPO | Common Stock
Organization And Basis Of Presentation [Line Items]
Number of shares issued	5,000,000
Shares issued, price per share | $ / shares	$ 15
Number of redeemable convertible preferred stock converted into common stock	14,969,118

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Investments with original maturities at date of purchase to be cash equivalents	3 months
Asset transfers Level 1 to Level 2	$ 0
Asset transfers Level 2 to Level 1	0
Asset transfers into Level 3	0
Asset transfers out of Level 3	0
Liabilities transfers Level 1 to Level 2	0
Liabilities transfers Level 2 to Level 1	0
Liability transfers into Level 3	0
Liability transfers out of Level 3	0
Property and equipment, net	631,000
Accumulated depreciation	100,000	$ 0
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net	$ 500,000	$ 0
Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of collateralized deposits in treasury securities	102.00%
Property and equipment, useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, useful life	5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details)	9 Months Ended
	Sep. 30, 2019shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	3,629,859
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	3,206,812
Common Stock Subject to Repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares of stock	423,047

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 ($)	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Fair value measurements	$ 151,461,000	$ 0
Money Market Fund
Financial assets:
Fair value measurements	28,716,000
U.S. Treasury Bills
Financial assets:
Fair value measurements	13,973,000
Corporate Debt Securities
Financial assets:
Fair value measurements	36,570,000
Commercial Paper
Financial assets:
Fair value measurements	24,777,000
U.S. Government Bonds
Financial assets:
Fair value measurements	22,482,000
Asset Backed Securities
Financial assets:
Fair value measurements	24,943,000
Level 1
Financial assets:
Fair value measurements	42,689,000
Level 1 | Money Market Fund
Financial assets:
Fair value measurements	28,716,000
Level 1 | U.S. Treasury Bills
Financial assets:
Fair value measurements	13,973,000
Level 2
Financial assets:
Fair value measurements	108,772,000
Level 2 | Corporate Debt Securities
Financial assets:
Fair value measurements	36,570,000
Level 2 | Commercial Paper
Financial assets:
Fair value measurements	24,777,000
Level 2 | U.S. Government Bonds
Financial assets:
Fair value measurements	22,482,000
Level 2 | Asset Backed Securities
Financial assets:
Fair value measurements	$ 24,943,000

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - Fair Value, Recurring Basis - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial assets	$ 151,461,000	$ 0
Financial liabilities		$ 0

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) $ in Thousands	Sep. 30, 2019USD ($)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents and investments, Amortized cost	$ 151,338
Cash equivalents and investments, Unrealized gain	127
Cash equivalents and investments, Unrealized loss	(4)
Cash equivalents and investments, Estimated Fair Value	151,461
Cash equivalents, Estimated Fair Value	28,716
Investments, Estimated Fair Value	122,745
Short-term investments	95,445
Long-term investments	27,300
Money Market Fund
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash equivalents, Amortized Cost	28,716
Cash equivalents, Estimated Fair Value	28,716
U.S. Treasury Bills
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	13,966
Investments, Unrealized Gain	7
Investments, Estimated Fair Value	13,973
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	36,471
Investments, Unrealized Gain	99
Investments, Estimated Fair Value	36,570
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	24,777
Investments, Estimated Fair Value	24,777
U.S. Government Bonds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	22,482
Investments, Unrealized Gain	1
Investments, Unrealized Loss	(1)
Investments, Estimated Fair Value	22,482
Asset Backed Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments, Amortized Cost	24,926
Investments, Unrealized Gain	20
Investments, Unrealized Loss	(3)
Investments, Estimated Fair Value	$ 24,943

Financial Instruments - Additio

Financial Instruments - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Financial Instruments Owned At Fair Value [Abstract]
Investments		$ 0
Realized gains or losses on available-for-sale investments	$ 0
Other-than-temporary impairment losses on available-for-sale investments	$ 0

Financial Instruments - Summa_2

Financial Instruments - Summary of Remaining Contractual Maturities of Available-for-sale Debt Securities (Details) $ in Thousands	Sep. 30, 2019USD ($)
Fair Value Disclosures [Abstract]
Due within one year	$ 95,445
One to two years	23,289
Three years or more	4,011
Total	$ 122,745

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued costs for Shire		$ 1,310
Accrued clinical trials	$ 3,484	785
Accrued professional service fees	1,099	70
Accrued contract manufacturing and non-clinical costs	2,035
Accrued compensation and related benefits	1,294	15
Total accrued expenses	$ 7,912	$ 2,180

Asset Acquisitions - Additional

Asset Acquisitions - Additional Information (Details)	Nov. 05, 2018USD ($)ProductCandidate$ / sharesshares	Jul. 31, 2019USD ($)	Sep. 30, 2019USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)shares	Dec. 31, 2018USD ($)shares
Asset Acquisitions [Line Items]
Research and development			$ 12,159,000	$ 3,000	$ 3,000	$ 28,611,000
Accrued expenses			$ 7,912,000			$ 7,912,000	$ 2,180,000
Redeemable Common Stock
Asset Acquisitions [Line Items]
Temporary equity, shares issued | shares			0			0	1,859,151
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 payment		$ 2,500,000
Accrued milestones			$ 0			$ 0
Shire Agreement | Shire | Maralixibat
Asset Acquisitions [Line Items]
Milestone payments, payable	109,500,000
Milestone payments, payable upon approval	25,000,000
Shire Agreement | Shire | Volixibat
Asset Acquisitions [Line Items]
Milestone payments, payable	$ 30,000,000
Shire Agreement | Shire | Redeemable Common Stock
Asset Acquisitions [Line Items]
Temporary equity, shares issued | shares	1,859,151
Issuance of Series A convertible preferred stock at $1.00259507 per share, net of issuance costs of $23	$ 7,000,000
Shares issued, price per share | $ / shares	$ 3.76
Transition Services Agreement | Shire
Asset Acquisitions [Line Items]
Accrued expenses			0			0	$ 2,100,000
Transition Services Agreement | Shire | Shire Provided Services
Asset Acquisitions [Line Items]
Research and development				0	0	400,000
Transition Services Agreement | Shire | Pass-through Cost
Asset Acquisitions [Line Items]
Research and development			0	$ 0	$ 0
Reduction of estimated expenses						100,000
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 payment		$ 500,000
Accrued milestones			0			0
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			$ 0

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) - Additional Information (Details)	Nov. 05, 2018shares	Jul. 31, 2019shares	Apr. 30, 2019shares	Oct. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2019Vote$ / sharesshares	Dec. 31, 2018$ / sharesshares	Aug. 31, 2018$ / shares
Class of Stock [Line Items]
Common stock, par value | $ / shares				$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Redeemable Common Stock
Class of Stock [Line Items]
Number of shares issued					0	1,859,151
Common Stock Agreement | Redeemable Common Stock
Class of Stock [Line Items]
Number of shares issued	1,859,151
Common Stock
Class of Stock [Line Items]
Number of shares issued upon conversion of series A preferred stock		14,969,118
Number of shares repurchased			25,782
Number of shares subject to repurchase					423,047	550,781
Common stock, voting rights					Each share of common stock is entitled to one voting right.
Number of vote entitled for each common stockholder | Vote					1
Common Stock | Founder
Class of Stock [Line Items]
Stock issued for services, shares				1,187,500
Stock issued for services, value | $				$ 950
Common Stock | Restricted Stock | Founder
Class of Stock [Line Items]
Stock issued for services, shares	562,500
Vesting period	4 years

Stockholders' Equity (Deficit_2

Stockholders' Equity (Deficit) - Schedule of Common Stock Reserved for Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock reserved for issuance	4,979,255	9,347,681
Series A Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Common stock reserved for issuance		7,488,530
Options to Purchase Common Stock
Class of Stock [Line Items]
Common stock reserved for issuance	3,206,812
Reserved for Future Stock Awards or Option Grants
Class of Stock [Line Items]
Common stock reserved for issuance	1,272,443	1,859,151
Reserved for Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock reserved for issuance	500,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Nov. 05, 2018	Jul. 31, 2019	Sep. 30, 2019	Oct. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Jul. 17, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock approved and reserved for issuance			4,979,255			4,979,255		9,347,681
Common Stock | Founder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock issued for services				1,187,500
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock approved and reserved for issuance			3,206,812			3,206,812
Expected dividend yield			0.00%			0.00%
Stock options granted					0	3,206,812
Total unrecognized stock-based compensation related to unvested stock option awards granted			$ 23.2			$ 23.2
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period						3 years 4 months 24 days
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						3 years 2 months 12 days
Shares vested			33,398			101,952
Total unrecognized compensation expense related to unvested restricted stock			$ 1.2			$ 1.2
Restricted Stock | Founder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Measurement date fair value for stock issued for services	$ 2.936
Restricted Stock | Common Stock | Founder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock issued for services	562,500
Total compensation cost	$ 1.7
Vesting period of stock issued for services	4 years
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock approved and reserved for issuance		1,401,443	1,272,443			1,272,443
Equity incentive plans, description		A total of 1,401,443 shares of common stock were approved to be initially reserved for issuance under the 2019 Plan, including 101,443 shares that remained available for issuance under the 2018 Plan as of July 17, 2019. 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, 2019, 1,272,443 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%
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock approved and reserved for issuance							101,443
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock approved and reserved for issuance		500,000	500,000			500,000
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 lessor 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. The Company had 500,000 shares available for future issuance under the 2019 ESPP as of September 30, 2019.
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%
Annual increase in common stock available for issuance, shares		1,500,000

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, 2019	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price, minimum	$ 9.86	$ 2.94
Exercise price, maximum	$ 15	$ 15
Expected volatility, minimum	75.31%	73.88%
Expected volatility, maximum	75.98%	75.98%
Risk-free interest rate, minimum	1.46%	1.46%
Risk-free interest rate, maximum	1.89%	2.46%
Expected dividend yield	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years	6 years
Grant date fair value of options granted	$ 6.62	$ 6.62
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 3 months 18 days	6 years 3 months 18 days
Grant date fair value of options granted	$ 10.01	$ 10.46

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - Stock Options - USD ($) $ / shares in Units, $ in Thousands	5 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares, Granted	0	3,206,812
Number of shares, Outstanding, Ending balance		3,206,812
Number of shares, Vested and exercisable		1,465
Weighted-average exercise price, Granted		$ 4.98
Weighted-average exercise price, Outstanding, Ending balance		4.98
Weighted-average exercise price, Vested and exercisable		$ 2.94
Weighted-average remaining contractual life, Outstanding		9 years 7 months 6 days
Weighted-average remaining contractual life, Granted		9 years 7 months 6 days
Weighted-average remaining contractual life, Vested and exercisable		9 years 6 months
Aggregate intrinsic value, Outstanding		$ 16,292	$ 0
Aggregate intrinsic value, Vested and exercisable		$ 10

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-based Compensation Expense Reflected in Unaudited Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,144	$ 4,003
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,314	2,464
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 830	$ 1,539

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2019USD ($)	Jan. 31, 2019USD ($)ft²
Leases [Abstract]
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 is 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
Tenant improvement allowance			$ 400
Operating lease right-of-use assets	$ 941	$ 941
Lease liability	$ 1,456	$ 1,456	$ 1,500
Estimated incremental borrowing rate	8.00%	8.00%
Operating lease remaining term	3 years 8 months 12 days	3 years 8 months 12 days
Rent expense	$ 100	$ 200
Short-term lease commitment	$ 400	$ 400

Leases - Schedule of Lease Liab

Leases - Schedule of Lease Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 31, 2019
Leases [Abstract]
2019 (remaining three months)	$ 108
2020	436
2021	448
2022	461
2023	215
2024	15
Total undiscounted lease payments	1,683
Less: imputed interest	(227)
Total lease liability	$ 1,456	$ 1,500

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax expense	$ 0	$ 0	$ 0	$ 0