Mirum Pharmaceuticals (MIRM) S-1 6 Jan 20 IPO registration Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2019
Document and Entity Information [Abstract]
Document Type	S-1
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Entity Registrant Name	Mirum Pharmaceuticals, Inc.
Entity Central Index Key	0001759425
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

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
Common stock	2	1
Additional paid-in capital	198,052	34
Accumulated deficit	(51,949)	(17,348)
Accumulated other comprehensive income	98
Total stockholders' 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 - USD ($) $ in Thousands	3 Months Ended		5 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Operating expenses:
Research and development, excluding in process cost				$ 2,331
In process research and development				14,490
Research and development	$ 12,159	$ 3	$ 3		$ 28,611
General and administrative	3,708	62	72	585	7,474
Total operating expenses	15,867	65	75	17,406	36,085
Loss from operations	(15,867)	(65)	(75)	(17,406)	(36,085)
Interest income (expense)	785	(2)	(2)	72	1,485
Other income (expense), net	(5)			(14)	(1)
Net loss	$ (15,087)	$ (67)	$ (77)	$ (17,348)	$ (34,601)
Net loss per common share, basic and diluted	$ (0.84)	$ (0.14)	$ (0.26)	$ (19.29)	$ (4.47)
Weighted-average common shares outstanding, basic and diluted	17,996,065	476,905	290,570	899,331	7,745,241

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - 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 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) - 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
Balance at May. 02, 2018	0	$ 0	0	0	0
Balance, Shares at May. 02, 2018		0
Net loss	(77)
Balance at Sep. 30, 2018	(76)	$ 1		(77)
Balance, Shares at Sep. 30, 2018		812,500
Balance at May. 02, 2018	0	$ 0	0	0	0
Balance, Shares at May. 02, 2018		0
Issuance of common stock	1	$ 1
Issuance of common stock, Shares		625,000
Restricted common stock vested in the period		11,719
Issuance of Series A redeemable convertible preferred stock, net of issuance costs						$ 59,785
Issuance of Series A redeemable convertible preferred stock, net of issuance costs, Shares						59,844,699
Shares issued in connection with asset acquisition							$ 6,990
Shares issued in connection with asset acquisition, Shares							1,859,151
Conversion of convertible notes and accrued interest to preferred stock						$ 64
Conversion of convertible notes and accrued interest to preferred stock, Shares						63,585
Stock-based compensation	34		34
Net loss	(17,348)			(17,348)
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
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	(17,313)	$ 1	34	(17,348)
Balance, Shares at Dec. 31, 2018		636,719
Restricted common stock vested in the period		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		33,398
Issuance of Series A redeemable convertible preferred stock, net of issuance costs						$ 59,977
Issuance of Series A redeemable convertible preferred stock, net of issuance costs, 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
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
Restricted common stock vested in the period		33,398
Stock-based compensation	2,144		2,144
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) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		8 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Series A Redeemable Convertible Preferred Stock
Issuance of stock at per share		$ 1.00259507	$ 1.00259507
Issuance of Series A redeemable convertible preferred stock, issuance costs		$ 23	$ 200
Common Stock
Issuance of stock at per share			$ 0.0001
Issuance of common stock upon initial public offering, issuance costs	$ 7,800

Condensed Consolidated Statem_5

Condensed Consolidated Statement of Cash Flows - USD ($) $ in Thousands	5 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Operating activities
Net loss	$ (77)	$ (17,348)	$ (34,601)
Reconciliation of net loss to net cash used in operating activities:
Stock-based compensation		34	4,003
Depreciation and amortization			80
Noncash interest expense on convertible note		14
Amortization of operating lease right-of-use assets			124
In process research and development		14,490
Amortization of premium on investments			(199)
Interest expense on convertible note	2
Change in operating assets and liabilities:
Prepaid and other current assets		(12)	(3,046)
Operating lease right-of-use assets			(33)
Other assets			(84)
Accounts payable, accrued expenses and other liabilities	56	2,449	8,079
Operating lease liabilities			13
Net cash used in operating activities	(19)	(373)	(25,664)
Investing activities
Purchase of investments			(130,923)
Research and development asset acquisition		(7,500)
Proceeds from maturities of investments			8,500
Purchase of property and equipment			(214)
Net cash used in investing activities		(7,500)	(122,637)
Financing activities
Proceeds from convertible note, related party	50	50
Proceeds from the sale of common stock	1	1
Proceeds from the issuance of Series A redeemable convertible preferred stock, net		59,785	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	59,836	127,177
Effect of exchange rate on cash and cash equivalents			(25)
Net (decrease) increase in cash and cash equivalents	32	51,963	(21,149)
Cash and cash equivalents at beginning of period			51,963
Cash and cash equivalents at end of period	32	51,963	30,814
Supplemental disclosure of cash flow information:
Operating lease right-of-use asset obtained in exchange for operating lease liability			1,444
Common stock issued in connection with asset acquisition		6,990
Landlord paid tenant improvements			455
Conversion of convertible note to preferred stock		$ 64
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 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. Following the IPO, there were no shares of Series A Preferred Stock or shares of redeemable common stock outstanding. 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. Management expects to continue to incur additional substantial losses in the foreseeable future as a result of the Company’s research and development activities.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and pursuant to Form 10-Q S-X S-1 No. 333-232251), 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 Investments The Company classifies all investments as available-for-sale, 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 available-for-sale available-for-sale available-for-sale 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 Leases In accordance with Accounting Standards Update (“ASU”) No. 2016-02, right-of-use 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 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) No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, No. 2019-05, Financial Instruments—Credit Losses (Topic 326), In August 2018, the FASB issued ASU No. 2018-13,	2. Summary of Significant Accounting Policies Basis of Presentation The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include all adjustments necessary for the fair presentation of the Company’s financial position for the period presented. Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in the financial statements and accompanying notes. The most significant estimates in the Company’s 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 on the 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. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment. 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. Cash and cash equivalents primarily represent funds invested in readily available checking and saving accounts. The carrying amounts reported in the consolidated balance sheet for cash and cash equivalents are valued at cost, which approximate their fair value. Concentrations of Credit Risk and Off-Balance The Company maintains its cash and cash equivalents deposited at financial institutions in excess of federally insured deposit limits. Cash is held on deposit in a major financial institution and is considered subject to minimal credit risk. 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, accounts payable and accrued liabilities are reasonable estimates of their fair value because of the short nature of these items. As of December 31, 2018, the Company had $51.5 million in Level 1 assets consisting of cash equivalents. There were no Level 2 or 3 balances nor transfers between levels within the fair value hierarchy during the period presented. 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, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, 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 Stock-Based Compensation Expense The Company recognizes stock-based compensation expense for all stock-based awards to employees based on the grant-date fair value of the award. All stock-based compensation costs are recorded in the consolidated statement of operations and comprehensive loss based upon the underlying employee’s role within the Company. Income Taxes Income taxes are recorded using the liability method, under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided when the expected realization of the deferred tax assets does not meet the more-likely-than-not The Company accounts for uncertain tax positions in accordance with the provisions of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification No. 740, Income Taxes (“ASC 740”). When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit would more likely than not be realized assuming examination by the taxing authority. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. Interest and penalties related to unrecognized tax benefits, if any, are recorded as a component of income tax expense. 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 Potentially dilutive securities as of December 31, 2018 that have been excluded in the calculation of diluted net loss per share because to do so would be anti-dilutive are 7,488,530 shares of Series A Preferred Stock (in common stock equivalent shares). Recent Accounting Pronouncements—To Be Adopted In February 2016, FASB issued Accounting Standards Update (“ASU”) 2016-02 , Leases (Topic 842) 2016-02”), right-of-use 2016-02 Recently Adopted Accounting Pronouncements In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business 2017-01”). 2017-01 In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers

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. Investments 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 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 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

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, 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 2,035 — Accrued compensation and related benefits 1,294 15 Total accrued expenses $ 7,912 $ 2,180

Asset Acquisitions

Asset Acquisitions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
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 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 product-by-licensed country-by-country 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 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, product-by-licensed country-by-country 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 product-by-licensed 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 product-by-licensed country-by-country	3. 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-Aventis”). 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 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 product-by-licensed country-by-country 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 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, product-by-licensed country-by-country 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 product-by-licensed Sanofi-Aventis License Through the Shire Agreement, the Company was assigned a license agreement with Sanofi-Aventis pursuant to which the Company obtained an exclusive, worldwide license to certain patents and know-how product-by-licensed country-by-country

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	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Share-based Payment Arrangement [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 Stock Options The fair value of each employee and non-employee stock 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 Nine Months Ended 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 Nine Months Ended 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	6. Stock Options Equity Incentive Plans On November 5, 2018, the Company adopted the 2018 Equity Incentive Plan (the “2018 Plan”) which authorizes up to 1,859,151 stock awards and incentive and nonstatutory stock options to employees, directors and consultants of the Company. Contingent upon the Second Closing of the Series A Preferred Stock, the shares reserved for issuance under the 2018 Plan will automatically increase by 1,320,104 shares for a total of 3,179,255 shares. As of December 31, 2018, no awards had been granted under the 2018 Plan. Restricted Stock On November 5, 2018, in connection with the issuance of the Series A Preferred Stock, the Company placed vesting conditions on 562,500 previously issued founder shares with four year vesting and a measurement date fair value of $2.936 per share. For the period of May 2, 2018 to December 31, 2018, 11,719 vested and the Company recognized $34,000 in stock-based compensation expense of which $26,000 and $8,000 were recorded as general and administrative and research and development expenses, respectively. At December 31, 2018, the total unrecognized compensation expense related to unvested restricted stock was $1.6 million expected to be recognized over a weighted-average period of approximately 3.9 years.

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	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
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.	7. Income Taxes The Company’s loss before income taxes was entirely generated from its U.S. operations. The net loss before taxes for the period of May 2, 2018 to December 31, 2018 is as follows (in thousands): Period from U.S. loss before taxes $ (17,348 ) Foreign loss before taxes — Loss before income taxes $ (17,348 ) A reconciliation of income tax expense for period of May 2, 2018 to December 31, 2018 is as follows (in thousands): Period from Current: Federal $ — State — Foreign — Total current income tax expense — Deferred: Federal $ — State — Foreign — Total deferred income tax expense — Total income tax expense $ — A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate is as follows: Period from Federal statutory income tax rate 21.0 % State tax 7.0 Tax credits 1.9 Change in valuation allowance (29.9 ) Total tax benefit — % Significant components of the Company’s deferred tax assets are as follows (in thousands): December 31, 2018 Deferred tax assets: Net operating losses $ 602 Tax credit carryforwards 346 Accrued expenses 148 Intangibles 4,085 Other 14 Net deferred tax assets before valuation allowance 5,195 Valuation allowance (5,195 ) Net deferred tax assets $ — The valuation allowance increased by $5.2 million for the period from May 2, 2018 to December 31, 2018. The tax benefit of deductible temporary differences or carryforwards is recorded as a deferred tax asset to the extent that management assesses the realization is “more likely than not.” Future realization of the tax benefit ultimately depends on the existence of sufficient taxable income within the period available under the tax law. At December 31, 2018, the Company has set up valuation allowances against all federal and state deferred tax assets because based on all available evidence, these deferred tax assets are not more likely than not to be realizable. At December 31, 2018, the Company had federal and California net operating loss carryforwards of approximately $2.2 million and $2.2 million, respectively. Federal losses do not expire, and California net operating losses will begin to expire in 2038. The Company also has federal and California research and development credit carryforwards totaling $0.5 million and $12,000, respectively. The federal research and development credit carryforwards will begin to expire in 2038, unless previously utilized. The California research credits do not expire. In general, if the Company experiences a greater than 50 percentage point aggregate change in ownership of certain significant stockholders over a three-year period (a “Section 382 ownership change”), utilization of its pre-change tax-exempt The Company recognizes the financial statements effects of a tax position when it is more likely than not, based on technical merits, that the position will be sustained upon examination. A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands): December 31, 2018 Balance at beginning of year $ — Increase related to current year tax provision 116 Balance at the end of the year $ 116 The Company has considered the amounts and probabilities of the outcomes that can be realized upon ultimate settlement with the tax authorities and determined unrecognized tax benefits primarily related to credits should be established as noted in the summary rollforward above. The unrecognized tax benefits, if recognized and in absence of full valuation allowance, would impact the income tax provision by $0.1 million and the Company’s effective income tax rate would not be impacted if the unrecognized tax benefits are recognized in the period from May 2, 2018 to December 31, 2018. Additional amounts in the summary rollforward could impact the Company’s effective tax rate if it did not maintain a full valuation allowance on its net deferred tax assets. As of December 31, 2018, the Company does not believe that it is reasonably possible that its unrecognized tax benefits would significantly change in the following 12 months. The Company is subject to taxation in the United States, California and Switzerland. Due to the Company’s losses incurred, the Company is subject to the income tax examination by authorities since inception on May 2, 2018. The Company’s policy is to recognize interest expense and penalties related to income tax matters as tax expense. As of December 31, 2018, there were no significant accruals for interest related to unrecognized tax benefits or tax penalties.

Subsequent Events

Subsequent Events	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events On January 1, 2020, the shares of common stock available under each of the 2019 Plan and ESPP were increased by 1,149,499 and 229,899 shares, respectively, pursuant to the automatic increase provisions of the respective plans. In December 2019, the Company entered into an agreement related to its corporate headquarters to add an additional 5,600 square feet to its existing leased office space for a term of approximately 5 years as well as extend the term of the existing lease space entered into in January 2019 by approximately 2 additional years. In October, November and December 2019 and January 2020, the Company granted an aggregate of 260,000 shares of common stock options to its employees at weighted average exercise price of $14.62 per share.	8. Subsequent Events For purposes of the financial statements as of December 31, 2018 and the year then ended, the Company evaluated subsequent events for recognition and measurement purposes through April 18, 2019, the date the financial statements were issued. Except as described below, the Company has concluded that no events or transactions have occurred that require disclosure. On January 22, 2019, the Company entered into a four-year agreement for the Company’s corporate headquarters for approximately 5,600 square feet of office space. The lease term, and allocated minimum lease payments which aggregate $1.6 million over the initial term of four years, is expected to commence mid-May In March and April 2019, the Company granted 1,426,875 shares of common stock options to its employees at an exercise price of $2.936 per share. The stock options generally vest over a four-year period. In connection with the Second Closing, the common stock reserved for issuance under the 2018 Plan increased by 1,320,104 shares to a total of 3,179,255 shares. In April 2019, the Company repurchased 25,782 shares of common stock from a former employee in connection with termination of employment.

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation 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 operations in November 2018. The 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 entity have been eliminated in consolidation. In July 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 Series A Preferred Stock conversion price to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying consolidated financial statements and notes to the consolidated financial statements give retroactive effect to the reverse stock split for all periods presented. Liquidity and Capital Resources 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 December 31, 2018, the Company had an accumulated deficit of $17.3 million. Since its inception, the Company has funded its operations primarily through equity financings. The Company raised net proceeds of $59.8 million in 2018 through an equity financing involving the sale of the Company’s Series A redeemable convertible preferred stock (the “Series A Preferred Stock”). Due to the Company’s continuing research and development activities, the Company expects to continue to incur net losses into the foreseeable future and may never become profitable. As a result, the Company will need to raise capital through public or private equity or debt financings, government or other third-party funding, collaborations, strategic alliances and licensing arrangements or a combination of these. Successful transition to attaining profitable operations is dependent upon achieving a level of revenues adequate to support the Company’s cost structure. Management believes that the Company has sufficient working capital on hand to fund operations through at least the next twelve months from the date these consolidated financial statements were available to be issued. There can be no assurance that the Company will be successful in acquiring additional funding, that the Company’s projections of its future working capital needs will prove accurate, or that any additional funding would be sufficient to continue operations in future years. If the Company is not able to secure adequate additional funding, the Company may be forced to make reductions in spending, extend payment terms with suppliers, liquidate assets where possible, and/or suspend or curtail planned programs. Any of these actions could materially harm the Company’s business, results of operations, and future prospects.

Convertible Note Financing

Convertible Note Financing	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Convertible Note Financing	4. Convertible Note Financing On August 3, 2018, the Company issued a convertible promissory note (the “Note”) in an amount of $50,000 to an affiliated entity of the Company’s then-Chief Executive Officer and member of its board of directors (the “Board”). The Note accrued interest at 8% per year and had a maturity date of December 31, 2019. The Note was subject to an automatic conversion upon a qualified equity financing defined as a raise in aggregate of at least $5.0 million, excluding the conversion of the Note and other indebtedness. The conversion was equal to the outstanding principal amount of the Note plus all accrued and previously unpaid interest thereon, divided by 80% of the lowest price per share paid by investor for qualified equity financing. On November 5, 2018, the Note converted into 63,585 shares of Series A Preferred Stock. For the period from May 2, 2018 to December 31, 2018, the Company recorded the accretion of the discount and aggregate interest expense of $14,000.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Deficit	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Deficit	5. Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders’ Deficit Series A Preferred Stock On November 5, 2018, the Company entered into a Series A Preferred Stock Purchase Agreement with various investors for issuance of up to 119,752,983 shares of Series A Preferred Stock at a purchase price of $1.00259507 per share. In an initial closing, the Company issued 59,908,284 shares of Series A Preferred Stock at $1.00259507 per share, for approximately $59.8 million in cash and the conversion of approximately $51,014 in principal and accrued interest under the Note (see Note 4, Convertible Note Financing). Certain investors agreed to purchase up to an additional 59,844,699 shares of Series A Preferred Stock at a subsequent closing under the same terms (the “Second Closing”) upon the approval of the Board, including a majority of the directors elected by the holders of the Series A Preferred Stock. One foreign investor has the right to defer its participation of 7,480,590 shares in the Second Closing until certain conditions are met. If any one of the investors fails to purchase these shares at the Second Closing, each share of Series A Preferred Stock held by such investor will automatically be converted into one-tenth The holders of the Series A Preferred Stock issued in the initial closing are precluded from a voluntary conversion into shares of common stock until the Second Closing except upon the occurrence of a deemed liquidation event. On April 12, 2019, the Second Closing of Series A Preferred Stock was completed for issuance of 59,844,699 shares and cash proceeds of $60.0 million. Dividend Provisions The holders of each outstanding share of Series A Preferred Stock are entitled to receive annual, noncumulative dividends of 8% of the applicable original issuance price per share when, as and if declared by the Board, in preference to any distribution to the holders of the common stock. Any additional dividends or distributions will be distributed among all holders of common stock and preferred stock in proportion to the number of shares of common stock held by each such holder if all shares of preferred stock were converted to common stock. No dividends have been declared as of December 31, 2018. Liquidation Preference In the event of a liquidation or winding-up Conversion Each share of preferred stock is convertible, at the option of the stockholder, into shares of common stock at any time after the date of issuance into a number of shares of common stock as is determined by dividing (a) the original issue price by (b) the applicable conversion price as such values are set forth in the Company’s certificate of incorporation. The resulting initial per share conversion rate is one share of common stock for each share of Series A Preferred Stock, as adjusted for stock splits, recapitalizations and certain other events. Each share shall automatically convert upon (1) the closing of a firm-commitment underwritten public offering pursuant to an effective registration statement under the Securities Act of 1933, as amended, resulting in at least $50.0 million of gross proceeds to the Company and a price of $16.0415 per share, as adjusted for stock splits, recapitalizations and certain other events or (2) an election of holders of a majority of the then outstanding shares of the Series A Preferred Stock on an as-converted Voting The holder of each of share of Series A Preferred Stock is entitled to one vote for each share of common stock into which it would convert. Presentation of Series A Preferred Stock The Series A Preferred Stock has been classified outside of stockholders’ equity (deficit) as temporary equity on the accompanying consolidated balance sheet because the shares contain certain redemption features that are not solely within the control of the Company. The Series A Preferred Stock is not generally redeemable; however, upon certain change in control events that are outside of the Company’s control, including liquidation, sale or transfer of control of the Company, holders of the Series A Preferred Stock may have the right to receive its liquidation preference under the terms of the Company’s certificate of incorporation. The Company is not adjusting the carrying value of the Series A Preferred Stock as it is uncertain whether or when a redemption event will occur. Redeemable Common Stock In connection with the Shire License Agreement, the Company entered into a common stock issuance agreement (the “Common Stock Agreement”) with Shire and issued 1,859,151 shares of the Company’s common stock (the “Shire Stock”), or 15% of the Company’s outstanding fully diluted shares issued as of December 31, 2018. Pursuant to the terms of the Common Stock Agreement, (1) in the event of an initial public offering prior to the occurrence of certain liquidation events, if Shire’s ownership of the Company’s common stock is less than 5% of the outstanding fully diluted shares of common stock immediately prior to the consummation of the IPO, the Company will issue additional shares of common stock to increase Shire’s ownership to 5% of the then-outstanding fully diluted shares, and (2) in the event that certain liquidation events occur prior to an initial public offering and Shire’s ownership is less than 5% of the then outstanding fully diluted shares of common stock, Shire is entitled to receive additional proceeds, if needed, to equal 5% of proceeds available for distribution to the Companies security holders. The Shire Stock has been classified outside of stockholders’ equity (deficit) as temporary equity on the accompanying consolidated balance sheet because these features are deemed for accounting purposes to contain certain redemption features that may be triggered in the event of certain liquidation events that are not solely in the control of the Company. The Company is not adjusting the carrying value of the redeemable common stock as it is uncertain whether or when a redemption event will occur. 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. As of December 31, 2018, 550,781 shares of common stock were subject to repurchase by the Company. The unvested stock liability related to these shares is immaterial as of December 31, 2018. Each share of common stock is entitled to one voting right. Common stock holders 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 Conversion of preferred stock 7,488,530 Authorized for future stock awards or option grants 1,859,151 9,347,681

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and pursuant to Form 10-Q S-X S-1 No. 333-232251),	Basis of Presentation The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include all adjustments necessary for the fair presentation of the Company’s financial position for the period presented.
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.	Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in the financial statements and accompanying notes. The most significant estimates in the Company’s 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 on the 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	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. Cash and cash equivalents primarily represent funds invested in readily available checking and saving accounts. The carrying amounts reported in the consolidated balance sheet for cash and cash equivalents are valued at cost, which approximate their fair value.
Investments	Investments The Company classifies all investments as available-for-sale, 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 available-for-sale available-for-sale available-for-sale
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.	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, accounts payable and accrued liabilities are reasonable estimates of their fair value because of the short nature of these items. As of December 31, 2018, the Company had $51.5 million in Level 1 assets consisting of cash equivalents. There were no Level 2 or 3 balances nor transfers between levels within the fair value hierarchy during the period 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.	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, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, 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	Research and Development Expenses Research and development expenses consist primarily of fees paid to contract research organizations and other vendors for clinical, non-clinical
Leases	Leases In accordance with Accounting Standards Update (“ASU”) No. 2016-02, right-of-use 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 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 —	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 Potentially dilutive securities as of December 31, 2018 that have been excluded in the calculation of diluted net loss per share because to do so would be anti-dilutive are 7,488,530 shares of Series A Preferred Stock (in common stock equivalent shares).
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.	Recently Adopted Accounting Pronouncements In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business 2017-01”). 2017-01 In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers
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) No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, No. 2019-05, Financial Instruments—Credit Losses (Topic 326), In August 2018, the FASB issued ASU No. 2018-13,	Recent Accounting Pronouncements—To Be Adopted In February 2016, FASB issued Accounting Standards Update (“ASU”) 2016-02 , Leases (Topic 842) 2016-02”), right-of-use 2016-02
Segment Reporting		Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment.
Concentrations of Credit Risk and Off-Balance Sheet Risk		Concentrations of Credit Risk and Off-Balance The Company maintains its cash and cash equivalents deposited at financial institutions in excess of federally insured deposit limits. Cash is held on deposit in a major financial institution and is considered subject to minimal credit risk.
Stock-Based Compensation Expense		Stock-Based Compensation Expense The Company recognizes stock-based compensation expense for all stock-based awards to employees based on the grant-date fair value of the award. All stock-based compensation costs are recorded in the consolidated statement of operations and comprehensive loss based upon the underlying employee’s role within the Company.
Income Taxes		Income Taxes Income taxes are recorded using the liability method, under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided when the expected realization of the deferred tax assets does not meet the more-likely-than-not The Company accounts for uncertain tax positions in accordance with the provisions of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification No. 740, Income Taxes (“ASC 740”). When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit would more likely than not be realized assuming examination by the taxing authority. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. Interest and penalties related to unrecognized tax benefits, if any, are recorded as a component of income tax expense.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [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 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 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, 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 2,035 — Accrued compensation and related benefits 1,294 15 Total accrued expenses $ 7,912 $ 2,180

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Deficit (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
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	Common stock reserved for issuance is as follows: As of Conversion of preferred stock 7,488,530 Authorized for future stock awards or option grants 1,859,151 9,347,681

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Share-based Payment Arrangement [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 Nine Months Ended 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 Nine Months Ended 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

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Net Loss before Taxes	The net loss before taxes for the period of May 2, 2018 to December 31, 2018 is as follows (in thousands): Period from U.S. loss before taxes $ (17,348 ) Foreign loss before taxes — Loss before income taxes $ (17,348 )
Summary of Reconciliation of Income Tax Expense	A reconciliation of income tax expense for period of May 2, 2018 to December 31, 2018 is as follows (in thousands): Period from Current: Federal $ — State — Foreign — Total current income tax expense — Deferred: Federal $ — State — Foreign — Total deferred income tax expense — Total income tax expense $ —
Summary of Reconciliation of Federal Statutory Income Tax Rate	A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate is as follows: Period from Federal statutory income tax rate 21.0 % State tax 7.0 Tax credits 1.9 Change in valuation allowance (29.9 ) Total tax benefit — %
Summary of Deferred Tax Assets	Significant components of the Company’s deferred tax assets are as follows (in thousands): December 31, 2018 Deferred tax assets: Net operating losses $ 602 Tax credit carryforwards 346 Accrued expenses 148 Intangibles 4,085 Other 14 Net deferred tax assets before valuation allowance 5,195 Valuation allowance (5,195 ) Net deferred tax assets $ —
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands): December 31, 2018 Balance at beginning of year $ — Increase related to current year tax provision 116 Balance at the end of the year $ 116

Description of Business - Addit

Description of Business - Additional Information (Detail) $ / 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	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2019USD ($)shares	Jul. 23, 2019shares	Jun. 30, 2019shares	Mar. 31, 2019shares
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	$ 1
Accumulated deficit | $				$ (51,949)			$ (17,348)	$ (51,949)
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		625,000
Shares issued, price per share | $ / shares			$ 0.0001		$ 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			59,908,284	0	0	119,752,983	59,908,284
Redeemable Common Stock
Organization And Basis Of Presentation [Line Items]
Redeemable stock, outstanding				0			1,859,151	0	0	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 (Detail)	8 Months Ended	9 Months Ended
	Dec. 31, 2018USD ($)Segmentshares	Sep. 30, 2019USD ($)shares
Summary Of Significant Accounting Policies [Line Items]
Investments with original maturities at date of purchase to be cash equivalents	3 months	3 months
Asset transfers Level 1 to Level 2		$ 0
Asset transfers Level 2 to Level 1		0
Asset transfers into Level 3	$ 0	0
Asset transfers out of Level 3	0	0
Liabilities transfers Level 1 to Level 2		0
Liabilities transfers Level 2 to Level 1		0
Liability transfers into Level 3	0	0
Liability transfers out of Level 3	0	0
Property and equipment, net		631,000
Accumulated depreciation	$ 0	100,000
Number of reporting segment | Segment	1
Cash and cash equivalents		$ 28,716,000
Anti-dilutive shares of stock | shares		3,629,859
Series A Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Anti-dilutive shares of stock | shares	7,488,530
Level 1
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 51,500,000
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net	$ 0	$ 500,000
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 (Detail)	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
Stock Options
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 (Detail) - 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 (Detail) - 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 (Detail) $ 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)
Investments, Estimated Fair Value	122,745
Cash equivalents, Estimated Fair Value	28,716
Short-term investments	95,445
Long-term investments	27,300
Cash equivalents and investments, Estimated Fair Value	151,461
Money Market Fund
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents, Estimated Fair Value	28,716
Cash equivalents, Amortized Cost	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 - Summa_2

Financial Instruments - Summary of Remaining Contractual Maturities of Available-for-sale Debt Securities (Detail) $ 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

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - 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

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - 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 (Detail)	Nov. 05, 2018USD ($)ProductCandidate$ / sharesshares	Jul. 31, 2019USD ($)	Dec. 31, 2018USD ($)shares	Sep. 30, 2019USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)shares
Asset Acquisitions [Line Items]
Research and development				$ 12,159,000	$ 3,000	$ 3,000	$ 28,611,000
Accrued expenses			$ 2,180,000	$ 7,912,000			$ 7,912,000
Redeemable Common Stock
Asset Acquisitions [Line Items]
Temporary equity, shares issued | shares	59,908,284		1,859,151	0			0
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 Provided Services
Asset Acquisitions [Line Items]
Research and development			$ 500,000
Transition Services Agreement | Pass-through Cost
Asset Acquisitions [Line Items]
Research and development			1,600,000
Transition Services Agreement | Shire
Asset Acquisitions [Line Items]
Accrued expenses			2,100,000	0			0
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			$ 0

Stockholders' Equity (Deficit)

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

Stockholders' Equity (Deficit_2

Stockholders' Equity (Deficit) - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock reserved for issuance	4,979,255	3,179,255	9,347,681
Series A Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Common stock reserved for issuance			7,488,530
Stock Options
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 (Detail) - USD ($)	Nov. 05, 2018	Jul. 31, 2019	Apr. 30, 2019	Mar. 30, 2019	Sep. 30, 2019	Oct. 31, 2018	Dec. 31, 2018	Sep. 30, 2019	Jul. 17, 2019	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock approved and reserved for issuance					4,979,255		9,347,681	4,979,255		3,179,255
Number of shares, Granted			1,426,875	1,426,875
Stock-based compensation expense					$ 2,144,000			$ 4,003,000
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense					830,000			1,539,000
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense					$ 1,314,000			$ 2,464,000
Common Stock | Founder
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued for services	562,500					1,187,500
Vesting period	4 years
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%
Total unrecognized stock-based compensation related to unvested stock option awards granted					$ 23,200,000			$ 23,200,000
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period								3 years 2 months 24 days
Number of shares, Granted								3,206,812
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
Share vested					33,398		11,719	101,952
Total unrecognized compensation expense related to unvested restricted stock					$ 1,200,000			$ 1,200,000
Stock-based compensation expense							$ 34,000
Restricted Stock | Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense							$ 26,000
Restricted Stock | General and Administrative Expense
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 10 months 24 days
Total unrecognized compensation expense related to unvested restricted stock							$ 1,600,000
Stock-based compensation expense							$ 8,000
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	$ 1,700,000
Vesting period	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	3,179,255								101,443
Stock option Additional reserve increase	1,320,104
Number of shares, Granted							0
2018 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option authorized	1,859,151
2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plans, description		A total of 500,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten-years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to the 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%
Stock issued for services		500,000			500,000			500,000
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 (Detail) - 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 (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Apr. 30, 2019	Mar. 30, 2019	Sep. 30, 2019
Options, Outstanding
Number of shares, Granted	1,426,875	1,426,875
Weighted-average exercise price, Outstanding
Weighted-average exercise price, Granted	$ 2.936	$ 2.936
Share-based Payment Award, Options, Additional Disclosures
Outstanding as of September 30, 2019	4 years	4 years
Stock Options
Options, Outstanding
Number of shares, Outstanding, Beginning balance
Number of shares, Granted			3,206,812
Number of shares, Exercised			0
Number of shares, Canceled and forfeited			0
Number of shares, Outstanding, Ending balance			3,206,812
Vested and exercisable as of September 30, 2019			1,465
Weighted-average exercise price, Outstanding
Weighted-average exercise price, Outstanding, Beginning balance
Weighted-average exercise price, Granted			$ 4.98
Weighted-average exercise price, Exercised			0
Weighted-average exercise price, Canceled and forfeited			0
Weighted-average exercise price, Outstanding, Ending balance			4.98
Weighted-average exercise price, Vested and exercisable			$ 2.94
Share-based Payment Award, Options, Additional Disclosures
Granted			9 years 7 months 6 days
Outstanding as of September 30, 2019			9 years 7 months 6 days
Vested and exercisable as of September 30, 2019			9 years 6 months
Aggregate intrinsic value, Outstanding			$ 16,292
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 (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 2,144	$ 4,003
General and Administrative Expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	1,314	2,464
Research and Development Expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 830	$ 1,539

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	Jan. 31, 2019USD ($)	Sep. 30, 2019USD ($)ft²	Sep. 30, 2019USD ($)ft²	Jan. 22, 2019USD ($)ft²
Leases [Abstract]
Area of office space | ft²		5,600	5,600	5,600
Term of lease		4 years	4 years	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	$ 1,600
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 (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 22, 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	$ 1,600
Total undiscounted lease payments	$ 1,683

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		5 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Income tax expense	$ 0	$ 0	$ 0	$ 0	$ 0
Valuation allowance, deferred tax asset, increase				5,200,000
Unrecognized tax benefits that would impact effective tax rate				100,000
Domestic Tax Authority [Member]
Operating loss carryforwards				2,200,000
Deferred tax assets, tax credit carryforwards, research				500,000
State and Local Jurisdiction [Member] | California Franchise Tax Board [Member]
Operating loss carryforwards				2,200,000
Deferred tax assets, tax credit carryforwards, research				$ 12,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Thousands

Dec. 31, 2018shares

Jan. 31, 2020$ / sharesshares

Dec. 31, 2019ft²$ / sharesshares

Nov. 30, 2019$ / sharesshares

Oct. 31, 2019$ / sharesshares

Jun. 30, 2019shares

Apr. 30, 2019$ / sharesshares

Mar. 30, 2019$ / sharesshares

Jan. 01, 2020shares

Sep. 30, 2019USD ($)ft²shares

Jul. 31, 2019shares

Jan. 22, 2019USD ($)ft²

Subsequent Event [Line Items]

Term of lease

4 years

4 years

Area of office space | ft²

5,600

5,600

Number of common stock options, granted

1,426,875

1,426,875

Common stock options granted, exercise price per share | $ / shares

$ 2.936

$ 2.936

Lease liability | $

$ 1,456

$ 1,600

Common stock options granded, vesting period

4 years

4 years

Shares reserved for issuance

9,347,681

3,179,255

4,979,255

Increase in shares reserved for issuance

1,320,104

Reserved for Employee Stock Purchase Plan

Subsequent Event [Line Items]

Shares reserved for issuance

500,000

2019 Equity Incentive Plan

Subsequent Event [Line Items]

Shares reserved for issuance

1,272,443

1,401,443

Common Stock

Subsequent Event [Line Items]

Number of shares repurchased

550,781

25,782

Subsequent Event

Subsequent Event [Line Items]

Term of lease

5 years

Area of office space | ft²

5,600

Number of common stock options, granted

260,000

260,000

260,000

260,000

Common stock options granted, exercise price per share | $ / shares

$ 14.62

$ 14.62

$ 14.62

$ 14.62

Subsequent Event | Reserved for Employee Stock Purchase Plan

Subsequent Event [Line Items]

Increase in number of common stock reserved for issuance

229,899

Subsequent Event | 2019 Equity Incentive Plan

Subsequent Event [Line Items]

Increase in number of common stock reserved for issuance

1,149,499

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Detail) $ in Thousands	Jul. 03, 2019	Dec. 31, 2018USD ($)	Sep. 30, 2019USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reverse stock split ratio	0.125
Accumulated deficit		$ (17,348)	$ (51,949)
Proceeds from the issuance of preferred stock, net		$ 59,785	$ 59,977

Convertible Note Financing - Ad

Convertible Note Financing - Additional Information (Detail) - USD ($)	Mar. 08, 2018	Dec. 31, 2019	Dec. 31, 2018	Nov. 05, 2018
Short-term Debt [Line Items]
Conversion upon a qualified equity financing	$ 5,000,000
Unpaid interest	80.00%
Convertible Notes Payable [Member]
Short-term Debt [Line Items]
Accured Interest		8.00%
Maturity date		Dec. 31, 2019
Accretion of the discount and aggregate interest expense			$ 14,000
Convertible Notes Payable [Member] | Series A Preferred Stock
Short-term Debt [Line Items]
Convertible preferred stock, shares issued upon conversion				63,585
Convertible Notes Payable [Member] | Chief Executive Officer [Member]
Short-term Debt [Line Items]
Promissory note issued	$ 50,000

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Deficit - Additional Information (Detail)	Dec. 31, 2018USD ($)$ / sharesshares	Nov. 05, 2018USD ($)$ / sharesshares	Apr. 30, 2019shares	Oct. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2018USD ($)Vote$ / sharesshares	Sep. 30, 2019USD ($)Vote$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Jun. 30, 2019USD ($)$ / shares	Apr. 12, 2019USD ($)shares	Mar. 31, 2019USD ($)
Class of Stock [Line Items]
Number of shares issued in cash | $		$ 59,800,000
Preferred stock share issuance	0				0	0	0
Share of common stock	1,187,500				1,187,500	22,989,987	1,187,500
Common stock, par value | $ / shares	$ 0.0001			$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Conversion [Member]
Class of Stock [Line Items]
Underwritten public offering | $	$ 50,000,000				$ 50,000,000		$ 50,000,000
Stock Split	16.0415				16.0415		16.0415
Common Stock
Class of Stock [Line Items]
Number of shares repurchased	550,781		25,782
Common stock voting right					Each share of common stock is entitled to one voting right.	Each share of common stock is entitled to one voting right.
Number of vote entitled for each common stockholder | Vote					1	1
Secured Convertible Promissory Note [Member]
Class of Stock [Line Items]
Stock converted into principal and accrued interest | $		$ 51,014
Founder | Common Stock
Class of Stock [Line Items]
Stock issued for services, shares		562,500		1,187,500
Stock issued for services, value | $				$ 950
Vesting period		4 years
Series A Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Number of shares issued	59,908,284	119,752,983			59,908,284	0	59,908,284
Issuance of stock at per share | $ / shares	$ 1.00259507	$ 1.00259507			$ 1.00259507		$ 1.00259507	$ 1.00259507
Number of shares issued in cash | $	$ 59,849,000				$ 59,849,000	$ 0	$ 59,849,000	$ 119,826,000		$ 59,849,000
Series A Redeemable Convertible Preferred Stock | Shire License Agreement [Member]
Class of Stock [Line Items]
Share of common stock	1,859,151				1,859,151		1,859,151
Outstanding diluted share					0.15
Ownership interest	5.00%				5.00%		5.00%
Ownership interest description					The Company will issue additional shares of common stock to increase Shire's ownership to 5% of the then-outstanding fully diluted shares, and (2) in the event that certain liquidation events occur prior to an initial public offering and Shire's ownership is less than 5% of the then outstanding fully diluted shares of common stock, Shire is entitled to receive additional proceeds, if needed, to equal 5% of proceeds available for distribution to the Companies security holders.
Series A Redeemable Convertible Preferred Stock | Shire License Agreement [Member] | Maximum
Class of Stock [Line Items]
Outstanding diluted share					0.05
Redeemable Common Stock
Class of Stock [Line Items]
Number of shares issued	1,859,151	59,908,284			1,859,151	0	1,859,151
Issuance of stock at per share | $ / shares		$ 1.00259507
Number of shares issued in cash | $	$ 6,990,000				$ 6,990,000	$ 0	$ 6,990,000	$ 6,990,000		$ 6,990,000
Series A Preferred Stock
Class of Stock [Line Items]
Additional number of shares issued	59,844,699				59,844,699		59,844,699
Preferred stock share issuance									59,844,699
Cash proceeds | $									$ 60,000,000
Preferred stock dividend rate							8.00%
Series A Preferred Stock | Foreign Investor [Member]
Class of Stock [Line Items]
Shares Distributed To Deferred Compensation Participants					7,480,590

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock, Redeemable Common Stock and Stockholders' Deficit - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock reserved for issuance	4,979,255	3,179,255	9,347,681
Reserved for Future Stock Awards or Option Grants
Class of Stock [Line Items]
Common stock reserved for issuance	1,272,443		1,859,151
Series A Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Common stock reserved for issuance			7,488,530

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Loss before Taxes (Detail) $ in Thousands	8 Months Ended
	Dec. 31, 2018USD ($)
Income Tax Disclosure [Abstract]
U.S. loss before taxes	$ (17,348)
Foreign loss before taxes	0
Loss before income taxes	$ (17,348)

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of Income Tax Expense (Detail) - USD ($)	3 Months Ended		5 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Current:
Federal				$ 0
State				0
Foreign				0
Total current income tax expense				0
Deferred:
Federal				0
State				0
Foreign				0
Total deferred income tax expense				0
Total income tax expense	$ 0	$ 0	$ 0	$ 0	$ 0

Income Taxes - Summary of Rec_2

Income Taxes - Summary of Reconciliation of Federal Statutory Income Tax Rate (Detail)	8 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%
State tax	7.00%
Tax credits	1.90%
Change in valuation allowance	(29.90%)
Total tax benefit	0.00%

Income Taxes - Significant Comp

Income Taxes - Significant Components of Deferred Tax Assets (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Deferred tax assets:
Net operating losses	$ 602
Tax credit carryforwards	346
Accrued expenses	148
Intangibles	4,085
Other	14
Net deferred tax assets before valuation allowance	5,195
Valuation allowance	(5,195)
Net deferred tax assets	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) $ in Thousands	8 Months Ended
	Dec. 31, 2018USD ($)
Income Tax Disclosure [Abstract]
Increase related to current year tax provision	$ 116
Balance at the end of the year	$ 116