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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2018	Nov. 12, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2018
Entity Registrant Name	resTORbio, Inc.
Entity Central Index Key	0001720580
Trading Symbol	TORC
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		28,055,344

Condensed Consolidated Balance Sheets (unaudited) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 22,856,000	$ 53,349,000
Marketable securities	92,522,000	0
Prepaid expenses	1,621,000	792,000
Deferred offering costs		929,000
Other current assets	24,000	84,000
Total current assets	117,023,000	55,154,000
Restricted cash	84,000	0
Property and equipment, net	319,000	39,000
Total assets	117,426,000	55,193,000
Current liabilities:
Accounts payable (including related party amounts of $37 and $32 as of September 30, 2018 and December 31, 2017, respectively)	4,201,000	1,515,000
Accrued liabilities	3,363,000	3,987,000
Funding advance	500,000
Total current liabilities	8,064,000	5,502,000
Other liabilities	22,000
Total liabilities	8,086,000	5,502,000
Commitments and contingencies (see Note 10)
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value, 10,000,000 and no shares authorized as of September 30, 2018 and December 31, 2017, respectively; no shares issued and outstanding as of September 30, 2018 and December 31, 2017
Common stock, $0.0001 par value, 150,000,000 and 30,000,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 28,048,315 and 5,659,089 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively; 28,046,815 and 4,562,640 shares vested as of September 30, 2018 and December 31, 2017, respectively	3,000	1,000
Additional paid-in capital	175,049,000	1,849,000
Accumulated deficit	(65,636,000)	(33,779,000)
Accumulated other comprehensive loss	(76,000)
Total stockholders’ equity (deficit)	109,340,000	(31,929,000)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 117,426,000	55,193,000
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock		41,674,000
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock		$ 39,946,000

Condensed Consolidated Balance Sheets (Parenthetical) (unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts payable to related parties	$ 37	$ 32
Preferred stock, shares authorized		20,320,667
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
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	30,000,000
Common stock, shares issued	28,048,315	5,659,089
Common stock, shares outstanding	28,048,315	5,659,089
Common stock, shares vested	28,046,815	4,562,640
Series A Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	15,527,951
Preferred stock, shares issued	0	15,527,951
Preferred stock, shares outstanding	0	15,527,951
Series B Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	4,792,716
Preferred stock, shares issued	0	4,792,716
Preferred stock, shares outstanding	0	4,792,716

Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 6,765	$ 3,333	$ 26,716	$ 10,047
General and administrative	2,267	612	6,629	1,312
Total operating expenses	9,032	3,945	33,345	11,359
Loss from operations	(9,032)	(3,945)	(33,345)	(11,359)
Other income, net	625	635	1,488	635
Loss before income taxes	(8,407)	(3,310)	(31,857)	(10,724)
Net loss	$ (8,407)	$ (3,310)	$ (31,857)	$ (10,724)
Net loss per share, basic and diluted	$ (0.30)	$ (0.75)	$ (1.23)	$ (2.79)
Weighted-average common shares used in computing net loss per share, basic and diluted	28,046,723	4,408,094	25,895,933	3,839,306
Other comprehensive loss:
Net loss	$ (8,407)	$ (3,310)	$ (31,857)	$ (10,724)
Unrealized losses on marketable securities, net of income tax expense	(46)		(76)
Comprehensive loss	$ (8,453)	$ (3,310)	$ (31,933)	$ (10,724)

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net loss	$ (31,857)	$ (10,724)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	(305)
Depreciation and amortization expense	55	3
Stock-based compensation expense	2,212	301
Change in fair value of tranche liability		(635)
Expense related to acquisition of intellectual property (see Note 6)		3,157
Changes in operating assets and liabilities:
Restricted cash	(84)
Prepaid expenses and other current assets	(769)	(214)
Accounts payable	2,807	1,094
Accrued liabilities	(386)	1,022
Funding advance	500
Other liabilities	22
Net cash used in operating activities	(27,805)	(5,996)
Investing activities:
Purchases of property and equipment	(333)	(39)
Purchases of marketable securities	(92,293)
Net cash used in investing activities	(92,626)	(39)
Financing activities:
Proceeds from issuance of Series A redeemable convertible preferred stock		10,000
Proceeds from initial public offering, net of issuance costs	89,938
Net cash provided by financing activities	89,938	10,000
Net (decrease) increase in cash and cash equivalents	(30,493)	3,965
Cash and cash equivalents at beginning of period	53,349
Cash and cash equivalents at end of period	22,856	$ 3,965
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	2
Conversion of redeemable convertible preferred stock into common stock	$ 81,620

Organization	9 Months Ended
Organization	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization resTORbio, Inc. (collectively referred to with its wholly-owned, controlled subsidiary, resTORbio Securities Corp. as “resTORbio” or the “Company”) was incorporated in the State of Delaware on July 5, 2016. The Company is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel therapeutics for the treatment of aging-related diseases and conditions. The Company’s principal operations are located in Boston, Massachusetts. Since inception, the Company has been primarily involved in research and development activities. The Company devotes substantially all of its efforts to product research and development, initial market development and raising capital. The Company has not generated any product revenue related to its primary business purpose to date and is subject to a number of risks similar to those of other early stage companies, including dependence on key individuals, competition from other companies, the need for development of commercially viable products and the need to obtain adequate additional financing to fund the development of its product candidates. The Company is also subject to a number of risks similar to other companies in the life sciences industry, including regulatory approval of products, uncertainty of market acceptance of products, competition from substitute products and larger companies, the need to obtain additional financing, compliance with government regulations, protection of proprietary technology, dependence on third parties, product liability and dependence on key individuals. Reverse Stock Split On January 12, 2018, the Company effected a one-for-1.2804 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 8). Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. Initial Public Offering On January 30, 2018, the Company completed its initial public offering (“IPO”), whereby the Company sold 6,516,667 shares of its common stock (inclusive of 850,000 shares of common stock sold by the Company pursuant to the full exercise of an overallotment option granted to the underwriters in connection with the offering) at a price of $15.00 per share. The shares began trading on The Nasdaq Global Select Market on January 26, 2018. The aggregate net proceeds received by the Company from the offering were approximately $89.4 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $8.4 million. Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock converted into 15,870,559 shares of common stock and all unvested shares of restricted stock automatically vested. Additionally, the Company is now authorized to issue 150,000,000 shares of common stock and 10,000,000 shares of preferred stock. Liquidity In the course of its development activities, the Company has sustained operating losses and expects such losses to continue over the next several years. The Company’s ultimate success depends on the outcome of its research and development activities. The Company has incurred net losses from operations since inception and has an accumulated deficit of $65.6 million as of September 30, 2018. On January 30, 2018, the Company completed its IPO whereby the Company sold 6,516,667 shares of its common stock for aggregate net proceeds of approximately $89.4 million. As of September 30, 2018, the Company had $115.4 million of cash, cash equivalents, and marketable securities, which the Company believes will be sufficient to fund the Company’s current operating plan through at least the next twelve months.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed consolidated financial statements are unaudited. The unaudited condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) for interim financial information and, in the opinion of management, reflect all adjustments of a normal recurring nature necessary for a fair statement of the Company's financial position as of September 30, 2018 and the results of operations and cash flows for the interim periods ended September 30, 2018 and 2017. The condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 that was filed with the Securities and Exchange Commission (“SEC”) on March 29, 2018 (the “2017 Form 10-K”). Interim results are not necessarily indicative of results for a full year or for any other interim period.

The condensed consolidated financial statements include the accounts of resTORbio, Inc. and its wholly owned subsidiary, resTORbio Securities Corp. All inter-company transactions and balances have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities, as of the date of the condensed consolidated financial statements, and the reported amounts of any expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to accrued liabilities, income taxes, and stock-based compensation expense. Management bases its estimates on historical experience, and on various other market-specific relevant assumptions that management believes to be reasonable, under the circumstances. Actual results may differ from those estimates or assumptions.

Summary of Significant Accounting Policies

The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the 2017 Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three and nine months ended September 30, 2018, except as noted below.

Marketable securities

The Company classifies marketable securities with remaining maturities when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. Available-for-sale securities are maintained by investment managers and consist of U.S. treasury securities and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expensed over the life of the instrument.

If any adjustment to fair value reflects a decline in the value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a change to the Company’s statement of operations and comprehensive loss.

Restricted Cash

The Company maintains a letter of credit for the benefit of the landlord in connection with the Company’s office lease. As of September 30, 2018 and December 31, 2017, restricted cash (non-current) related to this letter of credit consisted of $84,000 and $0, respectively.

Fair Value Measurements

Fair value is defined as the price at which an asset could be exchanged in a current transaction between knowledgeable, willing parties. The authoritative accounting guidance describes a fair value hierarchy based on three levels of inputs that may be used to measure fair value, of which the first two are considered observable and the last is considered unobservable. These levels of inputs are as follows:

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active.

Level 3—Unobservable inputs that reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model.

The following table summarizes assets measured at fair value on a recurring basis at September 30, 2018 (in thousands):

			Active		Observable		Unobservable
	September 30,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	14,791	$	14,791	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		8,065		8,065		—		—
U.S. treasury securities (included in marketable securities)		92,522		92,522		—		—
Total	$	115,378	$	115,378	$	—	$	—

The following table summarizes assets measured at fair value on a recurring basis at December 31, 2017 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	53,349	$	53,349	$	—	$	—
Total	$	53,349	$	53,349	$	—	$	—

Deferred Offering Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations. The Company recorded deferred offering costs of $0 and $0.9 million as of September 30, 2018 and December 31, 2017, respectively.

Property and Equipment

Property and equipment are stated at cost, less accumulated depreciation. Depreciation is recorded using the straight-line method over the estimated useful lives of the respective assets. Depreciation begins at the time the asset is placed in service. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statements of operations.

The estimated useful lives of property and equipment are as follows:

		Useful Life (in years)
Leasehold improvements		Lesser of useful life or remaining lease term
Laboratory and manufacturing equipment		2-8 years
Computer equipment		1-5 years
Furniture and fixtures		3-5 years
Office equipment		3-5 years
Software		3-5 years

Recently Adopted Accounting Pronouncements

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”). ASU 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The guidance is effective for annual periods beginning after December 15, 2017, with early adoption permitted, including adoption in any interim period for which financial statements have not yet been issued. The Company adopted the provisions of ASU 2017-09 on January 1, 2018. No modifications of share-based payment awards have occurred as of September 30, 2018.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (“ASU 2016-02”), which requires a lessee to recognize a right-of-use asset and a lease liability for operating leases, initially measured at the present value of the future lease payments, in the balance sheet. ASU 2016-02 also requires a lessee to recognize a single lease cost, calculated so that the cost of the lease is allocated over the lease term, generally on a straight-line basis. This new guidance is effective for fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the potential effects of adopting the provisions of ASU 2016-02 on its consolidated financial statements.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (“ASU 2016-18”), which requires that amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 is effective for fiscal years beginning after December 15, 2018 and interim periods in fiscal years beginning after December 15, 2019 and should be applied using a retrospective transition method to each period presented. Early adoption is permitted. The Company does not expect the impact of ASU 2016-18 to be material to its consolidated financial statements.

In July 2017, the FASB issued ASU 2017-11, Accounting for Certain Financial Instruments with Down Round Features (“ASU 2017-11”), which updates the guidance related to the classification analysis of certain equity-linked financial instruments (or embedded features) with down round features. Under ASU 2017-11, a down round feature no longer precludes equity classification when assessing whether the instrument is indexed to an entity’s own stock. As a result, a freestanding equity-linked financial instrument (or embedded conversion option) no longer would be accounted for as a derivative liability at fair value as a result of the existence of a down round feature. For freestanding equity classified financial instruments, the amendments require entities that present earnings per share (“EPS”) in accordance with Topic 260 to recognize the effect of the down round feature when it is triggered. That effect is treated as a dividend and as a reduction of income available to common shareholders in basic EPS. ASU 2017-11 is effective for public entities for all annual and interim periods beginning after December 15, 2019. Early adoption is permitted. The Company does not expect the impact of ASU 2017-11 to be material to its consolidated financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which intends to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities and emerging growth companies that choose to take advantage of the extended transition period, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities but no earlier than the Company’s adoption of ASU 2018-07. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its financial statements.

Marketable Securities

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

As of September 30, 2018, the fair value of marketable securities by type of security was as follows (in thousands):

	Amortized		Unrealized		Unrealized		Fair
Description	Cost		Gains		Losses		Value
U.S. government agency securities and treasuries	$	92,598	$	—	$	76	$	92,522
Total	$	92,598	$	—	$	76	$	92,522

The Company did not have any marketable securities as of December 31, 2017.

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows (in thousands):

	September 30, 2018
	Amortized		Fair
	Cost		Value
Due in one year or less	$	92,598	$	92,522
Total	$	92,598	$	92,522

Property and Equipment, Net

9 Months Ended

Sep. 30, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and equipment, net

Property and equipment, net consists of the following:

	September 30, 2018			December 31, 2017
	(In thousands)
Leasehold improvements	$	65		$	—
Machinery and equipment		38			38
Furniture and fixtures		176			—
Computers		69			6
Office equipment		9			—
Software		22			—
Total property and equipment		379			44
Less: accumulated depreciation		(60	)		(5	)
Property and equipment, net	$	319		$	39

Depreciation expense was $24,000 and $3,000 for the three months ended September 30, 2018 and 2017, respectively, and $55,000 and $3,000 for the nine months ended September 30, 2018 and 2017, respectively.

Accrued Liabilities

9 Months Ended

Sep. 30, 2018

Payables And Accruals [Abstract]

Accrued Liabilities

5. Accrued Liabilities

Accrued liabilities consist of the following:

	September 30, 2018		December 31, 2017
	(In thousands)
Accrued payroll and related expenses	$	789	$	394
Accrued research and development expenses		2,214		3,250
Deferred offering costs		—		238
Other		360		105
Total accrued liabilities	$	3,363	$	3,987

License Agreements	9 Months Ended
License Agreements	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	6. License Agreements Novartis License Agreement On March 23, 2017, the Company entered into an exclusive license agreement with Novartis International Pharmaceutical Ltd. (“Novartis”). Under the agreement, Novartis granted the Company an exclusive, field-restricted, worldwide license, to certain intellectual property rights owned or controlled by Novartis, to develop, commercialize and sell one or more therapeutic products comprising RTB101 or RTB101 in combination with everolimus in a fixed dose combination. The exclusive field under the license agreement is for the treatment, prevention and diagnosis of disease and other conditions in all indications in humans and animals. As initial consideration for the licensed rights, the Company issued Novartis Institutes for Biomedical Research (“NIBR”) 2,587,992 shares of the Company’s Series A Preferred Stock. The fair value of the Novartis license was $3.2 million based on the fair value of the Series A Preferred Stock which was determined to be $1.22 per share based on an independent third-party valuation, and is recorded as research and development expenses in the consolidated statements of operations. The agreement may be terminated by either party upon a material breach by the other party that is not cured within 60 days after written notice. The Company may terminate the agreement in its entirety or on a product-by-product or country-by-country basis with or without cause with 60 days’ prior written notice. Novartis may terminate the portion of the agreement related to everolimus if the Company fails to use commercially reasonable efforts to research, develop and commercialize a product utilizing everolimus for a period of three years. Novartis may terminate the license agreement upon the Company’s bankruptcy, insolvency, dissolution or winding up. As additional consideration for the license, the Company is required to pay up to an aggregate of $4.3 million upon the satisfaction of clinical milestones, up to an aggregate of $24 million upon the satisfaction of regulatory milestones for the first indication approved, and up to an aggregate of $18 million upon the satisfaction of regulatory milestones for the second indication approved. In addition, the Company is required to pay up to an aggregate of $125 million upon the satisfaction of commercial milestones, based on the amount of annual net sales. The Company is also required to pay tiered royalties ranging from a mid single-digit percentage to a low teen-digit percentage on annual net sales of products. These royalty obligations last on a product-by-product and country-by-country basis until the latest of (i) the expiration of the last valid claim of a Novartis patent covering a subject product, (ii) the expiration of any regulatory exclusivity for the subject product in a country, or (iii) the 10^th anniversary of the first commercial sale in the country, and are subject to a reduction after the expiration of the last valid claim of a Novartis patent or the introduction of a generic equivalent of a product in a country. Following the last visit of the 400^th subject in the Company’s Phase 2b clinical trial, Novartis is no longer entitled to sublicense revenue. Milestone payments to Novartis will be recorded as research and development expenses in the consolidated statements of operations once achievement of each associated milestone has occurred or the achievement is considered probable. In May 2017, the Company initiated a Phase 2b clinical trial for a first indication, triggering the first milestone payment under the agreement. Accordingly, the Company paid the related $0.3 million milestone in May 2017. As of September 30, 2018, none of the remaining development milestones, regulatory milestones, sales milestones, or royalties had been reached or were probable of achievement.

Research Funding Agreements	9 Months Ended
Research Funding Agreements	Sep. 30, 2018
Research Funding Agreement [Abstract]
Research Funding Agreement	7. Research Funding Agreement On March 6, 2018, the Company and the Silverstein Foundation for Parkinson’s with GBA (the “Silverstein Foundation”) entered into a research funding agreement (the “Funding Agreement”). One of the Company’s directors is a co-founder and current trustee of the Silverstein Foundation. Under the terms of the Funding Agreement, the Silverstein Foundation will partially fund the preclinical research, development work, and Phase 2 clinical trial expenses (the “Research”) to be conducted and borne by the Company in connection with the development of RTB101, alone or in combination with other products (the “Product”). Upon execution of the Funding Agreement, the Silverstein Foundation paid the Company an upfront sum of $0.5 million (the “Funding Amount”). The Company is entitled to use the Funding Amount solely to conduct the Research and is obligated to repay the Funding Amount in full to the Silverstein Foundation if it successfully conducts a positive Phase 3 clinical trial of the Product for Parkinson’s Disease. The Company is solely responsible for commencing and conducting the Research and will furnish periodic progress updates to the Silverstein Foundation throughout the term of the Funding Agreement. After completing the Research, the Company must provide the Silverstein Foundation with a formal report describing the work performed and the results of the Research. The Company recognizes proceeds received from the Silverstein Foundation as a reduction to research and development expenses, rather than as revenue, in the condensed consolidated statements of operations because the corresponding Funding Agreement does not contain specified performance obligations other than to conduct research on a particular program or in a particular field and contain no obligations to deliver specified products or technology. For funds received under the Funding Agreement, the Company recognizes a reduction in research and development expenses in an amount equal to the qualifying expenses incurred in each period up to the amount funded by the Silverstein Foundation. Funding that has been received by the Company in advance of incurring qualifying expenses is recorded in the condensed consolidated balance sheet as funding advance. As of September 30, 2018, no qualifying expenses have been incurred.

Redeemable Convertible Preferred Stock and Common Stock

9 Months Ended

Sep. 30, 2018

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock and Common Stock

8. Redeemable Convertible Preferred Stock and Common Stock

As of September 30, 2018, the Company had 10,000,000 shares of preferred stock authorized and none issued and outstanding. As of December 31, 2017, the Company had 20,320,667 shares of preferred stock authorized, of which 15,527,951 shares were issued and outstanding and were designated as $0.0001 par value Series A Preferred Stock and 4,792,716 shares were issued and outstanding and were designated as $0.0001 par value Series B Preferred Stock.

Upon completion of the Company’s initial public offering in January 2018, all the outstanding preferred stock of the Company automatically converted into 15,870,559 shares of the Company’s common stock. As of September 30, 2018, no shares of preferred stock were outstanding.

Reserve for future issuance

The Company has reserved the following number of shares of common stock for future issuance upon the conversion of preferred stock, exercise of options or grant of equity awards:

	September 30, 2018		December 31, 2017
Redeemable convertible preferred stock, on an as-converted basis		—		15,870,559
Options issued and outstanding		1,066,678		195,668
Unvested restricted stock units		24,960		—
Options available for future grants		1,413,610		1,670,341
Shares available for issuance under the 2018 ESPP		275,030		—
Total		2,780,278		17,736,568

Stock-Based Compensation

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9. Stock-based Compensation

In 2017, the Company adopted the 2017 Stock Incentive Plan (the “2017 Plan”). Under the 2017 Plan, a total of 537,914 shares of the Company’s common stock were reserved for the issuance of stock options to employees, directors, and consultants under terms and provisions established by the Board of Directors (the “Board”). Under the terms of the 2017 Plan, options were granted at an exercise price not less than fair market value. The terms of options granted under the 2017 Plan may not exceed ten years. The Board determined the terms and conditions of a Restricted Stock Award, including the conditions for vesting and repurchase (or forfeiture) and the issue price, if any. On October 11, 2017, the Company increased the number of shares of common stock available for issuance under the 2017 Plan from 537,914 shares to 630,662 shares. On November 29, 2017, the Company increased the number of shares of common stock available for issuance under the 2017 Plan from 630,662 shares to 1,866,009 shares.

In connection with the Company’s IPO, the Board adopted and the Company’s stockholders approved the 2018 Stock Incentive Plan (“2018 Plan”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units, stock appreciation rights, and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2018 Plan. The number of shares of common stock that are reserved for issuance under the 2018 Plan are 2,200,260 shares. The 2018 Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning on January 1, 2019, by 4% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Board.

Since the date of effectiveness of the 2018 Plan, the Company has not and will not grant any further awards under the 2017 Plan. However, any shares of common stock subject to awards under the 2017 Plan that expire, terminate, or otherwise are surrendered, canceled, forfeited or repurchased without having been fully exercised or resulting in any common stock being issued will become available for issuance under the 2018 Plan.

Stock-based Compensation Expense

Total stock-based compensation expense is recognized for stock options granted to employees and non-employees and has been reported in the Company’s consolidated statements of operations as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	229	$	47	$	955	$	152
General and administrative		400		44		1,257		149
Total stock-based compensation expense	$	629	$	91	$	2,212	$	301

Stock Options

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding		Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
										(In thousands)
Outstanding, December 31, 2017		1,670,341			195,668	$	4.49		9.67
Shares reserved for issuance		641,239
Options granted⁽¹⁾		(871,010	)		871,010		13.09
Restricted stock granted		(2,000	)
Restricted stock units granted		(24,960	)
Outstanding, September 30, 2018		1,413,610			1,066,678		11.52		9.43	$	3,901
Exercisable, September 30, 2018					25,487		0.79		8.70		366
Vested and expected to vest, September 30, 2018					1,066,678		11.52		9.43		3,901

(1)	The Company granted 7,200 stock options to non-employees during the nine months ended September 30, 2018.

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock as of September 30, 2018. No options were exercised, cancelled or forfeited during the nine months ended September 30, 2018.

During the nine months ended September 30, 2018, the Company granted options to employees and directors to purchase an aggregate of 863,810 common shares with a weighted-average grant date fair value of $9.20 per share. During the nine months ended September 30, 2018, the Company granted options to non-employees to purchase an aggregate of 7,200 common shares with a weighted-average grant date fair value of $12.51 per share. The expense related to options granted to employees and directors for the three and nine months ended September 30, 2018 was $0.6 million and $1.2 million, respectively. The expense related to options granted to non-employees for the three and nine months ended September 30, 2018 was $37,000 and $64,000, respectively. During the nine months ended September 30, 2017, the Company granted options to employees to purchase an aggregate of 64,460 common shares at a weighted-average grant date fair value of $0.53 per share. During the nine months ended September 30, 2017, the Company granted options to non-employees to purchase an aggregate of 46,860 common shares at a weighted-average grant date fair value of $0.67 per share. The expense related to options granted to employees and non-employees was $3,000 and $2,000, respectively, for the three and nine months ended September 30, 2017, respectively.

As of September 30, 2018, the total unrecognized compensation expense related to unvested options granted to employees and directors was $7.4 million, which the Company expects to recognize over an estimated weighted-average period of 3.40 years. As of September 30, 2018, the total unrecognized compensation expense related to unvested non-employee options was $0.4 million, which the Company expects to recognize over an estimated weighted-average period of 2.87 years.

The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Employees:
Fair value of common stock	$9.03 - $13.94		$0.79		$8.57 - $15.45		$0.79
Expected term (in years)	5.9 - 6.1			6.1	5.8 - 6.6			6.1
Expected volatility	89.8% - 90.6%		74.4%		75.9% - 90.6%		74.4%
Risk-free interest rate	2.8% - 2.9%		1.9%		2.4% - 2.9%		1.9%
Expected dividend yield	0.0%		0.0%		0.0%		0.0%
Non-employees:
Fair value of common stock		—	$1.00		$15.45		$0.79 - $1.00
Expected term (in years)		—		10.0		10.0		10.0
Expected volatility		—	76.9%		77.9%		74.6% - 76.9%
Risk-free interest rate		—	2.3%		2.7%		2.3%
Expected dividend yield		—	0.0%		0.0%		0.0%

Restricted Stock

On July 11, 2016, certain founders purchased 3,772,726 common shares that were subject to a repurchase right upon termination or cessation of services at the original purchase price of $0.0001 per share, or $483. The repurchase right lapsed as vesting occurred. Compensation expense of such unvested shares was remeasured at fair value until vested at each reporting date. On April 4, 2017, the non-employee directors became employees of the Company and as a result, compensation expense of the unvested shares was remeasured at fair value and fixed and was being recognized over the remaining vesting period. Upon the closing of the Series A preferred financing, a portion of the unvested shares accelerated and vested in full. Upon the Company’s IPO, the remaining unvested shares accelerated and vested in full.

On April 17, 2018, the Company granted 2,000 shares of restricted stock to a consultant. The restrictions will lapse in four equal quarterly installments and will be fully vested on the first anniversary of such grant. Compensation expenses of such unvested shares will be remeasured at fair value until vested at each reporting date.

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2017		1,096,449
Granted		2,000
Vested		(1,096,949	)
Unvested shares — September 30, 2018		1,500

The Company recognized $11,000, $86,000, $0.9 million and $0.3 million of stock-based compensation expense related to restricted shares during the three and nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, there was $12,000 of unrecognized stock-based compensation expense related to unvested restricted stock. This amount is expected to be recognized over a remaining weighted-average period of 0.55 years.

Restricted Stock Units

During the nine months ended September 30, 2018, the Company granted 24,960 restricted stock units to an employee with a weighted-average grant date fair value of $9.03 per share.

The summary of restricted stock unit activity and related information follows:

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2017		—
Granted		24,960
Unvested shares — September 30, 2018		24,960

The Company recognized $14,000 and $20,000 of stock-based compensation expense related to restricted stock units during the three and nine months ended September 30, 2018, respectively. As of September 30, 2018, there was $0.2 million of unrecognized stock-based compensation expense related to unvested restricted stock units. This amount is expected to be recognized over a remaining weighted-average period of 3.64 years. There were no restricted stock units granted to employees or non-employees during the three and nine months ended September 30, 2017.

2018 Employee Stock Purchase Plan

The Board adopted and the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 ESPP enables eligible employees to purchase shares of the Company’s Common Stock at a discount. The number of shares of common stock that are reserved for issuance under the 2018 ESPP are 275,030 shares. The 2018 ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and increasing each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31; (ii) 543,926 shares or (iii) such number of shares as determined by the ESPP administrator.

Commitments and Contingences	9 Months Ended
Commitments and Contingences	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingences	10. Commitments and Contingences Litigation The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities as of September 30, 2018 and December 31, 2017.

Net Loss Per Share

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Net Loss per Share

11. Net Loss per Share

The Company computes basic and diluted earnings (losses) per share using a methodology that gives effect to the impact of outstanding participating securities (the “two-class” method). Basic net loss per share is calculated by dividing net loss by the weighted-average number of common shares outstanding during the period and excludes any dilutive effects of share-based awards. Diluted net loss per share is computed giving effect to all potential dilutive common shares, including common stock issuable upon exercise of stock options, convertible preferred stock, and unvested restricted common stock. As the Company had net losses for the three and nine months ended September 30, 2018 and 2017, there is no income allocation required under the two-class method or dilution attributed to weighted average shares outstanding in the calculation of diluted loss per share.

The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive effect (in common stock equivalent shares):

	As of
	September 30,
	2018		2017
Redeemable convertible preferred stock		—		6,063,711
Options issued and outstanding		1,066,678		111,320
Total		1,066,678		6,175,031

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions Since the Company’s incorporation in July 2016, the Company has engaged in transactions with related parties. The Company is a party to an intellectual property license agreement with Novartis. In addition, NIBR, an affiliate of Novartis, is a shareholder of the Company (See Note 6). No payments have been made to Novartis during the three and nine months ended September 30, 2018. During the three and nine months ended September 30, 2017, the Company made payments to Novartis for milestones achieved pursuant to the license agreement. The Company is a party to a Funding Agreement with the Silverstein Foundation, an entity in which one of the Company’s directors is a co-founder and current trustee (See Note 7). The Company received $0 and $0.5 million from the Silverstein Foundation during the three and nine months ended September 30, 2018, respectively. No funds were received during the three and nine months ended September 30, 2017.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Summary of Significant Accounting Policies

Marketable Securities

Marketable securities

Restricted Cash

Fair Value Measurements

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.

The following table summarizes assets measured at fair value on a recurring basis at September 30, 2018 (in thousands):

			Active		Observable		Unobservable
	September 30,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	14,791	$	14,791	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		8,065		8,065		—		—
U.S. treasury securities (included in marketable securities)		92,522		92,522		—		—
Total	$	115,378	$	115,378	$	—	$	—

The following table summarizes assets measured at fair value on a recurring basis at December 31, 2017 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	53,349	$	53,349	$	—	$	—
Total	$	53,349	$	53,349	$	—	$	—

Deferred Offering Costs

Property and Equipment

The estimated useful lives of property and equipment are as follows:

		Useful Life (in years)
Leasehold improvements		Lesser of useful life or remaining lease term
Laboratory and manufacturing equipment		2-8 years
Computer equipment		1-5 years
Furniture and fixtures		3-5 years
Office equipment		3-5 years
Software		3-5 years

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of Assets Measured at Fair Value on Recurring Basis

The following table summarizes assets measured at fair value on a recurring basis at September 30, 2018 (in thousands):

			Active		Observable		Unobservable
	September 30,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	14,791	$	14,791	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		8,065		8,065		—		—
U.S. treasury securities (included in marketable securities)		92,522		92,522		—		—
Total	$	115,378	$	115,378	$	—	$	—

The following table summarizes assets measured at fair value on a recurring basis at December 31, 2017 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	53,349	$	53,349	$	—	$	—
Total	$	53,349	$	53,349	$	—	$	—

Schedule of Estimated Useful Lives of Property and Equipment

The estimated useful lives of property and equipment are as follows:

		Useful Life (in years)
Leasehold improvements		Lesser of useful life or remaining lease term
Laboratory and manufacturing equipment		2-8 years
Computer equipment		1-5 years
Furniture and fixtures		3-5 years
Office equipment		3-5 years
Software		3-5 years

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value Marketable Securities By Type of Security

As of September 30, 2018, the fair value of marketable securities by type of security was as follows (in thousands):

	Amortized		Unrealized		Unrealized		Fair
Description	Cost		Gains		Losses		Value
U.S. government agency securities and treasuries	$	92,598	$	—	$	76	$	92,522
Total	$	92,598	$	—	$	76	$	92,522

Schedule of Fair Value and Amortized Cost of Company's Available-for-Sale Securities by Contractual Maturity

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows (in thousands):

	September 30, 2018
	Amortized		Fair
	Cost		Value
Due in one year or less	$	92,598	$	92,522
Total	$	92,598	$	92,522

Property and Equipment, Net (Tables)

9 Months Ended

Sep. 30, 2018

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	September 30, 2018			December 31, 2017
	(In thousands)
Leasehold improvements	$	65		$	—
Machinery and equipment		38			38
Furniture and fixtures		176			—
Computers		69			6
Office equipment		9			—
Software		22			—
Total property and equipment		379			44
Less: accumulated depreciation		(60	)		(5	)
Property and equipment, net	$	319		$	39

Accrued Liabilities (Tables)

9 Months Ended

Sep. 30, 2018

Payables And Accruals [Abstract]

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	September 30, 2018		December 31, 2017
	(In thousands)
Accrued payroll and related expenses	$	789	$	394
Accrued research and development expenses		2,214		3,250
Deferred offering costs		—		238
Other		360		105
Total accrued liabilities	$	3,363	$	3,987

Redeemable Convertible Preferred Stock and Common Stock (Tables)

9 Months Ended

Sep. 30, 2018

Temporary Equity Disclosure [Abstract]

Schedule of Number of Shares of Common Stock Reserved for Future Issuance

The Company has reserved the following number of shares of common stock for future issuance upon the conversion of preferred stock, exercise of options or grant of equity awards:

	September 30, 2018		December 31, 2017
Redeemable convertible preferred stock, on an as-converted basis		—		15,870,559
Options issued and outstanding		1,066,678		195,668
Unvested restricted stock units		24,960		—
Options available for future grants		1,413,610		1,670,341
Shares available for issuance under the 2018 ESPP		275,030		—
Total		2,780,278		17,736,568

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	229	$	47	$	955	$	152
General and administrative		400		44		1,257		149
Total stock-based compensation expense	$	629	$	91	$	2,212	$	301

Summary of Stock Option Activity

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding		Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
										(In thousands)
Outstanding, December 31, 2017		1,670,341			195,668	$	4.49		9.67
Shares reserved for issuance		641,239
Options granted⁽¹⁾		(871,010	)		871,010		13.09
Restricted stock granted		(2,000	)
Restricted stock units granted		(24,960	)
Outstanding, September 30, 2018		1,413,610			1,066,678		11.52		9.43	$	3,901
Exercisable, September 30, 2018					25,487		0.79		8.70		366
Vested and expected to vest, September 30, 2018					1,066,678		11.52		9.43		3,901

(1)	The Company granted 7,200 stock options to non-employees during the nine months ended September 30, 2018.

Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model

The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Employees:
Fair value of common stock	$9.03 - $13.94		$0.79		$8.57 - $15.45		$0.79
Expected term (in years)	5.9 - 6.1			6.1	5.8 - 6.6			6.1
Expected volatility	89.8% - 90.6%		74.4%		75.9% - 90.6%		74.4%
Risk-free interest rate	2.8% - 2.9%		1.9%		2.4% - 2.9%		1.9%
Expected dividend yield	0.0%		0.0%		0.0%		0.0%
Non-employees:
Fair value of common stock		—	$1.00		$15.45		$0.79 - $1.00
Expected term (in years)		—		10.0		10.0		10.0
Expected volatility		—	76.9%		77.9%		74.6% - 76.9%
Risk-free interest rate		—	2.3%		2.7%		2.3%
Expected dividend yield		—	0.0%		0.0%		0.0%

Summary of Restricted Stock Activity

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2017		1,096,449
Granted		2,000
Vested		(1,096,949	)
Unvested shares — September 30, 2018		1,500

Summary of Restricted Stock Units Activity

The summary of restricted stock unit activity and related information follows:

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2017		—
Granted		24,960
Unvested shares — September 30, 2018		24,960

Net Loss Per Share (Tables)

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share

	As of
	September 30,
	2018		2017
Redeemable convertible preferred stock		—		6,063,711
Options issued and outstanding		1,066,678		111,320
Total		1,066,678		6,175,031

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended
	Jan. 30, 2018	Jan. 12, 2018	Sep. 30, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Entity incorporation date			Jul. 05, 2016
Reverse stock split effective date		Jan. 12, 2018
Reverse stock split description		one-for-1.2804
Net proceeds received from offering			$ 89,938
Common stock, shares authorized	150,000,000		150,000,000	30,000,000
Preferred stock, shares authorized	10,000,000		10,000,000	0
Accumulated deficit			$ 65,636	$ 33,779
Cash, cash equivalents and marketable securities			$ 115,400
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold	6,516,667
Issuance price per shares	$ 15.00
Net proceeds received from offering	$ 89,400
Payments for initial public offering after deducting underwriting discounts and commissions and other offering expenses	$ 8,400
Convertible preferred stock converted into common stock	15,870,559
Over-allotment Option
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold	850,000

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Restricted cash (non-current) related to letter of credit	$ 84,000	$ 0
Deferred offering costs	$ 0	$ 900,000

Summary of Significant Accounting Policies - Summary of Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 115,378	$ 53,349
Money Market Funds \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	14,791	53,349
U.S. Treasury Securities \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	8,065
U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	92,522
Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	115,378	53,349
Active Markets (Level 1) \| Money Market Funds \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	14,791	$ 53,349
Active Markets (Level 1) \| U.S. Treasury Securities \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	8,065
Active Markets (Level 1) \| U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 92,522

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	9 Months Ended
	Sep. 30, 2018
Laboratory and Manufacturing Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	2 years
Laboratory and Manufacturing Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	8 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	Lesser of useful life or remaining lease term
Computer Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	1 year
Computer Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	5 years
Furniture and Fixtures \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Furniture and Fixtures \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	5 years
Office Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Office Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	5 years
Software \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Software \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	5 years

Marketable Securities - Schedule of Fair Value Marketable Securities By Type of Security (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Marketable Securities [Line Items]
Amortized Cost	$ 92,598
Unrealized Losses	76
Fair Value	92,522
U.S. Government Agency Securities and Treasuries
Marketable Securities [Line Items]
Amortized Cost	92,598
Unrealized Losses	76
Fair Value	$ 92,522

Marketable Securities - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 92,522,000	$ 0

Marketable Securities - Schedule of Fair Value and Amortized Cost of Company's Available-for-Sale Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 92,598
Amortized Cost	92,598
Due in one year or less, Fair Value	92,522
Fair Value	$ 92,522

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 379	$ 44
Less: accumulated depreciation	(60)	(5)
Property and equipment, net	319	39
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	38	38
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	65
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	176
Computers
Property Plant And Equipment [Line Items]
Total property and equipment	69	$ 6
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	9
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 22

Property and Equipment, Net - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 24,000	$ 3,000	$ 55,000	$ 3,000

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 789	$ 394
Accrued research and development expenses	2,214	3,250
Deferred offering costs		238
Other	360	105
Total accrued liabilities	$ 3,363	$ 3,987

License Agreements - Additional Information (Details) - Novartis International Pharmaceutical Ltd. - License Agreement - USD ($)	1 Months Ended	9 Months Ended
	May 31, 2017	Sep. 30, 2018	Mar. 23, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination description		The agreement may be terminated by either party upon a material breach by the other party that is not cured within 60 days after written notice. The Company may terminate the agreement in its entirety or on a product-by-product or country-by-country basis with or without cause with 60 days’ prior written notice.
Milestone payment	$ 300,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount payable upon satisfaction of clinical milestones			$ 4,300,000
Aggregate amount payable upon satisfaction of regulatory milestones for first indication approved			24,000,000
Aggregate amount payable upon satisfaction of regulatory milestones for second indication approved			18,000,000
Aggregate amount payable upon satisfaction of commercial milestones			$ 125,000,000
Series A Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued			2,587,992
Fair value of license agreement			$ 3,200,000
Fair value of shares issued			$ 1.22

Research Funding Agreement - Additional Information (Details) - Silverstein Foundation - Funding Agreement - USD ($)	Mar. 06, 2018	Sep. 30, 2018
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront sum of funding amount	$ 500,000
Qualifying expenses		$ 0

Redeemable Convertible Preferred Stock and Common Stock - Additional Information (Details) - $ / shares	Sep. 30, 2018	Jan. 31, 2018	Jan. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred stock, shares authorized	10,000,000		10,000,000	0
Preferred stock, shares authorized				20,320,667
Preferred stock, shares issued	0			0
Preferred stock, shares outstanding	0			0
Initial Public Offering
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock			15,870,559
Initial Public Offering \| Common Stock
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock		15,870,559
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized	0			15,527,951
Preferred stock, shares issued	0			15,527,951
Preferred stock, shares outstanding	0			15,527,951
Preferred stock, par value	$ 0.0001			$ 0.0001
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized	0			4,792,716
Preferred stock, shares issued	0			4,792,716
Preferred stock, shares outstanding	0			4,792,716
Preferred stock, par value	$ 0.0001			$ 0.0001

Redeemable Convertible Preferred Stock and Common Stock -Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,780,278	17,736,568
Redeemable Convertible Preferred Stock, on an as-converted Basis
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance		15,870,559
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,066,678	195,668
Unvested Restricted Stock Units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,413,610	1,670,341
Shares Available for Issuance under the 2018 ESPP
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	275,030

Stock-Based Compensation - Additional Information (Details)					3 Months Ended		9 Months Ended
	Apr. 17, 2018 Installment shares	Nov. 29, 2017 shares	Oct. 11, 2017 shares	Jul. 11, 2016 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Recognized stock-based compensation expense \| $							$ 2,212,000	$ 301,000
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options exercised							0
Number of options cancelled or forfeited							0
Options granted, Number of Options							871,010
Options Issued and Outstanding \| Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted							863,810
Weighted-average grant date fair value of options granted \| $ / shares							$ 9.20
Expense related to options granted \| $					$ 600,000		$ 1,200,000
Unrecognized compensation expense \| $					7,400,000		$ 7,400,000
Unrecognized compensation expense, estimated weighted-average period for recognition							3 years 4 months 24 days
Options Issued and Outstanding \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of options granted \| $ / shares								$ 0.53
Expense related to options granted \| $						$ 3,000		$ 2,000
Options granted, Number of Options								64,460
Options Issued and Outstanding \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted							7,200
Weighted-average grant date fair value of options granted \| $ / shares							$ 12.51	$ 0.67
Expense related to options granted \| $					37,000	3,000	$ 64,000	$ 2,000
Options granted, Number of Options							7,200	46,860
Unrecognized compensation expense \| $					400,000		$ 400,000
Unrecognized compensation expense, estimated weighted-average period for recognition							2 years 10 months 13 days
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense, estimated weighted-average period for recognition							6 months 18 days
Shares granted	2,000						2,000
Number of quarterly installment in which restriction lapse \| Installment	4
Recognized stock-based compensation expense \| $					11,000	$ 86,000	$ 900,000	$ 300,000
Unrecognized stock-based compensation expense \| $					12,000		$ 12,000
Restricted Stock \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares issued, subject to repurchase right upon termination or cessation of service				3,772,726
Issuance price per shares \| $ / shares				$ 0.0001
Common shares issued, value \| $				$ 483
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense, estimated weighted-average period for recognition							3 years 7 months 20 days
Shares granted							24,960
Recognized stock-based compensation expense \| $					14,000		$ 20,000
Unrecognized stock-based compensation expense \| $					$ 200,000		$ 200,000
Restricted Stock Units (RSUs) \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted						0	24,960	0
Weighted-average grant date fair value \| $ / shares							$ 9.03
Restricted Stock Units (RSUs) \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted						0		0
2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		630,662	537,914		537,914		537,914
Number of additional shares reserved for issuance		1,866,009	630,662
2017 Stock Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Terms of stock based options granted							10 years
2018 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance					2,200,260		2,200,260
Percentage of additional shares added on outstanding shares							4.00%
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance					275,030		275,030
Number of additional shares reserved for issuance							543,926
Percentage of additional shares added on outstanding shares							1.00%

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - Employees And Non Employees - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 629	$ 91	$ 2,212	$ 301
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	229	47	955	152
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 400	$ 44	$ 1,257	$ 149

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning, Shares Available for Grant	1,670,341
Shares reserved for issuance, Shares Available for Grant	641,239
Options granted, Shares Available for Grant	(871,010)
Outstanding, ending, Shares Available for Grant	1,413,610	1,670,341
Outstanding, beginning, Number of Options	195,668
Options granted, Number of Options	871,010
Outstanding, ending, Number of Options	1,066,678	195,668
Exercisable, Number of Options	25,487
Vested and expected to vest, Number of Options	1,066,678
Outstanding, beginning, Weighted-Average Exercise Price per Option	$ 4.49
Options granted, Weighted-Average Exercise Price per Option	13.09
Outstanding, ending, Weighted-Average Exercise Price per Option	11.52	$ 4.49
Exercisable, Weighted-Average Exercise Price per Option	0.79
Vested and expected to vest, Weighted-Average Exercise Price per Option	$ 11.52
Outstanding, beginning, Weighted-Average Remaining Contract Term	9 years 5 months 4 days	9 years 8 months 1 day
Exercisable, Weighted-Average Remaining Contract Term	8 years 8 months 12 days
Vested and expected to vest, Weighted-Average Remaining Contract Term	9 years 5 months 4 days
Outstanding, ending, Aggregate Intrinsic Value	$ 3,901
Exercisable, Aggregate Intrinsic Value	366
Vested and expected to vest, Aggregate Intrinsic Value	$ 3,901
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options granted, Shares Available for Grant	(2,000)
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options granted, Shares Available for Grant	(24,960)

Stock-Based Compensation - Summary of Stock Option Activity (Parenthetical) (Details) - Options Issued and Outstanding - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	871,010
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	7,200	46,860

Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model (Details) - Options Issued and Outstanding - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock		$ 0.79		$ 0.79
Expected term (in years)		6 years 1 month 6 days		6 years 1 month 6 days
Expected volatility, minimum		74.40%		74.40%
Expected volatility, maximum			77.70%	77.70%
Risk-free interest rate, minimum		1.90%		1.90%
Risk-free interest rate, maximum			2.80%	2.80%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock		$ 1.00	$ 15.45
Expected term (in years)		10 years	10 years	10 years
Expected dividend yield		0.00%	0.00%	0.00%
Expected volatility		76.90%	77.90%
Risk-free interest rate		2.30%	2.70%	2.30%
Minimum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 9.03	$ 0.79	$ 8.57
Expected term (in years)	5 years 10 months 24 days		5 years 9 months 18 days
Expected volatility, minimum	89.80%		75.90%
Risk-free interest rate, minimum	2.80%		2.40%
Minimum \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock				$ 0.79
Expected volatility				74.60%
Maximum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 13.94		$ 15.45
Expected term (in years)	6 years 1 month 6 days		6 years 7 months 6 days
Expected volatility, minimum	90.60%		90.60%
Risk-free interest rate, minimum	2.90%		2.90%
Maximum \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock				$ 1.00
Expected volatility				76.90%

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock - shares		9 Months Ended
	Apr. 17, 2018	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Unvested shares, December 31, 2017		1,096,449
Number of Shares Outstanding, Granted	2,000	2,000
Number of Shares Outstanding, Vested		(1,096,949)
Number of Shares Outstanding, Unvested shares, September 30, 2018		1,500

Stock-Based Compensation - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs)	9 Months Ended
	Sep. 30, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Granted	24,960
Number of Shares Outstanding, Unvested shares, September 30, 2018	24,960

Commitment and Contingencies - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Estimated litigation liability	$ 0	$ 0

Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,066,678	6,175,031
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		6,063,711
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,066,678	111,320

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Novartis
Related Party Transaction [Line Items]
Payments to related parties	$ 0		$ 0
Silverstein Foundation \| Funding Agreement
Related Party Transaction [Line Items]
Fund received from related parties	$ 0	$ 0	$ 500,000	$ 0