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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 15, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Period Focus	FY
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 Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 132.4
Entity Common Stock, Shares Outstanding		28,055,344

Consolidated Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 7,042,000	$ 53,349,000
Marketable securities	100,986,000
Prepaid expenses	1,491,000	792,000
Deferred offering costs		929,000
Other current assets	15,000	84,000
Total current assets	109,534,000	55,154,000
Restricted cash	84,000	0
Property and equipment, net	321,000	39,000
Total assets	109,939,000	55,193,000
Current liabilities:
Accounts payable (including related party amounts of $0 and $32 as of December 31, 2018 and 2017, respectively)	2,989,000	1,515,000
Accrued liabilities	2,727,000	3,987,000
Total current liabilities	5,716,000	5,502,000
Other liabilities	19,000
Total liabilities	5,735,000	5,502,000
Commitments and contingencies (see Note 12)
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value, 10,000,000 and no shares authorized as of December 31, 2018 and 2017, respectively; no shares issued and outstanding as of December 31, 2018 and 2017
Common stock, $0.0001 par value, 150,000,000 and 30,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 28,055,344 and 5,659,089 shares issued and outstanding as of December 31, 2018 and 2017, respectively; 28,054,344 and 4,562,640 shares vested as of December 31, 2018 and 2017, respectively	3,000	1,000
Additional paid-in capital	175,635,000	1,849,000
Accumulated deficit	(71,393,000)	(33,779,000)
Other comprehensive loss	(41,000)
Total stockholders’ equity (deficit)	104,204,000	(31,929,000)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	109,939,000	55,193,000
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	0	41,674,000
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	$ 0	$ 39,946,000

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts payable to related parties	$ 0	$ 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	28,055,344	5,659,089
Common stock, shares outstanding	28,055,344	5,659,089
Common stock, shares vested	28,054,344	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
Preferred stock, liquidation preference	$ 0	$ 30,000
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
Preferred stock, liquidation preference	$ 0	$ 40,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development		$ 31,065	$ 16,839
General and administrative	$ 1	8,640	2,043
Total operating expenses	1	39,705	18,882
Loss from operations	(1)	(39,705)	(18,882)
Other Income, net		2,117	(14,896)
Loss before income taxes	(1)	(37,588)	(33,778)
Income tax expense	0	26	0
Net loss	$ (1)	$ (37,614)	$ (33,778)
Net loss per share, basic and diluted	$ 0.00	$ (1.42)	$ (8.42)
Weighted-average common shares used in computing net loss per share, basic and diluted	1,978,137	26,439,216	4,009,513
Other comprehensive loss:
Unrealized losses on marketable securities		$ (41)
Total other comprehensive loss		(41)
Comprehensive loss	$ (1)	$ (37,655)	$ (33,778)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Common Stock	Additional Pain In Capital	Accumulated Deficit	Comprehensive Income (Loss)
Balance at Jul. 04, 2016	$ 0			$ 0	$ 0	$ 0	$ 0
Balance, shares at Jul. 04, 2016				0
Vesting of restricted shares, value	1			$ 1
Vesting of restricted shares				2,082,860
Net loss	(1)					(1)
Balance at Dec. 31, 2016	0			$ 1		(1)
Balance, shares at Dec. 31, 2016				2,082,860
Issuance of common shares to PureTech, shares				1,886,363
Issuance of redeemable convertible preferred stock		$ 41,674	$ 39,946
Issuance of redeemable convertible preferred stock, shares		15,527,951	4,792,716
Issuance of redeemable convertible preferred stock, adjustments to additional paid in capital	1,379				1,379
Vesting of restricted shares				593,417
Stock-based compensation expense	470				470
Net loss	(33,778)					(33,778)
Balance at Dec. 31, 2017	(31,929)			$ 1	1,849	(33,779)
Balance, temporary equity, shares at Dec. 31, 2017		15,527,951	4,792,716
Balance, temporary equity at Dec. 31, 2017		$ 41,674	$ 39,946
Balance, shares at Dec. 31, 2017				4,562,640
Conversion of convertible preferred stock into common stock upon the closing of initial public offering	81,620	$ (41,674)	$ (39,946)	$ 1	81,619
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, Shares		(15,527,951)	(4,792,716)	15,870,559
Issuance of common stock	89,370			$ 1	89,369
Issuance of common stock, Shares				6,516,667
Vesting of restricted shares, value	865				865
Vesting of restricted shares				1,097,449
Exercise of stock options	5				5
Exercise of stock options, Shares				7,029
Stock-based compensation expense	1,928				1,928
Net loss	(37,614)					(37,614)
Unrealized losses on marketable securities	(41)						(41)
Balance at Dec. 31, 2018	$ 104,204			$ 3	$ 175,635	$ (71,393)	$ (41)
Balance, temporary equity, shares at Dec. 31, 2018		0	0
Balance, temporary equity at Dec. 31, 2018		$ 0	$ 0
Balance, shares at Dec. 31, 2018				28,054,344

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Series B Redeemable Convertible Preferred Stock
Stock issuance cost		$ 54
Common Stock
Stock issuance cost	$ 8,379

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net loss	$ (1)	$ (37,614)	$ (33,778)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities		(673)
Depreciation and amortization expense		80	5
Stock-based compensation expense		2,793	470
Change in fair value of tranche liability			14,896
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		(630)	(876)
Accounts payable		1,597	1,392
Accrued liabilities	1	(1,022)	3,749
Other liabilities		19
Net cash used in operating activities		(35,534)	(10,985)
Investing activities:
Purchases of property and equipment		(362)	(44)
Maturities of marketable securities		7,500
Purchase of marketable securities		(107,854)
Net cash used in investing activities		(100,716)	(44)
Financing activities:
Proceeds from initial public offering, net of issuance costs		90,908
Deferred offering costs		(970)	(568)
Proceeds from exercise of stock options		5
Net cash provided by financing activities		89,943	64,378
Net (decrease) increase in cash and cash equivalents		(46,307)	53,349
Cash and cash equivalents at beginning of period	0	53,349	0
Cash and cash equivalents at end of period	$ 0	7,042	53,349
Supplemental disclosure of non-cash investing and financing activities:
Conversion of redeemable convertible preferred stock into common stock		$ 81,620
Series A Redeemable Convertible Preferred Stock
Financing activities:
Proceeds from issuance of redeemable convertible preferred stock			25,000
Series B Redeemable Convertible Preferred Stock
Financing activities:
Proceeds from issuance of redeemable convertible preferred stock			$ 39,946

Organization

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1. Organization

resTORbio, Inc. (“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.

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. As of December 31, 2017, the Company had incurred $0.9 million of costs related to the IPO which have been deferred. 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 10,000,000 shares of preferred stock and 150,000,000 shares of common 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 $71.4 million as of December 31, 2018. The Company believes that its cash, cash equivalents, and marketable securities will be sufficient to fund the Company’s current operating plan through at least the next twelve months.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Use of Estimates

The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The Company’s fiscal year end is December 31^st. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). 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 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, fair value of tranche liabilities, 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.

The 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.

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 December 31, 2018 and 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. A liability’s fair value is defined as the amount that would be paid to transfer the liability to a new obligor, not the amount that would be paid to settle the liability with the creditor. Where available, fair value is based on observable market prices, or parameters derived from such prices. Where observable prices or inputs are not available, valuation models are applied. These valuation techniques involve some level of management estimation and judgment. The degree of management estimation and judgment is dependent on the price transparency for the instruments, or market, and the instruments’ complexity. 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 December 31, 2018 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	6,804	$	6,804	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		238		238		—		—
U.S. treasury securities (included in marketable securities)		100,986		100,986		—		—
Total	$	108,028	$	108,028	$	—	$	—

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	$	—	$	—

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

An entity may elect to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in net loss. The Company did not elect to measure any additional financial instruments or other items at fair value.

There have been no changes to the valuation methods utilized by the Company during the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of financial instruments between levels during the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and marketable securities. The Company’s cash, cash equivalents and marketable securities are held by financial institutions in the United States. Amounts on deposit may at times exceed federally insured limits. Management believes that the financial institution is financially sound, and accordingly, minimal credit risk exists with respect to the financial institution.

Concentration of Manufacturing Risk

As of December 31, 2018, the Company had manufacturing arrangements with vendors for the supply of materials for use in preclinical and clinical studies. If the Company were to experience any disruptions in either party’s ability or willingness to continue to provide manufacturing services, the Company may experience significant delays in its product development timelines and may incur substantial costs to secure alternative sources of manufacturing.

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 and comprehensive loss. The Company recorded deferred offering costs of $0 and $0.9 million as of December 31, 2018 and 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 sheets and the resulting gain or loss is reflected in the consolidated statements of operations and comprehensive loss.

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
Machinery and equipment		2-8 years
Furniture and fixtures		3-5 years
Computers		1-5 years
Office equipment		3-5 years
Software		3-5 years

Impairment of Long-Lived Assets

The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. The Company has recorded no impairment of any long-lived assets during any of the periods presented.

Accrued Research and Development Costs

The Company accrues for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical studies and clinical trials, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided and includes these costs in accrued liabilities in the consolidated balance sheets and within research and development expenses in the consolidated statements of operations and comprehensive loss. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company estimates the amount of work completed by its third-party service providers through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. The majority of the Company’s service providers invoice in arrears for services performed, on a pre-determined schedule or when contractual milestones are met; however, some require advance payments. The Company makes significant judgments and estimates in determining the accrued balance in each reporting period based on the facts and circumstances known at that time. As actual costs become known, the Company adjusts its accrued estimates. Although the Company does not expect its estimates to be materially different from amounts actually incurred, our understanding of the status and timing of services performed, the number of patients enrolled, and the rate of patient enrollment may vary from its estimates and could result in us reporting amounts that are too high or too low in any particular period. The Company’s accrued expenses are dependent, in part, upon the receipt of timely and accurate reporting from clinical research organizations, or CROs, clinical manufacturing organizations, or CMOs, and other third-party service providers. To date, there have been no material differences from its accrued expenses to actual expenses.

Research and Development Costs

Research and development costs are expensed as incurred and consist of personnel costs, lab supplies and other costs, as well as fees paid to third parties to conduct research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expenses. The Company records payments made to outside vendors for services performed or goods being delivered for use in research and development activities as either prepaid expenses or accrued expenses, depending on the timing of when services are performed or goods are delivered.

Equity-Based Compensation Expense

The Company recognizes equity-based compensation expense for awards of equity instruments to employees and non-employees based on the grant date fair value of those awards in accordance with FASB ASC Topic 718, Stock Compensation (“ASC 718”). ASC 718 requires all equity-based compensation awards to employees and non-employee directors, including grants of restricted shares and stock options, to be recognized as expense in the consolidated statements of operations and comprehensive loss based on their grant date fair values. The Company estimates the fair value of stock options using the Black-Scholes option pricing model. The Company uses the value of its common stock to determine the fair value of restricted shares.

The Company accounts for restricted stock and common stock options issued to non-employees under FASB ASC Topic 505-50, Equity- Based Payments to Non-Employees (“ASC 505-50”). As such, the value of such options is periodically remeasured and income or expense is recognized over their vesting terms. Compensation cost related to awards with service-based vesting schedules is recognized using the straight-line method. The Company determines the fair value of the restricted stock and common stock granted to non-employees as either the fair value of the consideration received or the fair value of the equity instruments issued.

The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the expected share price volatility, (ii) the expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The group of representative companies have characteristics similar to the Company, including stage of product development and focus on the life science industry. The Company uses the simplified method, which is the average of the final vesting tranche date and the contractual term, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

The Company expenses the fair value of its equity-based compensation awards granted to employees on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company measures equity-based compensation awards granted to non-employees at fair value as the awards vest and recognizes the resulting value as compensation expense at each financial reporting period. The Company accounts for award forfeitures as they occur.

Determination of Fair Value of Common and Preferred Shares and Tranche Rights Liability

Prior to the completion of the Company’s IPO, the Company was required to estimate the fair value of its common stock underlying its stock-based awards when performing the fair value calculations using the Black-Scholes option pricing model. The estimated fair value of the Company’s common and preferred shares has been determined by the board of directors as of the grant date, with input from management, considering the Company’s most recently available third-party valuations of common shares and the board of directors’ assessment of additional objective and subjective factors that it believed were relevant and which may have changed from the date of the most recent third-party valuation through the date of the grant. These third-party valuations were performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. The Company’s common and preferred share valuations were prepared using either an option-pricing method, or OPM, or a probability-weighted expected return method, or PWERM, which uses a combination of market approaches and an income approach to estimate the Company’s enterprise value. The OPM treats common securities and preferred securities as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, the common and preferred shares have value only if the funds available for distribution to members are expected to exceed the value of the preferred security liquidation preference at the time of the liquidity event, such as a strategic sale or a merger. The PWERM is a scenario-based methodology that estimates the fair value of common and preferred shares based upon an analysis of future values for the enterprise, assuming various outcomes. The common and preferred share values are based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the rights of each class of common and preferred securities. The future value of the common and preferred shares under each outcome is discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the common and preferred shares. The estimated fair value of the Tranche Rights was determined using the difference between the total purchase price of the Company’s preferred stock and the total fair value of the preferred stock using a risk-adjusted forward contract model.

Income Taxes

The Company uses the asset and liability method of accounting for income taxes in accordance with FASB ASC Topic 740, Income Taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the 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. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion, or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance.

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, the Company has no uncertain tax positions and there have been no interest charges or penalties related to unrecognized tax benefits.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period without consideration of common stock equivalents. Diluted net loss per common share is the same as basic net loss per common share for all periods presented, since the effects of potentially dilutive securities are antidilutive.

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 December 31, 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 ASC 606. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its financial statements.

Marketable Securities

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

As of December 31, 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 treasuries and securities	$	101,027	$	—	$	41	$	100,986
Total	$	101,027	$	—	$	41	$	100,986

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):

	December 31, 2018
	Amortized		Fair
	Cost		Value
Due in one year or less	$	101,027	$	100,986
Total	$	101,027	$	100,986

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment, net consists of the following:

	As of December 31,
	2018			2017
	(In thousands)
Leasehold improvements	$	65		$	—
Machinery and equipment		38			38
Furniture and fixtures		194			—
Computers		76			6
Office equipment		11			—
Software		22			—
Total property and equipment		406			44
Less: accumulated depreciation		(85	)		(5	)
Property and equipment, net	$	321		$	39

Depreciation expense was $80,000, $5,000, and $0 for the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016, respectively.

Accrued Liabilities

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accrued Liabilities

5. Accrued Liabilities

Accrued liabilities consist of the following:

	As of December 31,
	2018		2017
	(In thousands)
Accrued payroll and related expenses	$	1,189	$	394
Accrued research and development expenses		1,028		3,250
Deferred offering costs		—		238
Other		510		105
Total accrued liabilities	$	2,727	$	3,987

License Agreements

12 Months Ended

Dec. 31, 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 and comprehensive loss.

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.

Milestone payments to Novartis will be recorded as research and development expenses in the consolidated statements of operations and comprehensive loss 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 payment in May 2017. As of December 31, 2018, none of the remaining development milestones, regulatory milestones, sales milestones, or royalties had been reached or were probable of achievement.

Research Funding Agreements

12 Months Ended

Dec. 31, 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 consolidated statements of operations and comprehensive loss 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 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 consolidated balance sheet as funding advance. As of December 31, 2018, $0.5 million qualifying expenses have been incurred. Therefore, all amounts received have been recorded as a reduction of the research and development expense.

Preferred Stock and Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Preferred Stock and Redeemable Convertible Preferred Stock

8. Preferred Stock and Redeemable Convertible Preferred Stock

As of December 31, 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 stocks of the Company automatically converted into 15,870,559 shares of the Company’s common stock. As of December 31, 2018, no shares of preferred stock were outstanding.

The Company’s redeemable convertible preferred shares were classified as temporary or mezzanine equity on the accompanying consolidated balance sheets in accordance with U.S. GAAP for the classification and measurement of redeemable securities as the Series A and Series B Preferred Stock were contingently redeemable at the option of the holder for reasons outside of the Company’s control. As of December 31, 2017, there were no accretion of the redeemable convertible preferred shares to redemption value as at that date the shares were not redeemable or probable of being redeemed.

On March 23, 2017, the Company entered into a Series A Preferred Stock Purchase Agreement with PureTech Health LLC (“PureTech”) and NIBR. Under the agreement, in the initial March 2017 closing, PureTech purchased 2,846,791 shares of Series A Preferred Stock at a purchase price of $1.932 per share, resulting in aggregate gross proceeds of $5.5 million, and NIBR was issued 2,587,992 shares of Series A Preferred Stock as consideration for 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, alone or in combination with everolimus in a fixed dose combination (See Note 6). PureTech also agreed to purchase up to 4,917,185 additional shares, for total aggregate gross proceeds of $9.5 million (the “Tranche Rights”), at $1.932 per share at separate second and third closings to take place upon the occurrence of certain events as specified under the agreement. The fair value of the Series A Preferred Stock on the date of issuance was determined to be $1.22 per share based on an independent third-party valuation.

On March 23, 2017, the Company also entered into a side letter with PureTech under which PureTech agreed to purchase up to 5,175,984 additional shares at $1.932 per share at a fourth closing to take place on a future date based on the occurrence of certain events as specified under the letter. The Series A Tranche Rights were evaluated under ASC 480 – Distinguishing Liabilities from Equity and it was determined that they met the requirements for separate accounting from the initial issuance of Series A Preferred Stock as freestanding financial instruments and are accounted for as liabilities. The Company adjusted the carrying value of the Series A Tranche Rights to its estimated fair value at each reporting date up to the closing of each tranche financing. Increases or decreases in fair value of the Tranche Rights were recorded as other income (expense) in the consolidated statements of operations and comprehensive loss.

At the date of issuance, $2.0 million of the Series A Preferred Stock proceeds was allocated to the Series A Tranche Rights liability, which was recorded as a current liability on the consolidated balance sheets.

In September 2017, under the Series A Tranche Rights, the Company received gross proceeds of $4.5 million in exchange for the issuance of 2,329,193 shares of Series A Preferred Stock at $1.932 per share pursuant to the second closing on August 29, 2017. The fair value of the Series A Preferred Stock on the date of issuance was determined to be $1.34 per share based on an independent third-party valuation and the fair value of the Series A Tranche Rights liability was revalued to its estimated fair value of $1.4 million, resulting in other income of $0.6 million.

On October 12, 2017, the Company amended the Series A Preferred Stock Purchase Agreement to accelerate the third and fourth closings under the original agreement. The Company issued 7,763,975 shares of Series A Preferred Stock at $1.932 per share for aggregate gross proceeds of $15.0 million, of which $9.0 million was from PureTech and $6.0 million from a new investor. The fair value of the Series A Preferred Stock on the date of issuance was determined to be $4.11 per share based on an independent third-party valuation and the fair value of the Series A Tranche Rights liability was revalued to its estimated fair value of $10.1 million, resulting in other expense of $8.8 million. Following this closing of the Series A Preferred Stock financing, the Series A Tranche Rights were terminated.

In connection with the October 12, 2017 Series A closing, a new investor entered into a commitment to purchase up to $20 million dollars of Series B Preferred Stock at a purchase price of $8.346 (the “Series B Tranche Right”). The new investor’s commitment to purchase Series B Preferred Stock was also determined to meet the requirements for separate accounting. The Series B Tranche Right was determined to be a freestanding financial instrument accounted for as an asset. On October 12, 2017, $6.8 million of the Series A Preferred Stock proceeds were allocated to this liability.

On October 27, 2017, the Company entered into the Series B Preferred Stock Agreement for the issuance and sale of up to 4,792,716 shares of Series B Preferred Stock at $8.346 per share. On November 29, 2017, the Company issued and sold 4,792,716 shares of Series B Preferred Stock at $8.346 per share for aggregate gross proceeds of $40.0 million (the “Series B Financing”), of which $20.0 million satisfied the series B Tranche Right.

The Company measured the fair value of the Tranche Rights liabilities/assets from issuance on March 23, 2017 to settlement on November 29, 2017. The Tranche Rights liabilities/assets are considered a Level 3 liability/asset because its fair value measurement is based, in part, on significant inputs not observed in the market. Any reasonable changes in the assumptions used in the valuation could materially affect the financial results of the Company.

The following is a summary of the changes in fair value of the Tranche Rights and the impact on the consolidated financial statements:

	(In thousands)			Financial statement impacted
Beginning Balance, January 1, 2017	$	—
Establishment of Series A tranche right liability on March 23, 2017		2,014		Consolidated balance sheets
Change in fair value of Series A tranche right liability immediately prior to second closing on September 8, 2017		(605	)	Consolidated statements of operations and comprehensive loss
Mark-to-market of Series A tranche rights liability at September 30, 2017		(30	)	Consolidated statements of operations and comprehensive loss
Change in fair value of tranche right liability immediately prior to final closing of the Series A on October 12, 2017		8,767		Consolidated statements of operations and comprehensive loss
Settlement of Series A tranche right liability upon closing of Series A on October 12, 2017		(10,146	)	Consolidated balance sheets
Establishment of Series B tranche right asset on October 23, 2017		6,764		Consolidated statements of operations and comprehensive loss
Settlement of Series B tranche right asset upon closing of Series B on November 29, 2017		(6,764	)	Consolidated balance sheets
Ending Balance, December 31, 2017	$	—

The change in the fair value of the Tranche Rights is influenced primarily by the price of the underlying Series A and Series B Preferred Stock. The net change in fair value of $14.9 million for the year ended December 31, 2017 was recorded as other expense in the accompanying consolidated statements of operations and comprehensive loss.

Common Stock

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

Common Stock

9. Common Stock

General

The voting, dividend and liquidation rights of the holders of common stock are subject to and qualified by the rights, powers, and preferences of the holders of the shares of preferred stock. The common stock has the following characteristics:

Voting

The holders of the common stock are entitled to one vote for each share of common stock held at all meetings of stockholders and written actions in lieu of meetings, provided, however, that except as otherwise required by law, holders of common stock as such shall not be entitled to vote on any amendment to the Company’s Certificate of Incorporation that relates solely to the terms of one or more outstanding series of Preferred Stock if the holders of such affected series are entitled, either separately or together with the holders of one or more other such series, to vote thereon pursuant to the Company’s Certificate of Incorporation or pursuant to Delaware General Corporation Law. There shall be no cumulative voting.

Dividends

The holders of shares of common stock are entitled to receive dividends, if and when declared by the Board of Directors. Cash dividends may not be declared or paid to the holders of common stock until paid on the preferred stock. As of December 31, 2018, no dividends have been declared or paid since the Company’s inception.

Liquidation

After payment to the holders of shares of preferred stock of their liquidation preference, the holders of the common stock are entitled to share ratably in the Company’s assets available for distribution to stockholders, in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or upon the occurrence of a Deemed Liquidation Event.

Reserve for future issuance

As of December 31, 2016, the Company had not reserved any shares of common stock for future issuance. As of December 31, 2018 and 2017, 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:

	As of December 31,
	2018		2017
Redeemable convertible preferred stock, on an as-converted basis		—		15,870,559
Options issued and outstanding		1,122,677		195,668
Unvested restricted stock units		24,960		—
Options available for future grants		1,350,582		1,670,341
Shares available for issuance under the 2018 ESPP		275,030		—
Total		2,773,249		17,736,568

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10. Stock-based Compensation

In 2017, the Company adopted the 2017 Stock Incentive Plan (the “Plan”). Under the Plan, shares of the Company’s common stock have been reserved for the issuance of stock options to employees, directors, and consultants under terms and provisions established by the Board of Directors. A total of 537,914 shares were reserved for issuance under the Plan. Under the terms of the Plan, options may be granted at an exercise price not less than fair market value. The terms of options granted under the Plan may not exceed ten years. The Board shall determine 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 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 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 Option and 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, non-statutory 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. As of December 31, 2018, no such shares became available for issuance under the 2018 Plan.

Stock-based Compensation Expense

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

	Year Ended December 31, 2018		Year Ended December 31, 2017		July 5, 2016 (inception) through December 31, 2016
	(In thousands)
Research and development	$	1,236	$	246	$	—
General and administrative		1,557		224		—
Total stock-based compensation expense	$	2,793	$	470	$	—

Stock Options

The following table summarizes stock option activity under the Plan:

	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⁾		(934,038	)		934,038			13.05
Restricted stock granted		(2,000	)
Restricted stock units granted		(24,960	)
Options exercised					(7,029	)		0.79
Outstanding, December 31, 2018		1,350,582			1,122,677			11.63		9.22	$	818
Exercisable, December 31, 2018					67,613			6.32		8.74		241
Vested and expected to vest, December 31, 2018					1,122,677			11.63		9.22		818

(1)	The Company granted 7,200 stock options to non-employees during the year ended December 31, 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 December 31, 2018. The aggregate intrinsic values of options exercised during the year ended December 31, 2018 was $78,000. No options were exercised during the year ended December 31, 2017 and during the period from July 5, 2016 (inception) to December 31, 2016. No options were cancelled or forfeited during the years ended December 31, 2018 and 2017 and during the period from July 5, 2016 (inception) to December 31, 2016.

During the year ended December 31, 2018, the Company granted options to employees and directors to purchase an aggregate of 926,838 common shares with a weighted-average grant date fair value of $9.23. During the year ended December 31, 2017, the Company granted options to employees to purchase an aggregate of 148,808 common shares with a weighted-average grant date fair value of $3.75. During the year ended December 31, 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. During the year ended December 31, 2017, the Company granted options to non-employees to purchase an aggregate of 46,860 common shares with a weighted-average grant date fair value of $0.67. On December 1, 2017, a non-employee became an employee of the Company. The expense related to options granted to employees and directors was $1.7 million and $28,000 for the years ended December 31, 2018 and 2017, respectively. The expense related to options granted to non-employees was $0.1 million and $59,000 for the years ended December 31, 2018 and 2017, respectively. There were no stock options granted to employees or non-employees during the period from July 5, 2016 (inception) to December 31, 2016.

As of December 31, 2018, the total unrecognized compensation expense related to unvested employee options was $7.5 million which the Company expects to recognize over an estimated weighted-average period of 3.17 years. As of December 31, 2018, the total unrecognized compensation expense related to unvested non-employee options was $0.1 million which the Company expects to recognize over an estimated weighted-average period of 2.67 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:

	Year Ended December 31,
	2018	2017
Employees:
Fair value of common stock	$8.57 - $15.45	$0.79 - $9.33
Expected term (in years)	5.8 - 6.2	5.9 - 6.2
Expected volatility	75.9% - 90.6%	74.4% - 74.5%
Risk-free interest rate	2.4% - 3.1%	1.9% - 2.2%
Expected dividend yield	0.0%	0.0%
Non-employees:
Fair value of common stock	$8.62 - $15.45	$0.79 - $10.28
Expected term (in years)	10.0 - 8.50	10.0 - 9.4
Expected volatility	78.0% - 91.2%	74.6% - 77.0%
Risk-free interest rate	2.7% - 3.1%	2.3% - 2.4%
Expected dividend yield	0.0%	0.0%

Fair Value of Common Stock: Prior to the IPO, given the absence of a public trading market, the Board of Directors considered numerous objective and subjective factors to determine the fair value of common stock at each grant date. These factors included, but were not limited to, (i) contemporaneous valuations of common stock performed by independent third-party specialists; (ii) the prices for preferred stock sold to outside investors; (iii) the rights, preferences and privileges of preferred stock relative to common stock; (iv) the lack of marketability of common stock; (v) developments in the business; and (vi) the likelihood of achieving a liquidity event, such as an initial public offering or a merger or acquisition of the Company, given prevailing market conditions.

Restricted Stock

On July 11, 2016, certain founding non-employee directors purchased 3,772,726 common shares that are subject to a repurchase right upon termination or cessation of services at the original purchase price of $0.0001 per share, or $483. 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		(1,097,449	)
Unvested shares — December 31, 2018		1,000

The Company recognized $0.9 million, $0.4 million and $0 of stock-based compensation expense related to restricted shares during the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016, respectively. As of December 31, 2018, there was $3,000 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 0.29 years.

Restricted Stock Units

During the year ended December 31, 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 — December 31, 2018		24,960

The Company recognized $35,000, $0 and $0 of stock-based compensation expense related to restricted stock units during the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016, respectively. As of December 31, 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 3.39 years. There were no restricted stock units granted to employees or non-employees during the year ended December 31, 2017 and the period from July 5, 2016 (inception) to December 31, 2016.

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.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

Provision for Income Taxes

For the years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016, the Company did not record a current or deferred income tax expense. The Company’s consolidated loss before income taxes consists solely of a domestic loss.

A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:

	Year Ended December 31, 2018			Year Ended December 31, 2017			July 5, 2016 (inception) through December 31, 2016
	(In thousands)
Income tax expense (benefit) at federal statutory rate	$	(7,899	)	$	(11,484	)	$	—
State taxes		(2,340	)		(1,011	)		—
Tax credits		(817	)		(222	)		—
Stock-based compensation		764			140			—
Federal tax rate change		—			2,202			—
Change in fair value of tranche rights		3			5,065			—
Other		241						—
Change in valuation allowance		10,074			5,310			—
Income tax expense	$	26		$	—		$	—
Effective tax rate		0.0	%		0.0	%		0.0	%

On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 34% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of our deferred tax assets as of December 31, 2017, without an impact on the net amount of our deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA.

Deferred Tax Assets and Liabilities

Deferred income taxes reflect the net tax effects of loss and credit carryforwards and temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of deferred income taxes were as follows as of December 31, 2018 and 2017:

	As of December 31,
	2018			2017
	(In thousands)
Deferred tax assets:
Net operating losses	$	13,054		$	3,953
Capitalized license		869			896
Research credits		1,007			269
Accruals		514			176
Stock-based compensation		51			16
Total gross deferred tax assets		15,495			5,310
Less valuation allowance		(15,373	)		(5,310	)
Total deferred tax assets		122			—
Deferred tax liabilities:
Other comprehensive income - unrealized loss		11
Depreciation and amortization		111			—
Total gross deferred tax liability		122			—
Net deferred tax assets	$	—		$	—

Realization of deferred tax assets is dependent upon future earnings, if any, the timing and amount of which are uncertain. Due to the lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. A valuation allowance of $15.4 million and $5.3 million has been recorded for the years ended December 31, 2018 and 2017, respectively.

Net Operating Loss and Tax Credit Carryforwards

As of December 31, 2018 and 2017, the Company had net operating loss carryforwards for federal income tax purposes of approximately $47.3 million and $14.5 million, respectively which will begin to expire in 2036. As of December 31, 2018 and 2017, the Company had total state net operating loss carryforwards of approximately $49.4 million and $14.4 million, respectively which will begin to expire in 2036. Utilization of some of the federal and state net operating loss and credit carryforwards are subject to annual limitations due to the “change of ownership” provisions under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, and similar state provisions. The annual limitations may result in the expiration of net operating losses and credits before utilization. The Company has not performed an ownership change analysis.

As of December 31, 2018 and 2017, the Company had federal research credits of $0.9 million and $0.2 million, respectively, which will begin to expire in 2037 and state research credits of $0.2 million and $60,000, respectively, which will begin to expire in 2032. These tax credits are subject to the same limitations discussed above.

Unrecognized Tax Benefits

The Company has incurred net operating losses since inception and has no significant unrecognized tax benefits. If in the future the Company recognizes uncertain tax positions, the Company’s effective tax rate will be reduced. Currently, the Company has a full valuation allowance against its net deferred tax asset which would impact the timing of the effective tax rate benefit should any of these uncertain tax positions be favorably settled in the future. Any adjustments to uncertain tax positions would result in an adjustment of net operating loss or tax credit carry forwards rather than resulting in a cash outlay. As of December 31, 2018, the Company had no unrecognized tax benefits and no accrued interest or penalties related to uncertain tax positions.

Income tax returns are filed in the U.S. and Massachusetts. The Company is not currently under examination. Due to net operating losses and research credit carryovers, all of the tax years remain open to examination.

Commitments and Contingences

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingences

12. 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 December 31, 2018.

Lease

On January 3, 2018, the Company entered a lease for office space. The lease commencement date is January 1, 2018 and the lease expiration date is February 28, 2021. The Company paid $0.2 million and $0 in rent for years ended December 31, 2018 and 2017, respectively. Obligations to make future minimum lease payments at December 31, 2018, are as follows (in thousands):

Year ending December 31,	Minimum Lease Payments
2019	$	231
2020		236
2021		39
Total	$	506

Novartis License Agreement

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.

Silverstein Foundation

The Company 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 (see Note 7).

Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share

13. Net Loss per Share

As described in Note 2, 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 years ended December 31, 2018 and 2017 and the period from July 5, 2016 (inception) to December 31, 2016, 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.

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

14. Related Party Transactions

Since the Company’s incorporation in July 2016, the Company has engaged in transactions with related parties.

During the year ended December 31, 2017, the Company issued 1,886,363 shares of common stock and made payments to PureTech for certain founding services and cost reimbursements. PureTech is a founder of the Company and holds shares of common stock and preferred stock of the Company (See Note 8).

The Company is a party to an intellectual property license agreement with Novartis. In addition, NIBR is a preferred stock shareholder of the Company (See Note 6). During the year ended December 31, 2017, the Company made payments to Novartis for milestones achieved pursuant to the license agreement and for the purchases of materials for use in the Company’s clinical trials. No payments have been made to Novartis during the year ended December 31, 2018.

Aggregate payments for the above related party transactions totaled $0, $0.9 million and $0 for the years ended December 31, 2018 and 2017 and period from July 5, 2016 (inception) to December 31, 2016, respectively.

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 $0.5 million from the Silverstein Foundation during the year ended December 31, 2018. No funds were received during the year ended December 31, 2017 and the period from July 5, 2016 (inception) to December 31, 2016.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

15. Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share data)
Total operating expenses	$	10,200		$	14,113		$	9,032		$	6,360		$	39,705
Loss from operations		(10,200	)		(14,113	)		(9,032	)		(6,360	)		(39,705	)
Net loss		(9,859	)		(13,591	)		(8,407	)		(5,757	)		(37,614	)
Net loss applicable to common stockholders		(9,859	)		(13,591	)		(8,407	)		(5,757	)		(37,614	)
Net loss per share applicable to common stockholders—basic and diluted	$	(0.46	)	$	(0.48	)	$	(0.30	)	$	(0.21	)	$	(1.42	)

	2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share data)
Total operating expenses	$	3,357		$	4,057		$	3,945		$	7,523		$	18,882
Loss from operations		(3,357	)		(4,057	)		(3,945	)		(7,523	)		(18,882	)
Net loss		(3,357	)		(4,057	)		(3,310	)		(23,054	)		(33,778	)
Net loss applicable to common stockholders		(3,357	)		(4,057	)		(3,310	)		(23,054	)		(33,778	)
Net loss per share applicable to common stockholders—basic and diluted	$	(1.20	)	$	(0.94	)	$	(0.75	)	$	(5.11	)	$	(8.42	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation and Use of Estimates

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 December 31, 2018 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	6,804	$	6,804	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		238		238		—		—
U.S. treasury securities (included in marketable securities)		100,986		100,986		—		—
Total	$	108,028	$	108,028	$	—	$	—

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	$	—	$	—

Concentration of Credit Risk

Concentration of Manufacturing Risk

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
Machinery and equipment		2-8 years
Furniture and fixtures		3-5 years
Computers		1-5 years
Office equipment		3-5 years
Software		3-5 years

Impairment of Long-Lived Assets

Accrued Research and Development Costs

Research and Development Costs

Equity-Based Compensation Expense

Determination of Fair Value of Common and Preferred Shares and Tranche Rights Liability

Income Taxes

Net Loss Per Share

Net Loss per Share

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 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 December 31, 2018 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	6,804	$	6,804	$	—	$	—
U.S. treasury securities (included in cash and cash equivalents)		238		238		—		—
U.S. treasury securities (included in marketable securities)		100,986		100,986		—		—
Total	$	108,028	$	108,028	$	—	$	—

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
Machinery and equipment		2-8 years
Furniture and fixtures		3-5 years
Computers		1-5 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)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value Marketable Securities By Type of Security

As of December 31, 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 treasuries and securities	$	101,027	$	—	$	41	$	100,986
Total	$	101,027	$	—	$	41	$	100,986

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):

	December 31, 2018
	Amortized		Fair
	Cost		Value
Due in one year or less	$	101,027	$	100,986
Total	$	101,027	$	100,986

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	As of December 31,
	2018			2017
	(In thousands)
Leasehold improvements	$	65		$	—
Machinery and equipment		38			38
Furniture and fixtures		194			—
Computers		76			6
Office equipment		11			—
Software		22			—
Total property and equipment		406			44
Less: accumulated depreciation		(85	)		(5	)
Property and equipment, net	$	321		$	39

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	As of December 31,
	2018		2017
	(In thousands)
Accrued payroll and related expenses	$	1,189	$	394
Accrued research and development expenses		1,028		3,250
Deferred offering costs		—		238
Other		510		105
Total accrued liabilities	$	2,727	$	3,987

Preferred Stock and Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Summary of Change in Fair Value

The following is a summary of the changes in fair value of the Tranche Rights and the impact on the consolidated financial statements:

	(In thousands)			Financial statement impacted
Beginning Balance, January 1, 2017	$	—
Establishment of Series A tranche right liability on March 23, 2017		2,014		Consolidated balance sheets
Change in fair value of Series A tranche right liability immediately prior to second closing on September 8, 2017		(605	)	Consolidated statements of operations and comprehensive loss
Mark-to-market of Series A tranche rights liability at September 30, 2017		(30	)	Consolidated statements of operations and comprehensive loss
Change in fair value of tranche right liability immediately prior to final closing of the Series A on October 12, 2017		8,767		Consolidated statements of operations and comprehensive loss
Settlement of Series A tranche right liability upon closing of Series A on October 12, 2017		(10,146	)	Consolidated balance sheets
Establishment of Series B tranche right asset on October 23, 2017		6,764		Consolidated statements of operations and comprehensive loss
Settlement of Series B tranche right asset upon closing of Series B on November 29, 2017		(6,764	)	Consolidated balance sheets
Ending Balance, December 31, 2017	$	—

Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

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

	As of December 31,
	2018		2017
Redeemable convertible preferred stock, on an as-converted basis		—		15,870,559
Options issued and outstanding		1,122,677		195,668
Unvested restricted stock units		24,960		—
Options available for future grants		1,350,582		1,670,341
Shares available for issuance under the 2018 ESPP		275,030		—
Total		2,773,249		17,736,568

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

	Year Ended December 31, 2018		Year Ended December 31, 2017		July 5, 2016 (inception) through December 31, 2016
	(In thousands)
Research and development	$	1,236	$	246	$	—
General and administrative		1,557		224		—
Total stock-based compensation expense	$	2,793	$	470	$	—

Summary of Stock Option Activity

The following table summarizes stock option activity under the Plan:

	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⁾		(934,038	)		934,038			13.05
Restricted stock granted		(2,000	)
Restricted stock units granted		(24,960	)
Options exercised					(7,029	)		0.79
Outstanding, December 31, 2018		1,350,582			1,122,677			11.63		9.22	$	818
Exercisable, December 31, 2018					67,613			6.32		8.74		241
Vested and expected to vest, December 31, 2018					1,122,677			11.63		9.22		818

(1)	The Company granted 7,200 stock options to non-employees during the year ended December 31, 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:

	Year Ended December 31,
	2018	2017
Employees:
Fair value of common stock	$8.57 - $15.45	$0.79 - $9.33
Expected term (in years)	5.8 - 6.2	5.9 - 6.2
Expected volatility	75.9% - 90.6%	74.4% - 74.5%
Risk-free interest rate	2.4% - 3.1%	1.9% - 2.2%
Expected dividend yield	0.0%	0.0%
Non-employees:
Fair value of common stock	$8.62 - $15.45	$0.79 - $10.28
Expected term (in years)	10.0 - 8.50	10.0 - 9.4
Expected volatility	78.0% - 91.2%	74.6% - 77.0%
Risk-free interest rate	2.7% - 3.1%	2.3% - 2.4%
Expected dividend yield	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		(1,097,449	)
Unvested shares — December 31, 2018		1,000

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 — December 31, 2018		24,960

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Income Tax Expense Computed at Statutory Federal Income Tax Rate to Income Taxes

A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:

	Year Ended December 31, 2018			Year Ended December 31, 2017			July 5, 2016 (inception) through December 31, 2016
	(In thousands)
Income tax expense (benefit) at federal statutory rate	$	(7,899	)	$	(11,484	)	$	—
State taxes		(2,340	)		(1,011	)		—
Tax credits		(817	)		(222	)		—
Stock-based compensation		764			140			—
Federal tax rate change		—			2,202			—
Change in fair value of tranche rights		3			5,065			—
Other		241						—
Change in valuation allowance		10,074			5,310			—
Income tax expense	$	26		$	—		$	—
Effective tax rate		0.0	%		0.0	%		0.0	%

Schedule of Deferred Tax Assets and Liabilities

	As of December 31,
	2018			2017
	(In thousands)
Deferred tax assets:
Net operating losses	$	13,054		$	3,953
Capitalized license		869			896
Research credits		1,007			269
Accruals		514			176
Stock-based compensation		51			16
Total gross deferred tax assets		15,495			5,310
Less valuation allowance		(15,373	)		(5,310	)
Total deferred tax assets		122			—
Deferred tax liabilities:
Other comprehensive income - unrealized loss		11
Depreciation and amortization		111			—
Total gross deferred tax liability		122			—
Net deferred tax assets	$	—		$	—

Commitments and Contingences (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Obligations Future Minimum Lease Payments

Obligations to make future minimum lease payments at December 31, 2018, are as follows (in thousands):

Year ending December 31,	Minimum Lease Payments
2019	$	231
2020		236
2021		39
Total	$	506

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

	2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share data)
Total operating expenses	$	10,200		$	14,113		$	9,032		$	6,360		$	39,705
Loss from operations		(10,200	)		(14,113	)		(9,032	)		(6,360	)		(39,705	)
Net loss		(9,859	)		(13,591	)		(8,407	)		(5,757	)		(37,614	)
Net loss applicable to common stockholders		(9,859	)		(13,591	)		(8,407	)		(5,757	)		(37,614	)
Net loss per share applicable to common stockholders—basic and diluted	$	(0.46	)	$	(0.48	)	$	(0.30	)	$	(0.21	)	$	(1.42	)

	2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share data)
Total operating expenses	$	3,357		$	4,057		$	3,945		$	7,523		$	18,882
Loss from operations		(3,357	)		(4,057	)		(3,945	)		(7,523	)		(18,882	)
Net loss		(3,357	)		(4,057	)		(3,310	)		(23,054	)		(33,778	)
Net loss applicable to common stockholders		(3,357	)		(4,057	)		(3,310	)		(23,054	)		(33,778	)
Net loss per share applicable to common stockholders—basic and diluted	$	(1.20	)	$	(0.94	)	$	(0.75	)	$	(5.11	)	$	(8.42	)

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Entity incorporation date		Jul. 05, 2016
Net proceeds received from offering		$ 90,908
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		$ 71,393	$ 33,779
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
Deferred offering costs			$ 900
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 ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Restricted cash (non-current) related to letter of credit	$ 84,000	$ 0
Fair value assets transferred from level 1 to level 2	0	0
Fair value assets transferred from level 2 to level 1	0	0
Fair value liabilities transferred from level 1 to level 2	0	0
Fair value liabilities transferred from level 2 to level 1	0	0
Fair value measurement with unobservable inputs reconciliation recurring basis asset transfers net	0	0
Fair value measurement with unobservable inputs reconciliation recurring basis liabilities transfers net	0	0
Deferred offering costs	0	$ 900,000
Asset impairment charges on long-lived assets	0
Dividend yield	0
Uncertain tax positions	0
Interest charges or penalties related to unrecognized tax benefits	$ 0

Summary of Significant Accounting Policies - Summary of Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 108,028	$ 53,349
Money Market Funds \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	6,804	53,349
U.S. Treasury Securities \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	238
U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	100,986
Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	108,028	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	6,804	$ 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	238
Active Markets (Level 1) \| U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 100,986

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Leasehold Improvements
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	Lesser of useful life or remaining lease term
Machinery and Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	2 years
Machinery and Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	8 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
Computers \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	1 year
Computers \| 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	Dec. 31, 2018 USD ($)
Marketable Securities [Line Items]
Amortized Cost	$ 101,027
Unrealized Losses	41
Fair Value	100,986
U.S. Government Agency Treasuries and Securities
Marketable Securities [Line Items]
Amortized Cost	101,027
Unrealized Losses	41
Fair Value	$ 100,986

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

Marketable Securities - Schedule of Fair Value and Amortized Cost of Company's Available-for-Sale Securities by Contractual Maturity (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 101,027
Amortized Cost	101,027
Due in one year or less, Fair Value	100,986
Fair Value	$ 100,986

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 406	$ 44
Less: accumulated depreciation	(85)	(5)
Property and equipment, net	321	39
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	65
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	38	38
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	194
Computers
Property Plant And Equipment [Line Items]
Total property and equipment	76	$ 6
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	11
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 22

Property and Equipment, Net - Additional Information (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 0	$ 80,000	$ 5,000

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 1,189	$ 394
Accrued research and development expenses	1,028	3,250
Deferred offering costs		238
Other	510	105
Total accrued liabilities	$ 2,727	$ 3,987

License Agreements - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
	Mar. 23, 2017	May 31, 2017	Dec. 31, 2018	Sep. 30, 2017
Series A Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fair value of shares issued				$ 1.34
Novartis International Pharmaceutical Ltd.
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
Novartis International Pharmaceutical Ltd. \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount payable upon satisfaction of commercial milestones	$ 4,300,000		$ 4,300,000
Aggregate amount payable upon satisfaction of regulatory milestones for first indication approved	24,000,000		24,000,000
Aggregate amount payable upon satisfaction of commercial milestones	18,000,000		18,000,000
Aggregate amount payable upon satisfaction of commercial milestones	$ 125,000,000		$ 125,000,000
Novartis International Pharmaceutical Ltd. \| License Agreement \| 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 ($) $ in Millions	Mar. 06, 2018	Dec. 31, 2018
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront sum of funding amount	$ 0.5
Qualifying expenses		$ 0.5

Preferred Stock and Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)				1 Months Ended	12 Months Ended
	Nov. 29, 2017	Oct. 12, 2017	Mar. 23, 2017	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 31, 2018	Jan. 30, 2018	Oct. 27, 2017	Aug. 29, 2017
Temporary Equity [Line Items]
Preferred stock, shares authorized					10,000,000	0		10,000,000
Preferred stock, shares authorized						20,320,667
Preferred stock, shares issued					0	0
Preferred stock, shares outstanding					0	0
Accretion of redeemable convertible preferred shares to redemption value						$ 0
Issuance of common stock					$ 89,370,000
Fair value of tranche rights liabilities/assets issuance date					Mar. 23, 2017
Fair value of tranche rights liabilities/assets settlement date					Nov. 29, 2017
Other Income, net					$ 2,117,000	$ (14,896,000)
Series B Tranche Right
Temporary Equity [Line Items]
Issuance price per shares		$ 8.346
Side Letter \| PureTech Health LLC
Temporary Equity [Line Items]
Shares issued			5,175,984
Side Letter \| PureTech Health LLC \| Fourth Closing
Temporary Equity [Line Items]
Issuance price per shares			$ 1.932
Common Stock
Temporary Equity [Line Items]
Issuance of common stock					$ 1,000
Initial Public Offering
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock								15,870,559
Issuance price per shares								$ 15.00
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
Series A Preferred Stock
Temporary Equity [Line Items]
Issuance price per shares				$ 1.34
Issuance of common stock		$ 6,800,000
Series A Preferred Stock \| Series A Tranche Rights
Temporary Equity [Line Items]
Shares issued				2,329,193
Issuance price per shares		$ 4.11
Issuance of common stock			$ 2,000,000	$ 4,500,000
Estimated fair value		$ 10,100,000		1,400,000
Other income		$ 8,800,000		$ 600,000
Series A Preferred Stock \| Series A Tranche Rights \| Second Closing
Temporary Equity [Line Items]
Issuance price per shares										$ 1.932
Series A Preferred Stock \| Purchase Agreement
Temporary Equity [Line Items]
Shares issued		7,763,975
Issuance price per shares		$ 1.932	$ 1.22
Issuance of common stock		$ 15,000,000
Series A Preferred Stock \| Purchase Agreement \| New Investor
Temporary Equity [Line Items]
Issuance of common stock		6,000,000
Series A Preferred Stock \| Purchase Agreement \| Tranche Rights
Temporary Equity [Line Items]
Shares issued			4,917,185
Issuance of common stock			$ 9,500,000
Series A Preferred Stock \| Purchase Agreement \| Tranche Rights \| Third Closing
Temporary Equity [Line Items]
Issuance price per shares			$ 1.932
Series A Preferred Stock \| Purchase Agreement \| PureTech Health LLC
Temporary Equity [Line Items]
Shares issued			2,846,791
Issuance price per shares			$ 1.932
Issuance of common stock		9,000,000	$ 5,500,000
Series A Preferred Stock \| Purchase Agreement \| Novartis International Pharmaceutical Ltd.
Temporary Equity [Line Items]
Shares issued			2,587,992
Series B Preferred Stock
Temporary Equity [Line Items]
Shares issued	4,792,716								4,792,716
Issuance price per shares	$ 8.346								$ 8.346
Issuance of common stock		$ 20,000,000
Series B Preferred Stock \| Series B Tranche Right
Temporary Equity [Line Items]
Issuance of common stock	$ 20,000,000
Series B Preferred Stock \| Series B Financing
Temporary Equity [Line Items]
Issuance of common stock	$ 40,000,000

Preferred Stock and Redeemable Convertible Preferred Stock - Summary of Change in Fair Value (Details) - Tranche right asset/liability $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Fair Value [Line Items]
Tranche rights, beginning balance	$ 0
Tranche rights, ending balance	0
March 23, 2017 \| Series A
Fair Value [Line Items]
Establishment (Settlement) of tranche right asset/liability	2,014
September 8, 2017 \| Series A
Fair Value [Line Items]
Change in fair value of tranche right liability	(605)
September 30, 2017 \| Series A
Fair Value [Line Items]
Mark-to-market of Series A tranche rights liability	(30)
October 12, 2017 \| Series A
Fair Value [Line Items]
Establishment (Settlement) of tranche right asset/liability	(10,146)
Change in fair value of tranche right liability	8,767
October 23, 2017 \| Series B
Fair Value [Line Items]
Establishment (Settlement) of tranche right asset/liability	6,764
November 29, 2017 \| Series B
Fair Value [Line Items]
Establishment (Settlement) of tranche right asset/liability	$ (6,764)

Common Stock - Additional Information (Details)	12 Months Ended
Common Stock - Additional Information (Details)	Dec. 31, 2018 Vote $ / shares shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares
Stockholders Equity Note [Abstract]
Number of votes entitled for each share of common stock \| Vote	1
Common stock dividends declared	$ 0
Common stock dividends paid	$ 0
Number of shares of common stock reserved for future issuance \| shares	2,773,249	17,736,568	0

Common Stock - Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,773,249	17,736,568	0
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	1,122,677	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	1,350,582	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)					6 Months Ended	12 Months Ended
	Apr. 17, 2018 Installment shares	Nov. 29, 2017 shares	Oct. 11, 2017 shares	Jul. 11, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for issuance					0	2,773,249	17,736,568
Recognized stock-based compensation expense \| $						$ 2,793,000	$ 470,000
Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options exercised						7,029
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for issuance						1,122,677	195,668
Number of options exercised					0	78,000	0
Number of options cancelled or forfeited					0	0	0
Option granted						934,038
Options Issued and Outstanding \| Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted						926,838
Weighted-average grant date fair value of options granted \| $ / shares						$ 9.23
Expense related to options granted \| $						$ 1,700,000	$ 28,000
Options Issued and Outstanding \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted					0		148,808
Weighted-average grant date fair value of options granted \| $ / shares							$ 3.75
Unrecognized compensation expense \| $						$ 7,500,000
Unrecognized compensation expense, estimated weighted-average period for recognition						3 years 2 months 1 day
Options Issued and Outstanding \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted					0	7,200	46,860
Weighted-average grant date fair value of options granted \| $ / shares						$ 12.51	$ 0.67
Expense related to options granted \| $						$ 100,000	$ 59,000
Unrecognized compensation expense \| $						$ 100,000
Unrecognized compensation expense, estimated weighted-average period for recognition						2 years 8 months 1 day
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense, estimated weighted-average period for recognition						3 months 14 days
Shares granted	2,000					2,000
Number of quarterly installment in which restriction lapse \| Installment	4
Recognized stock-based compensation expense \| $					$ 0	$ 900,000	400,000
Unrecognized stock-based compensation expense \| $						$ 3,000
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]
Shares available for issuance						24,960
Unrecognized compensation expense, estimated weighted-average period for recognition						3 years 4 months 20 days
Shares granted						24,960
Recognized stock-based compensation expense \| $					$ 0	$ 35,000	$ 0
Unrecognized stock-based compensation expense \| $						$ 200,000
Restricted Stock Units (RSUs) \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted						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
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
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
Percentage of additional shares added on outstanding shares						4.00%
Shares available for issuance						0
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance						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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,793	$ 470
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	1,236	246
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,557	$ 224

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 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	(934,038)
Outstanding, ending, Shares Available for Grant	1,350,582	1,670,341
Outstanding, beginning, Number of Options	195,668
Options granted, Number of Options	934,038
Options exercised, Number of Options	(7,029)
Outstanding, ending, Number of Options	1,122,677	195,668
Exercisable, Number of Options	67,613
Vested and expected to vest, Number of Options	1,122,677
Outstanding, beginning, Weighted-Average Exercise Price per Option	$ 4.49
Options granted, Weighted-Average Exercise Price per Option	13.05
Options exercised, Weighted-Average Exercise Price per Option	0.79
Outstanding, ending, Weighted-Average Exercise Price per Option	11.63	$ 4.49
Exercisable, Weighted-Average Exercise Price per Option	6.32
Vested and expected to vest, Weighted-Average Exercise Price per Option	$ 11.63
Outstanding, Weighted-Average Remaining Contract Term	9 years 2 months 19 days	9 years 8 months 1 day
Exercisable, Weighted-Average Remaining Contract Term	8 years 8 months 26 days
Vested and expected to vest, Weighted-Average Remaining Contract Term	9 years 2 months 19 days
Outstanding, ending, Aggregate Intrinsic Value	$ 818
Exercisable, Aggregate Intrinsic Value	241
Vested and expected to vest, Aggregate Intrinsic Value	$ 818
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	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		934,038
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	0	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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	75.90%	74.40%
Expected volatility, maximum	90.60%	74.50%
Risk-free interest rate, minimum	2.40%	1.90%
Risk-free interest rate, maximum	3.10%	2.20%
Expected dividend yield	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	78.00%	74.60%
Expected volatility, maximum	91.20%	77.00%
Risk-free interest rate, minimum	2.70%	2.30%
Risk-free interest rate, maximum	3.10%	2.40%
Expected dividend yield	0.00%	0.00%
Minimum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.57	$ 0.79
Expected term (in years)	5 years 9 months 18 days	5 years 10 months 24 days
Minimum \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.62	$ 0.79
Expected term (in years)	8 years 6 months	9 years 4 months 24 days
Maximum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 15.45	$ 9.33
Expected term (in years)	6 years 2 months 12 days	6 years 2 months 12 days
Maximum \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 15.45	$ 10.28
Expected term (in years)	10 years	10 years

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock - shares		12 Months Ended
	Apr. 17, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Beginning, Unvested shares		1,096,449
Number of Shares Outstanding, Granted	2,000	2,000
Number of Shares Outstanding, Vested		(1,097,449)
Number of Shares Outstanding, Ending, Unvested shares		1,000

Stock-Based Compensation - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Granted	24,960
Number of Shares Outstanding, Ending, Unvested shares	24,960

Income Taxes - Schedule of Reconciliation of Income Tax Expense Computed at Statutory Federal Income Tax Rate to Income Taxes (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense (benefit) at federal statutory rate		$ (7,899)	$ (11,484)
State taxes		(2,340)	(1,011)
Tax credits		(817)	(222)
Stock-based compensation		764	140
Federal tax rate change			2,202
Change in fair value of tranche rights		3	5,065
Other		241
Change in valuation allowance		10,074	5,310
Income tax expense	$ 0	$ 26	$ 0
Effective tax rate	0.00%	0.00%	0.00%

Income Taxes - Additional Information (Details) - USD ($)		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
Federal statutory income tax rate, percent		21.00%	34.00%
Percentage of net operating loss carryforwards		80.00%
Tax cuts and jobs act of 2017 change in tax rate income tax expense benefit	$ 0
Valuation allowance		$ 15,373,000	$ 5,310,000
Uncertain tax positions		0
Penalties and interest accrued		0
Federal
Income Taxes Disclosure [Line Items]
Operating loss carryforwards		$ 47,300,000	14,500,000
Operating loss carryforwards, expiration date		Dec. 31, 2036
Federal \| Research Credits
Income Taxes Disclosure [Line Items]
Tax credit carryforward, amount		$ 900,000	200,000
Tax credit carryforward, expiration date		Dec. 31, 2037
State
Income Taxes Disclosure [Line Items]
Operating loss carryforwards		$ 49,400,000	14,400,000
Operating loss carryforwards, expiration date		Dec. 31, 2036
State \| Research Credits
Income Taxes Disclosure [Line Items]
Tax credit carryforward, amount		$ 200,000	$ 60,000
Tax credit carryforward, expiration date		Dec. 31, 2032

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating losses	$ 13,054	$ 3,953
Capitalized license	869	896
Research credits	1,007	269
Accruals	514	176
Stock-based compensation	51	16
Total gross deferred tax assets	15,495	5,310
Less valuation allowance	(15,373)	(5,310)
Total deferred tax assets	122	0
Deferred tax liabilities:
Other comprehensive income - unrealized loss	11
Depreciation and amortization	111
Total gross deferred tax liability	122	0
Net deferred tax assets	$ 0	$ 0

Commitment and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Mar. 23, 2017
Commitments and Contingences [Line Items]
Estimated litigation liability	$ 0
Lease commencement date	Jan. 01, 2018
Lease expiration date	Feb. 28, 2021
Lease payments	$ 200,000	$ 0
Novartis International Pharmaceutical Ltd. \| Maximum
Commitments and Contingences [Line Items]
Aggregate amount payable upon satisfaction of commercial milestones	4,300,000		$ 4,300,000
Aggregate amount payable upon satisfaction of regulatory milestones for first indication approved	24,000,000		24,000,000
Aggregate amount payable upon satisfaction of commercial milestones	18,000,000		18,000,000
Aggregate amount payable upon satisfaction of commercial milestones	$ 125,000,000		$ 125,000,000

Commitment and Contingencies - Summary of Obligations Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 231
2020	236
2021	39
Total	$ 506

Related Party Transactions - Additional Information (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Novartis International Pharmaceutical Ltd.
Related Party Transaction [Line Items]
Payments to related parties	$ 0	$ 0	$ 900,000
Silverstein Foundation \| Funding Agreement
Related Party Transaction [Line Items]
Fund received from related parties	$ 0	$ 500,000	$ 0
Common Stock
Related Party Transaction [Line Items]
Issuance of common shares to PureTech, shares			1,886,363

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 6,360	$ 9,032	$ 14,113	$ 10,200	$ 7,523	$ 3,945	$ 4,057	$ 3,357		$ 39,705	$ 18,882
Loss from operations	(6,360)	(9,032)	(14,113)	(10,200)	(7,523)	(3,945)	(4,057)	(3,357)	$ (1)	(39,705)	(18,882)
Net loss	(5,757)	(8,407)	(13,591)	(9,859)	(23,054)	(3,310)	(4,057)	(3,357)	$ (1)	(37,614)	(33,778)
Net loss applicable to common stockholders	$ (5,757)	$ (8,407)	$ (13,591)	$ (9,859)	$ (23,054)	$ (3,310)	$ (4,057)	$ (3,357)		$ (37,614)	$ (33,778)
Net loss per share, basic and diluted	$ (0.21)	$ (0.30)	$ (0.48)	$ (0.46)	$ (5.11)	$ (0.75)	$ (0.94)	$ (1.20)	$ 0.00	$ (1.42)	$ (8.42)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details