View Filing Data

Print Document View Excel Document

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Mar. 11, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2019
Entity Registrant Name	resTORbio, Inc.
Entity Central Index Key	0001720580
Entity File Number	001-38359
Trading Symbol	TORC
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Public Float			$ 277.5
Entity Common Stock, Shares Outstanding		36,445,751
Document Annual Report	true
Document Transition Report	false
Entity Tax Identification Number	81-3305277
Entity Incorporation, State or Country Code	DE
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Entity Address, Address Line One	500 Boylston Street
Entity Address, Address Line Two	13th Floor
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
City Area Code	857
Local Phone Number	315-5528
Entity Address, Postal Zip Code	02116
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer

Consolidated Balance Sheets - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 33,774,000	$ 7,042,000
Marketable securities	57,699,000	100,986,000
Prepaid expenses	1,707,000	1,491,000
Other current assets	73,000	15,000
Total current assets	93,253,000	109,534,000
Restricted cash	245,000	84,000
Property and equipment, net	414,000	321,000
Total assets	93,912,000	109,939,000
Current liabilities:
Accounts payable	6,716,000	2,989,000
Accrued liabilities	5,483,000	2,727,000
Total current liabilities	12,199,000	5,716,000
Other liabilities	15,000	19,000
Total liabilities	12,214,000	5,735,000
Commitments and contingencies (see Note 12)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2019 and 2018; none issued and outstanding as of December 31, 2019 and 2018
Common stock, $0.0001 par value, 150,000,000 shares authorized as of December 31, 2019 and 2018; 36,444,732 and 28,055,344 shares issued and outstanding as of December 31, 2019 and 2018, respectively; 36,444,732 and 28,054,344 shares vested as of December 31, 2019 and 2018, respectively	4,000	3,000
Additional paid-in capital	235,777,000	175,635,000
Accumulated deficit	(154,132,000)	(71,393,000)
Other comprehensive income (loss)	49,000	(41,000)
Total stockholders’ equity	81,698,000	104,204,000
Total liabilities and stockholders’ equity	$ 93,912,000	$ 109,939,000

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	36,444,732	28,055,344
Common stock, shares outstanding	36,444,732	28,055,344
Common stock, shares vested	36,444,732	28,054,344
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 73,634	$ 31,065	$ 16,839
General and administrative	11,823	8,640	2,043
Total operating expenses	85,457	39,705	18,882
Loss from operations	(85,457)	(39,705)	(18,882)
Interest income	2,817	2,124
Other expense, net	(63)	(7)	(14,896)
Loss before income taxes	(82,703)	(37,588)	(33,778)
Income tax expense	36	26	0
Net loss	$ (82,739)	$ (37,614)	$ (33,778)
Net loss per share, basic and diluted	$ (2.41)	$ (1.42)	$ (8.42)
Weighted-average common shares used in computing net loss per share, basic and diluted	34,306,374	26,439,216	4,009,513
Other comprehensive loss:
Net unrealized gains (losses) on marketable securities	$ 90	$ (41)
Total other comprehensive income (loss)	90	(41)
Comprehensive loss	$ (82,649)	$ (37,655)	$ (33,778)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Restricted stock units	Initial Public Offering	At-the-Market Offering	Common Stock	Common Stock Restricted stock units	Common Stock Initial Public Offering	Common Stock At-the-Market Offering	Additional Pain In Capital	Additional Pain In Capital Restricted stock units	Additional Pain In Capital Initial Public Offering	Additional Pain In Capital At-the-Market Offering	Accumulated Deficit	Comprehensive Income (Loss)	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock
Balance at Dec. 31, 2016	$ 0				$ 1				$ 0				$ (1)	$ 0
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 stock, 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				$ 1				81,619						$ (41,674)	$ (39,946)
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, Shares					15,870,559										(15,527,951)	(4,792,716)
Issuance of common stock			$ 89,370				$ 1				$ 89,369
Issuance of common stock, Shares							6,516,667
Vesting of restricted stock	865								865
Vesting of restricted stock, 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)
Net unrealized gains (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
Issuance of common stock			$ 49,747	$ 6,716			$ 1				$ 49,746	$ 6,716
Issuance of common stock, Shares							7,687,934	687,800
Vesting of restricted stock	1								1
Vesting of restricted stock, shares					1,000
Vesting of restricted stock units, net of shares withheld for taxes		$ (20)								$ (20)
Vesting of restricted stock units, net of shares withheld for taxes, shares						6,625
Exercise of stock options	6								6
Exercise of stock options, Shares					7,029
Stock-based compensation expense	3,693								3,693
Net loss	(82,739)												(82,739)
Net unrealized gains (losses) on marketable securities	90													90
Balance at Dec. 31, 2019	$ 81,698				$ 4				$ 235,777				$ (154,132)	$ 49
Balance, shares at Dec. 31, 2019					36,444,732

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Series B Redeemable Convertible Preferred Stock
Stock issuance cost			$ 54
Common Stock \| Initial Public Offering
Stock issuance cost	$ 3,683	$ 8,379
Common Stock \| At-the-Market Offering
Stock issuance cost	$ 285

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net loss	$ (82,739)	$ (37,614)	$ (33,778)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	(1,020)	(673)
Depreciation and amortization expense	125	80	5
Loss on disposal of property and equipment	53
Stock-based compensation expense	3,694	2,793	470
Change in fair value of tranche liability			14,896
Expense related to acquisition of intellectual property			3,157
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(274)	(630)	(876)
Accounts payable	3,727	1,597	1,392
Accrued liabilities	2,756	(1,022)	3,749
Other liabilities	(4)	19
Net cash used in operating activities	(73,682)	(35,450)	(10,985)
Investing activities:
Purchases of property and equipment	(271)	(362)	(44)
Maturities of marketable securities	141,500	7,500
Purchase of marketable securities	(97,103)	(107,854)
Net cash provided by (used in) investing activities	44,126	(100,716)	(44)
Financing activities:
Proceeds from public offering, net of issuance costs	49,747	90,908
Proceeds from at-the-market offering, net of issuance costs	6,716
Deferred offering costs		(970)	(568)
Taxes paid related to net share settlement of restricted stock units	(20)
Proceeds from exercise of stock options	6	5
Net cash provided by financing activities	56,449	89,943	64,378
Net increase (decrease) in cash, cash equivalents and restricted cash	26,893	(46,223)	53,349
Cash, cash equivalents and restricted cash at beginning of period	7,126	53,349
Cash, cash equivalents and restricted cash at end of period	34,019	7,126	53,349
Supplemental disclosure of non-cash investing and financing activities:
Cash paid for income taxes	$ 62
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, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1. Organization

resTORbio, Inc. (collectively referred to with its wholly-owned, controlled subsidiary, resTORbio Securities Corp. as “resTORbio” or the “the Company”) was incorporated in the State of Delaware on July 5, 2016. The Company is a clinical-stage biopharmaceutical company developing innovative medicines that target the biology of aging to prevent or treat aging-related diseases with the potential to extend healthy lifespan. The Company’s principal operations are located in Boston, Massachusetts.

In November 2019, the Company announced that top line data from the PROTECTOR 1 Phase 3 study, evaluating the safety and efficacy of RTB101 in preventing clinically symptomatic respiratory illness in adults age 65 and older, did not meet its primary endpoint and the Company has stopped the development of RTB101 for clinically symptomatic respiratory illness. In addition, in February 2020, the Company retained JMP Securities LLC as a financial advisor to assist it in its evaluation of a broad range of strategic alternatives to enhance stockholder value, including additional capital raising transactions, an acquisition, merger, business combination, licensing and/or other strategic transaction involving the Company. There is no assurance that the review of strategic alternatives will result in the Company changing its business plan, pursuing any particular transaction, or, if it pursues any such transaction, that it will be completed.

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.

Public Offering

On March 22, 2019, the Company completed an underwritten public offering, whereby the Company sold 7,200,000 shares of its common stock at a price of $6.95 per share. The aggregate net proceeds received by the Company from the offering were approximately $46.6 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $3.5 million. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 1,080,000 shares of common stock at the public offering price, less underwriting discounts and commissions. On April 10, 2019, the Company sold an additional 487,934 shares of its common stock at a price of $6.95 per share. The aggregate net proceeds received by the Company were approximately $3.2 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $0.2 million. The remainder of the option expired unexercised.

At-the-Market Offering

On February 1, 2019, the Company filed a Registration Statement on Form S-3 (the “Shelf”) with the Securities and Exchange Commission (the “SEC”) in relation to the registration of common stock, preferred stock, warrants and/or units of any combination thereof (collectively, the “Securities”). The Company also simultaneously entered into a Controlled Equity Offering Sales Agreement (the “Sales Agreement”) with SVB Leerink LLC and Cantor Fitzgerald & Co. (collectively, the “Sales Agents”), to provide for the offering, issuance and sale by the Company of up to an aggregate of $50.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf and subject to the limitations thereof. The Company will pay to the Sales Agents cash commissions of 3.0 percent of the gross proceeds of sales of common stock under the Sales Agreement. Beginning in June 2019 through September 19, 2019, based on settlement date, the Company sold approximately 688,000 shares of common stock at a weighted-average selling price of $10.18 per share in accordance with the Sales Agreement for aggregate net proceeds of $6.7 million, after payment of cash commissions of 3.0 percent of the gross proceeds to the Sales Agent and incurred issuance costs of approximately $75,000 related to legal, accounting, and other fees in connection with the sale. As of December 31, 2019, $43.0 million remained available for sale under the Sales Agreement.

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 $154.1 million as of December 31, 2019. As of December 31, 2019, the Company had $91.5 million of cash, cash equivalents, and marketable securities, which the Company believes will be sufficient to fund the Company’s current operating plan through at least the next twelve months from the date of filing this Annual Report on Form 10-K.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

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 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, 2019 and 2018, restricted cash (non-current) related to this letter of credit consisted of $245,000 and $84,000, 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, 2019 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	33,774	$	33,774	$	—	$	—
U.S. treasury securities (included in marketable securities)		57,699		57,699		—		—
Total	$	91,473	$	91,473	$	—	$	—

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	$	—	$	—

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, 2019 and 2018. 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, 2019 and 2018.

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, 2019, 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.

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 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, its 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 it 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 between estimated costs of research and development activities accrued by the Company each reporting period and amounts actually incurred.

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 stock, restricted stock units, 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 stock and restricted stock units.

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 awards 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 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.

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, 2019.

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, with early adoption permitted. The Company adopted ASU 2016-18 on December 31, 2019. Upon adoption of ASU 2016-18, the Company applied the retrospective transition method for each period presented and included $0 and $84,000 of restricted cash in the beginning-of-period and end-of-period cash, cash equivalents and restricted cash balance, respectively, in the consolidated statement of cash flows for the year ended 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, 2020. Early adoption is permitted. The adoption of this standard is expected to have an impact on the amount of the Company’s assets and liabilities presented. The Company expects to utilize the new transition method described in ASU No. 2018-11 and use the effective date as the Company’s date of initial application for the new standard. The Company expects to elect the available package of practical expedients in transition which would allow it to not re-assess whether existing or expired arrangements contain a lease, the lease classification of existing or expired leases, or whether previous initial direct costs would qualify for capitalization under the new lease standard. As of December 31, 2019, the Company has not elected to early adopt the guidance and is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments”. This ASU requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, this standard now requires allowances to be recorded instead of reducing the amortized cost of the investment. ASU 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. ASU 2016-13 will be effective for fiscal years beginning after December 15, 2020 with early adoption permitted, and requires adoption using a modified retrospective approach, with certain exceptions. Based on the composition of the Company’s investment portfolio as of December 31, 2019, current market conditions and historical credit loss activity, the adoption of this standard is not expected to have a material impact on the Company’s 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 does not expect the impact of ASU 2018-07 to be material to its consolidated financial statements.

Marketable Securities

12 Months Ended

Dec. 31, 2019

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

As of December 31, 2019, 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	$	57,650	$	49	$	—	$	57,699
Total	$	57,650	$	49	$	—	$	57,699

As of December 31, 2018, the fair value of marketable securities by type of security was as following (in thousands):

	Amortized		Unrealized		Unrealized			Fair
Description	Cost		Gains		Losses			Value
U.S. government agency treasuries and securities	$	101,027	$	4	$	(45	)	$	100,986
Total	$	101,027	$	4	$	(45	)	$	100,986

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

	December 31, 2019
	Amortized Cost		Fair Value
	(In thousands)
Due in one year or less	$	57,650	$	57,699
Total	$	57,650	$	57,699

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

Property and Equipment, Net

12 Months Ended

Dec. 31, 2019

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,
	2019			2018
	(In thousands)
Leasehold improvements	$	17		$	65
Machinery and equipment		—			38
Furniture and fixtures		397			194
Computers		125			76
Office equipment		11			11
Software		22			22
Total property and equipment		572			406
Less: accumulated depreciation		(158	)		(85	)
Property and equipment, net	$	414		$	321

Depreciation expense was $0.1 million, $80,000, and $5,000 for the years ended December 31, 2019, 2018, and 2017, respectively.

Accrued Liabilities

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Accrued Liabilities

5. Accrued Liabilities

Accrued liabilities consist of the following:

	As of December 31,
	2019		2018
	(In thousands)
Accrued payroll and related expenses	$	1,643	$	1,189
Accrued restructuring cost (see Note 15)		516		—
Accrued research and development expenses		3,171		1,028
Other		153		510
Total accrued liabilities	$	5,483	$	2,727

License Agreements

12 Months Ended

Dec. 31, 2019

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 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 are 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. In May 2019, the Company initiated a Phase 3 clinical trial for the first indication, triggering another milestone payment of $2.5 million under the agreement. As of December 31, 2019, none of the remaining clinical milestones, regulatory milestones, sales milestones, or royalties had been reached or were probable of achievement.

Research Funding Agreements

12 Months Ended

Dec. 31, 2019

Research Funding Agreement [Abstract]

Research Funding Agreement

7. Research Funding Agreement

Silverstein Foundation

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 “Silverstein Funding Agreement”). One of the Company’s directors is a co-founder and current trustee of the Silverstein Foundation. Under the terms of the Silverstein 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 Silverstein 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 PD. 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 Silverstein 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 Silverstein Funding Agreement does not contain specified performance obligations other than to conduct research on a particular program or in a particular field and there are no obligations to deliver specified products or technology.

For funds received under the Silverstein Funding Agreement, the Company recognizes a reduction in research and development expenses. During the year ended 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.

National Institute of Health

In May 2019, the Company was awarded a 5-year grant for up to $1.5 million from the National Institutes of Health (the “NIH”) to study RTB101 and the regulation of antiviral immunity in the elderly. The Company is entitled to use the award solely to conduct the research. The Company is solely responsible for commencing and conducting the research and will furnish periodic progress updates to the NIH throughout the term of the award. After completing the research, the Company must provide the NIH with a formal report describing the work performed and the results of the research.

For funds received under the NIH 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 NIH. Qualifying expenses incurred by the Company in advance of funding by the NIH are recorded in the consolidated balance sheets as other current assets. As of December 31, 2019, $0.1 million qualifying expenses have been incurred and $41,000 have been funded by the NIH. Therefore, $61,000 is included in other current assets on the accompanying balance sheet as of December 31, 2019.

Preferred Stock

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Preferred Stock

8. Preferred Stock

As of December 31, 2019 and 2018, the Company had 10,000,000 shares of preferred stock authorized and none was issued and outstanding.

Common Stock

12 Months Ended

Dec. 31, 2019

Stockholders Equity Note [Abstract]

Common Stock

9. Common Stock

General

As of December 31, 2019, the Company had 150,000,000 shares of common stock authorized, of which 36,444,732 shares were issued and outstanding. 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, 2019, 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, 2019 and 2018, the Company has reserved the following number of shares of common stock for future issuance upon the exercise of options, vesting of restricted stock units or grant of equity awards:

	As of December 31,
	2019		2018
Options issued and outstanding		2,562,800		1,122,677
Unvested restricted stock units		828,935		24,960
Options available for future grants		212,308		1,350,582
Shares available for issuance under the 2018 ESPP		555,583		275,030
Total		4,159,626		2,773,249

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

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 “2017 Plan”). Under the 2017 Plan, shares of the Company’s common stock have been reserved for the issuance of stock options, restricted stock awards and restricted stock units 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 2017 Plan. Under the terms of the 2017 Plan, options may be granted at an exercise price not less than fair market value. The terms of options granted under the 2017 Plan may not exceed ten years. The Board 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 2017 Plan from 537,914 shares to 630,662 shares. On November 29, 2017, the Company increased the number of shares of common stock available for issuance under the 2017 Plan from 630,662 shares to 1,866,009 shares.

In connection with the Company’s IPO, the Board adopted, and the Company’s stockholders approved the 2018 Stock 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 were initially reserved for issuance under the 2018 Plan was 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. On January 1, 2019, as a result of the foregoing evergreen provision, the number of common stock available for issuance under the 2018 Plan automatically increased from 2,200,260 to 3,322,473 shares.

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, 2019, 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,
	2019		2018		2017
	(In thousands)
Research and development	$	1,542	$	1,236	$	246
General and administrative		2,152		1,557		224
Total stock-based compensation expense	$	3,694	$	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 ($)		Weighted- Average Remaining Contract Term (Years)		Aggregate Intrinsic Value ($)
	(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677			11.63		9.22
Shares reserved for issuance		1,122,213
Options granted		(1,896,527	)		1,896,527			6.28
Restricted stock units granted		(813,335	)
Options exercised		—			(7,029	)		0.79
Options forfeited		449,375			(449,375	)		10.77
Outstanding, December 31, 2019		212,308			2,562,800			7.85		8.84		200
Exercisable, December 31, 2019					461,150			11.40		7.06		37
Vested and expected to vest, December 31, 2019					2,562,800			7.85		8.84		200

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, 2019. The aggregate intrinsic value of options exercised during the year ended December 31, 2019, was $67,000. The aggregate intrinsic values of options exercised during the year ended December 31, 2018 was $78,000.

During the year ended December 31, 2019, the Company granted options to employees and directors to purchase an aggregate of 1,886,687 common shares with a weighted-average grant date fair value of $4.83. During the year ended December 31, 2018, the Company granted options to employees 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, 2019, the Company granted options to non-employees to purchase an aggregate of 9,840 common shares with a weighted-average grant date fair value of $7.61. 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.

As of December 31, 2019, the total unrecognized compensation expense related to unvested employee options was $9.4 million which the Company expects to recognize over an estimated weighted-average period of 2.80 years. As of December 31, 2019, the total unrecognized compensation expense related to unvested non-employee options was $26,000 which the Company expects to recognize over an estimated weighted-average period of 2.14 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,
	2019	2018	2017
Employees:
Fair value of common stock	$1.27 - $10.66	$8.57 - $15.45	$0.79 - $9.33
Expected term (in years)	5.5 - 6.1	5.8 - 6.2	5.9 - 6.2
Expected volatility	92.0% - 104.9%	75.9% - 90.6%	74.4% - 74.5%
Risk-free interest rate	1.4% - 2.6%	2.4% - 3.1%	1.9% - 2.2%
Expected dividend yield	0.0%	0.0%	0.0%
Non-employees:
Fair value of common stock	$1.23 - $10.26	$8.62 - $15.45	$0.79 - $10.28
Expected term (in years)	7.4 - 10.0	8.4 - 10.0	9.4 - 10.0
Expected volatility	89.7% - 99.5%	78.0% - 91.2%	74.6% - 77.0%
Risk-free interest rate	1.7% - 2.8%	2.7% - 3.1%	2.3% - 2.4%
Expected dividend yield	0.0%	0.0%	0.0%

Restricted Stock

On April 17, 2018, the Company granted 2,000 shares of restricted stock to a consultant. The restrictions lapsed in four equal quarterly installments and was fully vested on the first anniversary of such grant. Compensation expenses of such unvested shares was 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, 2018		1,000
Vested		(1,000	)
Unvested shares — December 31, 2019		—

The Company recognized $4,000, $0.9 million and $0.4 million of stock-based compensation expense related to restricted shares during the years ended December 31, 2019, 2018 and 2017, respectively. As of December 31, 2019, there was no unrecognized stock-based compensation expense related to unvested restricted stock.

Restricted Stock Units

In May 2018, the Company granted 24,960 restricted stock units to an employee with a grant date fair value of $9.03 per share. In December 2019, the Company granted 813,335 restricted stock units to employees with a weighted-average grant date fair value of $1.27.

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

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2018		24,960
Granted		813,335
Vested		(9,360	)
Unvested shares — December 31, 2019		828,935

The Company recognized $74,000 and $35,000, of stock-based compensation expense related to restricted stock units during the years ended December 31, 2019 and 2018, respectively. As of December 31, 2019, there was $1.1 million of unrecognized stock-based compensation expense related to unvested restricted stock units which the Company expects to recognize over a remaining weighted-average period of 3.75 years.

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 were initially reserved for issuance under the 2018 ESPP was 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. On January 1, 2019, as a result of the foregoing evergreen provision, the number of common stock available for issuance under the 2018 ESPP automatically increased from 275,030 to 555,583 shares. No shares have been issued under the 2018 ESPP during the years ended December 31, 2019 and 2018.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

Provision for Income Taxes

For the years ended December 31, 2019, 2018, and 2017, the Company did not record a federal current or deferred income tax expense. For the years ended December 31, 2019 and 2018, the Company did record a state current 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,
	2019			2018			2017
	(In thousands)
Income tax benefit at federal statutory rate	$	(17,368	)	$	(7,899	)	$	(11,484	)
State taxes		(5,314	)		(2,340	)		(1,011	)
Tax credits		(2,910	)		(817	)		(222	)
Stock-based compensation		753			764			140
Federal tax rate change		—			—			2,202
Change in fair value of tranche rights		—			3			5,065
Other		25			241			—
Change in valuation allowance		24,850			10,074			5,310
Income tax expense	$	36		$	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,
	2019			2018
	(In thousands)
Deferred tax assets:
Net operating losses	$	34,938		$	13,054
Capitalized license		771			835
Research credits		4,151			1,007
Accruals		401			514
Stock-based compensation		53			51
Net unrealized loss		—			11
Total gross deferred tax assets		40,314			15,472
Less valuation allowance		(40,221	)		(15,395	)
Total deferred tax assets		93			77
Deferred tax liabilities:
Net unrealized gain		13			—
Depreciation and amortization		80			77
Total gross deferred tax liability		93			77
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 $40.2 million and $15.4 million has been recorded for the years ended December 31, 2019 and 2018, respectively.

Net Operating Loss and Tax Credit Carryforwards

As of December 31, 2019, the Company had net operating loss carryforwards for federal income tax purposes of approximately $127.0 million, of which $14.0 million will begin to expire in 2036 and $113.0 million can be carried forward indefinitely. As of December 31, 2019, the Company had total state net operating loss carryforwards of approximately $130.8 million 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, 2019 and 2018, the Company had federal research credits of $3.8 million and $0.9 million, respectively, which will begin to expire in 2037 and state research credits of $0.5 million and $0.2 million, respectively, which will begin to expire in 2032. These tax credits are subject to the same limitations discussed above. The Company has not yet conducted a study of its research and development credit carryforwards. This study may result in an increase or decrease to the Company's credit carryforwards, however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company's credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the statements of operations and comprehensive loss or statements of cash flows if an adjustment were required.

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, 2019, 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, 2019

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, 2019.

Lease

In April 2019, the Company amended its multi-year lease agreement to relocate its office space in Boston, Massachusetts under an operating lease agreement. The amended lease term is for a period of seven years from the date of relocation on August 1, 2019. The initial annual base rent of the relocation premises is $0.6 million per year, increasing 2% annually. In connection with the lease amendment, the Company issued a new cash-collateralized letter of credit for the benefit of the landlord in the amount of $0.2 million. Rent expense was $0.5 million, $0.3 million and $0 the years ended December 31, 2019, 2018, and 2017, respectively. Obligations to make future minimum lease payments as of December 31, 2019, are as follows:

Year ending December 31,	Minimum Lease Payments
	(In thousands)
2020	$	594
2021		606
2022		618
2023		630
2024		643
Years thereafter		1,043
Total	$	4,134

Novartis License Agreement

The Company is required to pay up to an aggregate of $1.5 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 PD (see Note 7).

Net Loss Per Share

12 Months Ended

Dec. 31, 2019

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, unvested restricted stock, and unvested restricted stock units. For periods in which the Company has reported net losses, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their affect is anti-dilutive.

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

	As of December 31,
	2019		2018
Options issued and outstanding		2,562,800		1,122,677
Unvested restricted stock		—		1,000
Unvested restricted stock units		828,935		24,960
Total		3,391,735		1,148,637

Related Party Transactions

12 Months Ended

Dec. 31, 2019

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.

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).

Aggregate payments for the above related party transactions totaled $2.5 million, $0, and $0.9 million for the years ended December 31, 2019, 2018 and 2017, 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). No funds were received from the Silverstein Foundation during the year ended December 31, 2019 and 2017. The Company received $0.5 million during the year ended December 31, 2018.

Reduction in Workforce

12 Months Ended

Dec. 31, 2019

Restructuring And Related Activities [Abstract]

Reduction in Workforce

15. Reduction in Workforce

In December 2019, the Company’s Board of Directors approved a restructuring plan to reduce operating costs and better align the Company’s workforce with its business needs following the Company’s November 2019 announcement regarding that top line data from the PROTECTOR 1 Phase 3 study, evaluating the safety and efficacy of RTB101 in preventing clinically symptomatic respiratory illness in adults age 65 and older, did not meet its primary endpoint, and that the Company has stopped the development of RTB101 in this indication.

Under the restructuring plan, the Company reduced its workforce by 8 employees (approximately 22% of total employees). Affected employees are eligible to receive severance payments and outplacement services in connection with the reduction. During the year ended December 31, 2019, the Company recorded aggregate restructuring charges of approximately $0.6 million related to severance payments and other employee-related costs. The Company does not expect to incur any additional significant costs associated with this restructuring. During the year ended December 31, 2019, $66,000 of the estimated restructuring charges were paid. The Company expects the remaining accrued restructuring costs of $0.5 million will be paid in the next 12 months.

The following table shows the total amount expected to be incurred and the liability related to the 2019 restructuring as of December 31, 2019:

	One-time Employee Termination Benefits
	(In thousands)
Accrued restructuring costs beginning balance	$	—
Restructuring charges incurred during the year		582
Amounts paid during the year		(66	)
Accrued restructuring costs as of December 31, 2019	$	516

The following table summarizes the restructuring charges reported in the consolidated statements of operations and comprehensive loss for the year ended December 31, 2019:

	Cash		Non-cash		Total Expenses
	(In thousands)
Research and development	$	306	$	—	$	306
General and administrative		276		—		276
Total	$	582	$	—	$	582

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

16. Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2019 and 2018. 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.

	2019
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(In thousands, except per share data)
Total operating expenses	$	11,691		$	19,169		$	25,161		$	29,436		$	85,457
Loss from operations		(11,691	)		(19,169	)		(25,161	)		(29,436	)		(85,457	)
Net loss		(11,069	)		(18,332	)		(24,448	)		(28,890	)		(82,739	)
Net loss per share—basic and diluted	$	(0.38	)	$	(0.51	)	$	(0.68	)	$	(0.79	)	$	(2.41	)

	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 per share—basic and diluted	$	(0.46	)	$	(0.48	)	$	(0.30	)	$	(0.21	)	$	(1.42	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

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, 2019 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	33,774	$	33,774	$	—	$	—
U.S. treasury securities (included in marketable securities)		57,699		57,699		—		—
Total	$	91,473	$	91,473	$	—	$	—

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	$	—	$	—

Concentration of Credit Risk

Concentration of Manufacturing Risk

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

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, 2019

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, 2019 (in thousands):

			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	33,774	$	33,774	$	—	$	—
U.S. treasury securities (included in marketable securities)		57,699		57,699		—		—
Total	$	91,473	$	91,473	$	—	$	—

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	$	—	$	—

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, 2019

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value Marketable Securities By Type of Security

As of December 31, 2019, 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	$	57,650	$	49	$	—	$	57,699
Total	$	57,650	$	49	$	—	$	57,699

As of December 31, 2018, the fair value of marketable securities by type of security was as following (in thousands):

	Amortized		Unrealized		Unrealized			Fair
Description	Cost		Gains		Losses			Value
U.S. government agency treasuries and securities	$	101,027	$	4	$	(45	)	$	100,986
Total	$	101,027	$	4	$	(45	)	$	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:

	December 31, 2019
	Amortized Cost		Fair Value
	(In thousands)
Due in one year or less	$	57,650	$	57,699
Total	$	57,650	$	57,699

	December 31, 2018
	Amortized Cost		Fair Value
	(In thousands)
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, 2019

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	As of December 31,
	2019			2018
	(In thousands)
Leasehold improvements	$	17		$	65
Machinery and equipment		—			38
Furniture and fixtures		397			194
Computers		125			76
Office equipment		11			11
Software		22			22
Total property and equipment		572			406
Less: accumulated depreciation		(158	)		(85	)
Property and equipment, net	$	414		$	321

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	As of December 31,
	2019		2018
	(In thousands)
Accrued payroll and related expenses	$	1,643	$	1,189
Accrued restructuring cost (see Note 15)		516		—
Accrued research and development expenses		3,171		1,028
Other		153		510
Total accrued liabilities	$	5,483	$	2,727

Common Stock (Tables)

12 Months Ended

Dec. 31, 2019

Stockholders Equity Note [Abstract]

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

	As of December 31,
	2019		2018
Options issued and outstanding		2,562,800		1,122,677
Unvested restricted stock units		828,935		24,960
Options available for future grants		212,308		1,350,582
Shares available for issuance under the 2018 ESPP		555,583		275,030
Total		4,159,626		2,773,249

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Research and development	$	1,542	$	1,236	$	246
General and administrative		2,152		1,557		224
Total stock-based compensation expense	$	3,694	$	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 ($)		Weighted- Average Remaining Contract Term (Years)		Aggregate Intrinsic Value ($)
	(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677			11.63		9.22
Shares reserved for issuance		1,122,213
Options granted		(1,896,527	)		1,896,527			6.28
Restricted stock units granted		(813,335	)
Options exercised		—			(7,029	)		0.79
Options forfeited		449,375			(449,375	)		10.77
Outstanding, December 31, 2019		212,308			2,562,800			7.85		8.84		200
Exercisable, December 31, 2019					461,150			11.40		7.06		37
Vested and expected to vest, December 31, 2019					2,562,800			7.85		8.84		200

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,
	2019	2018	2017
Employees:
Fair value of common stock	$1.27 - $10.66	$8.57 - $15.45	$0.79 - $9.33
Expected term (in years)	5.5 - 6.1	5.8 - 6.2	5.9 - 6.2
Expected volatility	92.0% - 104.9%	75.9% - 90.6%	74.4% - 74.5%
Risk-free interest rate	1.4% - 2.6%	2.4% - 3.1%	1.9% - 2.2%
Expected dividend yield	0.0%	0.0%	0.0%
Non-employees:
Fair value of common stock	$1.23 - $10.26	$8.62 - $15.45	$0.79 - $10.28
Expected term (in years)	7.4 - 10.0	8.4 - 10.0	9.4 - 10.0
Expected volatility	89.7% - 99.5%	78.0% - 91.2%	74.6% - 77.0%
Risk-free interest rate	1.7% - 2.8%	2.7% - 3.1%	2.3% - 2.4%
Expected dividend yield	0.0%	0.0%	0.0%

Summary of Restricted Stock Activity

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2018		1,000
Vested		(1,000	)
Unvested shares — December 31, 2019		—

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, 2018		24,960
Granted		813,335
Vested		(9,360	)
Unvested shares — December 31, 2019		828,935

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

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,
	2019			2018			2017
	(In thousands)
Income tax benefit at federal statutory rate	$	(17,368	)	$	(7,899	)	$	(11,484	)
State taxes		(5,314	)		(2,340	)		(1,011	)
Tax credits		(2,910	)		(817	)		(222	)
Stock-based compensation		753			764			140
Federal tax rate change		—			—			2,202
Change in fair value of tranche rights		—			3			5,065
Other		25			241			—
Change in valuation allowance		24,850			10,074			5,310
Income tax expense	$	36		$	26		$	—
Effective tax rate		0.0	%		0.0	%		0.0	%

Schedule of Deferred Tax Assets and Liabilities

	As of December 31,
	2019			2018
	(In thousands)
Deferred tax assets:
Net operating losses	$	34,938		$	13,054
Capitalized license		771			835
Research credits		4,151			1,007
Accruals		401			514
Stock-based compensation		53			51
Net unrealized loss		—			11
Total gross deferred tax assets		40,314			15,472
Less valuation allowance		(40,221	)		(15,395	)
Total deferred tax assets		93			77
Deferred tax liabilities:
Net unrealized gain		13			—
Depreciation and amortization		80			77
Total gross deferred tax liability		93			77
Net deferred tax assets	$	—		$	—

Commitments and Contingences (Tables)

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Summary of Obligations Future Minimum Lease Payments

Obligations to make future minimum lease payments as of December 31, 2019, are as follows:

Year ending December 31,	Minimum Lease Payments
	(In thousands)
2020	$	594
2021		606
2022		618
2023		630
2024		643
Years thereafter		1,043
Total	$	4,134

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

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

	As of December 31,
	2019		2018
Options issued and outstanding		2,562,800		1,122,677
Unvested restricted stock		—		1,000
Unvested restricted stock units		828,935		24,960
Total		3,391,735		1,148,637

Reduction in Workforce (Tables)

12 Months Ended

Dec. 31, 2019

Restructuring And Related Activities [Abstract]

Schedule of Amount Expected to be Incurred and Liability Related to Restructuring

The following table shows the total amount expected to be incurred and the liability related to the 2019 restructuring as of December 31, 2019:

	One-time Employee Termination Benefits
	(In thousands)
Accrued restructuring costs beginning balance	$	—
Restructuring charges incurred during the year		582
Amounts paid during the year		(66	)
Accrued restructuring costs as of December 31, 2019	$	516

Schedule of Restructuring Charges Reported in Consolidated Statements of Operations and Comprehensive Loss

The following table summarizes the restructuring charges reported in the consolidated statements of operations and comprehensive loss for the year ended December 31, 2019:

	Cash		Non-cash		Total Expenses
	(In thousands)
Research and development	$	306	$	—	$	306
General and administrative		276		—		276
Total	$	582	$	—	$	582

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

	2019
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(In thousands, except per share data)
Total operating expenses	$	11,691		$	19,169		$	25,161		$	29,436		$	85,457
Loss from operations		(11,691	)		(19,169	)		(25,161	)		(29,436	)		(85,457	)
Net loss		(11,069	)		(18,332	)		(24,448	)		(28,890	)		(82,739	)
Net loss per share—basic and diluted	$	(0.38	)	$	(0.51	)	$	(0.68	)	$	(0.79	)	$	(2.41	)

	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 per share—basic and diluted	$	(0.46	)	$	(0.48	)	$	(0.30	)	$	(0.21	)	$	(1.42	)

Organization - Additional Information (Details) - USD ($)				12 Months Ended
	Apr. 10, 2019	Mar. 22, 2019	Feb. 01, 2019	Dec. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Entity incorporation date				Jul. 05, 2016
Net proceeds received from offering				$ 49,747,000	$ 90,908,000
Accumulated deficit				154,132,000	71,393,000
Cash, cash equivalents and marketable securities				91,500,000
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold	487,934	7,200,000
Issuance price per shares	$ 6.95	$ 6.95
Net proceeds received from offering	$ 3,200,000	$ 46,600,000
Underwriting discounts and commissions and other offering expenses	$ 200,000	$ 3,500,000
Net proceeds received from offering				49,747,000	$ 89,370,000
Initial Public Offering \| Maximum
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Option to purchase of common stock at public offering price		1,080,000
At-the-Market Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds received from offering				$ 6,716,000
At-the-Market Offering \| Controlled Equity Offering Sales Agreement \| SVB Leerink LLC and Cantor Fitzgerald and Co
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold				688,000
Issuance price per shares				$ 10.18
Maximum aggregate value of common stock available for offering At-the-Market price			$ 50,000,000
Sales commission percentage payable to sales agents in cash			3.00%
Net proceeds received from offering				$ 6,700,000
Issuance costs				75,000
Common stock remained available for offering				$ 43,000,000

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Restricted cash (non-current) related to letter of credit	$ 245,000	$ 84,000
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
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
Cash, cash equivalents and restricted cash at beginning of period	7,126,000	53,349,000
Cash, cash equivalents and restricted cash at end of period	34,019,000	7,126,000
ASU 2016-18
Summary Of Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash at beginning of period	$ 84,000,000	0
Cash, cash equivalents and restricted cash at end of period		$ 84,000,000

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, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	$ 91,473	$ 108,028
Money Market Funds \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	33,774	6,804
U.S. Treasury Securities \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis		238
U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	57,699	100,986
Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	91,473	108,028
Active Markets (Level 1) \| Money Market Funds \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	33,774	6,804
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]
Assets measured at fair value on a recurring basis		238
Active Markets (Level 1) \| U.S. Treasury Securities \| Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	$ 57,699	$ 100,986

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
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 [Member]
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 [Member]
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 [Member]
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 [Member]
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 [Member]
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) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 57,650	$ 101,027
Unrealized Gains	49	4
Unrealized Losses		(45)
Marketable securities	57,699	100,986
U.S. Government Agency Treasuries and Securities
Marketable Securities [Line Items]
Amortized Cost	57,650	101,027
Unrealized Gains	49	4
Unrealized Losses		(45)
Marketable securities	$ 57,699	$ 100,986

Marketable Securities - Schedule of Fair Value and Amortized Cost of Company's Available-for-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 57,650	$ 101,027
Total, Amortized Cost	57,650	101,027
Due in one year or less, Fair Value	57,699	100,986
Total, Fair Value	$ 57,699	$ 100,986

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 572	$ 406
Less: accumulated depreciation	(158)	(85)
Property and equipment, net	414	321
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	17	65
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment		38
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	397	194
Computers
Property Plant And Equipment [Line Items]
Total property and equipment	125	76
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	11	11
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 22	$ 22

Property and Equipment, Net - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 100,000	$ 80,000	$ 5,000

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 1,643	$ 1,189
Accrued restructuring cost (see Note 15)	516
Accrued research and development expenses	3,171	1,028
Other	153	510
Total accrued liabilities	$ 5,483	$ 2,727

License Agreements - Additional Information (Details) - Novartis International Pharmaceutical Ltd. - USD ($)		1 Months Ended		12 Months Ended
	Mar. 23, 2017	May 31, 2019	May 31, 2017	Dec. 31, 2019
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.
Payment for development milestone		$ 2,500,000	$ 300,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount payable upon satisfaction of clinical milestones	$ 4,300,000
Aggregate amount payable upon satisfaction of regulatory milestones for first indication approved	24,000,000
Aggregate amount payable upon satisfaction of regulatory milestones for second indication approved	18,000,000
Aggregate amount payable upon satisfaction of commercial milestones	$ 125,000,000
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) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 06, 2018	May 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Silverstein Foundation \| Funding Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront sum of funding amount	$ 500
Amounts received for qualifying expenses incurred recorded as reduction of research and development expense				$ 500
National Institute of Health
Research And Development Arrangement Contract To Perform For Others [Line Items]
Grant term		5 years
Grant receivable		$ 1,500
Qualifying expenses			$ 41,000
Amounts received for qualifying expenses incurred recorded as other current assets			61,000
National Institute of Health \| Funding Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Qualifying expenses			$ 100

Preferred Stock - Additional Information (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Common Stock - Additional Information (Details)	12 Months Ended
Common Stock - Additional Information (Details)	Dec. 31, 2019 Vote $ / shares shares	Dec. 31, 2018 shares
Stockholders Equity Note [Abstract]
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	36,444,732	28,055,344
Common stock, shares outstanding	36,444,732	28,055,344
Number of votes entitled for each share of common stock \| Vote	1
Common stock dividends declared \| $ / shares	$ 0
Common stock dividends paid \| $ / shares	$ 0

Common Stock - Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	4,159,626	2,773,249
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	2,562,800	1,122,677
Restricted stock units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	828,935	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	212,308	1,350,582
Shares Available for Issuance under the 2018 ESPP
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	555,583	275,030

Stock-Based Compensation - Additional Information (Details)				1 Months Ended		12 Months Ended
	Apr. 17, 2018 Installment shares	Nov. 29, 2017 shares	Oct. 11, 2017 shares	Dec. 31, 2019 USD ($) $ / shares shares	May 31, 2018 $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for issuance				4,159,626		4,159,626	2,773,249
Recognized stock-based compensation expense \| $						$ 3,694,000	$ 2,793,000	$ 470,000
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for issuance				2,562,800		2,562,800	1,122,677
Aggregate intrinsic value of options exercised \| $						$ 67,000	$ 78,000
Option granted						1,896,527
Options Issued and Outstanding \| Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted						1,886,687
Weighted-average grant date fair value of options granted \| $ / shares						$ 4.83
Options Issued and Outstanding \| Employees
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
Unrecognized compensation expense \| $				$ 9,400,000		$ 9,400,000
Unrecognized compensation expense, estimated weighted-average period for recognition						2 years 9 months 18 days
Options Issued and Outstanding \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted						9,840	7,200
Weighted-average grant date fair value of options granted \| $ / shares						$ 7.61	$ 12.51
Unrecognized compensation expense \| $				26,000		$ 26,000
Unrecognized compensation expense, estimated weighted-average period for recognition						2 years 1 month 20 days
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted	2,000
Number of quarterly installment in which restriction lapse \| Installment	4
Recognized stock-based compensation expense \| $						$ 4,000,000	$ 900,000	$ 400,000
Unrecognized stock-based compensation expense \| $				$ 0		$ 0
Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for issuance				828,935		828,935	24,960
Unrecognized compensation expense, estimated weighted-average period for recognition						3 years 9 months
Shares granted						813,335
Recognized stock-based compensation expense \| $						$ 74,000	$ 35,000
Unrecognized stock-based compensation expense \| $				$ 1,100,000		$ 1,100,000
Restricted stock units \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted				813,335	24,960
Weighted-average grant date fair value \| $ / shares				$ 1.27	$ 9.03
2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		630,662	537,914	537,914		537,914
Number of additional shares reserved for issuance		1,866,009	630,662
2017 Stock Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Terms of stock based options granted						10 years
2018 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of additional shares added on outstanding shares						4.00%
Shares available for issuance				0		0
2018 Stock Incentive Plan \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance				2,200,260		2,200,260
2018 Stock Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance				3,322,473		3,322,473
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance							275,030
Percentage of additional shares added on outstanding shares							1.00%
Number of shares available for sale						0	0
2018 Employee Stock Purchase Plan \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of additional shares reserved for issuance							543,926
2018 Employee Stock Purchase Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of additional shares reserved for issuance							555,583

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - Employees And Non Employees - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 3,694	$ 2,793	$ 470
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	1,542	1,236	246
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,152	$ 1,557	$ 224

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning, Shares Available for Grant	1,350,582
Shares reserved for issuance, Shares Available for Grant	1,122,213
Options granted, Shares Available for Grant	(1,896,527)
Options forfeited, Shares Available for Grant	449,375
Outstanding, ending, Shares Available for Grant	212,308	1,350,582
Outstanding, beginning, Number of Options	1,122,677
Options granted, Number of Options	1,896,527
Options exercised, Number of Options	(7,029)
Options forfeited, Number of Options	(449,375)
Outstanding, ending, Number of Options	2,562,800	1,122,677
Exercisable, Number of Options	461,150
Vested and expected to vest, Number of Options	2,562,800
Outstanding, beginning, Weighted-Average Exercise Price per Option	$ 11.63
Options granted, Weighted-Average Exercise Price per Option	6.28
Options exercised, Weighted-Average Exercise Price per Option	0.79
Options forfeited, Weighted-Average Exercise Price per Option	10.77
Outstanding, ending, Weighted-Average Exercise Price per Option	7.85	$ 11.63
Exercisable, Weighted-Average Exercise Price per Option	11.40
Vested and expected to vest, Weighted-Average Exercise Price per Option	$ 7.85
Outstanding, Weighted-Average Remaining Contract Term	8 years 10 months 2 days	9 years 2 months 19 days
Exercisable, Weighted-Average Remaining Contract Term	7 years 21 days
Vested and expected to vest, Weighted-Average Remaining Contract Term	8 years 10 months 2 days
Outstanding, ending, Aggregate Intrinsic Value	$ 200
Exercisable, Aggregate Intrinsic Value	37
Vested and expected to vest, Aggregate Intrinsic Value	$ 200
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options granted, Shares Available for Grant	(813,335)

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	92.00%	75.90%	74.40%
Expected volatility, maximum	104.90%	90.60%	74.50%
Risk-free interest rate, minimum	1.40%	2.40%	1.90%
Risk-free interest rate, maximum	2.60%	3.10%	2.20%
Expected dividend yield	0.00%	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	89.70%	78.00%	74.60%
Expected volatility, maximum	99.50%	91.20%	77.00%
Risk-free interest rate, minimum	1.70%	2.70%	2.30%
Risk-free interest rate, maximum	2.80%	3.10%	2.40%
Expected dividend yield	0.00%	0.00%	0.00%
Minimum [Member] \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 1.27	$ 8.57	$ 0.79
Expected term (in years)	5 years 6 months	5 years 9 months 18 days	5 years 10 months 24 days
Minimum [Member] \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 1.23	$ 8.62	$ 0.79
Expected term (in years)	7 years 4 months 24 days	8 years 4 months 24 days	9 years 4 months 24 days
Maximum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 10.66	$ 15.45	$ 9.33
Expected term (in years)	6 years 1 month 6 days	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	$ 10.26	$ 15.45	$ 10.28
Expected term (in years)	10 years	10 years	10 years

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2019 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Beginning, Unvested shares	1,000
Number of Shares Outstanding, Vested	(1,000)

Stock-Based Compensation - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units	12 Months Ended
	Dec. 31, 2019 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Beginning, Unvested shares	24,960
Number of Shares Outstanding, Granted	813,335
Number of Shares Outstanding, Vested	(9,360)
Number of Shares Outstanding, Ending, Unvested shares	828,935

Income Taxes - Schedule of Reconciliation of Income Tax Expense Computed at Statutory Federal Income Tax Rate to Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax benefit at federal statutory rate	$ (17,368)	$ (7,899)	$ (11,484)
State taxes	(5,314)	(2,340)	(1,011)
Tax credits	(2,910)	(817)	(222)
Stock-based compensation	753	764	140
Federal tax rate change			2,202
Change in fair value of tranche rights		3	5,065
Other	25	241
Change in valuation allowance	24,850	10,074	5,310
Income tax expense	$ 36	$ 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, 2019	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		$ 40,221,000	$ 15,395,000
Operating loss carryforwards, limitations on use		$14.0 million will begin to expire in 2036 and $113.0 million can be carried forward indefinitely.
Operating loss carryforward expiration in two thousand thirty six		$ 14,000,000
Operating loss carryforward indefinitely		113,000,000
Uncertain tax positions		0
Penalties and interest accrued		0
Federal
Income Taxes Disclosure [Line Items]
Operating loss carryforwards		$ 127,000,000
Operating loss carryforwards, expiration date		Dec. 31, 2036
Federal \| Research Credits
Income Taxes Disclosure [Line Items]
Tax credit carryforward, amount		$ 3,800,000	900,000
Tax credit carryforward, expiration date		Dec. 31, 2037
State
Income Taxes Disclosure [Line Items]
Operating loss carryforwards		$ 130,800,000
Operating loss carryforwards, expiration date		Dec. 31, 2036
State \| Research Credits
Income Taxes Disclosure [Line Items]
Tax credit carryforward, amount		$ 500,000	$ 200,000
Tax credit carryforward, expiration date		Dec. 31, 2032

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating losses	$ 34,938	$ 13,054
Capitalized license	771	835
Research credits	4,151	1,007
Accruals	401	514
Stock-based compensation	53	51
Net unrealized loss		11
Total gross deferred tax assets	40,314	15,472
Less valuation allowance	(40,221)	(15,395)
Total deferred tax assets	93	77
Deferred tax liabilities:
Net unrealized gain	13
Depreciation and amortization	80	77
Total gross deferred tax liability	93	77
Net deferred tax assets	$ 0	$ 0

Commitment and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Aug. 01, 2019
Commitments and Contingences [Line Items]
Estimated litigation liability		$ 0
Amended lease term from the date of relocation					7 years
Amended base rent per year of the relocation premises	$ 600,000
Percentage increase in operating lease base rent	2.00%
Letters of Credit issued amount	$ 200,000
Lease payments		500,000	$ 300,000	$ 0
Novartis International Pharmaceutical Ltd. \| Maximum
Commitments and Contingences [Line Items]
Aggregate amount payable upon satisfaction of clinical milestones		1,500,000
Aggregate amount payable upon satisfaction of regulatory milestones for first indication approved		24,000,000
Aggregate amount payable upon satisfaction of regulatory milestones for second indication approved		18,000,000
Aggregate amount payable upon satisfaction of commercial milestones		$ 125,000,000

Commitment and Contingencies - Summary of Obligations Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 594
2021	606
2022	618
2023	630
2024	643
Years thereafter	1,043
Total	$ 4,134

Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	3,391,735	1,148,637
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		1,000
Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	828,935	24,960
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	2,562,800	1,122,677

Related Party Transactions - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Novartis International Pharmaceutical Ltd.
Related Party Transaction [Line Items]
Payments to related parties	$ 2,500,000	$ 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

Reduction in Workforce - Additional Information (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) Employee
Restructuring Charges [Abstract]
Reduction in workforce \| Employee		8
Aggregate restructuring charges	$ 600,000
Payments for Restructuring	66,000
Payments for accrued restructuring costs	$ 500,000	$ 500,000
Reduction in workforce, percentage		22.00%

Reduction in Workforce - Schedule of Amount Expected to be Incurred and Liability Related to Restructuring (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019
Restructuring charges incurred during the year	$ 600,000
Amounts paid during the year	(66,000)
Accrued restructuring costs as of December 31, 2019	516,000	$ 516,000
One-time Employee Termination Benefits
Restructuring charges incurred during the year		582,000
Amounts paid during the year		(66,000)
Accrued restructuring costs as of December 31, 2019	$ 516,000	$ 516,000

Reduction in Workforce - Schedule of Restructuring Charges Reported in Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Research and development		$ 73,634	$ 31,065	$ 16,839
General and administrative		11,823	$ 8,640	$ 2,043
Total	$ 600
Cash
Research and development		306
General and administrative		276
Total		582
Total Expenses
Research and development		306
General and administrative		276
Total		$ 582

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 29,436	$ 25,161	$ 19,169	$ 11,691	$ 6,360	$ 9,032	$ 14,113	$ 10,200	$ 85,457	$ 39,705
Loss from operations	(29,436)	(25,161)	(19,169)	(11,691)	(6,360)	(9,032)	(14,113)	(10,200)	(85,457)	(39,705)	$ (18,882)
Net loss	$ (28,890)	$ (24,448)	$ (18,332)	$ (11,069)	$ (5,757)	$ (8,407)	$ (13,591)	$ (9,859)	$ (82,739)	$ (37,614)	$ (33,778)
Net loss per share, basic and diluted	$ (0.79)	$ (0.68)	$ (0.51)	$ (0.38)	$ (0.21)	$ (0.30)	$ (0.48)	$ (0.46)	$ (2.41)	$ (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