View Filing Data

Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 14, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2019
Entity Registrant Name	resTORbio, Inc.
Entity Central Index Key	0001720580
Trading Symbol	TORC
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		35,743,278

Condensed Consolidated Balance Sheets (unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 7,181	$ 7,042
Marketable securities	135,910	100,986
Prepaid expenses	1,563	1,491
Other current assets	24	15
Total current assets	144,678	109,534
Restricted cash	84	84
Property and equipment, net	318	321
Total assets	145,080	109,939
Current liabilities:
Accounts payable	3,364	2,989
Accrued liabilities	1,245	2,727
Total current liabilities	4,609	5,716
Other liabilities	15	19
Total liabilities	4,624	5,735
Commitments and contingencies (see Note 10)
Stockholders’ equity:
Common stock, $0.0001 par value, 150,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 35,255,344 and 28,055,344 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively; 35,254,844 and 28,054,344 shares vested as of March 31, 2019 and December 31, 2018, respectively	4	3
Additional paid-in capital	222,882	175,635
Accumulated deficit	(82,462)	(71,393)
Accumulated other comprehensive gain (loss)	32	(41)
Total stockholders’ equity	140,456	104,204
Total liabilities and stockholders’ equity	$ 145,080	$ 109,939

Condensed Consolidated Balance Sheets (Parenthetical) (unaudited) - $ / shares	Mar. 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	35,255,344	28,055,344
Common stock, shares outstanding	35,255,344	28,055,344
Common stock, shares vested	35,254,844	28,054,344

Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 8,852	$ 8,106
General and administrative	2,839	2,094
Total operating expenses	11,691	10,200
Loss from operations	(11,691)	(10,200)
Other income, net	631	341
Loss before income taxes	(11,060)	(9,859)
Income tax expense	9
Net loss	$ (11,069)	$ (9,859)
Net loss per share, basic and diluted	$ (0.38)	$ (0.46)
Weighted-average common shares used in computing net loss per share, basic and diluted	29,014,750	21,523,072
Other comprehensive gain (loss):
Net loss	$ (11,069)	$ (9,859)
Unrealized gains on marketable securities	73
Comprehensive loss	$ (10,996)	$ (9,859)

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Common Stock	Additional Pain In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2017	$ (31,929)			$ 1	$ 1,849	$ (33,779)
Balance, shares at Dec. 31, 2017				4,562,640
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
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, Shares		(15,527,951)	(4,792,716)	15,870,559
Conversion of convertible preferred stock into common stock upon the closing of initial public offering	81,620	$ (41,674)	$ (39,946)	$ 1	81,619
Issuance of common stock	89,370			$ 1	89,369
Issuance of common stock, Shares				6,516,667
Vesting of restricted shares, value	865				865
Vesting of restricted shares				1,096,449
Stock-based compensation expense	316				316
Net loss	(9,859)					(9,859)
Balance at Mar. 31, 2018	130,383			$ 3	174,018	(43,638)
Balance, shares at Mar. 31, 2018				28,046,315
Balance, temporary equity, shares at Mar. 31, 2018		0	0
Balance, temporary equity at Mar. 31, 2018		$ 0	$ 0
Balance at Dec. 31, 2018	$ 104,204			$ 3	175,635	(71,393)	$ (41)
Balance, shares at Dec. 31, 2018	28,054,344			28,054,344
Issuance of common stock	$ 46,585			$ 1	46,584
Issuance of common stock, Shares				7,200,000
Vesting of restricted shares, value	1				1
Vesting of restricted shares				500
Stock-based compensation expense	662				662
Net loss	(11,069)					(11,069)
Unrealized gain (losses) on marketable securities	73						73
Balance at Mar. 31, 2019	$ 140,456			$ 4	$ 222,882	$ (82,462)	$ 32
Balance, shares at Mar. 31, 2019	35,254,844			35,254,844

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Common Stock
Stock issuance cost	$ 3,455	$ 8,379

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities:
Net loss	$ (11,069,000)	$ (9,859,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	(247,000)
Depreciation and amortization expense	27,000	9,000
Stock-based compensation expense	663,000	1,181,000
Changes in operating assets and liabilities:
Restricted cash		(84,000)
Prepaid expenses and other current assets	(81,000)	(264,000)
Accounts payable	144,000	80,000
Accrued liabilities	(1,482,000)	1,105,000
Funding advance		500,000
Other liabilities	(4,000)	28,000
Net cash used in operating activities	(12,049,000)	(7,304,000)
Investing activities:
Purchases of property and equipment	(24,000)	(278,000)
Maturities of marketable securities	42,500,000
Purchases of marketable securities	(77,104,000)
Net cash used in investing activities	(34,628,000)	(278,000)
Financing activities:
Proceeds from public offering, net of issuance costs	46,816,000	89,938,000
Net cash provided by financing activities	46,816,000	89,938,000
Net increase in cash and cash equivalents	139,000	82,356,000
Cash and cash equivalents at beginning of period	7,042,000	53,349,000
Cash and cash equivalents at end of period	7,181,000	135,705,000
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable		8,000
Conversion of redeemable convertible preferred stock into common stock		$ 81,620,000
Issuance costs associated with public offering included in accounts payable	$ 231,000

Organization

3 Months Ended

Mar. 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 “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. The Company’s principal operations are located in Boston, Massachusetts.

Since inception, the Company has been primarily involved in research and development activities. The Company devotes substantially all of its efforts to product research and development, initial market development and raising capital. The Company has not generated any product revenue related to its primary business purpose to date and is subject to a number of risks similar to those of other early stage companies, including dependence on key individuals, competition from other companies, the need for development of commercially viable products and the need to obtain adequate additional financing to fund the development of its product candidates. The Company is also subject to a number of risks similar to other companies in the life sciences industry, including regulatory approval of products, uncertainty of market acceptance of products, competition from substitute products and larger companies, the need to obtain additional financing, compliance with government regulations, protection of proprietary technology, dependence on third parties, product liability and dependence on key individuals.

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 $3.1 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $0.3 million.

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 $82.5 million as of March 31, 2019. As of March 31, 2019, the Company had $143.1 million of cash, cash equivalents, and marketable securities, which the Company believes will be sufficient to fund the Company’s current operating plan through at least the next twelve months.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed consolidated financial statements are unaudited. The unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) for interim financial information and, in the opinion of management, reflect all adjustments of a normal recurring nature necessary for a fair statement of the Company's financial position as of March 31, 2019 and the results of operations and cash flows for the interim periods ended March 31, 2019 and 2018. The condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 that was filed with the Securities and Exchange Commission (“SEC”) on March 18, 2019 (the “2018 Form 10-K”). Interim results are not necessarily indicative of results for a full year or for any other interim period. The condensed consolidated financial statements include the accounts of resTORbio, Inc. and its wholly owned subsidiary, resTORbio Securities Corp. All inter-company transactions and balances have been eliminated in consolidation.

Use of Estimates

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

Summary of Significant Accounting Policies

The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the 2018 Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2019.

Fair Value Measurements

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

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

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

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

The following table summarizes assets measured at fair value on a recurring basis at March 31, 2019 (in thousands):

			Active		Observable		Unobservable
	March 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Cash	$	31	$	31	$	—	$	—
Money market funds (included in cash and cash equivalents)		7,150		7,150		—		—
U.S. treasury securities (included in marketable securities)		135,910		135,910		—		—
Total	$	143,091	$	143,091	$	—	$	—

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	$	—	$	—

There have been no changes to the valuation methods utilized by the Company during the three months ended March 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 three months ended March 31, 2019 and 2018.

Recently Issued Accounting Pronouncements

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

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

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

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which intends to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities and emerging growth companies that choose to take advantage of the extended transition period, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities but no earlier than the Company’s adoption of ASU No. 2016-10, Revenue from Contracts with Customers (“ASC 606”). The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its consolidated financial statements.

Marketable Securities

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

Marketable Securities

3. Marketable Securities

As of March 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 securities and treasuries	$	135,878	$	32	$	—	$	135,910
Total	$	135,878	$	32	$	—	$	135,910

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

	Amortized		Unrealized		Unrealized		Fair
Description	Cost		Gains		Losses		Value
U.S. government agency treasuries and securities	$	101,027	$	—	$	41	$	100,986
Total	$	101,027	$	—	$	41	$	100,986

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

	March 31, 2019
	Amortized		Fair
	Cost		Value
Due in one year or less	$	135,878	$	135,910
Total	$	135,878	$	135,910

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

Property and Equipment, Net

3 Months Ended

Mar. 31, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and equipment, net

Property and equipment, net consists of the following:

	March 31, 2019			December 31, 2018
	(In thousands)
Leasehold improvements	$	68		$	65
Machinery and equipment		38			38
Furniture and fixtures		210			194
Computers		81			76
Office equipment		11			11
Software		22			22
Total property and equipment		430			406
Less: accumulated depreciation		(112	)		(85	)
Property and equipment, net	$	318		$	321

Depreciation and amortization expense was $27,000 and $9,000 for the three months ended March 31, 2019 and 2018, respectively.

Accrued Liabilities

3 Months Ended

Mar. 31, 2019

Payables And Accruals [Abstract]

Accrued Liabilities

5. Accrued Liabilities

Accrued liabilities consist of the following:

	March 31, 2019		December 31, 2018
	(In thousands)
Accrued payroll and related expenses	$	557	$	1,189
Accrued research and development expenses		524		1,028
Other		164		510
Total accrued liabilities	$	1,245	$	2,727

License Agreements

3 Months Ended

Mar. 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.

The agreement may be terminated by either party upon a material breach by the other party that is not cured within 60 days after written notice. The Company may terminate the agreement in its entirety or on a product-by-product or country-by-country basis with or without cause with 60 days’ prior written notice.

Novartis may terminate the portion of the agreement related to everolimus if the Company fails to use commercially reasonable efforts to research, develop and commercialize a product utilizing everolimus for a period of three years. Novartis may terminate the license agreement upon the Company’s bankruptcy, insolvency, dissolution or winding up.

As additional consideration for the license, the Company is required to pay up to an aggregate of $4.3 million upon the satisfaction of clinical milestones, up to an aggregate of $24 million upon the satisfaction of regulatory milestones for the first indication approved, and up to an aggregate of $18 million upon the satisfaction of regulatory milestones for the second indication approved. In addition, the Company is required to pay up to an aggregate of $125 million upon the satisfaction of commercial milestones, based on the amount of annual net sales. The Company is also required to pay tiered royalties ranging from a mid single-digit percentage to a low teen-digit percentage on annual net sales of products. These royalty obligations last on a product-by-product and country-by-country basis until the latest of (i) the expiration of the last valid claim of a Novartis patent covering a subject product, (ii) the expiration of any regulatory exclusivity for the subject product in a country, or (iii) the 10^th anniversary of the first commercial sale in the country, and are subject to a reduction after the expiration of the last valid claim of a Novartis patent or the introduction of a generic equivalent of a product in a country.

Milestone payments to Novartis will be recorded as research and development expenses in the condensed consolidated statements of operations once achievement of each associated milestone has occurred. In May 2017, the Company initiated a Phase 2b clinical trial for a first indication, triggering the first milestone payment under the agreement. Accordingly, the Company paid the related $0.3 million payment in May 2017. As of March 31, 2019, none of the remaining development milestones, regulatory milestones, sales milestones, or royalties had been reached. In May 2019, the Company initiated a Phase 3 clinical trial for the first indication, triggering a milestone payment of $2.5 million under the agreement.

Research Funding Agreements

3 Months Ended

Mar. 31, 2019

Research Funding Agreement [Abstract]

Research Funding Agreement

7. Research Funding Agreement

On March 6, 2018, the Company and the Silverstein Foundation for Parkinson’s with GBA (the “Silverstein Foundation”) entered into a research funding agreement (the “Funding Agreement”). One of the Company’s directors is a co-founder and current trustee of the Silverstein Foundation. Under the terms of the Funding Agreement, the Silverstein Foundation will partially fund the preclinical research, development work, and Phase 2 clinical trial expenses (the “Research”) to be conducted and borne by the Company in connection with the development of RTB101, alone or in combination with other products (the “Product”).

Upon execution of the Funding Agreement, the Silverstein Foundation paid the Company an upfront sum of $0.5 million (the “Funding Amount”). The Company is entitled to use the Funding Amount solely to conduct the Research and is obligated to repay the Funding Amount in full to the Silverstein Foundation if it successfully conducts a positive Phase 3 clinical trial of the Product for Parkinson’s Disease. The Company is solely responsible for commencing and conducting the Research and will furnish periodic progress updates to the Silverstein Foundation throughout the term of the Funding Agreement. After completing the Research, the Company must provide the Silverstein Foundation with a formal report describing the work performed and the results of the Research.

The Company recognizes proceeds received from the Silverstein Foundation as a reduction to research and development expenses, rather than as revenue, in the condensed consolidated statements of operations and comprehensive loss because the corresponding Funding Agreement does not contain specified performance obligations other than to conduct research on a particular program or in a particular field and no obligations to deliver specified products or technology.

For funds received under the Funding Agreement, the Company recognizes a reduction in research and development expenses in an amount equal to the qualifying expenses incurred in each period up to the amount funded by the Silverstein Foundation. Funding that has been received by the Company in advance of incurring qualifying expenses is recorded in the condensed consolidated balance sheet as funding advance. No qualifying expenses were incurred during the three months ended March 31, 2019 and 2018. As of March 31, 2019, $0.5 million qualifying expenses have been incurred. Therefore, all amounts received have been recorded as a reduction of the research and development expense.

Preferred Stock and Common Stock

3 Months Ended

Mar. 31, 2019

Temporary Equity Disclosure [Abstract]

Preferred Stock and Common Stock

8. Preferred Stock and Common Stock

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

Reserve for future issuance

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:

	March 31, 2019		December 31, 2018
Options issued and outstanding		1,706,317		1,122,677
Unvested restricted stock units		24,960		24,960
Options available for future grants		1,889,155		1,350,582
Shares available for issuance under the 2018 ESPP		555,583		275,030
Total		4,176,015		2,773,249

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9. Stock-based Compensation

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

In connection with the Company’s initial public offering completed in January 2018, the Board adopted and the Company’s stockholders approved the 2018 Stock Incentive Plan (“2018 Plan”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units, stock appreciation rights, and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2018 Plan. The number of shares of common stock that were reserved for issuance under the 2018 Plan were 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 shares 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.

Stock-based Compensation Expense

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

	Three Months Ended March 31,
	2019		2018
Research and development	$	277	$	625
General and administrative		386		556
Total stock-based compensation expense	$	663	$	1,181

Stock Options

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding			Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
											(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677		$	11.63		9.22
Shares reserved for issuance		1,122,213
Options granted⁽¹⁾		(604,440	)		604,440			8.58
Options cancelled		20,800			(20,800	)		9.91
Outstanding, March 31, 2019		1,889,155			1,706,317			10.57		9.30	$	627
Exercisable, March 31, 2019					194,221			11.62		8.70		202
Vested and expected to vest, March 31, 2019					1,706,317			10.57		9.30		627

(1)	The Company granted 9,840 stock options to non-employees during the three months ended March 31, 2019.

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 March 31, 2019. No options were exercised during the three months ended March 31, 2019.

During the three months ended March 31, 2019, the Company granted options to employees and directors to purchase an aggregate of 594,600 common shares with a weighted-average grant date fair value of $6.65 per share. During the three months ended March 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 $5.79 per share. The expense related to options granted to employees and directors for the three months ended March 31, 2019 was $0.6 million. The expense related to options granted to non-employees for the three months ended March 31, 2019 was $7,000. The expense related to options granted to employees, directors and non-employees was $0.3 million and $3,000, respectively, for the three months ended March 31, 2018.

As of March 31, 2019, the total unrecognized compensation expense related to unvested options granted to employees and directors was $10.7 million, which the Company expects to recognize over an estimated weighted-average period of 3.30 years. As of March 31, 2019, the total unrecognized compensation expense related to unvested non-employee options was $0.2 million, which the Company expects to recognize over an estimated weighted-average period of 2.99 years.

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

	Three Months Ended March 31,
	2019		2018
Employees:
Fair value of common stock	$8.53 - $8.90		$13.17 - $15.45
Expected term (in years)		6.1	5.8 - 6.6
Expected volatility	93.7% - 94.8%		76.1% - 77.7%
Risk-free interest rate	2.5% - 2.6%		2.4% - 2.7%
Expected dividend yield	0.0%		0.0%
Non-employees:
Fair value of common stock	$6.82 - $8.61		$9.58 - $15.45
Expected term (in years)	8.2 - 10.0		9.2 - 10.0
Expected volatility	91.3% - 94.9%		77.9% - 78.1%
Risk-free interest rate	2.4% - 2.6%		2.7% - 2.8%
Expected dividend yield	0.0%		0.0%

Restricted Stock

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

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2018		1,000
Vested		(500	)
Unvested shares — March 31, 2019		500

The Company recognized $1,000 and $0.9 million of stock-based compensation expense related to restricted shares during the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019, there was $159 of unrecognized stock-based compensation expense related to unvested restricted stock. This amount is expected to be recognized over a remaining weighted-average period of 0.05 years. There were no restricted stock awards during the three months ended March 31, 2019 and 2018.

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.

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		—
Vested		—
Unvested shares — March 31, 2019		24,960

The Company recognized $14,000 of stock-based compensation expense related to restricted stock units during the three months ended March 31, 2019. As of March 31, 2019, there was $0.2 million of unrecognized stock-based compensation expense related to unvested restricted stock units. This amount is expected to be recognized over a remaining weighted-average period of 3.14 years. There were no restricted stock units granted to employees or non-employees during the three months ended March 31, 2019 and 2018.

2018 Employee Stock Purchase Plan

The Board adopted and the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 ESPP enables eligible employees to purchase shares of the Company’s Common Stock at a discount. The number of shares of common stock that are reserved for issuance under the 2018 ESPP are 275,030 shares. The 2018 ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and increasing each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31; (ii) 543,926 shares or (iii) such number of shares as determined by the ESPP administrator. On January 1, 2019, as a result of the foregoing evergreen provision, the number of shares of common stock available for issuance under the 2018 ESPP automatically increased from 275,030 to 555,583 shares.

Commitments and Contingences

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingences

10. Commitments and Contingences

Litigation

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities as of March 31, 2019 and December 31, 2018.

Net Loss Per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Share

11. Net Loss per Share

The Company computes basic and diluted losses per share using a methodology that gives effect to the impact of outstanding participating securities (the “two-class” method). Basic net loss per share is calculated by dividing net loss by the weighted-average number of common shares outstanding during the period and excludes any dilutive effects of share-based awards. Diluted net loss per share is computed giving effect to all potential dilutive common shares, including common stock issuable upon exercise of stock options, convertible preferred stock, and unvested restricted common stock. As the Company had net losses for the three months ended March 31, 2019 and 2018, there is no income allocation required under the two-class method or dilution attributed to weighted average shares outstanding in the calculation of diluted loss per share.

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

	As of March 31,
	2019		2018
Options issued and outstanding		1,706,317		667,590
Total		1,706,317		667,590

Related Party Transactions

3 Months Ended

Mar. 31, 2019

Related Party Transactions

12. Related Party Transactions

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

The Company is a party to an intellectual property license agreement with Novartis. In addition, NIBR, an affiliate of Novartis, is a shareholder of the Company (See Note 6). No payments have been made to Novartis during the three months ended March 31, 2019 and 2018.

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. The Company received $0 and $0.5 million from the Silverstein Foundation during the three months ended March 31, 2019 and 2018, respectively.

Subsequent Event

3 Months Ended

Mar. 31, 2019

Subsequent Events [Abstract]

Subsequent Event

13. Subsequent Event

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. 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 letter of credit for the benefit of the landlord in the amount of $0.2 million.

In May 2019, the Company was awarded a 5-year grant for up to $1.5 million from the National Institutes of Health.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Summary of Significant Accounting Policies

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 March 31, 2019 (in thousands):

			Active		Observable		Unobservable
	March 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Cash	$	31	$	31	$	—	$	—
Money market funds (included in cash and cash equivalents)		7,150		7,150		—		—
U.S. treasury securities (included in marketable securities)		135,910		135,910		—		—
Total	$	143,091	$	143,091	$	—	$	—

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	$	—	$	—

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 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 March 31, 2019 (in thousands):

			Active		Observable		Unobservable
	March 31,		Markets		Inputs		Inputs
Description	2019		(Level 1)		(Level 2)		(Level 3)
Cash	$	31	$	31	$	—	$	—
Money market funds (included in cash and cash equivalents)		7,150		7,150		—		—
U.S. treasury securities (included in marketable securities)		135,910		135,910		—		—
Total	$	143,091	$	143,091	$	—	$	—

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	$	—	$	—

- 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)

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value Marketable Securities By Type of Security

As of March 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 securities and treasuries	$	135,878	$	32	$	—	$	135,910
Total	$	135,878	$	32	$	—	$	135,910

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

	Amortized		Unrealized		Unrealized		Fair
Description	Cost		Gains		Losses		Value
U.S. government agency treasuries and securities	$	101,027	$	—	$	41	$	100,986
Total	$	101,027	$	—	$	41	$	100,986

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

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

	March 31, 2019
	Amortized		Fair
	Cost		Value
Due in one year or less	$	135,878	$	135,910
Total	$	135,878	$	135,910

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

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2019

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	March 31, 2019			December 31, 2018
	(In thousands)
Leasehold improvements	$	68		$	65
Machinery and equipment		38			38
Furniture and fixtures		210			194
Computers		81			76
Office equipment		11			11
Software		22			22
Total property and equipment		430			406
Less: accumulated depreciation		(112	)		(85	)
Property and equipment, net	$	318		$	321

Accrued Liabilities (Tables)

3 Months Ended

Mar. 31, 2019

Payables And Accruals [Abstract]

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	March 31, 2019		December 31, 2018
	(In thousands)
Accrued payroll and related expenses	$	557	$	1,189
Accrued research and development expenses		524		1,028
Other		164		510
Total accrued liabilities	$	1,245	$	2,727

Preferred Stock and Common Stock (Tables)

3 Months Ended

Mar. 31, 2019

Temporary Equity Disclosure [Abstract]

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

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:

	March 31, 2019		December 31, 2018
Options issued and outstanding		1,706,317		1,122,677
Unvested restricted stock units		24,960		24,960
Options available for future grants		1,889,155		1,350,582
Shares available for issuance under the 2018 ESPP		555,583		275,030
Total		4,176,015		2,773,249

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

	Three Months Ended March 31,
	2019		2018
Research and development	$	277	$	625
General and administrative		386		556
Total stock-based compensation expense	$	663	$	1,181

Summary of Stock Option Activity

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding			Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
											(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677		$	11.63		9.22
Shares reserved for issuance		1,122,213
Options granted⁽¹⁾		(604,440	)		604,440			8.58
Options cancelled		20,800			(20,800	)		9.91
Outstanding, March 31, 2019		1,889,155			1,706,317			10.57		9.30	$	627
Exercisable, March 31, 2019					194,221			11.62		8.70		202
Vested and expected to vest, March 31, 2019					1,706,317			10.57		9.30		627

(1)	The Company granted 9,840 stock options to non-employees during the three months ended March 31, 2019.

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

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

	Three Months Ended March 31,
	2019		2018
Employees:
Fair value of common stock	$8.53 - $8.90		$13.17 - $15.45
Expected term (in years)		6.1	5.8 - 6.6
Expected volatility	93.7% - 94.8%		76.1% - 77.7%
Risk-free interest rate	2.5% - 2.6%		2.4% - 2.7%
Expected dividend yield	0.0%		0.0%
Non-employees:
Fair value of common stock	$6.82 - $8.61		$9.58 - $15.45
Expected term (in years)	8.2 - 10.0		9.2 - 10.0
Expected volatility	91.3% - 94.9%		77.9% - 78.1%
Risk-free interest rate	2.4% - 2.6%		2.7% - 2.8%
Expected dividend yield	0.0%		0.0%

Summary of Restricted Stock Activity

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2018		1,000
Vested		(500	)
Unvested shares — March 31, 2019		500

Summary of Restricted Stock Units Activity

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

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2018		24,960
Granted		—
Vested		—
Unvested shares — March 31, 2019		24,960

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

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

	As of March 31,
	2019		2018
Options issued and outstanding		1,706,317		667,590
Total		1,706,317		667,590

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Apr. 10, 2019	Mar. 22, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Entity incorporation date			Jul. 05, 2016
Net proceeds received from offering		$ 46,600	$ 46,816	$ 89,938
Underwriting discounts and commissions and other offering expenses		$ 3,500
Accumulated deficit			(82,462)		$ (71,393)
Cash, cash equivalents and marketable securities			$ 143,100
Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds received from offering	$ 3,100
Underwriting discounts and commissions and other offering expenses	$ 300
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold		7,200,000
Issuance price per shares		$ 6.95
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
Initial Public Offering \| Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of common stock shares sold	487,934
Issuance price per shares	$ 6.95

Summary of Significant Accounting Policies - Summary of Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 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	$ 143,091	$ 108,028
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	31
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	7,150	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	135,910	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	143,091	108,028
Active Markets (Level 1) \| Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	31
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	7,150	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	$ 135,910	$ 100,986

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accounting Policies [Abstract]
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

Marketable Securities - Schedule of Fair Value Marketable Securities By Type of Security (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 135,878	$ 101,027
Unrealized Gains	32
Unrealized Losses		41
Fair Value	135,910	100,986
U.S. Government Agency Securities and Treasuries
Marketable Securities [Line Items]
Amortized Cost	135,878	101,027
Unrealized Gains	32
Unrealized Losses		41
Fair Value	$ 135,910	$ 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	Mar. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 135,878	$ 101,027
Total, Amortized Cost	135,878	101,027
Due in one year or less, Fair Value	135,910	100,986
Total, Fair Value	$ 135,910	$ 100,986

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 430	$ 406
Less: accumulated depreciation	(112)	(85)
Property and equipment, net	318	321
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	68	65
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	38	38
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	210	194
Computers
Property Plant And Equipment [Line Items]
Total property and equipment	81	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 ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 27,000	$ 9,000

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued payroll and related expenses	$ 557	$ 1,189
Accrued research and development expenses	524	1,028
Other	164	510
Total accrued liabilities	$ 1,245	$ 2,727

License Agreements - Additional Information (Details) - Novartis International Pharmaceutical Ltd. - USD ($)	1 Months Ended		3 Months Ended
	May 31, 2019	May 31, 2017	Mar. 31, 2019	Mar. 23, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination description			The agreement may be terminated by either party upon a material breach by the other party that is not cured within 60 days after written notice. The Company may terminate the agreement in its entirety or on a product-by-product or country-by-country basis with or without cause with 60 days’ prior written notice.
Payment for development milestone		$ 300,000
Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment for development milestone	$ 2,500,000
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

Research Funding Agreement - Additional Information (Details) - USD ($)		3 Months Ended
	Mar. 06, 2018	Mar. 31, 2019	Mar. 31, 2018
Research And Development Arrangement Contract To Perform For Others [Line Items]
Amounts received for qualifying expenses incurred recorded as reduction of research and development expense		$ 500,000
Silverstein Foundation \| Funding Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront sum of funding amount	$ 500,000
Qualifying expenses		$ 0	$ 0

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

Preferred Stock and Common Stock -Schedule of Number of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	4,176,015	2,773,249
Options Issued and Outstanding
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,706,317	1,122,677
Unvested Restricted Stock Units
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	24,960	24,960
Options Available for Future Grants
Class Of Stock [Line Items]
Number of shares of common stock reserved for future issuance	1,889,155	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	3 Months Ended
	Apr. 17, 2018 Installment shares	Nov. 29, 2017 shares	Oct. 11, 2017 shares	May 31, 2018 $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Recognized stock-based compensation expense \| $					$ 663,000	$ 1,181,000
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options exercised					0
Options Issued and Outstanding \| Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted					594,600
Weighted-average grant date fair value of options granted \| $ / shares					$ 6.65
Expense related to options granted \| $					$ 600,000	300,000
Unrecognized compensation expense \| $					$ 10,700,000
Unrecognized compensation expense, estimated weighted-average period for recognition					3 years 3 months 18 days
Options Issued and Outstanding \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted					9,840
Weighted-average grant date fair value of options granted \| $ / shares					$ 5.79
Expense related to options granted \| $					$ 7,000	3,000
Unrecognized compensation expense \| $					$ 200,000
Unrecognized compensation expense, estimated weighted-average period for recognition					2 years 11 months 26 days
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense, estimated weighted-average period for recognition					18 days
Shares granted	2,000
Number of quarterly installment in which restriction lapse \| Installment	4
Recognized stock-based compensation expense \| $					$ 1,000	$ 900,000
Unrecognized stock-based compensation expense \| $					$ 159,000
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense, estimated weighted-average period for recognition					3 years 1 month 20 days
Shares granted					0
Recognized stock-based compensation expense \| $					$ 14,000
Unrecognized stock-based compensation expense \| $					$ 200,000
Restricted Stock Units \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted						0
Restricted Stock Units \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted				24,960		0
Weighted-average grant date fair value \| $ / shares				$ 9.03
2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		630,662	537,914		537,914
Number of additional shares reserved for issuance		1,866,009	630,662
2017 Stock Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Terms of stock based options granted					10 years
2018 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of additional shares added on outstanding shares					4.00%
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
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
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%
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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 663	$ 1,181
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	277	625
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 386	$ 556

Stock-Based Compensation - Summary of Stock Option Activity (Details) - Options Issued and Outstanding - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
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	(604,440)
Options cancelled, Shares Available for Grant	20,800
Outstanding, ending, Shares Available for Grant	1,889,155	1,350,582
Outstanding, beginning, Number of Options	1,122,677
Options granted, Number of Options	604,440
Options cancelled, Number of Options	(20,800)
Outstanding, ending, Number of Options	1,706,317	1,122,677
Exercisable, Number of Options	194,221
Vested and expected to vest, Number of Options	1,706,317
Outstanding, beginning, Weighted-Average Exercise Price per Option	$ 11.63
Options granted, Weighted-Average Exercise Price per Option	8.58
Options cancelled, Weighted-Average Exercise Price per Option	9.91
Outstanding, ending, Weighted-Average Exercise Price per Option	10.57	$ 11.63
Exercisable, Weighted-Average Exercise Price per Option	11.62
Vested and expected to vest, Weighted-Average Exercise Price per Option	$ 10.57
Outstanding, beginning, Weighted-Average Remaining Contract Term	9 years 3 months 18 days	9 years 2 months 19 days
Exercisable, Weighted-Average Remaining Contract Term	8 years 8 months 12 days
Vested and expected to vest, Weighted-Average Remaining Contract Term	9 years 3 months 18 days
Outstanding, ending, Aggregate Intrinsic Value	$ 627
Exercisable, Aggregate Intrinsic Value	202
Vested and expected to vest, Aggregate Intrinsic Value	$ 627

Stock-Based Compensation - Summary of Stock Option Activity (Parenthetical) (Details) - Options Issued and Outstanding	3 Months Ended
	Mar. 31, 2019 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	604,440
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	9,840

Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model (Details) - Options Issued and Outstanding - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days
Expected volatility, minimum	93.70%	76.10%
Expected volatility, maximum	94.80%	77.70%
Risk-free interest rate, minimum	2.50%	2.40%
Risk-free interest rate, maximum	2.60%	2.70%
Expected dividend yield	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	91.30%	77.90%
Expected volatility, maximum	94.90%	78.10%
Risk-free interest rate, minimum	2.40%	2.70%
Risk-free interest rate, maximum	2.60%	2.80%
Expected dividend yield	0.00%	0.00%
Minimum [Member] \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.53	$ 13.17
Expected term (in years)		5 years 9 months 18 days
Minimum [Member] \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 6.82	$ 9.58
Expected term (in years)	8 years 2 months 12 days	9 years 2 months 12 days
Maximum \| Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.90	$ 15.45
Expected term (in years)		6 years 7 months 6 days
Maximum \| Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.61	$ 15.45
Expected term (in years)	10 years	10 years

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 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	(500)
Number of Shares Outstanding, Ending, Unvested shares	500

Stock-Based Compensation - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units	3 Months Ended
	Mar. 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	0
Number of Shares Outstanding, Vested	0
Number of Shares Outstanding, Ending, Unvested shares	24,960

Commitment and Contingencies - Additional Information (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Estimated litigation liability	$ 0	$ 0

Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,706,317	667,590
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,706,317	667,590

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Novartis International Pharmaceutical Ltd.
Related Party Transaction [Line Items]
Payments to related parties	$ 0	$ 0
Silverstein Foundation \| Funding Agreement
Related Party Transaction [Line Items]
Fund received from related parties	$ 0	$ 500,000

Subsequent Event - Additional Information (Details) - Subsequent Event - USD ($) $ in Millions		1 Months Ended
	May 15, 2019	Apr. 30, 2019
Subsequent Event [Line Items]
Amended lease term from the date of relocation		7 years
Annual base rent per year of the relocation premises		$ 0.6
Percentage increase in operating lease base rent		2.00%
Letters of credit issued amount		$ 0.2
National Institutes of Health
Subsequent Event [Line Items]
Research and development grant term	5 years
National Institutes of Health \| Maximum
Subsequent Event [Line Items]
Grant awarded for research and development	$ 1.5