Tcr2 Therapeutics (TCRR) 10-Q 9 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	6 Months Ended
	Jun. 30, 2019	Jul. 30, 2019
Cover page.
Entity Shell Company	false
Title of 12(b) Security	Common Stock, $0.0001 Par Value
City Area Code	617
Entity Address, Address Line One	100 Binney Street
Entity Incorporation, State or Country Code	DE
Entity File Number	001-38811
Document Transition Report	false
Document Quarterly Report	true
Entity Registrant Name	TCR2 Therapeutics Inc.
Entity Central Index Key	0001750019
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		23,964,746
Entity Tax Identification Number	47-4152751
Entity Address, Address Line Two	Suite 710
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
Local Phone Number	949-5200
Trading Symbol	TCRR
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes

Unaudited Consolidated Balance

Unaudited Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 40,980	$ 47,674
Investments	128,646	75,493
Prepaid expenses and other current assets	6,051	2,326
Total current assets	175,677	125,493
Property and equipment, net	3,172	1,638
Investments, non-current	11,121	0
Restricted cash	290	290
Deferred offering costs	0	2,012
Total assets	190,260	129,433
Liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)
Accounts payable	4,044	2,663
Accrued expenses and other current liabilities	3,116	2,802
Total current liabilities	7,160	5,465
Other liabilities	464	434
Total liabilities	7,624	5,899
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock		209,230
Stockholders' equity (deficit)
Preferred stock, $0.0001 par value. 10,000,000 and no shares authorized, issued or outstanding at June 30, 2019 and December 31, 2018, respectively.	0
Common stock, $0.0001 par value; 150,000,000 and 20,988,730 shares authorized at June 30, 2019 and December 31, 2018, respectively; 23,964,746 and 914,602 shares issued at June 30, 2019 and December 31, 2018, respectively; 23,856,689 and 726,994 shares outstanding at June 30, 2019 and December 31, 2018, respectively.	2
Additional paid-in capital	338,380	0
Accumulated other comprehensive income (loss)	212	(106)
Accumulated deficit	(155,958)	(85,590)
Total stockholders’ equity (deficit)	182,636	(85,696)
Total liabilities, redeemable preferred stock and stockholders’ equity (deficit)	190,260	129,433
Series A redeemable convertible preferred stock
Liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)
Redeemable convertible preferred stock	0	72,980
Series B redeemable convertible preferred stock
Liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)
Redeemable convertible preferred stock	$ 0	$ 136,250

Unaudited Consolidated Balanc_2

Unaudited Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Preferred stock, par value (usd per share)	$ 0.0001	$ 0
Preferred stock, shares authorized (shares)	10,000,000	0
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Common stock, par value (usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (shares)	150,000,000	20,988,730
Common stock, shares issued (shares)	23,964,746	914,602
Common stock, shares outstanding (shares)	23,856,689	726,994
Series A redeemable convertible preferred stock
Redeemable convertible preferred stock, par value (usd per share)		$ 0.0001
Redeemable convertible preferred stock, shares authorized (shares)		45,000,000
Redeemable convertible preferred stock, shares issued (shares)		44,500,001
Redeemable convertible preferred stock, shares outstanding (shares)	0	44,500,001
Series B redeemable convertible preferred stock
Redeemable convertible preferred stock, par value (usd per share)		$ 0.0001
Redeemable convertible preferred stock, shares authorized (shares)		62,500,000
Redeemable convertible preferred stock, shares issued (shares)		62,500,000
Redeemable convertible preferred stock, shares outstanding (shares)	0	62,500,000

Unaudited Consolidated Statemen

Unaudited Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses
Research and development	$ 8,833	$ 5,175	$ 16,722	$ 8,068
General and administrative	3,307	1,634	6,193	2,854
Total operating expenses	12,140	6,809	22,915	10,922
Loss from operations	(12,140)	(6,809)	(22,915)	(10,922)
Interest income, net	1,077	622	1,949	749
Net loss	(11,063)	(6,187)	(20,966)	(10,173)
Accretion of redeemable convertible preferred stock to redemption value	0	(11,145)	(49,900)	(21,978)
Net loss attributable to common stockholders	$ (11,063)	$ (17,332)	$ (70,866)	$ (32,151)
Per share information
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.46)	$ (27.97)	$ (3.91)	$ (56.75)
Weighted average shares outstanding, basic and diluted (shares)	23,818,003	619,749	18,105,142	566,513

Unaudited Consolidated Statem_2

Unaudited Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (11,063)	$ (6,187)	$ (20,966)	$ (10,173)
Unrealized (loss) gain on investments	211	15
Comprehensive loss	$ (10,852)	$ (6,172)	$ (20,648)	$ (10,175)

Unaudited Consolidated Statem_3

Unaudited 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 Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Redeemable convertible preferred stock, beginning balance (shares) at Dec. 31, 2017		44,500,001	0
Redeemable convertible preferred stock beginning balance at Dec. 31, 2017		$ 47,102	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable preferred stock to redemption value		$ 9,413	$ 1,420
Sale of Series B preferred stock, net of issuance costs (shares)			60,000,000
Sale of Series B preferred stock, net of issuance costs			$ 119,830
Redeemable convertible preferred stock, ending balance (shares) at Mar. 31, 2018		44,500,001	60,000,000
Redeemable convertible preferred stock ending balance at Mar. 31, 2018		$ 56,515	$ 121,250
Beginning balance (shares) at Dec. 31, 2017				435,630
Beginning balance at Dec. 31, 2017	$ (26,324)			$ 0	$ 0	$ (26,324)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of shares issued and previously subject to repurchase (shares)				39,778
Exercise of stock options and warrants (shares)				123,270
Stock-based compensation expense	283				283
Unrealized gain (loss) on investments	(17)						(17)
Accretion of redeemable preferred stock to redemption value	(10,833)				(387)	(10,446)
Net loss	(3,986)					(3,986)
Reclassification of shares issued and previously subject to repurchase	13				13
Exercise of stock options and warrants	91				91
Ending balance (shares) at Mar. 31, 2018				598,678
Ending balance at Mar. 31, 2018	(40,773)			$ 0	0	(40,756)	(17)
Redeemable convertible preferred stock, beginning balance (shares) at Dec. 31, 2017		44,500,001	0
Redeemable convertible preferred stock beginning balance at Dec. 31, 2017		$ 47,102	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable preferred stock to redemption value	21,978
Redeemable convertible preferred stock, ending balance (shares) at Jun. 30, 2018		44,500,001	62,500,000
Redeemable convertible preferred stock ending balance at Jun. 30, 2018		$ 61,410	$ 132,500
Beginning balance (shares) at Dec. 31, 2017				435,630
Beginning balance at Dec. 31, 2017	(26,324)			$ 0	0	(26,324)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(10,173)
Ending balance (shares) at Jun. 30, 2018				641,227
Ending balance at Jun. 30, 2018	(57,725)			$ 0	0	(57,723)	(2)
Redeemable convertible preferred stock, beginning balance (shares) at Mar. 31, 2018		44,500,001	60,000,000
Redeemable convertible preferred stock beginning balance at Mar. 31, 2018		$ 56,515	$ 121,250
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable preferred stock to redemption value		$ 4,895	$ 6,250
Sale of Series B preferred stock, net of issuance costs (shares)			2,500,000
Sale of Series B preferred stock, net of issuance costs			$ 5,000
Redeemable convertible preferred stock, ending balance (shares) at Jun. 30, 2018		44,500,001	62,500,000
Redeemable convertible preferred stock ending balance at Jun. 30, 2018		$ 61,410	$ 132,500
Beginning balance (shares) at Mar. 31, 2018				598,678
Beginning balance at Mar. 31, 2018	(40,773)			$ 0	0	(40,756)	(17)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of shares issued and previously subject to repurchase (shares)				39,775
Exercise of stock options and warrants (shares)				2,774
Stock-based compensation expense	350				350
Unrealized gain (loss) on investments	15						15
Accretion of redeemable preferred stock to redemption value	(11,145)				(365)	(10,780)
Net loss	(6,187)					(6,187)
Reclassification of shares issued and previously subject to repurchase	13				13
Exercise of stock options and warrants	2				2
Ending balance (shares) at Jun. 30, 2018				641,227
Ending balance at Jun. 30, 2018	(57,725)			$ 0	0	(57,723)	(2)
Redeemable convertible preferred stock, beginning balance (shares) at Dec. 31, 2018		44,500,001	62,500,000
Redeemable convertible preferred stock beginning balance at Dec. 31, 2018	$ 209,230	$ 72,980	$ 136,250
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable preferred stock to redemption value		$ 34,789	$ 15,111
Conversion of shares upon IPO (in shares)		(44,500,001)	(62,500,000)
Conversion of shares upon IPO		$ (107,769)	$ (151,361)
Redeemable convertible preferred stock, ending balance (shares) at Mar. 31, 2019		0	0
Redeemable convertible preferred stock ending balance at Mar. 31, 2019		$ 0	$ 0
Beginning balance (shares) at Dec. 31, 2018	726,994			726,994
Beginning balance at Dec. 31, 2018	$ (85,696)			$ 0	0	(85,590)	(106)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of shares issued and previously subject to repurchase (shares)				39,776
Exercise of stock options and warrants (shares)				124
Stock-based compensation expense	1,141				1,141
Unrealized gain (loss) on investments	107						107
Accretion of redeemable preferred stock to redemption value	(49,900)				(498)	(49,402)
Conversion of shares upon IPO (shares)				17,275,299
Conversion of shares upon IPO	259,130			$ 2	259,128
Initial public offering, net of issuance costs (in shares)				5,750,000
Initial public offering, net of issuance costs	77,168
Net loss	(9,903)					(9,903)
Ending balance (shares) at Mar. 31, 2019				23,792,193
Ending balance at Mar. 31, 2019	192,047			$ 2	336,939	(144,895)	1
Redeemable convertible preferred stock, beginning balance (shares) at Dec. 31, 2018		44,500,001	62,500,000
Redeemable convertible preferred stock beginning balance at Dec. 31, 2018	209,230	$ 72,980	$ 136,250
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable preferred stock to redemption value	$ 49,900
Redeemable convertible preferred stock, ending balance (shares) at Jun. 30, 2019		0	0
Redeemable convertible preferred stock ending balance at Jun. 30, 2019		$ 0	$ 0
Beginning balance (shares) at Dec. 31, 2018	726,994			726,994
Beginning balance at Dec. 31, 2018	$ (85,696)			$ 0	0	(85,590)	(106)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ (20,966)
Ending balance (shares) at Jun. 30, 2019	23,856,689			23,856,689
Ending balance at Jun. 30, 2019	$ 182,636			$ 2	338,380	(155,958)	212
Redeemable convertible preferred stock, beginning balance (shares) at Mar. 31, 2019		0	0
Redeemable convertible preferred stock beginning balance at Mar. 31, 2019		$ 0	$ 0
Redeemable convertible preferred stock, ending balance (shares) at Jun. 30, 2019		0	0
Redeemable convertible preferred stock ending balance at Jun. 30, 2019		$ 0	$ 0
Beginning balance (shares) at Mar. 31, 2019				23,792,193
Beginning balance at Mar. 31, 2019	192,047			$ 2	336,939	(144,895)	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of shares issued and previously subject to repurchase (shares)				39,775
Exercise of stock options and warrants (shares)				24,721
Stock-based compensation expense	1,444				1,444
Unrealized gain (loss) on investments	211						211
Initial public offering, net of issuance costs	(34)				(34)
Net loss	(11,063)					(11,063)
Reclassification of shares issued and previously subject to repurchase	10				10
Exercise of stock options and warrants	$ 21				21
Ending balance (shares) at Jun. 30, 2019	23,856,689			23,856,689
Ending balance at Jun. 30, 2019	$ 182,636			$ 2	$ 338,380	$ (155,958)	$ 212

Unaudited Consolidated Statem_4

Unaudited Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018
Redeemable Convertible Preferred Stock | Series B redeemable convertible preferred stock
Redeemable convertible preferred stock issuance costs	$ 170	$ 170

Unaudited Consolidated Statem_5

Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities:
Net loss	$ (20,966)	$ (10,173)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	300	188
Stock-based compensation expense	2,585	633
Loss on fixed asset disposal	0	2
Accretion on investments	(252)	(76)
Changes in operating assets and liabilities:
Interest receivable on investments	(325)	(78)
Prepaid expenses and other current assets	(3,158)	153
Accounts payable	180	1,165
Accrued expenses and other liabilities	646	493
Cash used in operating activities	(20,990)	(7,693)
Investing activities:
Purchase of investments	(106,566)	(32,343)
Proceeds from sale or maturity of investments	42,619	5,530
Purchases of equipment	(941)	(772)
Cash used in investing activities	(64,888)	(27,585)
Financing activities:
Proceeds from the sale of Series B preferred stock	0	125,000
Proceeds from initial public offering, net of issuance costs	80,213	0
Proceeds from the exercise of stock options	18	219
Deferred offering costs	(1,047)	(57)
Payment of issuance costs	0	(150)
Cash provided by financing activities	79,184	125,012
Net change in cash, cash equivalents, and restricted cash	(6,694)	89,734
Cash, cash equivalents, and restricted cash at beginning of year	47,964	20,101
Cash, cash equivalents, and restricted cash at end of period	41,270	109,835
Supplemental disclosure of noncash investing and financing activities:
Conversion of redeemable convertible preferred stock to common stock	259,130	0
Accretion of redeemable preferred stock to redemption value	49,900	21,978
Deferred offering costs included in accounts payable	309	254
Property and equipment additions in accounts payable	893	19
Reclassification of early exercise liability upon vesting of options	$ 26	$ 26

Organization and Description of

Organization and Description of Business	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	Organization and Description of Business TCR 2 Therapeutics Inc. (the Company) is a clinical-stage immunotherapy company developing the next generation of novel T cell therapies for patients suffering from cancer. The Company was incorporated under the laws of the State of Delaware on May 29, 2015 under the name TCR 2 , Inc. In November 2016, the Company changed its name to TCR 2 Therapeutics Inc. The Company’s principal operations are located in Cambridge, Massachusetts. Initial Public Offering In February 2019, the Company completed the initial public offering of its common stock (the IPO) pursuant to which it issued and sold 5,750,000 shares of its common stock at a price to the public of $15.00 per share. The shares began trading on The Nasdaq Global Select Market on February 14, 2019. The aggregate net proceeds received by the Company from the offering were approximately $77,134 , after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $9,116 . Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock converted into 17,275,299 shares of common stock. Additionally, as of the closing of the IPO, the Company is authorized to issue 150,000,000 shares of common stock and 10,000,000 shares of preferred stock. Reverse Stock Split

Liquidity

Liquidity	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Liquidity	Liquidity The Company’s operations to date have focused on organization and staffing, business planning, raising capital, acquiring technology and assets, manufacturing and conducting preclinical studies. The Company does not have any product candidates approved for sale and has not generated any revenue from product sales. The Company’s product candidates are subject to long development cycles and the Company may be unsuccessful in its efforts to develop, obtain regulatory approval for or market its product candidates. The Company is subject to a number of risks including, but not limited to, the need to obtain adequate additional funding for the ongoing and planned clinical development of its product candidates. Because of the numerous risks and uncertainties associated with pharmaceutical products and development, the Company is unable to accurately predict the timing or amount of funds required to complete development of its product candidates, and costs could exceed the Company’s expectations for a number of reasons, including reasons beyond the Company’s control. The Company expects to continue to generate losses for the foreseeable future. The Company expects that its cash, cash equivalents and investments as of June 30, 2019 of $180,747 will be sufficient to fund its operating expenses and capital expenditure requirements through at least twelve months from the date of issuance of these unaudited consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of consolidation and basis of presentation The unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (U.S. GAAP) for interim financial information and in accordance with Article 10 of Regulation S-X of the U.S. Securities and Exchange Commission (SEC), and reflect the financial position, results of operations and cash flows of the Company's business. Accordingly, they do not include all of the disclosures required by U.S. GAAP for a complete set of annual audited financial statements. All significant intercompany accounts and transactions are eliminated in consolidation. The unaudited consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. The accompanying financial information should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company's Annual Report on Form 10-K filed with the SEC on March 29, 2019 for the year ended December 31, 2018 (the 2018 Form 10-K). In the opinion of the Company's management, all adjustments (consisting of normal and recurring adjustments) considered necessary for a fair statement of the results for the interim periods presented have been included. Use of estimates The preparation of the accompanying unaudited consolidated 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 disclosure of contingent assets and liabilities at the date of the unaudited consolidated financial statements and reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these unaudited consolidated financial statements include, but are not limited to, the fair value of the royalty transfer agreement obligations, the valuation of redeemable convertible preferred and common stock prior to the IPO, and the fair value of stock-based compensation awards granted under the Company’s equity-based compensation plans. Due to the uncertainty of factors surrounding the estimates or judgments used in the preparation of the unaudited consolidated financial statements, actual results may materially vary from these estimates. Estimates and assumptions are periodically reviewed and the effects of revisions are reflected in the consolidated financial statements in the period they are determined to be necessary. Concentrations of credit risk and of manufacturing risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and investments. The Company’s cash, cash equivalents and investments are held by financial institutions in the United States. Amounts on deposit may at times exceed federally insured limits. Management believes that the financial institutions are financially sound, and accordingly, minimal credit risk exists with respect to the financial institutions. As of June 30, 2019 , the Company has 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. Fair value of financial instruments At June 30, 2019 and December 31, 2018 , the Company’s financial instruments consist of money market funds, commercial paper, asset-backed securities, agency, and corporate bonds and are included in investments. The carrying value of investments is the estimated fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Cash equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. As of June 30, 2019 and December 31, 2018 , cash equivalents consisted of U.S treasuries, corporate bonds and government-backed money market funds. Investments As of June 30, 2019 , all investments were classified as available-for-sale and were carried at their estimated fair value. Unrealized gains and losses are recorded as a component of accumulated other comprehensive income (loss) until realized. The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company periodically reviews its investments in debt securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. If losses on these securities are considered to be other than temporary, the loss is recognized in earnings. The Company classifies its available-for-sale marketable securities as current or non-current based on each instrument’s underlying effective maturity date and for which the Company has the intent and ability to hold the investment for a period of greater than 12 months. Marketable securities with maturities of less than 12 months are classified as current and are included in investments in the consolidated balance sheets. Marketable securities with maturities greater than 12 months for which the Company has the intent and ability to hold the investment for greater than 12 months are classified as non-current and are included in investments, non-current in the consolidated balance sheets. Deferred offering costs The Company capitalizes costs that are directly associated with in-process equity financings until such financings are consummated at which time such costs are recorded against the gross proceeds of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the unaudited consolidated statements of operations. Restricted cash Cash accounts that are restricted as to withdrawal or usage are presented as restricted cash. Restricted cash includes amounts held as a security deposit in the form of a letter of credit for the Company’s leased facility. Classification and accretion of redeemable convertible preferred stock Through the date of the IPO, the Company had classified redeemable convertible preferred stock outstanding and classified outside of stockholders’ equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. The carrying value of the redeemable convertible preferred stock was accreted to redemption value at the end of each reporting period, up to the date of the IPO, as if the end of the reporting period were the redemption date. Increases to the carrying value of redeemable convertible preferred stock were charged to additional paid-in capital or, in the absence of additional paid-in capital, charged to accumulated deficit. Upon completion of the IPO during February 2019, all redeemable convertible preferred stock was converted to common stock. Stock-based compensation The Company measures employee stock-based awards at grant-date fair value and records compensation expense on a straight-line basis over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues awards with only service-based vesting conditions. The Company accounts for forfeitures as they occur. The Company measures the fair value of stock-based awards granted to non-employees on the date at which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such non-employee consultants until completed. In the second quarter of 2019, the Company adopted ASU 2018-07 on a retrospective basis effective January 1, 2019, the beginning of the fiscal year of adoption. Prior to the adoption of ASU 2018-07, share-based payments awards granted to non-employees were measured at fair value on their grant date, subject to periodic remeasurement at each reporting period, and share-based compensation expense was recognized over their vesting terms. After the adoption of ASU 2018-07, the fair value of all outstanding and unvested previously granted non-employee awards was established on January 1, 2019, the effective date of adoption, and share-based compensation expense will continue to be recorded on a straight-line basis over their remaining vesting period, consistent with share-based payment awards granted to employees. Common shares issued and stock-options exercised prior to vesting are subject to repurchase by the Company until vested by the Company at the lesser of the initial exercise price and the fair market value of the Company’s common stock at the time of repurchase. The proceeds from the shares subject to repurchase are classified as a liability and reclassified to equity as the shares vest. Estimating the fair value of stock-based awards requires the input of subjective assumptions, including the fair value of the Company’s common stock, and, for stock options and warrants, the expected life of the options and stock price volatility. The Company uses the Black-Scholes option pricing model to value its stock option awards and warrants. The assumptions used in calculating the fair value of stock-based awards represent management’s estimates and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, stock-based compensation expense could be materially different for future awards. The Company classifies stock-based compensation expense in its statements of operations in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Research and development expenses Research and development costs are expensed as incurred and consist primarily of funds paid to third parties for the provision of services for product candidate development, clinical and preclinical development and related supply and manufacturing costs, and regulatory compliance costs. At the end of the reporting period, the Company compares payments made to third party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. Upfront milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Income taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A reduction in the carrying value of the deferred tax assets is required when it is not more likely than not that such deferred tax assets are not realizable. Net loss per share Basic and diluted net loss per common share is determined by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period. The Company’s outstanding redeemable convertible preferred stock contractually entitles the holders of such shares to participate in distributions but contractually does not require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding shares of common stock at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. Therefore, the weighted-average shares used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities, on an as converted basis have been excluded from the computation of diluted weighted-average shares outstanding as of June 30, 2019 and 2018 , as they would be antidilutive: As of June 30, 2019 2018 Series A redeemable convertible preferred stock — 7,184,588 Series B redeemable convertible preferred stock — 10,090,711 Stock options outstanding 3,343,357 1,167,172 Employee stock purchase plan 7,408 — Unvested shares of restricted stock 3,320 92,602 Common stock warrants 203,676 203,678 Total 3,557,761 18,738,751 Comprehensive loss Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources (which excludes investments from owners). The Company’s only element of other comprehensive loss is unrealized gains and losses on investments. Reconciliation of cash and cash equivalents and restricted cash as presented in the statements of cash flows The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the unaudited consolidated balance sheets to the total of the same such amounts shown in the unaudited consolidated statements of cash flows for the six months ended June 30, 2019 and 2018 . As of June 30, 2019 2018 Cash and cash equivalents $ 40,980 $ 109,542 Restricted cash 290 293 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,270 $ 109,835 JOBS Act accounting election The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the JOBS Act). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it is (i) no longer an emerging growth company or (ii) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these consolidated financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recently issued accounting pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) which will require lessees to record most operating leases on their balance sheets, but recognize the expenses in the statements of operations in a manner similar to current practice. Under the new standard, lessees will be required to recognize a lease liability for the obligation to make lease payments, and an asset for the right to use the underlying asset for the lease term, for all leases with terms longer than 12 months. Leases will be classified as finance or operating, with classification affecting the pattern and classification of expense recognition in the statements of operations. Expenses related to operating leases will be recognized on a straight-line basis, while those determined to be financing leases will be recognized following a front-loaded expense profile, in which interest and amortization are presented separately in the statements of operations. The principal effect on the Company’s financial statements will be an increase in assets and liabilities. The standard will be effective for public business entities for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. The Company will adopt the new standard beginning January 1, 2020. A modified retrospective transition approach is required, applying the new standard to all leases existing at the date of initial application. An entity may choose to apply the standard either (1) on the January 1, 2020 effective date, or (2) the beginning of the earliest comparative period presented in its financial statements. The standard includes a number of practical expedients that the Company is evaluating and may elect to apply. Early adoption is permitted. The Company is currently evaluating the potential impact of the adoption of this standard on its consolidated financial statements and related disclosures. Recently adopted accounting pronouncements Beginning January 1, 2019, the Company adopted ASU 2016-18, Statement of Cash Flows (Topic 230) Restricted Cash, which requires entities to show the changes in the total of cash, cash equivalents, restricted cash and restricted cash equivalents in the statement of cash flows. Entities will no longer present transfers between cash and cash equivalents and restricted cash and restricted cash equivalents in the statement of cash flows. The standard requires retrospective application in the consolidated statements of cash flows. In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718) Improvements to Non-employee Share-Based Payment Accounting. The amendments in this update expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from non-employees. Under this ASU, an entity should apply the requirements of Topic 718 to non-employee awards except for specific guidance on inputs to an option pricing model and the attribution of costs (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The guidance is applicable to public business entities for fiscal years beginning after December 15, 2018 including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020.

Investments and Fair Value Meas

Investments and Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value And Debt Securities, Available-For-Sale [Abstract]
Investments and Fair Value Measurements	Investments and Fair Value Measurements As of June 30, 2019 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 123,410 $ 205 $ (4 ) $ 123,611 $ 112,490 $ 11,121 Agency bonds 6,450 10 — 6,460 6,460 — Commercial paper 7,447 1 — 7,448 7,448 — Asset-backed securities 2,248 — — 2,248 2,248 — $ 139,555 $ 216 $ (4 ) $ 139,767 $ 128,646 $ 11,121 As of December 31, 2018 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 58,029 $ 1 $ (94 ) $ 57,936 $ 57,936 $ — Agency bonds 9,966 — (9 ) 9,957 9,957 — Commercial paper 7,214 — (4 ) 7,210 7,210 — Asset-backed securities 390 — — 390 390 — $ 75,599 $ 1 $ (107 ) $ 75,493 $ 75,493 $ — The amortized cost and estimated fair value of marketable securities, by contractual maturity: June 30, 2019 Amortized Cost Fair Value Due within one year $ 128,455 $ 128,646 Due after one year through five years 11,100 11,121 $ 139,555 $ 139,767 December 31, 2018 Amortized Cost Fair Value Due within one year $ 75,599 $ 75,493 Due after one year through five years — — $ 75,599 $ 75,493 The Company believes that the decline in value of its debt securities is temporary and primarily related to the change in market interest rates since purchase. The Company believes it is more likely than not that it will be able to hold these securities to maturity. Therefore, the Company anticipates full recovery of its debt securities’ amortized cost basis at maturity. The Company follows FASB’s accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. Fair value is defined as the price at which an asset could be exchanged in a current transaction between knowledgeable, willing parties. 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. This hierarchy requires the use of observable market data when available and to minimize the use of unobservable inputs when determining fair value. 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 guidance requires fair value measurements to be classified and disclosed in one of the following three categories: Level 1—Quoted prices (unadjusted in active markets for identical assets or liabilities) Level 2—Inputs other than quoted prices in active markets that are observable either directly or indirectly Level 3—Unobservable inputs in which there is little or no market data, which require the Company to develop its own assumptions As of June 30, 2019 , the Company has classified assets measured at fair value on a recurring basis as follows: Fair Value Measurement Based On Amortized Cost Fair Value Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs Significant Unobservable Inputs Cash equivalents (1) $ 39,685 $ 39,685 $ 39,685 $ — $ — Corporate bonds 123,410 123,611 — 123,611 — Agency bonds 6,450 6,460 — 6,460 — Commercial paper 7,447 7,448 — 7,448 — Asset-backed securities 2,248 2,248 — 2,248 — $ 179,240 $ 179,452 $ 39,685 $ 139,767 $ — During the six months ended June 30, 2019 , there were no transfers among the Level 1, Level 2 and Level 3 categories. As of December 31, 2018 , the Company has classified assets measured at fair value on a recurring basis as follows: Fair Value Measurement Based On Amortized Cost Fair Value Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs Significant Unobservable Inputs Cash equivalents (1) $ 45,974 $ 45,974 $ 45,108 $ 866 $ — Corporate bonds 58,029 57,936 — 57,936 — Agency bonds 9,966 9,957 — 9,957 — Commercial paper 7,214 7,210 — 7,210 — Asset-backed securities 390 390 — 390 — $ 121,573 $ 121,467 $ 45,108 $ 76,359 $ — During the twelve months ended December 31, 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories. (1)

Property and Equipment

Property and Equipment	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net, consisted of: June 30, December 31, Laboratory equipment $ 3,198 $ 2,118 Computer hardware and equipment 105 105 Furniture and fixtures 326 326 Leasehold improvements 84 34 Laboratory equipment not placed in service 704 — 4,417 2,583 Less: Accumulated depreciation and amortization (1,245 ) (945 ) $ 3,172 $ 1,638 Depreciation expense was $165 and $112 for the three months ended June 30, 2019 , and 2018, respectively, and $300 and $188 for the six months ended June 30, 2019 and 2018, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	6 Months Ended
	Jun. 30, 2019
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of: June 30, December 31, Employee compensation and related benefits $ 1,088 $ 1,676 Professional fees 276 342 Contract manufacturing organization fees 361 173 Contract research organization fees 459 232 University partnerships 315 162 Property received not yet invoiced 415 103 Other 202 114 $ 3,116 $ 2,802

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company recognizes rent expense on a straight-line basis over the lease period and has accrued for rent expense incurred but not yet paid. Landlord allowances for tenant improvements are deferred and recognized as a reduction to rent expense on a straight-line basis and over the remaining lease term. In March 2018, the Company entered into a lease for office and laboratory facilities that expires in July 2025. Under the terms of the lease, the Company placed $290 letter of credit into a restricted cash account as security for the facility. In December 2018, the Company signed a collaboration agreement (the Collaboration Agreement) with Cell Therapy Catapult Limited (Catapult) to establish the Company's manufacturing process in Catapult’s GMP manufacturing facility in the United Kingdom. The initial term of the Collaboration Agreement is three years which began March 1, 2019. The Company can terminate the Collaboration Agreement earlier with twelve months ’ notice and continued payment for contributions during the twelve -month termination period. The Collaboration Agreement provides for Catapult to provide identified space, called a module, and other specified services. The Company has concluded that the Collaboration Agreement contains an embedded lease as the Company has the right to operate the module in a manner it determines. The Company also concluded that it is not the deemed owner during modification of the module nor does the agreement represent a capital lease under ASC 840, “Leases”. As a result, the Collaboration Agreement will be accounted for as an operating lease. The Company determined the amounts to be representative of rent to be £300 per year based on the relative selling prices of the services being provided. This amount will be amortized annually on a straight-line basis as rent expense over the term of the embedded lease, commencing March 1, 2019. The following table presents future minimum rent payments under non-cancellable operating leases with initial terms in excess of one year at June 30, 2019 : Minimum Rent Payments Remainder of 2019 $ 1,340 2020 2,735 2021 2,447 2022 2,008 2023 2,002 Thereafter 3,116 Total minimum payments required $ 13,648 Rent expense was $694 and $574 for the three months ended June 30, 2019 , and 2018, respectively, and $1,325 and $949 for the six months ended June 30, 2019 and 2018, respectively. Litigation The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings. Royalty transfer agreement In connection with the sale of Series A redeemable convertible preferred stock (see Note 8), certain investors are entitled to receive, in the aggregate, a royalty from the Company equal to one percent of (i) all global net sales of any Company products and (ii) any license income on intellectual property that was in existence at the time of the Series A preferred stock financing. The Company has elected to account for this liability at fair value with changes recognized in earnings. Given the early stage nature of the underlying technology and inherent risks associated with obtaining regulatory approval and achieving commercialization, the Company ascribed no value to the royalty agreement at inception and at June 30, 2019 and December 31, 2018 . The Company currently does no t have any net sales or license income and as a result has paid no royalties under this obligation for the six months ended June 30, 2019 or 2018 no r has the Company accrued any liability as of June 30, 2019 or December 31, 2018 .

Common Stock and Redeemable Con

Common Stock and Redeemable Convertible Preferred Stock	6 Months Ended
	Jun. 30, 2019
Temporary Equity And Stockholders' Equity Note [Abstract]
Common Stock and Redeemable Convertible Preferred Stock	Common Stock and Redeemable Convertible Preferred Stock Common stock Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Subject to the rights of holders of redeemable convertible preferred stock, common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any. No dividends had been declared through June 30, 2019 . Redeemable convertible preferred stock Upon completion of the IPO during February 2019, all redeemable convertible preferred stock was converted to common stock. Prior to the IPO, the Company elected to accrete the carrying value of the Series A and B preferred stock to redemption value at the end of each reporting period as if the end of the reporting period were the redemption date, increases to the carrying value of redeemable convertible preferred stock are charged to additional paid-in capital or, in the absence of additional paid-in capital, charged to accumulated deficit. Series A redeemable convertible preferred stock Prior to the IPO, there were 44,500,001 Series A preferred shares issued and outstanding. Included in the Series A preferred stock purchase agreement, the investor is required to purchase additional shares upon the achievement of certain Company milestones. The Company evaluated the future commitment obligations at original issuance and determined they were not freestanding instruments as they were not legally detachable. The future commitment obligations were also evaluated as embedded derivatives and determined they did not meet the definition of a derivative instrument for which bifurcation would be required. Upon completion of the IPO in February 2019, all Series A preferred stock was converted to 7,184,588 shares of common stock. Conversion Prior to the IPO, each share of Series A preferred stock was convertible, at the option of the holder, into shares of common stock. Prior to the common stock reverse stock split in February 2019, the shares were convertible on a one -to-one basis. Post-split the Series A stock were convertible at 1-to- 0.1615 basis. The Series A conversion rights were subject to adjustment for certain dilutive events. The conversion price could have been adjusted to prevent dilution of the Series A preferred stock. The preferred stock was also mandatorily convertible upon the closing of an initial public offering resulting in gross proceeds to the Company exceeding $50,000 or by a written election by the majority of the Series A stockholders. Redemption Prior to the IPO, at the election of a majority of the Series A stockholders, the Series A preferred stock was redeemable at any time on or after October 16, 2020. The Series A preferred stock may be redeemed at a price equal to the greater of (a) the original issuance price, plus any cumulative dividends accrued but unpaid thereon, whether or not declared, or (b) the fair market value as of the date of the redemption. Dividends Prior to the IPO, the holders of shares of Series A preferred stock were entitled to receive cumulative dividends of 6% from the date of issuance. Accumulated dividends were payable only when and if declared by the Board of Directors, in preference to dividends paid to holders of common stock. The dividend preference for Series A preferred stock is $0.06 per share, as adjusted for recapitalizations. No dividends have been declared through June 30, 2019 . Liquidation Prior to the IPO, in the event of a liquidation, dissolution or winding up of the Company, either voluntary or involuntary, or in the event of a deemed liquidation event, which included a sale of the Company as defined in the Company’s certificate of incorporation, holders of Series A preferred stock were entitled to receive, subject to the preference of the Series B holders but in preference to common stockholders, an amount equal to their original investment amount plus any declared and unpaid dividends. If upon the occurrence of such event, the assets and funds available for distribution were insufficient to pay such holders the full amount to which they are entitled, then the entire assets and funds legally available for distribution would have been distributed ratably among the holders of the Series A preferred stock in proportion to the full amounts to which they would otherwise be entitled. After payment of the liquidation preference on shares of Series A preferred stock had been made, any remaining assets would have been distributed ratably to common, Series B stockholders and Series A stockholders, on an as-converted basis. Series B redeemable convertible preferred stock Prior to the IPO, there were 62,500,000 Series B preferred shares issued and outstanding. The Series B preferred stock is classified outside of stockholders’ equity (deficit) as the preferred holders may, at their option, elect to have their shares redeemed upon written notice by a majority of the preferred shareholders and at any time after February 2023. Upon completion of the IPO in February 2019, all Series B preferred stock was converted to 10,090,711 shares of common stock. Conversion Prior to the IPO, each share of Series B preferred stock was convertible, at the option of the holder, into shares of common stock. Prior to the common stock reverse stock split in February 2019, the shares were convertible on a one -to-one basis. Post-split the Series B stock were convertible at 1-to- 0.1615 basis. The Series B conversion rights were subject to adjustment for certain dilutive events. The conversion price could have been adjusted to prevent dilution of the Series B preferred stock. The Series B preferred stock was also mandatorily convertible upon the closing of an initial public offering resulting in gross proceeds to the Company exceeding $50,000 or by a written election by the majority of the Series B stockholders. Redemption Prior to the IPO, at the election of a majority of the Series B stockholders, the Series B preferred stock was redeemable at any time on or after February 2023. The Series B preferred stock could have been redeemable at a price equal to the greater of (a) the original issuance price, plus any cumulative dividends accrued but unpaid thereon, whether or not declared, or (b) the fair market value as of the date of the redemption. Dividends Prior to the IPO, the holders of shares of Series B preferred stock were entitled to receive cumulative dividends of 6% from the date of issuance. Accumulated dividends were payable only when and if declared by the Board of Directors, in preference to dividends paid to holders of Series B preferred stock and common stock. The dividend preference for Series B preferred stock was $0.12 per share, as adjusted for recapitalizations. No dividends were declared through June 30, 2019 . Liquidation Prior to the IPO, in the event of a liquidation, dissolution or winding up of the Company, either voluntary or involuntary, or in the event of a deemed liquidation event, which includes a sale of the Company as defined in the Company’s articles of incorporation, holders of Series B preferred stock were entitled to receive, in preference to the holders of Series A preferred stock or Common Stock, an amount equal to their original investment amount plus any declared and unpaid dividends. If upon the occurrence of such event, the assets and funds available for distribution were insufficient to pay such holders the full amount to which they are entitled, then the entire assets and funds legally available for distribution would have been distributed ratably among the holders of the Series B preferred stock in proportion to the full amounts to which they would otherwise be entitled. After payment of the liquidation preference on shares of Series B preferred stock has been made, any remaining assets would have been distributed ratably to Series A stockholders in an amount equal to their original investment amount plus any accrued dividends, whether or not declared, together with any other dividends declared but unpaid thereon. After payment of the liquidation preference on shares of Series A preferred stock had been made, any remaining assets would have been distributed ratably to common, Series B stockholders and Series A stockholders, on an as-converted basis.

Stock-based Compensation

Stock-based Compensation	6 Months Ended
	Jun. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based Compensation	Stock-based Compensation In February 2019, the Company’s Board of Directors and stockholders approved the 2018 Stock Option and Incentive Plan (the 2018 Plan), which replaced the 2015 Plan. The shares under the 2015 Plan which were not issued, were rolled into the 2018 Plan. 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 officers, employees, directors and other key persons (including consultants) are eligible to receive awards under the 2018 Plan. The maximum number of authorized shares to be issued under the Plan is 3,000,000 shares of common stock. The number of shares of common stock reserved for issuance under the 2018 Plan shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 4% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year or a lesser number of shares determined by the Company's Board of Directors. The amount, terms of grants, and exercisability provisions are determined and set by the Compensation Committee of the Company’s Board of Directors. The term of the options may be up to 10 years , and options are exercisable in cash or as otherwise determined by the Board of Directors. As of June 30, 2019 , there were 1,726,613 shares of common stock available for future issuance under the 2018 Plan. Generally, options and restricted stock awards vest over a four -year period. The Company recorded stock-based compensation expense in the following expense categories of its accompanying unaudited consolidated statements of operations for the three and six months ended June 30, 2019 and 2018 : Three Months Ended June 30, Six Months Ended 2019 2018 2019 2018 Research and development 616 76 914 132 General and administrative 828 274 1,671 501 $ 1,444 $ 350 $ 2,585 $ 633 Stock options During the six months ended June 30, 2018, there were 242,458 grants of stock options, 124,909 options forfeited, and 126,044 options exercised, respectively. The following table summarizes the activity related to stock option grants to employees and non-employees for the six months ended June 30, 2019 : SHARES WEIGHTED WEIGHTED (IN YEARS) Balance at January 1, 2019 2,094,815 $ 3.78 9.1 Granted 1,348,290 16.08 Exercised (24,845 ) 0.74 Forfeited (74,903 ) 3.77 Outstanding at June 30, 2019 3,343,357 $ 8.77 9.2 Exercisable at June 30, 2019 490,574 Vested and expected to vest at June 30, 2019 3,343,357 As of June 30, 2019 , there was $18,212 in unrecognized compensation cost that is expected to be recognized over an estimated weighted-average amortization period of 3.3 years . The fair value of options is estimated using the Black-Scholes option pricing model, which takes into account inputs such as the exercise price, the value of the underlying common stock at the grant date, expected term, expected volatility, risk-free interest rate and dividend yield. The fair value of each grant of options six months ended June 30, 2019 and 2018 was determined using the methods and assumptions discussed below: ▪ The expected term of employee options is determined using the “simplified” method, as prescribed in the SEC Staff Accounting Bulletin (SAB) No. 107, whereby the expected life equals the arithmetic average of the vesting term and the original contractual term of the option due to the Company’s lack of sufficient historical data. The expected term of non-employee options is equal to the contractual term. ▪ The expected volatility is based on historical volatilities of similar entities within the Company’s industry which were commensurate with the expected term assumption as described in SAB No. 107. ▪ The estimated annual dividend yield is 0% because the Company has not historically paid, and does not expect for the foreseeable future to pay, a dividend on its common stock. ▪ The Company considered numerous objective and subjective factors in estimating the fair value of its common stock, including the estimated fair value of the Company’s Series A and Series B preferred stock for periods prior to our IPO. For the six months ended June 30, 2019 and 2018, the grant date fair value of all option grants was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted-average assumptions: Six Months Ended June 30, 2019 2018 Risk free interest rate 2.33 % 2.8 % Expected term (in years) 5.9 6.1 Expected volatility 70.86 % 65.0 % Annual dividend yield — % — % Fair value of common stock $ 10.27 $ 3.67 Restricted stock During the six months ended June 30, 2019 and 2018, the Company granted no restricted stock and there were no forfeitures. As of June 30, 2019 , there was $30 in unrecognized compensation cost that is expected to be recognized by December 31, 2019. Warrants During the six months ended June 30, 2019 and 2018, the Company granted no warrants and there were no forfeitures. As of June 30, 2019 , there was $944 in unrecognized compensation cost that is expected to be recognized over an estimated weighted-average amortization period of 1.5 years . Employee stock purchase plan (ESPP) In February 2019, the Company’s Board of Directors adopted and the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (2018 ESPP). The 2018 ESPP enables eligible employees to purchase shares of the Company's common stock at the end of each six -month offering period at a price equal to 85% of the fair market value of the shares on the first business day or the last business day of the offering period, whichever is lower. Eligible employees generally included all employees. Offering periods began on the first trading day September 1 and March 1 of each year and ended on the last trading day in February and August of each year. Share purchases are funded through payroll deductions of up to 15% of an employee’s eligible compensation for each payroll period, or $25 each calendar year. During the six months ended June 30, 2019 and 2018, there were no shares issued under the 2018 ESPP.

Related party transactions

Related party transactions	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related party transactions	Related party transactions Consulting arrangements On October 1, 2015, the Company entered into a consulting agreement with Dr. Patrick Baeuerle. Pursuant to the consulting agreement, Dr. Baeuerle agreed to perform such consulting, advisory and related services to and for the Company as may be reasonably requested. In exchange, the Company agreed to pay Dr. Baeuerle a consulting fee of €15 per month. On November 1, 2016, the Company amended the consulting agreement to revise Dr. Baeuerle’s consulting fee to be €3 per month. Dr. Baeuerle is also eligible for an annual bonus equal to 33% of the annual fees paid under the consulting agreement, subject to the discretion of the Company's Board of Directors based on Dr. Baeuerle’s performance and the Company's performance. The term of the agreement is one year , and automatically extends for additional one -year periods unless terminated. During the six months ended June 30, 2019 and 2018 , the Company incurred fees and travel related expenses to Dr. Baeuerle in the amount of $36 and $38 , respectively, under the consulting agreement. Dr. Baeuerle is a member of the Company's Board of Directors and is a managing director at MPM Capital, the beneficial owner of more than 5% of the Company's voting securities. On March 2, 2016, the Company entered into a consulting agreement with Dr. Mitchell Finer (the Original Finer Agreement), which was amended and restated on May 9, 2017 to, among other things, add Pattern Recognition Ventures as a party. Pursuant to the amended and restated consulting agreement, Pattern Recognition Ventures agreed to perform scientific consulting, advisory and related services to and for the Company as may be reasonably requested, including making Dr. Finer available to serve as Chairman of the Company's Scientific Advisory Board. Pursuant to the amended and restated consulting agreement, the Company agreed (i) to pay Pattern Recognition Ventures a consulting fee of $19 per quarter for services provided under the agreement, commencing on July 1, 2017. During the six months ended June 30, 2019 and 2018 , the Company incurred fees and travel-related expenses to Pattern Recognition Ventures in the amount of $38 and $38 , respectively. Dr. Finer has a financial interest in Pattern Recognition Ventures and is its managing member. Dr. Finer is also a member of the Company's Board of Directors and is an executive partner at MPM Capital, the beneficial owner of more than 5% of the Company's voting securities. The majority investor in the Company is MPM Capital (MPM). During 2018 and 2019, the Company continued to receive advisory services related to intellectual property from an executive partner at MPM. For the six months ended June 30, 2019 and 2018, the Company incurred approximately $171 and $143 , respectively, for intellectual property services. In addition, for the six months ended June 30, 2019 and 2018, the Company incurred approximately $24 and $20 , respectively, for travel-related expenses incurred by two MPM executive partners that serve on the Company's Board of Directors. These amounts were recorded in general and administrative expenses in the unaudited consolidated statements of operations. On October 1, 2017, the Company entered into a consulting agreement with Globeways Holdings Limited. Dr. Morana Jovan-Embiricos has financial interests in Globeways Holdings Limited and is its founding director. Pursuant to the consulting agreement, Globeways Holdings Limited provides consulting, advisory and related services in exchange for consulting fees of $0.1 per year. During the six months ended June 30, 2019 and 2018 , the Company incurred fees and travel-related expenses to Globeways Holdings Limited in the amount of $52 and $60 , respectively. Dr. Jovan was a member of the Company's Board of Directors until her resignation on May 13, 2019. Dr. Jovan is the founding director of Globeways Holdings Limited. Globeways Holdings Limited is the appointed manager of certain affiliates of F2 Capital that collectively beneficially own more than 5% of the Company's voting securities. The consulting agreement with Globeways Holdings Limited was terminated in connection with Dr. Jovan-Embiricos's resignation from the Company's Board of Directors.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of presentation	The unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (U.S. GAAP) for interim financial information and in accordance with Article 10 of Regulation S-X of the U.S. Securities and Exchange Commission (SEC), and reflect the financial position, results of operations and cash flows of the Company's business.
Principles of consolidation	All significant intercompany accounts and transactions are eliminated in consolidation.
Use of estimates	The preparation of the accompanying unaudited consolidated 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 disclosure of contingent assets and liabilities at the date of the unaudited consolidated financial statements and reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these unaudited consolidated financial statements include, but are not limited to, the fair value of the royalty transfer agreement obligations, the valuation of redeemable convertible preferred and common stock prior to the IPO, and the fair value of stock-based compensation awards granted under the Company’s equity-based compensation plans. Due to the uncertainty of factors surrounding the estimates or judgments used in the preparation of the unaudited consolidated financial statements, actual results may materially vary from these estimates. Estimates and assumptions are periodically reviewed and the effects of revisions are reflected in the consolidated financial statements in the period they are determined to be necessary.
Concentration of credit risk	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and investments. The Company’s cash, cash equivalents and investments are held by financial institutions in the United States. Amounts on deposit may at times exceed federally insured limits.
Concentration of manufacturing risk	As of June 30, 2019 , the Company has 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.
Fair value of financial instruments	At June 30, 2019 and December 31, 2018 , the Company’s financial instruments consist of money market funds, commercial paper, asset-backed securities, agency, and corporate bonds and are included in investments. The carrying value of investments is the estimated fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date.
Cash equivalents	The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. As of June 30, 2019 and December 31, 2018 , cash equivalents consisted of U.S treasuries, corporate bonds and government-backed money market funds.
Investments	As of June 30, 2019 , all investments were classified as available-for-sale and were carried at their estimated fair value. Unrealized gains and losses are recorded as a component of accumulated other comprehensive income (loss) until realized. The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company periodically reviews its investments in debt securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. If losses on these securities are considered to be other than temporary, the loss is recognized in earnings. The Company classifies its available-for-sale marketable securities as current or non-current based on each instrument’s underlying effective maturity date and for which the Company has the intent and ability to hold the investment for a period of greater than 12 months. Marketable securities with maturities of less than 12 months are classified as current and are included in investments in the consolidated balance sheets. Marketable securities with maturities greater than 12 months for which the Company has the intent and ability to hold the investment for greater than 12 months are classified as non-current and are included in investments, non-current in the consolidated balance sheets.
Deferred offering costs	The Company capitalizes costs that are directly associated with in-process equity financings until such financings are consummated at which time such costs are recorded against the gross proceeds of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the unaudited consolidated statements of operations.
Restricted cash	Cash accounts that are restricted as to withdrawal or usage are presented as restricted cash. Restricted cash includes amounts held as a security deposit in the form of a letter of credit for the Company’s leased facility.
Classification and accretion of redeemable convertible preferred stock	he Company had classified redeemable convertible preferred stock outstanding and classified outside of stockholders’ equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. The carrying value of the redeemable convertible preferred stock was accreted to redemption value at the end of each reporting period, up to the date of the IPO, as if the end of the reporting period were the redemption date. Increases to the carrying value of redeemable convertible preferred stock were charged to additional paid-in capital or, in the absence of additional paid-in capital, charged to accumulated deficit.
Stock-based compensation	The Company measures employee stock-based awards at grant-date fair value and records compensation expense on a straight-line basis over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues awards with only service-based vesting conditions. The Company accounts for forfeitures as they occur. The Company measures the fair value of stock-based awards granted to non-employees on the date at which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such non-employee consultants until completed. In the second quarter of 2019, the Company adopted ASU 2018-07 on a retrospective basis effective January 1, 2019, the beginning of the fiscal year of adoption. Prior to the adoption of ASU 2018-07, share-based payments awards granted to non-employees were measured at fair value on their grant date, subject to periodic remeasurement at each reporting period, and share-based compensation expense was recognized over their vesting terms. After the adoption of ASU 2018-07, the fair value of all outstanding and unvested previously granted non-employee awards was established on January 1, 2019, the effective date of adoption, and share-based compensation expense will continue to be recorded on a straight-line basis over their remaining vesting period, consistent with share-based payment awards granted to employees. Common shares issued and stock-options exercised prior to vesting are subject to repurchase by the Company until vested by the Company at the lesser of the initial exercise price and the fair market value of the Company’s common stock at the time of repurchase. The proceeds from the shares subject to repurchase are classified as a liability and reclassified to equity as the shares vest. Estimating the fair value of stock-based awards requires the input of subjective assumptions, including the fair value of the Company’s common stock, and, for stock options and warrants, the expected life of the options and stock price volatility. The Company uses the Black-Scholes option pricing model to value its stock option awards and warrants. The assumptions used in calculating the fair value of stock-based awards represent management’s estimates and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, stock-based compensation expense could be materially different for future awards. The Company classifies stock-based compensation expense in its statements of operations in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Research and development expenses	Research and development costs are expensed as incurred and consist primarily of funds paid to third parties for the provision of services for product candidate development, clinical and preclinical development and related supply and manufacturing costs, and regulatory compliance costs. At the end of the reporting period, the Company compares payments made to third party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. Upfront milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered.
Income taxes	Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A reduction in the carrying value of the deferred tax assets is required when it is not more likely than not that such deferred tax assets are not realizable.
Net loss per share	Basic and diluted net loss per common share is determined by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period. The Company’s outstanding redeemable convertible preferred stock contractually entitles the holders of such shares to participate in distributions but contractually does not require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding shares of common stock at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. Therefore, the weighted-average shares used to calculate both basic and diluted loss per share are the same.
Comprehensive loss	Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources (which excludes investments from owners). The Company’s only element of other comprehensive loss is unrealized gains and losses on investments.
Recently issued and adopted accounting pronouncements	In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) which will require lessees to record most operating leases on their balance sheets, but recognize the expenses in the statements of operations in a manner similar to current practice. Under the new standard, lessees will be required to recognize a lease liability for the obligation to make lease payments, and an asset for the right to use the underlying asset for the lease term, for all leases with terms longer than 12 months. Leases will be classified as finance or operating, with classification affecting the pattern and classification of expense recognition in the statements of operations. Expenses related to operating leases will be recognized on a straight-line basis, while those determined to be financing leases will be recognized following a front-loaded expense profile, in which interest and amortization are presented separately in the statements of operations. The principal effect on the Company’s financial statements will be an increase in assets and liabilities. The standard will be effective for public business entities for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. The Company will adopt the new standard beginning January 1, 2020. A modified retrospective transition approach is required, applying the new standard to all leases existing at the date of initial application. An entity may choose to apply the standard either (1) on the January 1, 2020 effective date, or (2) the beginning of the earliest comparative period presented in its financial statements. The standard includes a number of practical expedients that the Company is evaluating and may elect to apply. Early adoption is permitted. The Company is currently evaluating the potential impact of the adoption of this standard on its consolidated financial statements and related disclosures. Recently adopted accounting pronouncements Beginning January 1, 2019, the Company adopted ASU 2016-18, Statement of Cash Flows (Topic 230) Restricted Cash, which requires entities to show the changes in the total of cash, cash equivalents, restricted cash and restricted cash equivalents in the statement of cash flows. Entities will no longer present transfers between cash and cash equivalents and restricted cash and restricted cash equivalents in the statement of cash flows. The standard requires retrospective application in the consolidated statements of cash flows. In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718) Improvements to Non-employee Share-Based Payment Accounting. The amendments in this update expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from non-employees. Under this ASU, an entity should apply the requirements of Topic 718 to non-employee awards except for specific guidance on inputs to an option pricing model and the attribution of costs (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The guidance is applicable to public business entities for fiscal years beginning after December 15, 2018 including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. In June 2019, the Company adopted ASU 2018-07 on a retrospective basis effective January 1, 2019, the beginning of the fiscal year of adoption. Prior to the adoption of ASU 2018-07, share-based payments awards granted to non-employees were measured at fair value on their grant date, subject to periodic remeasurement at each reporting period, and share-based compensation expense was recognized over their vesting terms. After the adoption of ASU 2018-07,the fair value of all outstanding and unvested previously granted non-employee awards was established on January 1, 2019, the effective date of adoption, and share-based compensation expense will continue to be recorded on a straight-line basis over their remaining vesting period, consistent with share-based payment awards granted to employees. As a result of the adoption of ASU 2018-07, there was no material impact to the financial statements.
Fair value measurement	The Company follows FASB’s accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. Fair value is defined as the price at which an asset could be exchanged in a current transaction between knowledgeable, willing parties. 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. This hierarchy requires the use of observable market data when available and to minimize the use of unobservable inputs when determining fair value. 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 guidance requires fair value measurements to be classified and disclosed in one of the following three categories: Level 1—Quoted prices (unadjusted in active markets for identical assets or liabilities) Level 2—Inputs other than quoted prices in active markets that are observable either directly or indirectly Level 3—Unobservable inputs in which there is little or no market data, which require the Company to develop its own assumptions

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations	The following potentially dilutive securities, on an as converted basis have been excluded from the computation of diluted weighted-average shares outstanding as of June 30, 2019 and 2018 , as they would be antidilutive: As of June 30, 2019 2018 Series A redeemable convertible preferred stock — 7,184,588 Series B redeemable convertible preferred stock — 10,090,711 Stock options outstanding 3,343,357 1,167,172 Employee stock purchase plan 7,408 — Unvested shares of restricted stock 3,320 92,602 Common stock warrants 203,676 203,678 Total 3,557,761 18,738,751
Schedule of Cash and Cash Equivalents	The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the unaudited consolidated balance sheets to the total of the same such amounts shown in the unaudited consolidated statements of cash flows for the six months ended June 30, 2019 and 2018 . As of June 30, 2019 2018 Cash and cash equivalents $ 40,980 $ 109,542 Restricted cash 290 293 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,270 $ 109,835
Schedule of Restricted Cash and Cash Equivalents	The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the unaudited consolidated balance sheets to the total of the same such amounts shown in the unaudited consolidated statements of cash flows for the six months ended June 30, 2019 and 2018 . As of June 30, 2019 2018 Cash and cash equivalents $ 40,980 $ 109,542 Restricted cash 290 293 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,270 $ 109,835

Investments and Fair Value Me_2

Investments and Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value And Debt Securities, Available-For-Sale [Abstract]
Summary of Amortized Cost and Fair Values of Investment Securities Available-for-sale	As of June 30, 2019 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 123,410 $ 205 $ (4 ) $ 123,611 $ 112,490 $ 11,121 Agency bonds 6,450 10 — 6,460 6,460 — Commercial paper 7,447 1 — 7,448 7,448 — Asset-backed securities 2,248 — — 2,248 2,248 — $ 139,555 $ 216 $ (4 ) $ 139,767 $ 128,646 $ 11,121 As of December 31, 2018 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 58,029 $ 1 $ (94 ) $ 57,936 $ 57,936 $ — Agency bonds 9,966 — (9 ) 9,957 9,957 — Commercial paper 7,214 — (4 ) 7,210 7,210 — Asset-backed securities 390 — — 390 390 — $ 75,599 $ 1 $ (107 ) $ 75,493 $ 75,493 $ —
Summary of Maturities	The amortized cost and estimated fair value of marketable securities, by contractual maturity: June 30, 2019 Amortized Cost Fair Value Due within one year $ 128,455 $ 128,646 Due after one year through five years 11,100 11,121 $ 139,555 $ 139,767 December 31, 2018 Amortized Cost Fair Value Due within one year $ 75,599 $ 75,493 Due after one year through five years — — $ 75,599 $ 75,493
Summary of Classified Assets Measured at Fair Value on a Recurred Basis	As of June 30, 2019 , the Company has classified assets measured at fair value on a recurring basis as follows: Fair Value Measurement Based On Amortized Cost Fair Value Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs Significant Unobservable Inputs Cash equivalents (1) $ 39,685 $ 39,685 $ 39,685 $ — $ — Corporate bonds 123,410 123,611 — 123,611 — Agency bonds 6,450 6,460 — 6,460 — Commercial paper 7,447 7,448 — 7,448 — Asset-backed securities 2,248 2,248 — 2,248 — $ 179,240 $ 179,452 $ 39,685 $ 139,767 $ — During the six months ended June 30, 2019 , there were no transfers among the Level 1, Level 2 and Level 3 categories. As of December 31, 2018 , the Company has classified assets measured at fair value on a recurring basis as follows: Fair Value Measurement Based On Amortized Cost Fair Value Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs Significant Unobservable Inputs Cash equivalents (1) $ 45,974 $ 45,974 $ 45,108 $ 866 $ — Corporate bonds 58,029 57,936 — 57,936 — Agency bonds 9,966 9,957 — 9,957 — Commercial paper 7,214 7,210 — 7,210 — Asset-backed securities 390 390 — 390 — $ 121,573 $ 121,467 $ 45,108 $ 76,359 $ — During the twelve months ended December 31, 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories. (1) Includes cash sweep accounts, U.S. Treasury money market mutual fund, bank certificates of deposit, U.S. Treasury bills and corporate bonds that have a maturity of three months or less from the original acquisition date.

Property and Equipment (Tables)

Property and Equipment (Tables)	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment, net, consisted of: June 30, December 31, Laboratory equipment $ 3,198 $ 2,118 Computer hardware and equipment 105 105 Furniture and fixtures 326 326 Leasehold improvements 84 34 Laboratory equipment not placed in service 704 — 4,417 2,583 Less: Accumulated depreciation and amortization (1,245 ) (945 ) $ 3,172 $ 1,638

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	6 Months Ended
	Jun. 30, 2019
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of: June 30, December 31, Employee compensation and related benefits $ 1,088 $ 1,676 Professional fees 276 342 Contract manufacturing organization fees 361 173 Contract research organization fees 459 232 University partnerships 315 162 Property received not yet invoiced 415 103 Other 202 114 $ 3,116 $ 2,802

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Rental Payments under Non-cancellable Operating Leases	The following table presents future minimum rent payments under non-cancellable operating leases with initial terms in excess of one year at June 30, 2019 : Minimum Rent Payments Remainder of 2019 $ 1,340 2020 2,735 2021 2,447 2022 2,008 2023 2,002 Thereafter 3,116 Total minimum payments required $ 13,648

Stock-based Compensation (Table

Stock-based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-based Compensation Expense	The Company recorded stock-based compensation expense in the following expense categories of its accompanying unaudited consolidated statements of operations for the three and six months ended June 30, 2019 and 2018 : Three Months Ended June 30, Six Months Ended 2019 2018 2019 2018 Research and development 616 76 914 132 General and administrative 828 274 1,671 501 $ 1,444 $ 350 $ 2,585 $ 633
Schedule of Stock Option Activity	The following table summarizes the activity related to stock option grants to employees and non-employees for the six months ended June 30, 2019 : SHARES WEIGHTED WEIGHTED (IN YEARS) Balance at January 1, 2019 2,094,815 $ 3.78 9.1 Granted 1,348,290 16.08 Exercised (24,845 ) 0.74 Forfeited (74,903 ) 3.77 Outstanding at June 30, 2019 3,343,357 $ 8.77 9.2 Exercisable at June 30, 2019 490,574 Vested and expected to vest at June 30, 2019 3,343,357
Schedule of Weighted-Average Assumptions	For the six months ended June 30, 2019 and 2018, the grant date fair value of all option grants was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted-average assumptions: Six Months Ended June 30, 2019 2018 Risk free interest rate 2.33 % 2.8 % Expected term (in years) 5.9 6.1 Expected volatility 70.86 % 65.0 % Annual dividend yield — % — % Fair value of common stock $ 10.27 $ 3.67

Organization and Description _2

Organization and Description of Business - Narrative (Details) $ / shares in Units, $ in Thousands	Feb. 14, 2019USD ($)$ / sharesshares	Feb. 01, 2019	Jun. 30, 2019USD ($)shares	Mar. 31, 2019USD ($)shares	Jun. 30, 2019USD ($)shares	Jun. 30, 2018USD ($)	Dec. 31, 2018shares
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $			$ (34)	$ 77,168
Underwriting discounts and commissions and other offering expenses | $					$ 0	$ 150
Common stock, shares authorized (shares) | shares			150,000,000		150,000,000		20,988,730
Preferred stock, shares authorized (shares) | shares			10,000,000		10,000,000		0
Reverse stock split		0.1615
Initial Public Offering (IPO)
Subsidiary, Sale of Stock [Line Items]
Shares sold during initial public offering (shares) | shares	5,750,000
Share price (usd per share) | $ / shares	$ 15
Underwriting discounts and commissions and other offering expenses | $	$ 9,116
Additional Paid-In Capital
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $			$ (34)
Additional Paid-In Capital | Initial Public Offering (IPO)
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $	$ 77,134			$ 77,168
Common Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares) | shares				17,275,299
Common Stock | Initial Public Offering (IPO)
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares) | shares	17,275,299

Liquidity - Narrative (Details)

Liquidity - Narrative (Details) $ in Thousands	Jun. 30, 2019USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash, cash equivalents and investments	$ 180,747

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations (Details) - shares	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	3,557,761	18,738,751
Series A redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	0	7,184,588
Series B redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	0	10,090,711
Stock options outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	3,343,357	1,167,172
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	7,408	0
Unvested shares of restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	3,320	92,602
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	203,676	203,678

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash and Cash Equivalents and Restricted Cash as presented in the Statements of Cash Flows (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 40,980	$ 47,674	$ 109,542
Restricted cash	290	290	293
Cash, cash equivalents, and restricted cash shown in the statements of cash flows	$ 41,270	$ 47,964	$ 109,835	$ 20,101

Investments and Fair Value Me_3

Investments and Fair Value Measurements - Summary of Amortized Cost and Fair Values of Investment Securities Available-for-sale (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 139,555	$ 75,599
Unrealized Gains	216	1
Unrealized Losses	(4)	(107)
Fair Value	139,767	75,493
Investments	128,646	75,493
Investments, non-current	11,121	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	123,410	58,029
Unrealized Gains	205	1
Unrealized Losses	(4)	(94)
Fair Value	123,611	57,936
Investments	112,490	57,936
Investments, non-current	11,121	0
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,450	9,966
Unrealized Gains	10	0
Unrealized Losses	0	(9)
Fair Value	6,460	9,957
Investments	6,460	9,957
Investments, non-current	0	0
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,447	7,214
Unrealized Gains	1	0
Unrealized Losses	0	(4)
Fair Value	7,448	7,210
Investments	7,448	7,210
Investments, non-current	0	0
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,248	390
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	2,248	390
Investments	2,248	390
Investments, non-current	$ 0	$ 0

Investments and Fair Value Me_4

Investments and Fair Value Measurements - Summary of Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due within one year	$ 128,455	$ 75,599
Due after one year through five years	11,100	0
Amortized Cost	139,555	75,599
Fair Value
Due within one year	128,646	75,493
Due after one year through five years	11,121	0
Fair Value	$ 139,767	$ 75,493

Investments and Fair Value Me_5

Investments and Fair Value Measurements - Summary of Classified Assets Measured at Fair Value on a Recurred Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 40,980	$ 47,674	$ 109,542
Available-for-sale securities, amortized cost	139,555	75,599
Available-for-sale securities, fair value	139,767	75,493
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	39,685	45,974
Cash equivalents, fair value	39,685	45,974
Total, amortized cost	179,240	121,573
Total, fair value	179,452	121,467
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, fair value	39,685	45,108
Total, fair value	39,685	45,108
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, fair value	0	866
Total, fair value	139,767	76,359
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, fair value	0	0
Total, fair value	0	0
Corporate bonds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	123,410	58,029
Available-for-sale securities, fair value	123,611	57,936
Corporate bonds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Corporate bonds | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	123,611	57,936
Corporate bonds | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	6,450	9,966
Available-for-sale securities, fair value	6,460	9,957
Agency bonds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	6,450	9,966
Available-for-sale securities, fair value	6,460	9,957
Agency bonds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Agency bonds | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	6,460	9,957
Agency bonds | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Commercial paper | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	7,447	7,214
Available-for-sale securities, fair value	7,448	7,210
Commercial paper | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Commercial paper | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	7,448	7,210
Commercial paper | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	2,248	390
Available-for-sale securities, fair value	2,248	390
Asset-backed securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, amortized cost	2,248	390
Available-for-sale securities, fair value	2,248	390
Asset-backed securities | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	0	0
Asset-backed securities | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	2,248	390
Asset-backed securities | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	$ 0	$ 0

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 4,417		$ 4,417		$ 2,583
Less: Accumulated depreciation and amortization	(1,245)		(1,245)		(945)
Property and equipment, net	3,172		3,172		1,638
Depreciation expense	165	$ 112	300	$ 188
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,198		3,198		2,118
Computer hardware and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	105		105		105
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	326		326		326
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	84		84		34
Laboratory equipment not placed in service
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 704		$ 704		$ 0

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Employee compensation and related benefits	$ 1,088	$ 1,676
Professional fees	276	342
Contract manufacturing organization fees	361	173
Contract research organization fees	459	232
University partnerships	315	162
Property received not yet invoiced	415	103
Other	202	114
Accrued expenses and other current liabilities	$ 3,116	$ 2,802

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) £ in Thousands	Mar. 01, 2019	Dec. 31, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019GBP (£)	Jun. 30, 2019USD ($)	Mar. 31, 2018USD ($)
Other Commitments [Line Items]
Restricted cash									$ 290,000
Initial term	3 years
Termination period notice requirement		12 months
Termination period		12 months
Operating lease liability per year | £							£ 300
Rent expense			$ 694,000	$ 574,000	$ 1,325,000	$ 949,000
Royalty payments, percentage of net sales						1.00%
Royalty payments, percentage of license income						1.00%
Royalty agreement value		$ 0						$ 0
Royalties paid						$ 0
Accrued liabilities		$ 0						$ 0
Global Net Sales
Other Commitments [Line Items]
Revenue					0	0
Intellectual License Income
Other Commitments [Line Items]
Revenue					$ 0	$ 0

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Rental Payments under Non-cancellable Operating Leases (Details) $ in Thousands	Jun. 30, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Remainder of 2019	$ 1,340
2020	2,735
2021	2,447
2022	2,008
2023	2,002
Thereafter	3,116
Total minimum payments required	$ 13,648

Common Stock and Redeemable C_2

Common Stock and Redeemable Convertible Preferred Stock - Narrative (Details)	Feb. 19, 2019shares	Feb. 14, 2019shares	Feb. 13, 2019$ / sharesshares	Mar. 31, 2019shares	Jun. 30, 2019USD ($)voteshares	Feb. 01, 2019shares	Jan. 31, 2019shares	Dec. 31, 2018shares	Jun. 30, 2018shares	Mar. 31, 2018shares	Dec. 31, 2017shares
Class of Stock [Line Items]
Votes per each common stock share | vote					1
Series A redeemable convertible preferred stock
Class of Stock [Line Items]
Redeemable convertible preferred stock, shares issued (shares)			44,500,001					44,500,001
Redeemable convertible preferred stock, shares outstanding (shares)			44,500,001	0	0			44,500,001	44,500,001	44,500,001	44,500,001
Shares issued upon conversion (in shares)						0.1615	1
Minimum proceeds from initial public offering | $					$ 50,000
Preferred stock dividend rate			6.00%
Preferred stock dividend rate (in dollars per share) | $ / shares			$ 0.06
Series B redeemable convertible preferred stock
Class of Stock [Line Items]
Redeemable convertible preferred stock, shares issued (shares)			62,500,000					62,500,000
Redeemable convertible preferred stock, shares outstanding (shares)			62,500,000	0	0			62,500,000	62,500,000	60,000,000	0
Shares issued upon conversion (in shares)						0.1615	1
Minimum proceeds from initial public offering | $					$ 50,000
Preferred stock dividend rate			6.00%
Preferred stock dividend rate (in dollars per share) | $ / shares			$ 0.12
Common Stock
Class of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares)				17,275,299
Initial Public Offering (IPO) | Common Stock
Class of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares)		17,275,299
Initial Public Offering (IPO) | Common Stock | Series A redeemable convertible preferred stock
Class of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares)	7,184,588
Initial Public Offering (IPO) | Common Stock | Series B redeemable convertible preferred stock
Class of Stock [Line Items]
Redeemable convertible preferred stock converted into shares of common stock (shares)	10,090,711

Stock-based Compensation - Narr

Stock-based Compensation - Narrative (Details) - USD ($)	1 Months Ended	6 Months Ended
	Feb. 28, 2019	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		4 years
Stock options granted (in shares)		1,348,290	242,458
Stock options forfeited (in shares)		74,903	124,909
Stock options exercised (in shares)		24,845	126,044
Unrecognized compensation costs		$ 18,212,000
Unrecognized compensation cost, period of recognition		3 years 3 months 18 days
Annual dividend yield		0.00%	0.00%
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)		0	0
Restricted stock forfeited (in shares)		0	0
Unrecognized compensation cost		$ 30,000
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, period of recognition		1 year 6 months
Unrecognized compensation cost		$ 944,000
Warrants granted (in shares)		0	0
Warrants forfeited (in shares)		0	0
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock purchase offering period		6 months
Maximum stock purchase value per employee, percent of eligible compensation	15.00%
Maximum stock purchase value per employee	$ 25,000
ESPP shares issued (in shares)		0	0
2018 Stock Option and Incentive Plan | Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)		3,000,000
Shares available for issusance as a percentage of outstanding shares, maximum		4.00%
Term of options		10 years
Number of shares available for issuance (in shares)		1,726,613
Common Stock | Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock, percentage of fair market value	85.00%

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,444	$ 350	$ 2,585	$ 633
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	616	76	914	132
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 828	$ 274	$ 1,671	$ 501

Stock-based Compensation - St_2

Stock-based Compensation - Stock Option Activity (Details) - $ / shares	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, beginning balance (in shares)	2,094,815
Options granted (in shares)	1,348,290	242,458
Options exercised (in shares)	(24,845)	(126,044)
Options forfeited (in shares)	(74,903)	(124,909)
Options outstanding, ending balance (in shares)	3,343,357		2,094,815
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Options outstanding, weighted average exercise price per share, beginning balance (in dollars per share)	$ 3.78
Options granted, weighted average exercise price per share (in dollars per share)	16.08
Options exercised, weighted average exercise price per share (in dollars per share)	0.74
Options forfeited, weighted average exercise price per share (in dollars per share)	3.77
Options outstanding, weighted average exercise price per share, ending balance (in dollars per share)	$ 8.77		$ 3.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, weighted average remaining contractual life (in years)	9 years 2 months 12 days		9 years 1 month 6 days
Options exercisable (in shares)	490,574
Options vested and expected to vest (in shares)	3,343,357

Stock-based Compensation - St_3

Stock-based Compensation - Stock Options Valuation Assumptions (Details) - $ / shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate	2.33%	2.80%
Expected term (in years)	5 years 10 months 24 days	6 years 1 month 6 days
Expected volatility	70.86%	65.00%
Annual dividend yield	0.00%	0.00%
Fair value of common stock (in dollars per share)	$ 10.27	$ 3.67

Related party transactions - Na

Related party transactions - Narrative (Details) € in Thousands, $ in Thousands	Oct. 01, 2017USD ($)	May 09, 2017USD ($)	Nov. 01, 2016EUR (€)	Oct. 01, 2015EUR (€)	Sep. 30, 2015director	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)
MPM Capital
Related Party Transaction [Line Items]
Related party transaction, number of directors of related party in agreement | director					2
MPM Capital | Consulting Agreement, Intellectual Property Services
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						$ 171	$ 143
MPM Capital | Consulting Agreement, Travel-Related Expenses
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						24	20
Dr. Patrick Baeuerle | Director | Consulting Agreement, Monthly Fee
Related Party Transaction [Line Items]
Related party transaction, amount of transaction | €			€ 3	€ 15
Dr. Patrick Baeuerle | Director | Consulting Agreement
Related Party Transaction [Line Items]
Related party transaction, eligible bonus as percentage of annual fees				33.00%
Related party transaction, term of agreement				1 year
Related party transaction, term of agreement, extension option				1 year
Dr. Patrick Baeuerle | Director | Consulting Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						36	38
Dr. Mitchell Finer / Pattern Recognition Ventures | Director | Consulting Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						38	38
Dr. Mitchell Finer / Pattern Recognition Ventures | Director | Consulting Agreement, Quarterly Fee
Related Party Transaction [Line Items]
Related party transaction, amount of transaction		$ 19
Globeways Holdings Limited | Director | Consulting Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						$ 52	$ 60
Globeways Holdings Limited | Director | Consulting Agreement, Yearly Fee
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	$ 100
MPM Capital | TCR2 Therapeutics Inc.
Related Party Transaction [Line Items]
Shareholder ownership, percentage						5.00%
F2 Capital | TCR2 Therapeutics Inc.
Related Party Transaction [Line Items]
Shareholder ownership, percentage						5.00%