Tcr2 Therapeutics (TCRR) 10-Q 13 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	3 Months Ended
	Mar. 31, 2019	May 06, 2019
Document And Entity Information [Abstract]
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	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		23,940,025

Unaudited Consolidated Balance

Unaudited Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 46,405	$ 47,674
Investments	115,251	75,493
Prepaid expenses and other current assets	3,929	2,326
Total current assets	165,585	125,493
Property and equipment, net	1,936	1,638
Investments, non-current	30,048	0
Restricted cash	290	290
Deferred offering costs	0	2,012
Total assets	197,859	129,433
Liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)
Accounts payable	2,830	2,663
Accrued expenses and other current liabilities	2,527	2,802
Total current liabilities	5,357	5,465
Other liabilities	455	434
Total liabilities	5,812	5,899
Commitments and contingencies (Note 6)
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 March 31, 2019 and December 31, 2018, respectively.	0
Common stock, $0.0001 par value; 150,000,000 and 20,988,730 shares authorized at March 31, 2019 and December 31, 2018, respectively; 23,940,025 and 914,602 shares issued at March 31, 2019 and December 31, 2018, respectively; 23,792,193 and 726,994 shares outstanding at March 31, 2019 and December 31, 2018, respectively.	2
Additional paid-in capital	336,939	0
Accumulated other comprehensive income (loss)	1	(106)
Accumulated deficit	(144,895)	(85,590)
Total stockholders’ equity (deficit)	192,047	(85,696)
Total liabilities, redeemable preferred stock and stockholders’ equity (deficit)	197,859	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 Unaudited Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Preferred stock, par value (usd per share)	$ 0.0001	$ 0.0001
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)	23,940,025	914,602
Common stock, shares outstanding (shares)	23,792,193	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
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses
Research and development	$ 7,889	$ 2,893
General and administrative	2,886	1,220
Total operating expenses	10,775	4,113
Loss from operations	(10,775)	(4,113)
Interest income, net	872	127
Net loss	(9,903)	(3,986)
Accretion of redeemable convertible preferred stock to redemption value	(49,900)	(10,833)
Net loss attributable to common stockholders	$ (59,803)	$ (14,819)
Per share information
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (4.85)	$ (28.90)
Weighted average shares outstanding, basic and diluted (shares)	12,328,805	512,685

Unaudited Consolidated Statem_2

Unaudited Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (9,903)	$ (3,986)
Unrealized (loss) gain on investments	107	(17)
Comprehensive loss	$ (9,796)	$ (4,003)

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]
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
Accretion of redeemable preferred stock to redemption value	$ 10,833	$ 9,413	$ 1,420
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
Exercise of stock options and warrants	104				104
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)
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, 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	$ 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				77,168
Net loss	$ (9,903)					(9,903)
Ending balance (shares) at Mar. 31, 2019	23,792,193			23,792,193
Ending balance at Mar. 31, 2019	$ 192,047			$ 2	$ 336,939	$ (144,895)	$ 1

Unaudited Consolidated Statem_4

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

Unaudited Consolidated Statem_5

Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities:
Net loss	$ (9,903)	$ (3,986)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	135	76
Stock-based compensation expense	1,141	283
Accretion on investments	(131)	(18)
Changes in operating assets and liabilities:
Interest receivable on investments	(157)	(51)
Prepaid expenses and other current assets	(1,207)	583
Accounts payable	(401)	221
Accrued expenses and other liabilities	(310)	(120)
Cash used in operating activities	(10,833)	(3,012)
Investing activities:
Purchase of investments	(86,626)	(13,369)
Proceeds from maturity of investments	16,819	0
Purchases of equipment	(188)	(368)
Cash used in investing activities	(69,995)	(13,737)
Financing activities:
Proceeds from the sale of Series B preferred stock		120,000
Proceeds from initial public offering, net of issuance costs	80,213	0
Proceeds from the exercise of stock options	0	219
Deferred offering costs	(654)	(3)
Payment of issuance costs	0	(150)
Cash provided by financing activities	79,559	120,066
Net change in cash, cash equivalents, and restricted cash	(1,269)	103,317
Cash, cash equivalents, and restricted cash at beginning of year	47,964	20,101
Cash, cash equivalents, and restricted cash at end of period	46,695	123,418
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	10,833
Deferred Offering Costs Incurred But Not Yet Paid	392	0
Property and equipment additions in accounts payable	245	253
Reclassification of early exercise liability upon vesting of options	13	13
Series B redeemable convertible preferred stock
Financing activities:
Proceeds from the sale of Series B preferred stock	0
Supplemental disclosure of noncash investing and financing activities:
Accretion of redeemable preferred stock to redemption value	$ 15,111	$ 1,420

Organization and Description of

Organization and Description of Business	3 Months Ended
	Mar. 31, 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 $77,168 , after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $9,082 . 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	3 Months Ended
	Mar. 31, 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 March 31, 2019 of $191,704

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 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 institution is financially sound, and accordingly, minimal credit risk exists with respect to the financial institution. As of March 31, 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 March 31, 2019 and December 31, 2018 , the Company’s financial instruments consist of money market funds, commercial paper, 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 March 31, 2019 and December 31, 2018 , cash equivalents consisted of U.S treasuries, corporate bonds and government-backed money market funds. Investments As of March 31, 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 The Company has classified redeemable convertible preferred stock 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 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. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of its common stock and updated assumption inputs in the Black-Scholes option-pricing model for options or the then current fair value of its common stock for restricted stock. Exercised but unvested stock-based awards are subject to repurchase 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 March 31, 2019 and 2018 , as they would be antidilutive: Three Months Ended March 31, 2019 2018 Series A redeemable convertible preferred stock — 7,184,588 Series B redeemable convertible preferred stock — 9,687,106 Stock options 2,072,650 1,077,519 Unvested shares of restricted stock 25,639 114,924 Common stock warrants 203,676 203,678 Total 2,301,965 18,267,815 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 three months ended March 31, 2019 and 2018 . March 31, 2019 2018 Cash and cash equivalents $ 46,405 $ 123,128 Restricted cash 290 290 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 46,695 $ 123,418 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 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. The Company is currently evaluating the effect that this guidance will have on its consolidated financial statements and related disclosures. 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

Investments and Fair Value Meas

Investments and Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
Fair Value And Debt Securities, Available-For-Sale [Abstract]
Investments and Fair Value Measurements	Investments and Fair Value Measurements As of March 31, 2019 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 122,158 $ 36 $ (44 ) $ 122,150 $ 92,102 $ 30,048 Agency bonds 9,984 8 — 9,992 9,992 — Commercial paper 10,912 1 — 10,913 10,913 — Asset-backed securities 2,244 — — 2,244 2,244 — $ 145,298 $ 45 $ (44 ) $ 145,299 $ 115,251 $ 30,048 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: March 31, 2019 Amortized Cost Fair Value Due within one year $ 115,242 $ 115,251 Due after one year through five years 30,056 30,048 $ 145,298 $ 145,299 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 March 31, 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 Assets Cash equivalents (1) $ 7,959 $ 7,959 $ 3,715 $ 4,244 $ — Corporate bonds 122,158 122,150 — 122,150 — Agency bonds 9,984 9,992 — 9,992 — Commercial paper 10,912 10,913 — 10,913 — Asset-backed securities 2,244 2,244 — 2,244 — $ 153,257 $ 153,258 $ 3,715 $ 149,543 $ — During the three months ended March 31, 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 Assets 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 three months ended March 31, 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories. (1)

Property and Equipment

Property and Equipment	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net, consisted of: March 31, 2019 December 31, 2018 Laboratory equipment 2,398 2,118 Computer hardware and equipment 105 105 Furniture and fixtures 326 326 Leasehold improvements 84 34 Construction in-process 104 — 3,017 2,583 Less: Accumulated depreciation and amortization (1,081 ) (945 ) $ 1,936 $ 1,638 Depreciation expense was $135 and $76 for the three months ended March 31,

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 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: March 31, 2019 December 31, 2018 Employee compensation and related benefits $ 625 $ 1,676 Professional fees 576 342 Contract manufacturing organization fees 315 173 Contract research organization fees 265 232 University partnerships 257 162 Other 489 217 $ 2,527 $ 2,802

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 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 £300,000 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 March 31, 2019 : Minimum Rent Payments Remainder of 2019 $ 2,017 2020 2,745 2021 2,457 2022 2,009 2023 2,002 Thereafter 3,116 Total minimum payments required $ 14,346 Rent expense was $631 and $375 for the three months ended March 31, 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 7), 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 March 31, 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 three months ended March 31, 2019 or 2018 no r has the Company accrued any liability as of March 31, 2019 or December 31, 2018

Common Stock and Redeemable Con

Common Stock and Redeemable Convertible Preferred Stock	3 Months Ended
	Mar. 31, 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 March 31, 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 March 31, 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 March 31, 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.

Stock-based Compensation

Stock-based Compensation	3 Months Ended
	Mar. 31, 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 March 31, 2019 , there were 3,045,491 shares of common stock available for future issuance. 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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Research and development $ 298 $ 56 General and administrative 843 227 $ 1,141 $ 283 Stock options During the three months ended March 31, 2018, there were no grants of stock options, 12,731 options forfeited, and 122,171 options exercised, respectively. The following table summarizes the activity related to stock option grants to employees and non-employees for the three months ended March 31, 2019 : SHARES WEIGHTED WEIGHTED (IN YEARS) Balance at January 1, 2019 2,121,551 $ 3.78 9.1 Granted — — Exercised (3,464 ) 0.74 Forfeited (45,438 ) 0.74 Outstanding at March 31, 2019 2,072,649 $ 3.43 8.9 Exercisable at March 31, 2019 562,900 Vested and expected to vest at March 31, 2019 2,072,649 As of March 31, 2019 , there was $6,839 in unrecognized compensation cost that is expected to be recognized over an estimated weighted-average amortization period of 3.0 years . Restricted stock During the three months ended March 31, 2018, the Company granted no restricted stock and there were no forfeitures. During the three months ended March 31, 2019, the Company granted no restricted stock and there were no forfeitures. As of March 31, 2019 , there was $400 in unrecognized compensation cost that is expected to be recognized by December 31, 2019. Warrants During the three months ended March 31, 2019 and 2018, the Company granted no warrants and there were no forfeitures. As of March 31, 2019 , there was $1,800 in unrecognized compensation cost that is expected to be recognized over an estimated weighted-average amortization period of 1.7 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 three months ended March 31, 2019 and 2018, there were no

Related party transactions

Related party transactions	3 Months Ended
	Mar. 31, 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 three months ended March 31, 2019 and 2018 , the Company incurred fees and travel related expenses to Dr. Baeuerle in the amount of $18 and $19 , 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 three months ended March 31, 2019 and 2018 , the Company incurred fees and travel-related expenses to Pattern Recognition Ventures in the amount of $19 and $19 , 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. 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 three months ended March 31, 2019 and 2018 , the Company incurred fees and travel-related expenses to Globeways Holdings Limited in the amount of $30 for each period. Dr. Jovan is also a member of the Company's Board of Directors and 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 majority investor in the Company is MPM Capital (MPM). In September 2015, the Company began receiving consulting and management services pursuant to agreements with three Managing Directors at MPM. For the three months ended March 31, 2019 and 2018 , the Company incurred approximately $82 and $102

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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 March 31, 2019
Fair value of financial instruments	At March 31, 2019 and December 31, 2018 , the Company’s financial instruments consist of money market funds, commercial paper, agency and corporate bonds and are included in investments. The
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 March 31, 2019 and December 31, 2018
Investments	As of March 31, 2019
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	The Company has classified redeemable convertible preferred stock 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 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. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of its common stock and updated assumption inputs in the Black-Scholes option-pricing model for options or the then current fair value of its common stock for restricted stock. Exercised but unvested stock-based awards are subject to repurchase 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.
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.
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 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. The Company is currently evaluating the effect that this guidance will have on its consolidated financial statements and related disclosures. 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
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

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2019 and 2018 , as they would be antidilutive: Three Months Ended March 31, 2019 2018 Series A redeemable convertible preferred stock — 7,184,588 Series B redeemable convertible preferred stock — 9,687,106 Stock options 2,072,650 1,077,519 Unvested shares of restricted stock 25,639 114,924 Common stock warrants 203,676 203,678 Total 2,301,965 18,267,815
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 three months ended March 31, 2019 and 2018 . March 31, 2019 2018 Cash and cash equivalents $ 46,405 $ 123,128 Restricted cash 290 290 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 46,695 $ 123,418
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 three months ended March 31, 2019 and 2018 . March 31, 2019 2018 Cash and cash equivalents $ 46,405 $ 123,128 Restricted cash 290 290 Cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 46,695 $ 123,418

Investments and Fair Value Me_2

Investments and Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2019 , investments were comprised of the following: Amortized Cost Unrealized Gains Unrealized Losses Fair Value Investments Investments, non-current Corporate bonds $ 122,158 $ 36 $ (44 ) $ 122,150 $ 92,102 $ 30,048 Agency bonds 9,984 8 — 9,992 9,992 — Commercial paper 10,912 1 — 10,913 10,913 — Asset-backed securities 2,244 — — 2,244 2,244 — $ 145,298 $ 45 $ (44 ) $ 145,299 $ 115,251 $ 30,048 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: March 31, 2019 Amortized Cost Fair Value Due within one year $ 115,242 $ 115,251 Due after one year through five years 30,056 30,048 $ 145,298 $ 145,299 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 March 31, 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 Assets Cash equivalents (1) $ 7,959 $ 7,959 $ 3,715 $ 4,244 $ — Corporate bonds 122,158 122,150 — 122,150 — Agency bonds 9,984 9,992 — 9,992 — Commercial paper 10,912 10,913 — 10,913 — Asset-backed securities 2,244 2,244 — 2,244 — $ 153,257 $ 153,258 $ 3,715 $ 149,543 $ — During the three months ended March 31, 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 Assets 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 three months ended March 31, 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories. (1)

Property and Equipment (Tables)

Property and Equipment (Tables)	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment, net, consisted of: March 31, 2019 December 31, 2018 Laboratory equipment 2,398 2,118 Computer hardware and equipment 105 105 Furniture and fixtures 326 326 Leasehold improvements 84 34 Construction in-process 104 — 3,017 2,583 Less: Accumulated depreciation and amortization (1,081 ) (945 ) $ 1,936 $ 1,638

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2019
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of: March 31, 2019 December 31, 2018 Employee compensation and related benefits $ 625 $ 1,676 Professional fees 576 342 Contract manufacturing organization fees 315 173 Contract research organization fees 265 232 University partnerships 257 162 Other 489 217 $ 2,527 $ 2,802

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2019 : Minimum Rent Payments Remainder of 2019 $ 2,017 2020 2,745 2021 2,457 2022 2,009 2023 2,002 Thereafter 3,116 Total minimum payments required $ 14,346

Stock-based Compensation (Table

Stock-based Compensation (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 2018 Research and development $ 298 $ 56 General and administrative 843 227 $ 1,141 $ 283
Share-based Compensation, Stock Options, Activity [Table Text Block]	The following table summarizes the activity related to stock option grants to employees and non-employees for the three months ended March 31, 2019 : SHARES WEIGHTED WEIGHTED (IN YEARS) Balance at January 1, 2019 2,121,551 $ 3.78 9.1 Granted — — Exercised (3,464 ) 0.74 Forfeited (45,438 ) 0.74 Outstanding at March 31, 2019 2,072,649 $ 3.43 8.9 Exercisable at March 31, 2019 562,900 Vested and expected to vest at March 31, 2019 2,072,649

Organization and Description _2

Organization and Description of Business - Narrative (Details) $ / shares in Units, $ in Thousands	Feb. 14, 2019USD ($)$ / sharesshares	Feb. 01, 2019	Mar. 31, 2019USD ($)shares	Mar. 31, 2018USD ($)	Dec. 31, 2018shares
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $			$ 77,168
Underwriting discounts and commissions and other offering expenses | $			$ 0	$ 150
Common stock, shares authorized (shares) | shares			150,000,000		20,988,730
Preferred stock, shares authorized (shares) | shares			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 | $	$ 9,082
Additional Paid-In Capital
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $			$ 77,168
Additional Paid-In Capital | Initial Public Offering (IPO)
Subsidiary, Sale of Stock [Line Items]
Initial public offering, net of issuance costs | $			$ 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	Mar. 31, 2019USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash, cash equivalents and investments	$ 191,704

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
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	2,301,965	18,267,815
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	9,687,106
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	2,072,650	1,077,519
Unvested shares of restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	25,639	114,924
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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 46,405	$ 47,674	$ 123,128
Restricted cash	290	290	290
Cash, cash equivalents, and restricted cash shown in the statements of cash flows	$ 46,695	$ 47,964	$ 123,418	$ 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	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 145,298	$ 75,599
Unrealized Gains	45	1
Unrealized Losses	(44)	(107)
Fair Value	145,299	75,493
Investments	115,251	75,493
Investments, non-current	30,048	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	122,158	58,029
Unrealized Gains	36	1
Unrealized Losses	(44)	(94)
Fair Value	122,150	57,936
Investments	92,102	57,936
Investments, non-current	30,048	0
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,984	9,966
Unrealized Gains	8	0
Unrealized Losses	0	(9)
Fair Value	9,992	9,957
Investments	9,992	9,957
Investments, non-current	0	0
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,912	7,214
Unrealized Gains	1	0
Unrealized Losses	0	(4)
Fair Value	10,913	7,210
Investments	10,913	7,210
Investments, non-current	0	0
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,244	390
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	2,244	390
Investments	2,244	390
Investments, non-current	$ 0	$ 0

Investments and Fair Value Me_4

Investments and Fair Value Measurements - Summary of Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Due within one year	$ 115,242	$ 75,599
Due after one year through five years	30,056	0
Amortized Cost	145,298	75,599
Fair Value
Due within one year	115,251	75,493
Due after one year through five years	30,048	0
Fair Value	$ 145,299	$ 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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 46,405	$ 47,674	$ 123,128
Available-for-sale securities, amortized cost	145,298	75,599
Available-for-sale securities, fair value	145,299	75,493
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	7,959	45,974
Cash equivalents, fair value	7,959	45,974
Total, amortized cost	153,257	121,573
Total, fair value	153,258	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	3,715	45,108
Total, fair value	3,715	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	4,244	866
Total, fair value	149,543	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	122,158	58,029
Available-for-sale securities, fair value	122,150	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	122,150	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	9,984	9,966
Available-for-sale securities, fair value	9,992	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	9,984	9,966
Available-for-sale securities, fair value	9,992	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	9,992	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	10,912	7,214
Available-for-sale securities, fair value	10,913	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	10,913	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	2,244	390
Available-for-sale securities, fair value	2,244	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	2,244	390
Available-for-sale securities, fair value	2,244	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	2,244	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
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,017		$ 2,583
Less: Accumulated depreciation and amortization	(1,081)		(945)
Property and equipment, net	1,936		1,638
Depreciation expense	135	$ 76
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,398		2,118
Computer hardware and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	105		105
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	326		326
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	84		34
Construction in-process
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 104		$ 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	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Employee compensation and related benefits	$ 625	$ 1,676
Professional fees	576	342
Contract manufacturing organization fees	315	173
Contract research organization fees	265	232
University partnerships	257	162
Other	489	217
Accrued expenses and other current liabilities	$ 2,527	$ 2,802

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) £ in Thousands	Mar. 01, 2019	Dec. 31, 2018USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2019GBP (£)	Mar. 31, 2019USD ($)
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 | £					£ 300,000
Rent expense			$ 631,000
Rent expense				$ 375,000
Royalty payments, percentage of net sales				1.00%
Royalty payments, percentage of license income				1.00%
Royalty agreement value		$ 0				$ 0
Royalties paid			0	$ 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	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Remainder of 2019	$ 2,017
2020	2,745
2021	2,457
2022	2,009
2023	2,002
Thereafter	3,116
Total minimum payments required	$ 14,346

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, 2019USD ($)voteshares	Feb. 01, 2019shares	Jan. 31, 2019shares	Dec. 31, 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)				0			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)				0			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	3 Months Ended
	Feb. 28, 2019	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		4 years
Stock options granted (in shares)		0	0
Stock options forfeited (in shares)		45,438	12,731
Stock options exercised (in shares)		3,464	122,171
Unrecognized compensation costs		$ 6,839,000
Unrecognized compensation cost, period of recognition		3 years
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		$ 400,000
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, period of recognition		1 year 8 months 12 days
Unrecognized compensation cost		$ 1,800,000
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)		3,045,491
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
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,141	$ 283
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	298	56
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 843	$ 227

Stock-based Compensation - St_2

Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, beginning balance (in shares)	2,121,551
Options granted (in shares)	0	0
Options exercised (in shares)	(3,464)	(122,171)
Options forfeited (in shares)	(45,438)	(12,731)
Options outstanding, ending balance (in shares)	2,072,649		2,121,551
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)	0
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)	0.74
Options outstanding, weighted average exercise price per share, ending balance (in dollars per share)	$ 3.43		$ 3.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, weighted average remaining contractual life (in years)	8 years 10 months 24 days		9 years 1 month 6 days
Options exercisable (in shares)	562,900
Options vested and expected to vest (in shares)	2,072,649

Related party transactions - Na

Related party transactions - Narrative (Details) € in Thousands	Oct. 01, 2017USD ($)	May 09, 2017USD ($)	Nov. 01, 2016EUR (€)	Oct. 01, 2015EUR (€)	Sep. 30, 2015director	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)
MPM Capital
Related Party Transaction [Line Items]
Related party transaction, number of directors of related party in agreement | director					3
MPM Capital | Consulting And Management Services Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						$ 82,000	$ 102,000
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						18,000	19,000
Dr. Mitchell Finer / Pattern Recognition Ventures | Director | Consulting Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						19,000	19,000
Dr. Mitchell Finer / Pattern Recognition Ventures | Director | Consulting Agreement, Quarterly Fee
Related Party Transaction [Line Items]
Related party transaction, amount of transaction		$ 19,000
Globeways Holdings Limited | Director | Consulting Agreement, Fees
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions						$ 30,000	$ 30,000
Globeways Holdings Limited | Director | Consulting Agreement, Yearly Fee
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	$ 100,000
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%