Neon Therapeutics 10-Q 6 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document and Entity Information
Entity Registrant Name	Neon Therapeutics, Inc.
Entity Central Index Key	0001694187
Document Type	10-Q
Document Period End Date	Jun. 30, 2019
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		28,381,094
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 46,285	$ 52,700
Marketable securities	14,714	50,611
Prepaid expenses and other current assets	1,998	2,116
Total current assets	62,997	105,427
Operating lease, right-of-use assets	8,148
Property and equipment, net	8,046	8,205
Other long-term assets	491	456
Total assets	79,682	114,088
Current liabilities:
Accounts payable	2,751	4,268
Accrued expenses	6,371	8,422
Operating lease liabilities, current	1,153
Total current liabilities	10,275	12,690
Operating lease liabilities, net of current portion	7,185
Other liabilities	16	149
Total liabilities	17,476	12,839
Commitments and contingencies (Note 8)
Stockholders’ equity:
Common stock, $0.001 par value; 150,000,000 shares authorized as of June 30, 2019 and December 31, 2018; 28,356,344 and 28,314,274 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	28	28
Additional paid-in capital	278,896	275,058
Accumulated other comprehensive loss	0	(75)
Accumulated deficit	(216,718)	(173,762)
Total stockholders’ equity	62,206	101,249
Total liabilities and stockholders’ equity	$ 79,682	$ 114,088

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares issued (in shares)	28,356,344	28,314,274
Common stock, shares outstanding (in shares)	28,356,344	28,314,274

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 16,735	$ 14,804	$ 32,907	$ 27,962
General and administrative	5,580	4,313	10,988	7,912
Total operating expenses	22,315	19,117	43,895	35,874
Loss from operations	(22,315)	(19,117)	(43,895)	(35,874)
Other income (expense), net
Interest income	418	218	974	464
Other expense	(35)	0	(35)	(10)
Total other income, net	383	218	939	454
Net loss	(21,932)	(18,899)	(42,956)	(35,420)
Accretion of redeemable convertible preferred stock to redemption value	0	(3,185)	0	(6,371)
Net loss attributable to common stockholders	$ (21,932)	$ (22,084)	$ (42,956)	$ (41,791)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.79)	$ (7.84)	$ (1.55)	$ (17.05)
Weighted average common shares outstanding, basic and diluted (in shares)	27,787,184	2,815,708	27,719,501	2,450,380
Comprehensive loss:
Net loss	$ (21,932)	$ (18,899)	$ (42,956)	$ (35,420)
Other comprehensive income:
Unrealized gains on marketable securities	8	6	75	11
Total other comprehensive income	8	6	75	11
Comprehensive loss	$ (21,924)	$ (18,893)	$ (42,881)	$ (35,409)

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONTINGENTLY REDEEMABLE RESTRICTED COMMON STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Redeemable Convertible Preferred Stock	Contingently Redeemable Restricted Common Stock
Balance at the beginning of the period (in shares) at Dec. 31, 2017						93,222,418
Balance at the beginning of the period at Dec. 31, 2017						$ 174,895	$ 355
Changes in temporary equity
Stock-based compensation expense							99
Accretion of redeemable convertible preferred stock to redemption value						$ 3,186
Balance at the end of the period (in shares) at Mar. 31, 2018						93,222,418
Balance at the end of the period at Mar. 31, 2018						$ 178,081	454
Balance at the beginning of the period (in shares) at Dec. 31, 2017		3,302,927
Balance at the beginning of the period at Dec. 31, 2017	$ (93,572)	$ 3	$ 0	$ (13)	$ (93,562)
Changes in equity
Stock-based compensation expense	1,551		1,551
Accretion of redeemable convertible preferred stock to redemption value	(3,186)		(1,557)		(1,629)
Vesting of restricted common stock	6		6
Unrealized gains on marketable securities	5			5
Net loss	(16,520)				(16,520)
Balance at the end of the period (in shares) at Mar. 31, 2018		3,302,927
Balance at the end of the period at Mar. 31, 2018	(111,716)	$ 3	0	(8)	(111,711)
Balance at the beginning of the period (in shares) at Dec. 31, 2017						93,222,418
Balance at the beginning of the period at Dec. 31, 2017						$ 174,895	355
Balance at the end of the period (in shares) at Jun. 30, 2018						0
Balance at the end of the period at Jun. 30, 2018						$ 0	0
Balance at the beginning of the period (in shares) at Dec. 31, 2017		3,302,927
Balance at the beginning of the period at Dec. 31, 2017	(93,572)	$ 3	0	(13)	(93,562)
Changes in equity
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock	181,831
Unrealized gains on marketable securities	11
Net loss	(35,420)
Balance at the end of the period (in shares) at Jun. 30, 2018		28,279,208
Balance at the end of the period at Jun. 30, 2018	139,248	$ 28	271,470	(2)	(132,248)
Balance at the beginning of the period (in shares) at Mar. 31, 2018						93,222,418
Balance at the beginning of the period at Mar. 31, 2018						$ 178,081	454
Changes in temporary equity
Stock-based compensation expense							111
Accretion of redeemable convertible preferred stock to redemption value						$ 3,185
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock (in shares)						(93,222,418)
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock						$ (181,266)	(565)
Balance at the end of the period (in shares) at Jun. 30, 2018						0
Balance at the end of the period at Jun. 30, 2018						$ 0	0
Balance at the beginning of the period (in shares) at Mar. 31, 2018		3,302,927
Balance at the beginning of the period at Mar. 31, 2018	(111,716)	$ 3	0	(8)	(111,711)
Changes in equity
Stock-based compensation expense	1,311		1,311
Exercise of stock options (in shares)		84,444
Exercise of stock options	228		228
Accretion of redeemable convertible preferred stock to redemption value	(3,186)		(1,548)		(1,638)
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock (in shares)		18,644,462
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock	181,831	$ 19	181,812
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs (in shares)		6,250,000
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	89,667	$ 6	89,661
Cancellation of restricted common stock (in shares)		(2,625)
Cancellation of restricted common stock	0
Vesting of restricted common stock	6		6
Unrealized gains on marketable securities	6			6
Net loss	(18,899)				(18,899)
Balance at the end of the period (in shares) at Jun. 30, 2018		28,279,208
Balance at the end of the period at Jun. 30, 2018	$ 139,248	$ 28	271,470	(2)	(132,248)
Balance at the beginning of the period (in shares) at Dec. 31, 2018						0
Balance at the beginning of the period at Dec. 31, 2018						$ 0	0
Balance at the end of the period (in shares) at Mar. 31, 2019						0
Balance at the end of the period at Mar. 31, 2019						$ 0	0
Balance at the beginning of the period (in shares) at Dec. 31, 2018	28,314,274	28,314,274
Balance at the beginning of the period at Dec. 31, 2018	$ 101,249	$ 28	275,058	(75)	(173,762)
Changes in equity
Stock-based compensation expense	1,729		1,729
Exercise of stock options (in shares)		17,070
Exercise of stock options	45		45
Vesting of restricted common stock	5		5
Unrealized gains on marketable securities	67			67
Net loss	(21,024)				(21,024)
Balance at the end of the period (in shares) at Mar. 31, 2019		28,331,344
Balance at the end of the period at Mar. 31, 2019	$ 82,071	$ 28	276,837	(8)	(194,786)
Balance at the beginning of the period (in shares) at Dec. 31, 2018						0
Balance at the beginning of the period at Dec. 31, 2018						$ 0	0
Balance at the end of the period (in shares) at Jun. 30, 2019						0
Balance at the end of the period at Jun. 30, 2019						$ 0	0
Balance at the beginning of the period (in shares) at Dec. 31, 2018	28,314,274	28,314,274
Balance at the beginning of the period at Dec. 31, 2018	$ 101,249	$ 28	275,058	(75)	(173,762)
Changes in equity
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock	0
Unrealized gains on marketable securities	75
Net loss	$ (42,956)
Balance at the end of the period (in shares) at Jun. 30, 2019	28,356,344	28,356,344
Balance at the end of the period at Jun. 30, 2019	$ 62,206	$ 28	278,896	0	(216,718)
Balance at the beginning of the period (in shares) at Mar. 31, 2019						0
Balance at the beginning of the period at Mar. 31, 2019						$ 0	0
Balance at the end of the period (in shares) at Jun. 30, 2019						0
Balance at the end of the period at Jun. 30, 2019						$ 0	$ 0
Balance at the beginning of the period (in shares) at Mar. 31, 2019		28,331,344
Balance at the beginning of the period at Mar. 31, 2019	82,071	$ 28	276,837	(8)	(194,786)
Changes in equity
Stock-based compensation expense	2,054		2,054
Issuance of restricted common stock (in shares)		25,000
Issuance of restricted common stock	0
Vesting of restricted common stock	5		5
Unrealized gains on marketable securities	8			8
Net loss	$ (21,932)				(21,932)
Balance at the end of the period (in shares) at Jun. 30, 2019	28,356,344	28,356,344
Balance at the end of the period at Jun. 30, 2019	$ 62,206	$ 28	$ 278,896	$ 0	$ (216,718)

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (42,956)	$ (35,420)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	833	692
Non-cash lease expense	580
Net amortization of premiums and discounts on marketable securities	(8)	196
Stock-based compensation expense	3,783	3,072
Loss on disposal of property and equipment	35	10
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	118	22
Other long-term assets	(36)	125
Accounts payable	(1,262)	(1,132)
Accrued expenses and other liabilities	(1,809)	2,883
Lease liabilities	(514)
Net cash used in operating activities	(41,236)	(29,552)
Cash flows from investing activities:
Purchases of marketable securities	0	(11,990)
Sales and maturities of marketable securities	35,981	25,450
Purchases of property and equipment	(1,205)	(2,275)
Net cash provided by investing activities	34,776	11,185
Cash flows from financing activities:
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	0	93,000
Payment of initial public offering costs	0	(2,307)
Proceeds from exercise of stock options	45	228
Net cash provided by financing activities	45	90,921
Net (decrease) increase in cash, cash equivalents and restricted cash	(6,415)	72,554
Cash, cash equivalents and restricted cash, beginning of period	53,156	58,857
Cash, cash equivalents and restricted cash, end of period	46,741	131,411
Supplemental disclosure of non-cash items:
Accretion of redeemable convertible preferred stock to redemption value	0	6,371
Purchases of property and equipment included in accounts payable and accrued expenses	0	72
Deferred offering costs included in accounts payable and accrued expenses	0	1,003
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock upon closing of the initial public offering	$ 0	$ 181,831

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of the cash, cash equivalents and restricted cash balances
Cash and cash equivalents	$ 46,285	$ 130,804
Restricted cash included in other long-term assets	456	607
Total cash, cash equivalents and restricted cash	$ 46,741	$ 131,411

Nature of the Business

Nature of the Business	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business	Nature of the Business Neon Therapeutics, Inc. (the "Company") is a clinical-stage immuno-oncology company and a leader in the field of neoantigen-targeted therapies, dedicated to transforming the treatment of cancer by directing the immune system towards neoantigens. The Company is leveraging its neoantigen platform and over a decade of insights from its founders to develop neoantigen-targeted therapies that use two distinct approaches, NEON / ONE and NEON / SELECT. These approaches focus on targeting a prioritized set of what the Company believes are the most therapeutically-relevant neoantigens. In NEON / ONE, these neoantigens are specific to each individual. In NEON / SELECT, these neoantigens are shared across subsets of patients or tumor types. The Company is applying these two approaches to develop neoantigen-targeted product candidates using multiple treatment modalities. NEO-PV-01, the Company's most advanced product candidate, is a personal neoantigen vaccine that is custom-designed and manufactured based on the unique mutational fingerprint of each individual patient. The neoantigen-targeted peptides in NEO-PV-01 are intended to generate an immune response that trains each patient's immune system to target his or her individual tumor's particular neoantigens and kill the cancer cells. NEO-PV-01 is currently being evaluated in multiple Phase 1b clinical trials. In April 2019, the Company completed enrollment in NT-002, the Company’s Phase 1b clinical trial evaluating NEO-PV-01 in combination with the current standard of care, KEYTRUDA ® (pembrolizumab) and chemotherapy, in first-line patients with untreated advanced or metastatic smoking-associated non-small cell lung cancer ("NSCLC"). The Company is conducting its NT-003 trial in melanoma to evaluate NEO-PV-01 and OPDIVO ® (nivolumab) in combination with other agents, including a CD40 agonist or a CTLA-4 antagonist, to potentially further enhance NEO-PV-01-induced neoantigen immune response and improve clinical outcomes. In July 2019, the Company reported top-line results, including at least 12-month median follow-up from NT-001, the Company’s ongoing, multi-center Phase 1b clinical trial evaluating NEO-PV-01 in combination with OPDIVO in patients with advanced or metastatic melanoma, NSCLC and bladder cancer. Across all three distinct tumor types, patients demonstrated prolonged and consistent improvements in progression-free survival compared to that observed in checkpoint inhibitor monotherapy, based on historical benchmark data. NEO-PTC-01, the Company's personal neoantigen T cell therapy, consists of multiple T cell populations targeting what the Company predicts to be the most therapeutically-relevant neoantigens from each patient’s tumor. NEO-PTC-01 is currently in preclinical development, and the Company expects to file a clinical trial application in Europe in the second half of 2019 to evaluate NEO-PTC-01 in solid tumors in patients who are refractory to checkpoint inhibitors. NEON / SELECT is the Company's precision medicine approach to neoantigen-targeted therapies. The Company's first product candidate using this approach, NEO-SV-01, is a neoantigen vaccine for the treatment of a genetically defined subset of hormone-receptor-positive breast cancer, for which an Investigational New Drug application was approved by the U.S. Food and Drug Administration in August 2019. The Company is subject to risks common to early-stage companies in the biotechnology industry including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the ability to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company's development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Initial Public Offering On June 29, 2018, the Company completed an initial public offering ("IPO") of its common stock and issued and sold 6,250,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $89.9 million after deducting underwriting discounts, commissions and other offering costs. Upon the closing of the IPO in June 2018, all shares of the Company’s outstanding redeemable convertible preferred stock converted into an aggregate of 18,644,462 shares of common stock (see Note 9). In advance of the IPO, the board of directors and the stockholders of the Company approved a one-for-five reverse split of the Company’s issued and outstanding common stock that became effective on June 13, 2018. All common share and per share amounts in these condensed consolidated financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split. Liquidity In accordance with Accounting Standards Update ("ASU") No. 2014-15, Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern (Subtopic 205-40) , management must evaluate whether there are conditions or events, when considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. This evaluation initially does not take into consideration the potential mitigating effect of management’s plans that have not been fully implemented as of the date the financial statements are issued. When substantial doubt exists under this methodology, management evaluates whether the mitigating effect of its plans sufficiently alleviates substantial doubt about the company’s ability to continue as a going concern. The mitigating effect of management’s plans, however, is only considered if both (1) it is probable that the plans will be effectively implemented within one year after the date that the financial statements are issued, and (2) it is probable that the plans, when implemented, will mitigate the relevant conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Generally, to be considered probable of being effectively implemented, the plans must have been approved before the date that the financial statements are issued. The Company's financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through June 30, 2019 , the Company has funded its operations with net proceeds of $89.9 million from its IPO, as well as an aggregate of $161.1 million of net proceeds from sales of the Company’s preferred stock and convertible debt. Since inception, the Company has incurred recurring losses and negative cash flows from operations in each period and on an aggregate basis. As of June 30, 2019 and December 31, 2018 , the Company had an accumulated deficit of $216.7 million and $173.8 million , respectively. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to develop, manufacture and commercialize its products. As of June 30, 2019 , the Company had cash, cash equivalents and marketable securities of $61.0 million . The Company expects that, based on its current operating plan, its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements into June 2020. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. The Company expects that it will continue to incur significant expenses in connection with its ongoing business activities. As a result, the Company will need substantial additional funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of equity, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may be unable to raise additional funds or enter into such other agreements or arrangements when needed on favorable terms, or at all. If the Company is unable to obtain funding on a timely basis, the Company may be required to curtail, delay or discontinue one or more of its research and development programs or may be unable to expand its operations or otherwise capitalize on its business opportunities, as desired, which could materially affect the Company's business, financial condition and results of operations. The Company has determined that its cash runway of less than twelve months, along with its accumulated deficit, history of losses and future expected losses, raises substantial doubt about the Company's ability to continue as a going concern within one year from the issuance date of these interim condensed consolidated financial statements. While the Company has plans in place to mitigate this risk, which primarily consist of raising additional capital through a combination of equity or debt financings, and, depending on the availability and level of additional financings, potentially new collaborations and reducing cash expenditures, there is no guarantee that the Company will be successful in these mitigation efforts.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements and the related disclosures are unaudited and have been prepared in conformity with the accounting principles generally accepted in the United States of America ("GAAP") and include the accounts of Neon Therapeutics, Inc. and its wholly owned subsidiary, Neon Securities Corporation. All intercompany transactions and balances have been eliminated. The Company consolidates entities in which it has a controlling financial interest. Additionally, certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Accordingly, these interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2018 , and notes thereto, which are contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the Securities and Exchange Commission (the "SEC") on March 11, 2019 (the "Annual Report on Form 10-K"). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements and, in the opinion of management, reflect all normal recurring adjustments considered necessary for a fair presentation of the Company’s financial position as of June 30, 2019 , and the results of its operations for the three and six months ended June 30, 2019 and 2018 , and its cash flows for the six months ended June 30, 2019 and 2018 . The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results that may be expected for the full year or any other subsequent interim period. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2018 , and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the six months ended June 30, 2019 , except as discussed below with respect to the adoption of ASU No. 2016-02, Leases (Topic 842) ("ASU 2016-02"), as amended. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates of accounting reflected in these condensed consolidated financial statements include, but are not limited to, estimates related to accrued expenses, the valuation of common stock prior to the completion of the Company's IPO, stock-based compensation, the present value of lease liabilities and the corresponding right-of-use assets and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions. Recently Adopted Accounting Pronouncements ASU No. 2016-02, Leases In February 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-02, which requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability, as well as provide disclosures with respect to certain qualitative and quantitative information related to a company's leasing arrangements. Leases are classified as either operating or finance based on criteria similar to existing lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. The FASB subsequently issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company adopted these amendments with ASU 2016-02 (collectively, the "New Leasing Standards") effective January 1, 2019. The Company adopted the New Leasing Standards as of the effective date of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. Comparative periods in the Company's financial statements will be presented in accordance with the existing guidance under Accounting Standards Codification ("ASC") Topic 840, Leases . Upon adoption, the Company took advantage of the transition package of practical expedients permitted within ASU 2016-02, which allowed the Company not to reassess previous accounting conclusions around whether arrangements are, or contain, leases, as well as to carry forward both the historical classification of leases and the treatment of initial direct costs for existing leases. In addition, the Company also has made an accounting policy election to exclude leases with an initial term of twelve months or less from its balance sheet. Under the New Leasing Standards, the Company determines whether an arrangement is or contains a lease at the inception of the contract based on the unique facts and circumstances around identified assets, if present, and control over those identified assets. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that it will exercise that option. The Company uses the implicit rate when readily determinable and uses its estimated incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments. The incremental borrowing rate is determined using a secured borrowing rate for the same currency and term as the associated lease. Adoption of the New Leasing Standards resulted in the recognition of operating lease right-of-use assets and operating lease liabilities of approximately $8.7 million and $8.9 million , respectively, as of January 1, 2019. Upon adoption and as of June 30, 2019 , the Company did not have any finance leases. The adoption of the New Leasing Standards did not materially impact the Company's condensed consolidated statement of operations. ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"). The standard expands the scope of ASC 718 to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. Under the amended guidance, equity-classified share-based payment awards issued to nonemployees will be measured at grant date fair value. Upon transition, the entity is required to remeasure these nonemployee awards at fair value as of the adoption date. The Company adopted this standard as of the effective date of January 1, 2019. Prior to the adoption of ASU 2018-07, for share-based awards granted to nonemployees, compensation expense was recognized over the period during which services were rendered by such nonemployees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards was remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model, as applicable. After the adoption of ASU 2018-07, equity-classified share-based payment awards issued to nonemployees are measured at grant date fair value similarly to those of employees and are no longer revalued as the equity instruments vest. The new standard allows entities to use the expected term to measure nonemployee options or elect to use the contractual term as the expected term, on an award-by-award basis. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). The new standard requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. ASU 2016-13 is effective for the Company on January 1, 2020. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of this standard will have on its condensed consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"), which modifies the disclosure requirements on fair value measurements. ASU 2018-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this standard. In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract ("ASU 2018-15") , which clarifies the accounting for implementation costs in cloud computing arrangements. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of this amendment will have on its condensed consolidated financial statements.

Fair Value Measurement

Fair Value Measurement	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurement	Fair Value Measurement The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at June 30, 2019 Using: Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 46,165 $ 46,165 $ — $ — Marketable securities: Corporate debt securities 14,714 — 14,714 — $ 60,879 $ 46,165 $ 14,714 $ — Fair Value Measurements at December 31, 2018 Using: Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 53,188 $ 53,188 $ — $ — Marketable securities: Corporate debt securities 46,122 — 46,122 — Commercial paper 4,489 — 4,489 — $ 103,799 $ 53,188 $ 50,611 $ — There were no changes in valuation techniques or transfers between the fair value measurement levels during the three and six months ended June 30, 2019 or 2018 . There were no liabilities measured at fair value on a recurring basis as of June 30, 2019 or December 31, 2018 .

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2019
Marketable Securities [Abstract]
Marketable Securities	Marketable Securities Marketable securities consisted of the following at June 30, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments: Corporate debt securities $ 14,714 $ 1 $ (1 ) $ 14,714 $ 14,714 $ 1 $ (1 ) $ 14,714 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments: Corporate debt securities $ 46,197 $ — $ (75 ) $ 46,122 Commercial paper 4,489 — — 4,489 $ 50,686 $ — $ (75 ) $ 50,611 The contractual maturities of all securities held at June 30, 2019 are one year or less. As of June 30, 2019 and December 31, 2018 , the aggregate fair value of securities that were in an unrealized loss position for less than twelve months was $4.5 million and $46.1 million , respectively. The Company does not intend to sell the investments, and it is not more likely than not that the Company will be required to sell the investments, before recovery of their amortized cost bases. As a result, the Company determined that it did no t hold any securities with any other-than-temporary impairment as of June 30, 2019 . There were no sales of available-for-sale securities during the three or six months ended June 30, 2019 or 2018 . Net unrealized holding gains or losses for the period that have been included in accumulated other comprehensive loss were not material to the Company’s condensed consolidated results of operations.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): June 30, 2019 December 31, 2018 Software $ 1,181 $ 1,180 Laboratory equipment 8,928 8,230 Computer equipment 102 102 Furniture and fixtures 317 371 Leasehold improvements 674 592 Assets under construction 439 511 11,641 10,986 Less: Accumulated depreciation and amortization (3,595 ) (2,781 ) $ 8,046 $ 8,205 Depreciation and amortization expense for the three and six months ended June 30, 2019 was $0.4 million and $0.8 million , respectively, and for three and six months ended June 30, 2018 was $0.4 million and $0.7 million , respectively.

Accrued Expenses

Accrued Expenses	6 Months Ended
	Jun. 30, 2019
Accrued Liabilities, Current [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consisted of the following (in thousands): June 30, 2019 December 31, 2018 Accrued compensation costs $ 2,442 $ 3,364 Accrued professional services 807 1,262 Accrued external research and manufacturing costs 2,536 3,001 Accrued additions of property and equipment — 243 Other accrued expenses 586 552 $ 6,371 $ 8,422

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	Leases On January 21, 2016, the Company entered into an operating lease agreement for office and laboratory space at its current headquarters in Cambridge, Massachusetts. The lease commenced on September 28, 2016 and expires on September 27, 2024. The Company has the right to extend the lease for one additional five -year period at a market rental rate as determined by the landlord and agreed to by the Company. Per the terms of the lease agreement, the Company does not have any residual value guarantees. In connection with the lease agreement, the Company issued a letter of credit to the landlord for $0.5 million . The Company secured the letter of credit for the full amount of the letter with cash on deposit, which is reported as restricted cash, and which is classified within other long-term assets. The Company identified and assessed the following significant assumptions in recognizing the right-of-use asset and corresponding liability related to the lease: • Expected lease term - The expected lease term includes the contractual lease period. The lease agreement contains a renewal option, which was not included in the calculation of the right-of-use asset and lease liabilities as the renewal is not reasonably certain. • Incremental borrowing rate - As the Company’s lease does not provide a readily determinable implicit rate, nor is it available from the lessor, the Company estimated the incremental borrowing rate based on information available at the commencement date in determining the present value of lease payments. The Company used the incremental borrowing rate on January 1, 2019 for operating leases that commenced prior to that date. The Company recognized the right-of-use asset and corresponding lease liability on January 1, 2019 by calculating the present value of lease payments, discounted at 10% , the Company’s estimated incremental borrowing rate, over the 5.7 years expected remaining lease term. Amortization of the operating lease right-of-use asset for the lease was $0.3 million and $0.6 million for the three and six months ended June 30, 2019 and was included in operating expenses. The variable lease expense, which includes common area maintenance, utility charges and management fees was $0.2 million and $0.5 million for the three and six months ended June 30, 2019 . As of June 30, 2019 the remaining lease term was 5.20 years . The Company also, from time to time, enters into short-term operating lease arrangements for certain laboratory and office equipment. Leases with a term of twelve months or less are not recorded on the balance sheet and the Company recognizes lease expense for these leases on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASU 2016-02, the Company has elected to combine lease and non-lease components for all classes of underlying assets. The components of lease expense and related cash flows were as follows (in thousands): Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Lease cost Operating lease cost $ 502 $ 1,004 Variable lease cost 234 477 Short-term lease cost 105 213 Total lease cost $ 841 $ 1,694 Cash paid for amounts included in the measurement of lease liabilities $ 469 $ 938 Future lease payments for the Company's operating leases as of June 30, 2019 were as follows (in thousands): Year Ending December 31, 2019 (remaining six months) $ 953 2020 1,948 2021 2,006 2022 2,066 2023 2,128 Thereafter 1,632 Total future minimum lease payments $ 10,733 Less: interest (2,395 ) Present value of operating lease liabilities $ 8,338 Under the prior lease guidance, future minimum lease payments for the Company's operating leases as of December 31, 2018 were as follows (in thousands): Year Ending December 31 2019 $ 1,891 2020 1,948 2021 2,006 2022 2,066 2023 2,129 Thereafter 1,632 Total future minimum lease payments $ 11,672

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Significant Agreements Manufacturing Agreement In December 2015, the Company entered into a manufacturing agreement (the "Manufacturing Agreement") with an independent third party (the "vendor") whereby the vendor performs manufacturing, analytical testing and quality assurance services related to the manufacture of drug product for use in the Company's preclinical and clinical activities. The Manufacturing Agreement included the development and establishment of a manufacturing suite (a "Cell") at the vendor's facility that would be used in the manufacturing process to fill orders of peptides ordered by the Company. All amounts incurred under the Manufacturing Agreement and subsequent amendments are recognized as research and development expense as incurred. In October 2016, the Company and the vendor amended the Manufacturing Agreement (hereinafter the "2016 Manufacturing Agreement") to modify the payment due for the establishment of a second Cell and amend the fixed pricing for drug product produced by the vendor. The 2016 Manufacturing Agreement had a three -year term and was able to be terminated by the Company for convenience with six -months' notice. In July 2017, the Company and the vendor further amended the 2016 Manufacturing Agreement (hereinafter the "2017 Manufacturing Agreement"). Under the 2017 Manufacturing Agreement, the Company will reimburse the vendor for specified manufacturing costs incurred in the manufacture of the peptides, plus a fixed profit margin. The 2017 Manufacturing Agreement has a five -year term and can be terminated by the Company for convenience with three -months' notice. Other Agreements License Agreement with the Broad Institute, Inc. On November 13, 2015, the Company entered into a license agreement with the Broad Institute, Inc. (the "Broad"), a related party (see Note 12) and, in January and November 2018, the Company entered into amendments to the license agreement (as amended to date, the "Broad Agreement"). Under the Broad Agreement, the Company has been granted an exclusive worldwide license to certain intellectual property rights owned or controlled by the Broad, Dana-Farber Cancer Institute (the "DFCI") and The General Hospital Corporation d/b/a Massachusetts General Hospital ("MGH") to develop and commercialize any diagnostic, prognostic, preventative or therapeutic product for humans, including any neoantigen vaccine product. In particular, the Company has been granted both exclusive and non-exclusive licenses to a patent portfolio comprised of twelve patent families, including certain granted patents and pending patent applications in the United States and foreign jurisdictions. Pursuant to the terms of the Broad Agreement, the Company has also been granted (i) a non-exclusive license under each institution's respective interest in certain of its patent rights to exploit the licensed products in the field in the territory during the term of the license and (ii) a non-exclusive license under each institution's licensed know-how, to exploit any diagnostic, prognostic, preventative or therapeutic product in the field in the territory during the term of the license. The Company is also entitled to sub-license the rights granted to it under the Broad Agreement. In connection with the Broad Agreement, the Company has also entered into a non-exclusive software license with the Broad under which it licenses certain object and source codes for several software programs. These licenses and rights are subject to certain limitations and retained rights, including field restrictions. As consideration for the license, the Company paid the Broad a non-refundable license fee of $0.1 million . As additional consideration for the license, the Company must pay the Broad immaterial annual license maintenance fees. Additionally, the Company granted 60,000 shares of restricted common stock to each of the Broad, DFCI and MGH, which were determined to have an aggregate fair value of $0.2 million , and reimbursed the Broad $0.6 million for a portion of its past patent expenses related to the in-licensed patent rights. In June 2018, to align with institutional policies in place between the Broad, DFCI and MGH, DFCI and MGH transferred certain of the shares of restricted common stock that they had previously received to the Broad. Under the Broad Agreement, the Company agreed to reimburse the Broad for future patent expenses related to the patents covered by the license agreement. The Company could be obligated to make up to $12.6 million of developmental milestone payments to the Broad if certain development milestones are achieved over the term of the license agreement. Additionally, under the terms of the license agreement, the Company could be obligated to make up to an aggregate of $97.5 million of payments upon the achievement of specified sales milestones and to pay tiered royalties of low to mid single-digit percentages on net sales of products licensed under the agreement. The Company is required to pay the Broad a low double-digit percentage of any consideration received by the Company from a sublicensee in consideration for a sublicense. No developmental or commercial milestones have been achieved to date. The Company has the right to terminate the agreement for any reason, with or without cause. License Agreement with the Dana-Farber Cancer Institute On August 5, 2016, the Company entered into a license agreement with the DFCI to grant the Company an exclusive, royalty-free license to provide certain licensed know-how. The know-how in this agreement has particular utility in connection with the development of the licensed products referred to in the Broad Agreement. The agreement also grants a non-exclusive, royalty free right to certain clinical data being generated by the DFCI. In consideration for the licenses, the Company granted 120,000 shares of common stock to each of the Broad and the DFCI. The shares issued to the Broad were unrestricted and fully vested. The 120,000 shares issued to the DFCI contained a contingent repurchase option whereby, if the DFCI failed to achieve three specific milestones, the Company could repurchase the shares (one-third for each milestone) at the original purchase price, which is at zero cost. The Company has accounted for these awards consistent with equity awards with performance-based vesting conditions and, upon it being probable that the Company would not repurchase the award associated with a milestone, the expense associated with the equity grant would be recognized. During the six months ended June 30, 2018, the repurchase option associated with one-third of the shares expired due to the achievement of the specified criteria and the Company recognized $0.4 million of incremental stock-based compensation expense, which was reflected within research and development expenses in the accompanying condensed consolidated financial statements. Through June 30, 2019 , the repurchase option on 80,000 of these shares has expired. The Company has the right to terminate the license agreement with the DFCI for any reason, with or without cause. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters arising out of the relationship between the parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and certain executive officers and other employees that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of these indemnification obligations. The Company does not believe that the outcome of any existing claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it had not accrued any liabilities related to its obligations under these agreements in its condensed consolidated financial statements as of June 30, 2019 or December 31, 2018 . Legal Proceedings 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 its legal proceedings.

Preferred Stock and Common Stoc

Preferred Stock and Common Stock	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Preferred Stock and Common Stock	referred Stock and Common Stock Preferred Stock The Company's amended and restated certificate of incorporation authorizes the Company to issue up to 10,000,000 shares of undesignated preferred stock, $0.001 par value per share, none of which was issued or outstanding as of June 30, 2019 or December 31 2018. Upon completion of the Company’s IPO on June 29, 2018, all shares of the Company's previously issued Redeemable Convertible Preferred Stock converted into an aggregate of 18,644,462 shares of common stock. As of June 30, 2019 and December 31, 2018, there were no shares of Redeemable Convertible Preferred Stock issued or outstanding. Common Stock As of June 30, 2019 and December 31, 2018 , the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 150,000,000 shares of common stock with a par value of $0.001 per share. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the Company’s board of directors, if any. No dividends have been declared or paid during the three or six months ended June 30, 2019 or 2018 . As of June 30, 2019 and December 31, 2018 the Company has reserved for future issuance the following number of shares of common stock: June 30, 2019 December 31, 2018 Shares reserved for exercise of outstanding stock options 3,686,242 2,548,073 Shares reserved for vesting of restricted stock units 471,119 — Shares reserved for future issuance under the 2018 Stock Option and Grant Plan 241,840 760,628 Shares reserved for future issuance under the 2018 Employee Stock Purchase Plan 553,142 270,000 4,952,343 3,578,701

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2019
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation 2015 Stock Option and Grant Plan The Company’s 2015 Stock Option and Grant Plan, as amended (the "2015 Plan"), provided for the Company to grant incentive or nonqualified stock options, restricted stock awards, unrestricted stock awards or restricted stock units to employees, directors and consultants of the Company. As of June 26, 2018, the effective date of the 2018 Stock Option and Incentive Plan, and as of June 30, 2019 and December 31, 2018, no shares remained available for future issuance under the 2015 Plan. 2018 Stock Option and Incentive Plan On June 13, 2018, the Company’s stockholders approved the 2018 Stock Option and Incentive Plan (the "2018 Plan"), which became effective on June 26, 2018. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards and dividend equivalent rights to the Company’s officers, employees, directors and other key persons (including consultants). The number of shares initially reserved for issuance under the 2018 Plan was 1,215,000 shares, which was cumulatively increased on January 1, 2019 and which will be cumulatively increased each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s compensation committee. Effective January 1, 2019, 1,132,570 additional shares were automatically added to the shares authorized for issuance under the 2018 Plan and these shares were subsequently registered on a Registration Statement on Form S-8. As of the effective date of the 2018 Plan, the Company will not grant any further awards under the 2015 Plan. However, the shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Plan and the 2015 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. The terms of stock options and restricted stock awards, including vesting requirements, are determined by the board of directors or its delegates, subject to the provisions of the 2018 Plan. As of June 30, 2019 , there were 241,840 shares available for future issuance under the 2018 Plan. 2018 Employee Stock Purchase Plan On June 13, 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the "ESPP"), which became effective on June 26, 2018. A total of 270,000 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2019, and each January 1 thereafter through January 1, 2028, by the lesser of (i) 405,000 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares determined by the administrator of the Company’s ESPP. Effective January 1, 2019, 283,142 additional shares were automatically added to the shares authorized for issuance under the ESPP and these shares were subsequently registered on a Registration Statement on Form S-8. No offering periods under the 2018 ESPP had been initiated as of June 30, 2019 . Stock Options The following table summarizes changes in stock option activity during the six months ended June 30, 2019: Number of Shares Weighted Average Exercise Price Weighted Average Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2018 2,548,073 $ 7.11 8.68 $ 1,965 Granted 1,231,128 6.01 Exercised (17,070 ) 2.65 Forfeited (75,889 ) 7.29 Outstanding as of June 30, 2019 3,686,242 $ 6.76 8.60 $ 1,689 Options vested or expected to vest as of June 30, 2019 3,686,242 $ 6.76 8.60 $ 1,689 Options exercisable as of June 30, 2019 1,148,127 $ 6.34 7.76 $ 1,025 The weighted average grant-date fair value per share of stock options granted during the three and six months ended June 30, 2019 was $3.73 per share and $4.65 per share, respectively. The weighted average grant-date fair value per share of stock options granted during the three and six months ended June 30, 2018 was $9.71 per share and $8.33 per share, respectively. There were no stock options exercised during the three months ended June 30, 2019 . The aggregate intrinsic value of stock options exercised during the six months ended June 30, 2019 was $0.1 million . The aggregate intrinsic value of stock options exercised during the three and six months ended June 30, 2018 was $0.8 million . Stock Option Valuation The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Three Months Ended Six Months Ended 2019 2018 2019 2018 Expected volatility 84.92 % 103.62 % 94.77 % 102.49 % Risk-free interest rate 1.95 % 2.84 % 2.37 % 2.57 % Expected dividend yield — % — % — % — % Expected life (in years) 5.71 6.09 6.01 6.07 There were no stock option awards granted to nonemployees during the three or six months ended June 30, 2019 or 2018 . Restricted Stock Units During the six months ended June 30, 2019 , under the 2018 Plan, the Company granted restricted stock units ("RSUs"), as part of the Company's equity compensation program it provides to its employees. Pursuant to the terms of the applicable award agreements, each RSU represents the right to receive one share of the Company’s common stock and the RSUs generally vest in equal annual installments over three years, provided the employee remains continuously employed with the Company through the vesting period. Upon vesting, shares of the Company's common stock are delivered to the employee, subject to the payment of applicable withholding taxes. The fair value of RSUs is based on the market value of the Company's common stock on the date of grant. Compensation expense is recognized over the applicable service period. The following table summarizes RSU activity for the six months ended June 30, 2019 : Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested as of December 31, 2018 — $ — Granted 484,339 $ 6.25 Vested — $ — Cancelled (13,220 ) $ 6.46 Unvested as of June 30, 2019 471,119 $ 6.24 Restricted Stock Awards Restricted stock awards originally issued under the terms of the 2015 Plan allow the Company, at its discretion, to repurchase unvested shares at the initial purchase price if the employee or nonemployee terminates his or her service relationship with the Company. No restricted stock awards were issued under the 2015 Plan during the three and six months ended June 30, 2019 or June 30, 2018 . The 2018 Plan provides for the grant of restricted stock awards to the Company’s officers, employees, directors and other key persons (including consultants). During the three months ended June 30, 2019 , the Company issued restricted stock awards for 25,000 shares of common stock to certain nonemployee founders and collaborators. The shares were granted under the terms of the 2018 Plan and the respective award agreements governing these awards. These awards vest quarterly over a one -year period. The following table summarizes the Company’s restricted common stock activity since December 31, 2018 : Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 383,964 $ 1.97 Granted 25,000 $ 4.64 Vested (149,318 ) $ 1.78 Unvested restricted common stock as of June 30, 2019 259,646 $ 2.34 The aggregate fair value of restricted common stock awards that vested during the three and six months ended June 30, 2019 , based upon the fair values of the stock underlying the restricted stock awards on the applicable vesting dates, was $0.4 million and $0.8 million , respectively. The aggregate fair value of restricted common stock awards that vested during the three and six months ended June 30, 2018 , based upon the fair values of the stock underlying the restricted stock awards on the applicable vesting dates, was $0.9 million and $1.7 million , respectively. Restricted Stock Awards Issued Outside of Equity Plans From May 2015 through July 2016, the Company issued 1,510,000 shares of restricted common stock outside of the 2015 Plan to nonemployee founders and collaborators. The shares were issued under the terms of the respective restricted common stock agreements and unvested shares are subject to repurchase by the Company upon the holder’s termination of their relationship with the Company. The unvested shares of restricted common stock are subject to the Company’s right to repurchase at the original purchase price per share. The Company did no t issue any shares of restricted common stock outside of the Company's 2015 Plan and 2018 Plan during the three or six months ended June 30, 2019 and 2018 . Of the total shares of restricted common stock awarded to founders and collaborators, 300,000 shares vested immediately upon grant; 910,000 shares vest quarterly over a four -year period based on each grantee’s continued service relationship with the Company in varying advisory capacities; and 180,000 shares vest upon the achievement of specified performance milestones. Additionally, 120,000 shares were issued as fully vested awards, but are subject to a repurchase option that expires upon the achievement of specified milestones. Through June 30, 2019 , the repurchase option on 80,000 of these shares has expired (see Note 8). Of these awards, the underlying restricted common stock agreement for 180,000 shares of restricted common stock provided for a put option whereby the recipient was able to sell its vested shares back to the Company at a price per share equal to the fair value of the Company’s common stock upon both (i) the termination of the consulting agreement between the recipient and the Company for any reason and (ii) the determination by the recipient’s employer that the ownership of the restricted common stock was in violation of the employer’s conflict of interest policy. Prior to the closing of the Company’s IPO, these awards were classified in the consolidated balance sheet as contingently redeemable common stock and were presented outside of permanent equity. As of December 31, 2017 , $0.4 million was recorded in temporary equity related to these awards. Upon the closing of the Company’s IPO, this put option expired and the amount recorded in temporary equity was recorded to additional paid in capital. A summary of the changes in the Company’s unvested restricted common stock awards granted to founders and collaborators outside of the Company's 2015 Plan or 2018 Plan since December 31, 2018 is as follows: Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 350,625 $ 1.29 Vested (113,750 ) $ 1.29 Unvested restricted common stock as of June 30, 2019 236,875 $ 1.29 The aggregate fair value of restricted common stock awards issued outside of the Company's 2015 Plan or 2018 Plan that vested during the three and six months ended June 30, 2019 , based upon the fair values of the stock underlying the restricted stock awards on the applicable vesting dates, was $0.3 million and $0.6 million , respectively. The aggregate fair value of restricted common stock awards issued outside of the Company's 2015 Plan or 2018 Plan that vested during the three and six months ended June 30, 2018 , based upon the fair values of the stock underlying the restricted stock awards on the applicable vesting dates, was $0.7 million and $1.3 million , respectively. Stock-Based Compensation Expense The Company recorded stock-based compensation expense related to all stock-based awards in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands): Three Months Six Months 2019 2018 2019 2018 Research and development expenses $ 1,008 $ 947 $ 1,828 $ 2,175 General and administrative expenses 1,046 476 1,955 897 $ 2,054 $ 1,423 $ 3,783 $ 3,072 During the six months ended June 30, 2018, the Company recognized stock-based compensation expense of $0.4 million for awards with performance-based vesting conditions related to the expiration of an additional repurchase option on a portion of the unvested restricted common shares issued to DFCI (see Note 8). As of June 30, 2019 , the Company had an aggregate of $13.1 million of unrecognized stock-based compensation expense related to unvested stock option awards, excluding awards with performance-based vesting conditions, which is expected to be recognized over a weighted-average period of approximately 2.69 years . As of June 30, 2019 , the Company also had an aggregate of $0.7 million of unrecognized stock-based compensation expense related to unvested restricted common stock awards, excluding awards with performance-based vesting conditions, which is expected to be recognized over a weighted-average period of approximately 0.81 years . Additionally as of June 30, 2019 , the Company had an aggregate of $2.6 million of unrecognized stock-based compensation expense related to unvested RSUs, which is expected to be recognized over a weighted-average period of approximately 2.70 years .

Net Loss per Share

Net Loss per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share The Company excluded 496,521 shares of restricted common stock for the three and six months ended June 30, 2019 and 1,010,152 shares of restricted common stock for the three and six months ended June 30, 2018 from the calculation of basic net loss per share because these shares had not vested. The Company’s potential dilutive securities, which include stock options, unvested restricted common stock and redeemable convertible preferred stock, have been excluded from the computation of diluted net loss per share attributable to common stockholders whenever the effect of including them would be to reduce the net loss per share. In periods where there is a net loss, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Outstanding stock options 3,686,242 2,223,147 3,686,242 2,223,147 Unvested restricted stock units 471,119 — 471,119 — Unvested restricted common stock 496,521 1,010,152 496,521 1,010,152 4,653,882 3,233,299 4,653,882 3,233,299

Related Parties

Related Parties	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related Parties	Related Parties A member of the Company’s board of directors is a founding director and the current president of the Broad. In November 2015, the Company entered into the Broad Agreement with the Broad (see Note 8) and, as consideration, the Company granted 60,000 shares of restricted common stock to the Broad, which were determined to have a fair value of $0.1 million . Additionally, the Company must pay the Broad immaterial annual license maintenance fees. At the time the Company entered into the Broad Agreement, the Company reimbursed the Broad $0.6 million for a portion of past patent expenses and, under the terms of the license agreement, the Company is required to reimburse Broad for future patent expenses related to patents covered by the license agreement. The Company could be obligated to make up to $12.6 million of developmental milestone payments to the Broad if certain development milestones are achieved over the term of the license agreement. Additionally, under the terms of the license agreement, the Company could be obligated to make up to an aggregate of $97.5 million of payments upon the achievement of specified sales milestones and to pay tiered royalties of low to mid single-digit percentages on net sales of products licensed under the agreement. The Company is required to pay the Broad a low double-digit percentage of any consideration received by the Company from a sublicensee in consideration for a sublicense. No developmental or commercial milestones have been achieved to date. In August 2016, the Company entered into a license agreement with the DFCI in connection with the development of licensed products referred to in the 2015 Broad Agreement. As consideration, the Company granted 120,000 shares of restricted common stock to the Broad, which were determined to have a fair value of $0.2 million . In June 2018, to align with institutional policies in place between the Broad, the DFCI and MGH, the DFCI and MGH transferred certain of the shares of restricted common stock that they had previously received to the Broad. The Company recorded expenses related to payments to the Broad of $0.6 million and $1.1 million during the three and six months ended June 30, 2019 , respectively, and $0.3 million and $0.5 million during the three and six months ended June 30, 2018 , respectively. At June 30, 2019 and December 31, 2018 , the Company had $0.6 million and $2.0 million in accounts payable and accrued expenses due to the Broad, respectively.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On July 1, 2019, the Company filed a Registration Statement on Form S-3 (Registration No. 333-232487) with the SEC, which was declared effective on July 8, 2019, pursuant to which it registered for sale up to $200.0 million of any combination of common stock, preferred stock, warrants and/or units from time to time and at prices and on terms that the Company may determine, including up to $50.0 million of its common stock available for issuance pursuant to a Controlled Equity Offering SM Sales Agreement (the "Sales Agreement") that it entered into with Cantor Fitzgerald & Co. ("Cantor"). Under the Sales Agreement, Cantor may sell shares of the Company’s common stock by any method permitted by law deemed to be an "at the market" offering as defined in Rule 415 of the Securities Act, subject to the terms of the Sales Agreement. The Company will pay to Cantor cash commissions of 3.0 percent of the aggregate gross proceeds of sales of common stock under the Sales Agreement. No shares have been sold under this "at the market" program.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation and Consolidation	Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements and the related disclosures are unaudited and have been prepared in conformity with the accounting principles generally accepted in the United States of America ("GAAP") and include the accounts of Neon Therapeutics, Inc. and its wholly owned subsidiary, Neon Securities Corporation. All intercompany transactions and balances have been eliminated. The Company consolidates entities in which it has a controlling financial interest. Additionally, certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Accordingly, these interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2018 , and notes thereto, which are contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the Securities and Exchange Commission (the "SEC") on March 11, 2019 (the "Annual Report on Form 10-K"). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements and, in the opinion of management, reflect all normal recurring adjustments considered necessary for a fair presentation of the Company’s financial position as of June 30, 2019 , and the results of its operations for the three and six months ended June 30, 2019 and 2018 , and its cash flows for the six months ended June 30, 2019 and 2018 . The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results that may be expected for the full year or any other subsequent interim period.
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2018 , and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the six months ended June 30, 2019 , except as discussed below with respect to the adoption of ASU No. 2016-02, Leases (Topic 842) ("ASU 2016-02"), as amended.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates of accounting reflected in these condensed consolidated financial statements include, but are not limited to, estimates related to accrued expenses, the valuation of common stock prior to the completion of the Company's IPO, stock-based compensation, the present value of lease liabilities and the corresponding right-of-use assets and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions.
Recently Adopted and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements ASU No. 2016-02, Leases In February 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-02, which requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability, as well as provide disclosures with respect to certain qualitative and quantitative information related to a company's leasing arrangements. Leases are classified as either operating or finance based on criteria similar to existing lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. The FASB subsequently issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company adopted these amendments with ASU 2016-02 (collectively, the "New Leasing Standards") effective January 1, 2019. The Company adopted the New Leasing Standards as of the effective date of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. Comparative periods in the Company's financial statements will be presented in accordance with the existing guidance under Accounting Standards Codification ("ASC") Topic 840, Leases . Upon adoption, the Company took advantage of the transition package of practical expedients permitted within ASU 2016-02, which allowed the Company not to reassess previous accounting conclusions around whether arrangements are, or contain, leases, as well as to carry forward both the historical classification of leases and the treatment of initial direct costs for existing leases. In addition, the Company also has made an accounting policy election to exclude leases with an initial term of twelve months or less from its balance sheet. Under the New Leasing Standards, the Company determines whether an arrangement is or contains a lease at the inception of the contract based on the unique facts and circumstances around identified assets, if present, and control over those identified assets. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that it will exercise that option. The Company uses the implicit rate when readily determinable and uses its estimated incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments. The incremental borrowing rate is determined using a secured borrowing rate for the same currency and term as the associated lease. Adoption of the New Leasing Standards resulted in the recognition of operating lease right-of-use assets and operating lease liabilities of approximately $8.7 million and $8.9 million , respectively, as of January 1, 2019. Upon adoption and as of June 30, 2019 , the Company did not have any finance leases. The adoption of the New Leasing Standards did not materially impact the Company's condensed consolidated statement of operations. ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"). The standard expands the scope of ASC 718 to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. Under the amended guidance, equity-classified share-based payment awards issued to nonemployees will be measured at grant date fair value. Upon transition, the entity is required to remeasure these nonemployee awards at fair value as of the adoption date. The Company adopted this standard as of the effective date of January 1, 2019. Prior to the adoption of ASU 2018-07, for share-based awards granted to nonemployees, compensation expense was recognized over the period during which services were rendered by such nonemployees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards was remeasured using the then-current fair value of the Company's common shares and updated assumption inputs in the Black-Scholes option-pricing model, as applicable. After the adoption of ASU 2018-07, equity-classified share-based payment awards issued to nonemployees are measured at grant date fair value similarly to those of employees and are no longer revalued as the equity instruments vest. The new standard allows entities to use the expected term to measure nonemployee options or elect to use the contractual term as the expected term, on an award-by-award basis. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). The new standard requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. ASU 2016-13 is effective for the Company on January 1, 2020. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of this standard will have on its condensed consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"), which modifies the disclosure requirements on fair value measurements. ASU 2018-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this standard. In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract ("ASU 2018-15") , which clarifies the accounting for implementation costs in cloud computing arrangements. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of this amendment will have on its condensed consolidated financial statements.

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Schedule Assets Measured at Fair Value on a Recurring Basis	The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at June 30, 2019 Using: Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 46,165 $ 46,165 $ — $ — Marketable securities: Corporate debt securities 14,714 — 14,714 — $ 60,879 $ 46,165 $ 14,714 $ — Fair Value Measurements at December 31, 2018 Using: Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 53,188 $ 53,188 $ — $ — Marketable securities: Corporate debt securities 46,122 — 46,122 — Commercial paper 4,489 — 4,489 — $ 103,799 $ 53,188 $ 50,611 $ —

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2019
Marketable Securities [Abstract]
Schedule of Marketable Securities	Marketable securities consisted of the following at June 30, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments: Corporate debt securities $ 14,714 $ 1 $ (1 ) $ 14,714 $ 14,714 $ 1 $ (1 ) $ 14,714 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments: Corporate debt securities $ 46,197 $ — $ (75 ) $ 46,122 Commercial paper 4,489 — — 4,489 $ 50,686 $ — $ (75 ) $ 50,611

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): June 30, 2019 December 31, 2018 Software $ 1,181 $ 1,180 Laboratory equipment 8,928 8,230 Computer equipment 102 102 Furniture and fixtures 317 371 Leasehold improvements 674 592 Assets under construction 439 511 11,641 10,986 Less: Accumulated depreciation and amortization (3,595 ) (2,781 ) $ 8,046 $ 8,205

Accrued Expenses (Tables)

Accrued Expenses (Tables)	6 Months Ended
	Jun. 30, 2019
Accrued Liabilities, Current [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): June 30, 2019 December 31, 2018 Accrued compensation costs $ 2,442 $ 3,364 Accrued professional services 807 1,262 Accrued external research and manufacturing costs 2,536 3,001 Accrued additions of property and equipment — 243 Other accrued expenses 586 552 $ 6,371 $ 8,422

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Schedule of Components of Lease Expense	The components of lease expense and related cash flows were as follows (in thousands): Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Lease cost Operating lease cost $ 502 $ 1,004 Variable lease cost 234 477 Short-term lease cost 105 213 Total lease cost $ 841 $ 1,694 Cash paid for amounts included in the measurement of lease liabilities $ 469 $ 938
Schedule of Future Lease Payments of the Company's Operating Leases	Future lease payments for the Company's operating leases as of June 30, 2019 were as follows (in thousands): Year Ending December 31, 2019 (remaining six months) $ 953 2020 1,948 2021 2,006 2022 2,066 2023 2,128 Thereafter 1,632 Total future minimum lease payments $ 10,733 Less: interest (2,395 ) Present value of operating lease liabilities $ 8,338
Schedule of Future Lease Payments of the Company's Operating Leases Under the Prior Lease Guidance	Under the prior lease guidance, future minimum lease payments for the Company's operating leases as of December 31, 2018 were as follows (in thousands): Year Ending December 31 2019 $ 1,891 2020 1,948 2021 2,006 2022 2,066 2023 2,129 Thereafter 1,632 Total future minimum lease payments $ 11,672

Preferred Stock and Common St_2

Preferred Stock and Common Stock (Tables)	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	As of June 30, 2019 and December 31, 2018 the Company has reserved for future issuance the following number of shares of common stock: June 30, 2019 December 31, 2018 Shares reserved for exercise of outstanding stock options 3,686,242 2,548,073 Shares reserved for vesting of restricted stock units 471,119 — Shares reserved for future issuance under the 2018 Stock Option and Grant Plan 241,840 760,628 Shares reserved for future issuance under the 2018 Employee Stock Purchase Plan 553,142 270,000 4,952,343 3,578,701

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of stock-based compensation expense related to all stock-based awards	The Company recorded stock-based compensation expense related to all stock-based awards in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands): Three Months Six Months 2019 2018 2019 2018 Research and development expenses $ 1,008 $ 947 $ 1,828 $ 2,175 General and administrative expenses 1,046 476 1,955 897 $ 2,054 $ 1,423 $ 3,783 $ 3,072
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of stock option activity	The following table summarizes changes in stock option activity during the six months ended June 30, 2019: Number of Shares Weighted Average Exercise Price Weighted Average Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2018 2,548,073 $ 7.11 8.68 $ 1,965 Granted 1,231,128 6.01 Exercised (17,070 ) 2.65 Forfeited (75,889 ) 7.29 Outstanding as of June 30, 2019 3,686,242 $ 6.76 8.60 $ 1,689 Options vested or expected to vest as of June 30, 2019 3,686,242 $ 6.76 8.60 $ 1,689 Options exercisable as of June 30, 2019 1,148,127 $ 6.34 7.76 $ 1,025
Schedule of assumptions used to determine fair value of the stock options granted to employees and directors	The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Three Months Ended Six Months Ended 2019 2018 2019 2018 Expected volatility 84.92 % 103.62 % 94.77 % 102.49 % Risk-free interest rate 1.95 % 2.84 % 2.37 % 2.57 % Expected dividend yield — % — % — % — % Expected life (in years) 5.71 6.09 6.01 6.07
2018 Plan | Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of restricted stock unit activity	The following table summarizes RSU activity for the six months ended June 30, 2019 : Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested as of December 31, 2018 — $ — Granted 484,339 $ 6.25 Vested — $ — Cancelled (13,220 ) $ 6.46 Unvested as of June 30, 2019 471,119 $ 6.24
2018 Plan | Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of restricted common stock activity	The following table summarizes the Company’s restricted common stock activity since December 31, 2018 : Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 383,964 $ 1.97 Granted 25,000 $ 4.64 Vested (149,318 ) $ 1.78 Unvested restricted common stock as of June 30, 2019 259,646 $ 2.34
Outside of 2015 or 2018 Plans | Founders and Collaborators Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of restricted common stock activity	A summary of the changes in the Company’s unvested restricted common stock awards granted to founders and collaborators outside of the Company's 2015 Plan or 2018 Plan since December 31, 2018 is as follows: Number of Shares Weighted Average Grant- Date Fair Value per Share Unvested restricted common stock as of December 31, 2018 350,625 $ 1.29 Vested (113,750 ) $ 1.29 Unvested restricted common stock as of June 30, 2019 236,875 $ 1.29

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Computation of Net Loss per Share	The following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Outstanding stock options 3,686,242 2,223,147 3,686,242 2,223,147 Unvested restricted stock units 471,119 — 471,119 — Unvested restricted common stock 496,521 1,010,152 496,521 1,010,152 4,653,882 3,233,299 4,653,882 3,233,299

Nature of the Business (Details

Nature of the Business (Details) $ / shares in Units, $ in Thousands	Jun. 29, 2018USD ($)$ / sharesshares	Jun. 13, 2018	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Proceeds from IPO			$ 0	$ 93,000
Proceeds from sale of preferred stock and convertible debt						$ 161,100
Accumulated deficit			216,718		$ 216,718	216,718	$ 173,762
Cash, cash equivalents and marketable securities			$ 61,000		61,000	$ 61,000
IPO
Common stock issued and sold (in shares) | shares	6,250,000
Common stock public offering price (in dollars per share) | $ / shares	$ 16
Proceeds from IPO after deducting underwriting discounts, commissions and other offering costs	$ 89,900
One-to-five reverse split stock conversion ratio		0.20
Proceeds from IPO					$ 89,900
IPO | Redeemable Convertible Preferred Stock
Common stock issued resulting from conversion (in shares) | shares	18,644,462

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use assets	$ 8,148
Operating lease liabilities	$ 8,338
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use assets		$ 8,700
Operating lease liabilities		$ 8,900

Fair Value Measurement (Details

Fair Value Measurement (Details) - Fair value measurement, Recurring - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurement
Asset at fair value	$ 60,879,000	$ 103,799,000
Liabilities at fair value	0	0
Money market funds
Fair Value Measurement
Asset at fair value	46,165,000	53,188,000
Corporate debt securities
Fair Value Measurement
Asset at fair value	14,714,000	46,122,000
Commercial paper
Fair Value Measurement
Asset at fair value		4,489,000
Level 1
Fair Value Measurement
Asset at fair value	46,165,000	53,188,000
Level 1 | Money market funds
Fair Value Measurement
Asset at fair value	46,165,000	53,188,000
Level 1 | Corporate debt securities
Fair Value Measurement
Asset at fair value	0	0
Level 1 | Commercial paper
Fair Value Measurement
Asset at fair value		0
Level 2
Fair Value Measurement
Asset at fair value	14,714,000	50,611,000
Level 2 | Money market funds
Fair Value Measurement
Asset at fair value	0	0
Level 2 | Corporate debt securities
Fair Value Measurement
Asset at fair value	14,714,000	46,122,000
Level 2 | Commercial paper
Fair Value Measurement
Asset at fair value		4,489,000
Level 3
Fair Value Measurement
Asset at fair value	0	0
Level 3 | Money market funds
Fair Value Measurement
Asset at fair value	0	0
Level 3 | Corporate debt securities
Fair Value Measurement
Asset at fair value	$ 0	0
Level 3 | Commercial paper
Fair Value Measurement
Asset at fair value		$ 0

Marketable Securities (Details)

Marketable Securities (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019USD ($)security	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)security	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Marketable Securities
Amortized Cost	$ 14,714		$ 14,714		$ 50,686
Gross Unrealized Gains	1		1		0
Gross Unrealized Losses	(1)		(1)		(75)
Fair Value	14,714		14,714		50,611
Aggregate fair value of securities in unrealized loss position	$ 4,500		$ 4,500		46,100
Securities with other-than-temporary impairment | security	0		0
Proceeds from sale of available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Corporate debt securities
Marketable Securities
Amortized Cost	14,714		14,714		46,197
Gross Unrealized Gains	1		1		0
Gross Unrealized Losses	(1)		(1)		(75)
Fair Value	$ 14,714		$ 14,714		46,122
Commercial paper
Marketable Securities
Amortized Cost					4,489
Gross Unrealized Gains					0
Gross Unrealized Losses					0
Fair Value					$ 4,489

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 11,641		$ 11,641		$ 10,986
Less: Accumulated depreciation and amortization	(3,595)		(3,595)		(2,781)
Property and equipment, net	8,046		8,046		8,205
Depreciation and amortization expense	400	$ 400	833	$ 692
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,181		1,181		1,180
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,928		8,928		8,230
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	102		102		102
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	317		317		371
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	674		674		592
Assets under construction
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 439		$ 439		$ 511

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities, Current [Abstract]
Accrued compensation costs	$ 2,442	$ 3,364
Accrued professional services	807	1,262
Accrued external research and manufacturing costs	2,536	3,001
Accrued additions of property and equipment	0	243
Other accrued expenses	586	552
Total accrued expenses	$ 6,371	$ 8,422

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	Sep. 28, 2016USD ($)renewal_option	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)
Property, Plant and Equipment [Line Items]
Amortization of operating lease right-of-use assets				$ 580
Variable lease expenses		$ 234		$ 477
Office and Laboratory Space
Property, Plant and Equipment [Line Items]
Number lease renewal options | renewal_option	1
Term of lease contract	5 years
Operating lease discount rate	10.00%
Expected remaining lease term	5 years 8 months 12 days	5 years 2 months 12 days		5 years 2 months 12 days
Amortization of operating lease right-of-use assets		$ 300		$ 600
Variable lease expenses			$ 200		$ 500
Office and Laboratory Space | Restricted Cash Noncurrent
Property, Plant and Equipment [Line Items]
Letters of credit issued to the landlord in connection with lease agreement	$ 500

Leases - Components of Lease Ex

Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating lease cost	$ 502	$ 1,004
Variable lease cost	234	477
Short-term lease cost	105	213
Total lease cost	841	1,694
Cash paid for amounts included in the measurement of lease liabilities	$ 469	$ 938

Leases - Future Operating Lease

Leases - Future Operating Lease Payments (Details) $ in Thousands	Jun. 30, 2019USD ($)
Leases [Abstract]
2019 (remaining six months)	$ 953
2020	1,948
2021	2,006
2022	2,066
2023	2,128
Thereafter	1,632
Total future minimum lease payments	10,733
Less: interest	(2,395)
Present value of operating lease liabilities	$ 8,338

Leases - Future Minimum Operati

Leases - Future Minimum Operating Lease Payments Under Prior Lease Guidance (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 1,891
2020	1,948
2021	2,006
2022	2,066
2023	2,129
Thereafter	1,632
Total future minimum lease payments	$ 11,672

Commitments and Contingencies -

Commitments and Contingencies - Significant Agreements (Details) - Vendor	1 Months Ended
	Jul. 31, 2017	Oct. 31, 2016
2016 Manufacturing Agreement
Significant Agreements
Term		3 years
Notice period for termination		6 months
2017 Manufacturing Agreement
Significant Agreements
Term	5 years
Notice period for termination	3 months

Commitments and Contingencies_2

Commitments and Contingencies - Other Agreements (Details)	Aug. 05, 2016item$ / sharesshares	Nov. 13, 2015USD ($)patentshares	Aug. 31, 2016USD ($)shares	Nov. 30, 2015USD ($)shares	Jun. 30, 2019USD ($)shares	Jun. 30, 2018USD ($)shares	Jun. 30, 2019USD ($)shares	Jun. 30, 2018USD ($)shares	Jul. 31, 2016shares	Jun. 30, 2019shares	Jun. 30, 2019USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Incremental share-based compensation expenses recognized					$ 2,054,000	$ 1,423,000	$ 3,783,000	$ 3,072,000
Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Incremental share-based compensation expenses recognized					$ 1,008,000	$ 947,000	$ 1,828,000	$ 2,175,000
Founders and Collaborators Awards | Outside of 2015 or 2018 Plans
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Awards granted (in shares) | shares					0	0	0	0	1,510,000
Founders and Collaborators Awards | Fully vested subject to repurchase option | Outside of 2015 or 2018 Plans
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Shares expired (in shares) | shares										80,000
License Agreement | Restricted Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate fair value		$ 200,000
License Agreement | Broad
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of patent families | patent		12
Non-refundable license fee paid		$ 100,000
Awards granted (in shares) | shares			120,000	60,000
Aggregate fair value			$ 200,000	$ 100,000
Reimbursement amount		600,000		600,000
License Agreement | Broad | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Developmental milestone payments to be paid on achievements		12,600,000		12,600,000
Payments upon achievement of specified sales milestones and tiered royalties on net sales		$ 97,500,000		$ 97,500,000
Developmental and commercial milestones achieved to date under collaborative arrangement											$ 0
License Agreement | Broad | Restricted Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Awards granted (in shares) | shares	120,000	60,000
License Agreement | DFCI
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of specific milestones with contingent repurchase option | item	3
Percentage of each milestone shares to be repurchased with contingent repurchase option if specific milestones not achieved	0.33
Percentage of shares with contingent repurchase option expired on achievement of specific criteria	0.33							0.33
License Agreement | DFCI | Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Incremental share-based compensation expenses recognized								$ 400,000
License Agreement | DFCI | Restricted Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Awards granted (in shares) | shares	120,000	60,000
Shares granted with contingent repurchase option (in shares) | shares	120,000
Original purchase price of shares (in dollars per share) | $ / shares	$ 0
License Agreement | MGH | Restricted Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Awards granted (in shares) | shares		60,000

Preferred Stock and Common St_3

Preferred Stock and Common Stock - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Jun. 30, 2018USD ($)shares	Jun. 30, 2019USD ($)Vote / shares$ / sharesshares	Jun. 30, 2018USD ($)shares	Mar. 31, 2019shares	Dec. 31, 2018$ / sharesshares	Jun. 29, 2018shares	Mar. 31, 2018shares	Dec. 31, 2017shares
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	10,000,000		10,000,000
Preferred stock par value per share (in dollars per share) | $ / shares	$ 0.001		$ 0.001
Preferred stock, shares issued (in shares)	0		0			0
Preferred stock, shares outstanding (in shares)	0		0			0
Common stock, shares authorized (in shares)	150,000,000		150,000,000			150,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001			$ 0.001
Votes granted per share of common stock | Vote / shares			1
Dividends declared and paid | $	$ 0	$ 0	$ 0	$ 0
Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Number of convertible preferred shares issued (in shares)	0		0			0
Number of convertible preferred shares outstanding (in shares)	0	0	0	0	0	0		93,222,418	93,222,418
Redeemable Convertible Preferred Stock | IPO
Class of Stock [Line Items]
Number of shares of common stock issued in conversion (in shares)							18,644,462

Preferred Stock and Common St_4

Preferred Stock and Common Stock - Stock Reserved for Future Issuance (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Shares reserved for exercise of outstanding stock options (in shares)	3,686,242	2,548,073
Shares reserved for vesting of restricted stock units (in shares)	471,119	0
Shares reserved for future issuance under the 2018 Stock Option and Grant Plan (in shares)	241,840	760,628
Shares reserved for future issuance under the 2018 Employee Stock Purchase Plan (in shares)	553,142	270,000
Shares reserves for future issuance (in shares)	4,952,343	3,578,701

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Options Plans (Details)	Jan. 01, 2019shares	Jun. 30, 2019itemshares	Dec. 31, 2018shares	Jun. 26, 2018shares	Jun. 13, 2018shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares remained available future issuance (in shares)		4,952,343	3,578,701
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares remained available future issuance (in shares)		0	0	0
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares remained available future issuance (in shares)		241,840
Total number of shares authorized for issuance (in shares)					1,215,000
Annual increase as a percentage of common stock outstanding (in percent)		4.00%
Number of additional shares authorized (in shares)	1,132,570
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares authorized for issuance (in shares)					270,000
Number of additional shares authorized (in shares)	283,142
Number of offering periods initiated | item		0
ESPP | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase as a percentage of common stock outstanding (in percent)		1.00%
Annual increase to common stock (in shares)		405,000

Stock-Based Compensation - St_2

Stock-Based Compensation - Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Outstanding stock options
Number of Shares
Outstanding at beginning of period (in shares)			2,548,073
Granted (in shares)			1,231,128
Exercised (in shares)	0		(17,070)
Forfeited (in shares)			(75,889)
Outstanding at end of period (in shares)	3,686,242		3,686,242		2,548,073
Options vested or expected to vest at end of period (in shares)	3,686,242		3,686,242
Options exercisable at end of period (in shares)	1,148,127		1,148,127
Weighted Average Exercise Price
Outstanding at end of period (in dollars per share)			$ 7.11
Granted (in dollars per share)			6.01
Exercised (in dollars per share)			2.65
Forfeited (in dollars per share)			7.29
Outstanding at end of period (in dollars per share)	$ 6.76		6.76		$ 7.11
Options vested or expected to vest at end of period (in dollars per share)	6.76		6.76
Options exercisable at end of period (in dollars per share)	$ 6.34		$ 6.34
Weighted Average Contractual Term
Outstanding at beginning of period			8 years 7 months 6 days		8 years 8 months 5 days
Outstanding at end of period			8 years 7 months 6 days		8 years 8 months 5 days
Options vested or expected to vest at end of period			8 years 7 months 6 days
Options exercisable at end of period			7 years 9 months 4 days
Aggregate Intrinsic Value
Outstanding at beginning of period			$ 1,965
Outstanding at end of period	$ 1,689		1,689		$ 1,965
Options vested or expected to vest at end of period	1,689		1,689
Options exercisable at end of period	$ 1,025		$ 1,025
Weighted average grant-date fair value per share of stock options granted (in dollars per share)	$ 3.73	$ 9.71	$ 4.65	$ 8.33
Aggregate intrinsic value of stock options exercised		$ 800	$ 100	$ 800
Non Employee Stock Options
Number of Shares
Granted (in shares)	0	0	0	0

Stock-Based Compensation - St_3

Stock-Based Compensation - Stock Options Valuation (Details) - Outstanding stock options	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	84.92%	103.62%	94.77%	102.49%
Risk-free interest rate	1.95%	2.84%	2.37%	2.57%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected life (in years)	5 years 8 months 16 days	6 years 1 month 2 days	6 years 4 days	6 years 26 days

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Units (Details) - 2018 Plan - Unvested restricted stock units	6 Months Ended
	Jun. 30, 2019$ / sharesshares	Jun. 30, 2018$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares to be received per each award (in shares)	1
Award vesting period	3 years
Number of Shares
Unvested at beginning of period (in shares)	0
Granted (in shares)	484,339
Vested (in shares)	0
Cancelled (in shares)	(13,220)
Unvested at end of period (in shares)	471,119
Weighted Average Grant- Date Fair Value per Share
Unvested at beginning of period (in dollars per share) | $ / shares	$ 0
Granted (in dollars per share) | $ / shares	6.25
Vested (in dollars per share) | $ / shares	0
Cancelled (in dollars per share) | $ / shares		$ 6.46
Unvested at end of period (in dollars per share) | $ / shares	$ 6.24

Stock-Based Compensation - Re_2

Stock-Based Compensation - Restricted Stock Awards (Details) - Restricted Common Stock - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	0	0	0	0
Number of Shares
Granted (in shares)	0	0	0	0
Aggregate intrinsic value
Aggregate fair value of restricted common stock awards vested	$ 0.4	$ 0.9	$ 0.8	$ 1.7
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	25,000		25,000
Award vesting period	1 year
Number of Shares
Unvested at beginning of period (in shares)			383,964
Granted (in shares)	25,000		25,000
Vested (in shares)			(149,318)
Unvested at end of period (in shares)	259,646		259,646
Weighted Average Grant- Date Fair Value per Share
Unvested at beginning of period (in dollars per share)			$ 1.97
Granted (in dollars per share)			4.64
Vested (in dollars per share)			1.78
Unvested at end of period (in dollars per share)	$ 2.34		$ 2.34

Stock-Based Compensation - Foun

Stock-Based Compensation - Founder and Collaborator Awards Narrative (Details) - Outside of 2015 or 2018 Plans - Founders and Collaborators Awards - USD ($) $ in Millions	3 Months Ended		6 Months Ended		15 Months Ended	35 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jul. 31, 2016	Jun. 30, 2019	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	0	0	0	0	1,510,000
Shares vested (in shares)			113,750
Contingently redeemable common stock							$ 0.4
Vested immediately upon grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested (in shares)					300,000
Vest quarterly based on continued service
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested (in shares)					910,000
Award vesting period					4 years
Vest upon achievement of specified performance milestones
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested (in shares)					180,000
Fully vested subject to repurchase option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested (in shares)					120,000
Shares expired (in shares)						80,000

Stock-Based Compensation - Fo_2

Stock-Based Compensation - Founder and Collaborator Awards Activity (Details) - Outside of 2015 or 2018 Plans - Founders and Collaborators Awards - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Number of Shares
Unvested at beginning of period (in shares)			350,625
Vested (in shares)			(113,750)
Unvested at end of period (in shares)	236,875		236,875
Weighted Average Grant-Date Fair Value per Share
Unvested at beginning of period (in dollars per share)			$ 1.29
Vested (in dollars per share)			1.29
Unvested at end of period (in dollars per share)	$ 1.29		$ 1.29
Aggregate intrinsic value
Aggregate fair value of restricted common stock awards vested	$ 0.3	$ 0.7	$ 0.6	$ 1.3

Stock-Based Compensation - St_4

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-based compensation expense
Stock-based compensation expense	$ 2,054	$ 1,423	$ 3,783	$ 3,072
Performance-based vesting
Stock-based compensation expense
Stock-based compensation expense		400
Outstanding stock options
Stock-based compensation expense
Unrecognized stock-based compensation expense	13,100		$ 13,100
Weighted average period for recognition of shared-based compensation not yet recognized			2 years 8 months 9 days
Restricted stock, excluding performance-based vesting
Stock-based compensation expense
Unrecognized stock-based compensation expense	700		$ 700
Weighted average period for recognition of shared-based compensation not yet recognized			9 months 22 days
Unvested restricted stock units
Stock-based compensation expense
Unrecognized stock-based compensation expense	2,600		$ 2,600
Weighted average period for recognition of shared-based compensation not yet recognized			2 years 8 months 12 days
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense	1,008	947	$ 1,828	2,175
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense	$ 1,046	$ 476	$ 1,955	$ 897

Net Loss per Share (Details)

Net Loss per Share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive securities excluded from computation of earnings per share
Potential common shares excluded from calculation of diluted net loss per share	4,653,882	3,233,299	4,653,882	3,233,299
Restricted common stock
Antidilutive securities excluded from computation of earnings per share
Potential common shares excluded from calculation of diluted net loss per share	496,521	1,010,152	496,521	1,010,152
Outstanding stock options
Antidilutive securities excluded from computation of earnings per share
Potential common shares excluded from calculation of diluted net loss per share	3,686,242	2,223,147	3,686,242	2,223,147
Unvested restricted stock units
Antidilutive securities excluded from computation of earnings per share
Potential common shares excluded from calculation of diluted net loss per share	471,119	0	471,119	0
Unvested restricted common stock
Antidilutive securities excluded from computation of earnings per share
Potential common shares excluded from calculation of diluted net loss per share	496,521	1,010,152	496,521	1,010,152

Related Parties (Details)

Related Parties (Details) - Broad - License Agreement - USD ($)	Nov. 13, 2015	Aug. 31, 2016	Nov. 30, 2015	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Related Parties
Awards granted (in shares)		120,000	60,000
Aggregate fair value		$ 200,000	$ 100,000
Reimbursement amount	$ 600,000		600,000
Amount of expenses in relation to related party				$ 600,000	$ 300,000	$ 1,100,000	$ 500,000
Accounts payable and accrued expenses due to related party				$ 600,000		$ 600,000		$ 600,000	$ 2,000,000
Maximum
Related Parties
Developmental milestone payments to be paid on achievements	12,600,000		12,600,000
Payments upon achievement of specified sales milestones and tiered royalties on net sales	$ 97,500,000		$ 97,500,000
Developmental and commercial milestones achieved to date under collaborative arrangement								$ 0

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event - Cantor - At the market $ in Millions	Jul. 01, 2019USD ($)shares
Subsequent Event [Line Items]
Aggregate amount authorized under controlled equity offering program	$ 200
Sale of stock commission percentage	3.00%
Shares issued under controlled equity offering program (in shares) | shares	0
Common Stock
Subsequent Event [Line Items]
Aggregate amount authorized under controlled equity offering program	$ 50