Constellation Pharmaceuticals (CNST) 10-Q 8 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	CNST
Entity Registrant Name	CONSTELLATION PHARMACEUTICALS INC
Entity Central Index Key	0001434418
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true
Entity Common Stock, Shares Outstanding		25,807,229

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 52,447	$ 114,592
Marketable securities	63,344
Prepaid expenses and other current assets	2,695	2,711
Total current assets	118,486	117,303
Property and equipment, net	1,312	1,210
Restricted cash	425	425
Operating lease, right-of-use assets	4,700
Total assets	124,923	118,938
Current liabilities:
Accounts payable	7,523	5,723
Accrued expenses and other current liabilities	6,628	8,937
Current portion of lease liabilities - operating lease	2,970
Total current liabilities	17,121	14,660
Long-term debt, net of current portion and discount	19,529
Operating lease liabilities, net of current portion	2,185
Deferred rent, net of current portion		118
Other long-term liabilities	13	2
Total liabilities	38,848	14,780
Stockholders' equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding at March 31, 2019 and December 31, 2018
Common stock, $0.0001 par value; 200,000,000 shares authorized at March 31, 2019 and December 31, 2018, respectively; 25,807,229 and 25,803,475 shares issued at March 31, 2019 and December 31, 2018, respectively; 25,806,974 and 25,803,135 shares outstanding at March 31, 2019 and December 31, 2018, respectively	3	3
Additional paid-in capital	339,326	337,992
Accumulated other comprehensive gain	9
Accumulated deficit	(253,263)	(233,837)
Total stockholders' equity	86,075	104,158
Total liabilities and stockholders' equity	$ 124,923	$ 118,938

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	25,807,229	25,803,475
Common stock, shares outstanding	25,806,974	25,803,135

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 15,677	$ 9,874
General and administrative	4,429	2,303
Total operating expenses	20,106	12,177
Loss from operations	(20,106)	(12,177)
Other income (expense):
Interest income	755	109
Interest expense	(75)	(34)
Total other income (expense), net	680	75
Net loss attributable to common stockholders	(19,426)	(12,102)
Other comprehensive gain:
Unrealized gain on marketable securities	9
Other comprehensive gain	9
Comprehensive loss	$ (19,417)	$ (12,102)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.75)	$ (12.44)
Weighted average common shares outstanding, basic and diluted	25,804,595	972,981

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock (Series A, B, D, E, E-1 and F) [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Gain [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ (165,833)			$ 8,079		$ (173,912)
Beginning balance, shares at Dec. 31, 2017		118,867,177
Beginning balance at Dec. 31, 2017		$ 173,228
Beginning balance, shares at Dec. 31, 2017			962,898
Issuance of stock, net of issuance costs		$ 68,368
Issuance of stock, net of issuance costs, shares		68,500,000
Stock-based compensation expense	585			585
Vesting of common stock issued upon early exercise of unvested options	3			3
Vesting of common stock issued upon early exercise of unvested options, shares			408
Repayment of promissory notes issued upon early exercise of unvested options	290			290
Repayment of promissory notes issued upon early exercise of unvested options, shares			229,357
Net loss	(12,102)					(12,102)
Ending balance at Mar. 31, 2018	(177,057)	$ 241,596		8,957		(186,014)
Ending balance, shares at Mar. 31, 2018		187,367,177
Ending balance, shares at Mar. 31, 2018			1,192,663
Beginning balance at Dec. 31, 2017	(165,833)			8,079		(173,912)
Beginning balance, shares at Dec. 31, 2017		118,867,177
Beginning balance at Dec. 31, 2017		$ 173,228
Beginning balance, shares at Dec. 31, 2017			962,898
Net loss	(59,900)
Ending balance at Dec. 31, 2018	104,158		$ 3	337,992		(233,837)
Ending balance, shares at Dec. 31, 2018			25,803,135
Stock-based compensation expense	1,313			1,313
Vesting of common stock issued upon early exercise of unvested options, shares			85
Stock option exercises	21			21
Stock option exercises, shares			3,754
Unrealized gain on marketable securities	9				$ 9
Net loss	(19,426)					(19,426)
Ending balance at Mar. 31, 2019	$ 86,075		$ 3	$ 339,326	$ 9	$ (253,263)
Ending balance, shares at Mar. 31, 2019			25,806,974

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)
Issuance of stock, net of issuance costs	$ 21
Series F Convertible Preferred Stock [Member]
Issuance of stock, net of issuance costs	$ 132

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (19,426)	$ (12,102)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	168	135
Stock-based compensation expense	1,313	585
Non-cash interest expense	18	33
Amortization and accretion on marketable securities	(212)
Change in fair value of preferred stock warrant liability		(65)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	128	(571)
Operating lease, right-of-use assets	690
Accounts payable	1,768	562
Accrued expenses and other current liabilities	(2,116)	(333)
Operating lease liabilities	(664)
Deferred rent		(41)
Other assets		18
Net cash used in operating activities	(18,333)	(11,779)
Cash flows from investing activities:
Purchase of marketable securities	(63,123)
Purchases of property and equipment	(234)	(21)
Net cash used in investing activities	(63,357)	(21)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	19,649
Payment of debt issuance costs	(125)
Proceeds from issuance of convertible preferred stock, net of issuance costs		68,368
Payments on long-term debt		(1,739)
Proceeds from repayment of promissory notes issued upon early exercise of stock options		290
Payments of initial public offering costs		(21)
Proceeds from issuance of common stock upon stock option exercises	21
Net cash provided by financing activities	19,545	66,898
Net increase (decrease) in cash, cash equivalents and restricted cash	(62,145)	55,098
Cash, cash equivalents and restricted cash at beginning of period	115,017	16,646
Cash, cash equivalents and restricted cash at end of period	52,872	71,744
Supplemental disclosure of cash flow information:
Interest paid		67
Supplemental disclosure of noncash investing and financing information:
Purchases of property and equipment included in accounts payable	$ 216	15
Vesting of common stock subject to repurchase		3
Deferred offering costs included in accounts payable and accrued expenses		$ 385

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Constellation Pharmaceuticals, Inc. (“Constellation” or the “Company”) is a clinical-stage biopharmaceutical company using its expertise in epigenetics to discover and develop novel therapeutics that address serious unmet medical needs in patients with cancers associated with abnormal gene expression or drug resistance. The Company was incorporated in January 2008 as EpiGenetiX, Inc. under the laws of the State of Delaware. On March 31, 2008, the Company changed its name to Constellation Pharmaceuticals, Inc. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital 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 and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with the sales of convertible preferred stock, payments received in connection with collaboration agreements, borrowings under loan agreements, and proceeds from the initial public offering (“IPO”) completed in July 2018. On March 20, 2019, the Company entered into a Loan and Security Agreement (the “Loan Agreement”) with Hercules Capital, Inc. (“Hercules”) pursuant to which Hercules agreed to provide the Company with up to $40.0 million in funding, to be made available in four tranches. As of March 31, 2019, the Company had drawn down on the first of the four tranches and in connection with the draw down received net proceeds of $19.5 million. The Company has incurred losses since inception, including net losses of $19.4 million for the three months ended March 31, 2019, and $59.9 million for the year ended December 31, 2018. As of March 31, 2019, the Company had an accumulated deficit of $253.3 million. The Company expects to continue to generate operating losses in the foreseeable future. As of May 8, 2019, the issuance date of the interim financial statements, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of the interim financial statements. The Company’s financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Consolidated Financial Information The accompanying unaudited condensed consolidated financial statements as of March 31, 2019 and for the three months ended March 31, 2019 and 2018 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim consolidated financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 (the “Annual Report”). The unaudited condensed consolidated financial statements include the accounts of Constellation Pharmaceuticals, Inc. and its wholly owned subsidiary, Constellation Securities Corporation. All intercompany transactions and balances of the subsidiary have been eliminated in consolidation. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of March 31, 2019 and results of operations for the three months ended March 31, 2019 and 2018, and cash flows for the three months ended March 31, 2019 and 2018 have been made. The Company’s results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains most of its cash, cash equivalents and marketable securities at two accredited financial institutions in amounts that could exceed federally insured limits. Cash equivalents are invested in an institutional money market fund. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Summary of Significant Accounting Policies The Company’s significant accounting policies are described in Note 2, “Summary of Significant Accounting Policies,” to the Consolidated Financial Statements included in the Annual Report. There have been no material changes to the significant accounting policies previously disclosed in the Annual Report other than as noted below. Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days. The Company classifies its investments with maturities beyond one year as short term based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of marketable securities as available-for-sale. Accordingly, these marketable securities are recorded at fair value and unrealized gains and losses are reported as a component of accumulated other comprehensive loss in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Comprehensive Loss Comprehensive loss consists of net loss and unrealized gain (losses) on available-for-sale marketable securities. Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents consists of all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. As of March 31, 2019, the Company classified $425,000 as restricted cash related to a letter of credit issued as a security deposit in connection with Company's lease of its corporate office facilities (Note 12). Cash, cash equivalents and restricted cash consists of the following: March 31, 2019 December 31, 2018 Cash and cash equivalents $ 52,447 $ 114,592 Restricted cash 425 425 Cash, cash equivalents and restricted cash $ 52,872 $ 115,017 Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Quoted prices in active markets for identical assets or liabilities. • Level 2 – Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding debt as of March 31, 2019 (see Note 7) approximated fair value (a Level 3 measurement) based on interest rates currently available to the Company. Revenue Recognition On January 1, 2018, the Company adopted the new revenue standard, discussed below under the heading “Recently Adopted Accounting Pronouncements”, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries (“ASC 606”). Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the contract(s) with the customer, (ii) identification of the promised goods or services in the contract and determination of whether the promised goods or services are performance obligations, (iii) measurement of the transaction price, (iv) allocation of the transaction price to the performance obligations, and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that it will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The Company accounts for a contract with a customer that is within the scope of ASC 606 when all of the following criteria are met: (i) the arrangement has been approved by the parties and the parties are committed to perform their respective obligations, (ii) each party’s rights regarding the goods or services to be transferred can be identified, (iii) the payment terms for the goods or services to be transferred can be identified, (iv) the arrangement has commercial substance and (v) collection of substantially all of the consideration to which the Company will be entitled in exchange for the goods or services that will be transferred to the customer is probable. The Company estimates the transaction price based on the amount of consideration the Company expects to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of the potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the transaction price based on which method better predicts the amount of consideration expected to be received. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. For arrangements that include development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such milestones and any related constraint, and if necessary, adjusts the estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue and net income (loss) in the period of adjustment. For sales-based royalties, including milestone payments based on the level of sales, the Company determines whether the sole or predominant item to which the royalties relate is a license. When the license is the sole or predominant item to which the sales-based royalty relates, the Company recognizes revenue at the later of: (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). The Company allocates the transaction price based on the estimated standalone selling price. The Company must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Certain variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated to each performance obligation are consistent with the amounts the Company would expect to receive for each performance obligation. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation in order to determine whether the combined performance obligation is satisfied over time or at a point in time. The Company determines the appropriate method of measuring progress of combined performance obligations satisfied over time for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company receives payments from customers based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. Recently adopted accounting pronouncements In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which requires lessees to recognize leases on the balance sheet and disclose key information about leasing arrangements. The Company adopted ASU 2016-02 as of January 1, 2019, using the modified retrospective approach. Prior period amounts have not been adjusted. The main difference between previous GAAP (“Topic 840”) and Topic 842 is the recognition of right-of-use lease assets and lease liabilities by lessees for those leases classified as operating leases under Topic 840. In addition, the Company elected the following practical expedients: • the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed the Company to carry forward the historical lease classification; • the short-term lease practical expedient, which allowed the Company to exclude short-term leases of less than 12 months from recognition in the unaudited consolidated balance sheets; and • the bifurcation of lease and non-lease components practical expedient, which did not require the Company to bifurcate lease and non-lease components for all classes of assets. The adoption of this accounting standard resulted in the recording of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months of $3.1 million and $3.5 million, respectively, as of January 1, 2019. The difference between the operating lease assets and liabilities was recorded as an adjustment to “Other liabilities” on the consolidated and condensed balance sheets, primarily related to deferred rent (lease incentives). The adoption of ASU 2016-02 had no impact on Retained earnings. For additional information regarding how the Company is accounting for leases under Topic 842, refer to Note 12, Leases Recently issued accounting pronouncements In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception . In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities 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): March 31, 2019 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds included in cash and cash equivalents $ 47,059 $ — $ — $ 47,059 Commercial paper — 5,388 — 5,388 Total $ 47,059 $ 5,388 $ — $ 52,447 Marketable securities: Corporate debt securities $ — $ 10,661 $ — $ 10,661 Commercial paper — $ 34,781 — 34,781 Government securities — 17,902 — 17,902 Total $ — $ 63,344 $ — $ 63,344 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds included in cash and cash equivalents $ 114,592 $ — $ — $ 114,592 $ 114,592 $ — $ — $ 114,592 Money market funds were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 1 measurement within the fair value hierarchy. During the three months ended March 31, 2019 and the year ended December 31, 2018, there were no transfers between Level 1 and Level 2. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities were determined through third-party pricing services.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2019
Cash And Cash Equivalents [Abstract]
Marketable Securities	4. Marketable Securities The following table summarizes the Company’s marketable securities and cash equivalents as of March 31, 2019. The Company did not hold any marketable securities as of December 31, 2018. March 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Cash equivalents: Money market funds $ 47,059 $ — $ — $ 47,059 Commercial paper 5,388 — — 5,388 Total cash equivalents $ 52,447 $ — $ — $ 52,447 Marketable securities: Corporate debt securities $ 10,654 $ 7 $ — $ 10,661 Commercial paper 34,781 — — 34,781 Government securities 17,900 2 — 17,902 Total marketable securities $ 63,335 $ 9 $ — $ 63,344 Total cash equivalents and marketable securities $ 115,782 $ 9 $ — $ 115,791

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, 2019 December 31, 2018 Accrued employee compensation and benefits $ 1,569 $ 2,726 Accrued external research and development expense 4,576 5,610 Accrued professional fees 302 255 Other 181 346 $ 6,628 $ 8,937

Collaboration Agreement

Collaboration Agreement	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Agreement	6. Collaboration Agreement The Company has a collaboration agreement (the “LLS Agreement”) with the Leukemia and Lymphoma Society, (“LLS”) pursuant to which LLS committed to provide funding to the Company for research and development services, conditional on (i) the achievement of milestones in accordance with the LLS Agreement and (ii) equal funding being provided by the Company. Through each of March 31, 2019 and December 31, 2018, the Company received funding totaling $7.3 million from LLS upon the achievement of specified milestones, which were recorded as a reduction of research and development expense. The LLS Agreement requires the Company to make payments to LLS upon the Company’s achievement of specified milestones that could total up to $25.0 million in aggregate (see Note 11).

Debt

Debt	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Debt	7. Debt The Company previously had outstanding amounts due under an agreement of $11.8 million (the “2016 Loan Agreement”). Borrowings under the 2016 Loan Agreement bore interest at an annual rate of 7.6% and were repaid in full on July 3, 2018. In addition, a final payment equal to 5% of the original principal amount was paid upon the final principal payment. On March 20, 2019, the Company entered into the Loan Agreement with Hercules as administrative and collateral agent, and various other lenders, pursuant to which the Company may borrow under a term loan up to an aggregate principal amount of $40.0 million, to be made available in four tranches. The outstanding principal balance as of March 31, 2019 is $20 million. As of March 31, 2019, the Company had drawn down on the first of the four tranches, and its ability to draw down the remainder of the tranches is subject to certain time limitations, achievement of performance milestones and lender approval. The term loan bears interest at an annual rate equal to the greater of 8.55% and the prime rate of interest plus 2.55%. The Loan Agreement provides for interest-only payments until April 30, 2021, and repayment of the aggregate outstanding principal balance of the term loan in monthly installments starting on May 1, 2021 and continuing through April 1, 2023 (the “Maturity Date”). In addition, the Company paid a fee of $255,000 upon closing and is required to pay a fee of 6.35% of the aggregate amount of advances under the Loan Agreement at maturity. At its option, the Company may elect to prepay all or a portion of the outstanding advances by paying the entire principal balance (or a portion thereof) and all accrued and unpaid interest thereon plus a prepayment charge equal to the following percentage of the principal amount being prepaid: 2% if an advance is prepaid during the first 12 months following the applicable advance date, 1% if an advance is prepaid after 12 months but prior to 24 months following the applicable advance date, and 0.5% if an advance is prepaid any time after 24 months following the applicable advance date but prior to the Maturity Date. In connection with the Loan Agreement, the Company granted Hercules a security interest in all of its personal property now owned or hereafter acquired, excluding intellectual property (but including the rights to payment and proceeds from the sale, licensing or disposition of intellectual property), and a negative pledge on intellectual property. The Loan Agreement also contains certain events of default, representations, warranties and non-financial covenants of the Company. If the Company fails to make payments when due, or breaches any operational covenant or has any event of default, this could have a material adverse effect on its business and financial condition As of March 31, 2019, notes payable consisted of the following: March 31, 2019 Principal amount of term loans $ 20,000 Debt discount current portion — Less: Current portion — Long-term debt, net of current portion 20,000 Debt discount net of current portion (471 ) Long-term debt, net of discount and current portion $ 19,529

Convertible Preferred Stock

Convertible Preferred Stock	3 Months Ended
	Mar. 31, 2019
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock As of April 5, 2018, the Company had issued Series A, Series B, Series D, Series E, Series E-1, and Series F convertible preferred stock (collectively the “Preferred Stock”). On July 23, 2018, upon the closing of the Company’s IPO, all outstanding convertible preferred stock automatically converted into shares of common stock.

Warrants to Purchase Convertibl

Warrants to Purchase Convertible Preferred Stock	3 Months Ended
	Mar. 31, 2019
Warrants And Rights Note Disclosure [Abstract]
Warrants to Purchase Convertible Preferred Stock	9. Warrants to Purchase Convertible Preferred Stock The Company issued warrants to purchase convertible preferred stock in 2013 and 2014 for the purchase of 375,000 shares of Series B Preferred Stock. Upon the closing of the IPO in July 2018, these warrants became warrants to purchase 34,062 shares of common stock at which time the Company reclassified the carrying value of the warrants to additional paid-in capital. Prior to the warrants becoming warrants to purchase common stock, the Company was required to remeasure the fair value of the liability for these preferred stock warrants at each reporting date since their grant date, with any adjustments recorded in interest expense. The warrants outstanding at each reporting date were remeasured using the Black-Scholes option-pricing model, and the resulting change in fair value was recorded in interest expense in the Company’s statements of operations and comprehensive loss.

Equity

Equity	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Equity	10. Equity Common Stock As of March 31, 2019, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 200,000,000 shares of common stock, $0.0001 par value 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 not entitled to receive dividends, unless declared by the Company’s board of directors. No dividends have been declared or paid by the Company since its inception. Warrants to Purchase Common Stock As of March 31, 2019, the Company had outstanding warrants to purchase common stock as follows: Issuance Date Term (in years) Exercise Price Number of Common Shares Issuable under Warrant May 23, 2011 10 $ 1.55 61,868 June 28, 2013 10 $ 13.22 11,354 September 30, 2014 10 $ 13.22 22,708 95,930

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation 2008 Stock Incentive Plan The Company’s 2008 Stock Incentive Plan (the “2008 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors, consultants and advisors of the Company. The 2008 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors could also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2008 Plan with service-based vesting conditions generally vest over four years and expire after ten years. The total number of shares of common stock that were authorized for issuance under the 2008 Plan was 4,039,829 shares. Upon effectiveness of the Company’s 2018 Equity Incentive Plan, the (“2018 Plan”) in July 2018, the remaining 245,557 shares available under the 2008 Plan became available for issuance under the 2018 Plan and no future issuance will be made under the 2008 Plan. Additionally, outstanding options under the 2008 Plan that expired, terminated, are surrendered or canceled without having been fully exercised will be available for future awards under the 2018 Plan. The exercise price for stock options granted is not less than the fair value of common shares as determined by the board of directors as of the date of grant. The Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant. 2018 Equity Incentive Plan In June 2018, the Company’s stockholders approved the 2018 Plan, which became effective on July 18, 2018. The 2018 Plan provides for the grant of incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2018 Plan is 2,779,544 plus the 245,557 shares of common stock remaining available for issuance under the 2008 Plan as of that date. The number of shares reserved shall be annually increased on January 1, 2019 and each January 1 thereafter through January 1, 2028 by the least of (i) 2,216,368 shares, (ii) 4% of the number of shares of the Company’s common stock outstanding on the first day of the year or (iii) an amount determined by the Company’s board of directors. The shares of common stock underlying any awards that are expired, forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan or the 2008 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. In January 2019, the shares available for issuance under the 2018 Plan were increased by 1,032,125 shares pursuant to the annual increase described above. As of March 31, 2019, 1,919,540 shares remained available for future issuance under the 2018 Plan. 2018 Employee Stock Purchase Plan In June 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan which became effective on July 18, 2018. A total of 272,504 shares of common stock were reserved for issuance under this plan. The number of shares reserved shall be annually increased on January 1, 2020 and each January 1 thereafter through January 1, 2028 by the least of (i) 545,008 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the first day of the year or (iii) an amount determined by the Company’s board of directors. Stock Option Issuances The following is a summary of stock option activity for the three months ended March 31, 2019, Weighted- Average Remaining Aggregate Stock Weighted Average Contractual Term Intrinsic Value Options Exercise Price (in years) (in thousands) Outstanding as of December 31, 2018 3,779,403 $ 7.78 8.80 $ 110 Granted 1,115,273 $ 8.90 Exercised (3,839 ) $ 5.58 Forfeited (178,783 ) $ 8.60 Outstanding as of March 31, 2019 4,712,054 $ 8.01 8.87 $ 26,092 Vested and expected to vest at March 31, 2019 4,712,054 $ 8.01 8.87 $ 26,094 Exercisable at March 31, 2019 1,174,589 $ 6.43 7.74 $ 8,367 During the three months ended March 31, 2019, the Company granted options to employees, consultants and directors for the purchase of 1,115,273 shares of common stock with a weighted average exercise price of $8.90 per share and a weighted average grant-date fair value of $6.33 per share. The Company estimated the fair value of each stock option award using the Black-Scholes option-pricing model based on the following assumptions: Three Months Ended March 31, 2019 2018 Risk-free interest rate 2.49 % 2.71 % Expected volatility 82.13 % 80.66 % Expected dividend yield — — Expected term (in years) 6.06 6.04 As of March 31, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $20.5 million, which is expected to be recognized over a weighted average period of 3.19 years. Stock-Based Compensation The Company recorded stock-based compensation expense in the following expense categories of its statements of operations and comprehensive loss (in thousands): Three Months Ended March 31, 2019 2018 Research and development expenses $ 524 $ 211 General and administrative expenses 789 374 Total $ 1,313 $ 585

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	12. Leases The Company has leases for office and laboratory space. The Company occupies approximately 36,309 square feet of office and laboratory space in Cambridge, Massachusetts under a lease that currently expires in June 2020 and an additional 11,237 of office space in the same facility which expires in February 2022. We determined that these leases are operating leases. We recognize our minimum rental expense on a straight-line basis over the term of the lease beginning with the date of initial control of the asset. With the adoption of ASC 842 we recognized all leases with terms greater than 12 months in duration on our Consolidated Balance Sheets as right-of-use assets and lease liabilities as of January 1, 2019. We adopted the standard using the modified retrospective approach. Upon adoption of ASC 842 on January 1, 2019, we recorded operating lease assets of $3.1 million and operating lease liabilities of $3.5 million. The adoption of ASC 842 did not have a material impact on our condensed consolidated statements of operations. Prior periods are presented in accordance with ASC 840, Leases We have made certain assumptions and judgments when applying ASC 842, the most significant of which are: • We elected the package of practical expedients available for transition which allow us to not (i) reassess whether expired or existing contracts contain leases under the new definition of a lease, (ii) determine lease classification for expired or existing leases and (iii) determine whether previously capitalized initial direct costs would qualify for capitalization under ASC 842. • We did not elect to use hindsight when considering judgments and estimates such as assessments of lessee options to extend or terminate a lease or purchase the underlying asset. • For all asset classes, we elected to not recognize a right-of-use asset and lease liability for short-term leases of less than 12 months. • For all asset classes, we elected to not separate non-lease components from lease components to which they relate and have accounted for the combined lease and non-lease components as a single lease component. • We use our incremental borrowing rate to calculate the present value of our lease payments, as the implicit rates in our leases are not readily determinable. As of March 31, 2019, assets under operating lease were $4.7 million. The elements of lease expense were as follows (in thousands): For Three Months Ended March 31, 2019 Lease cost: Operating lease cost 695 Variable lease cost (1) 90 Total Lease cost 785 Other information: Operating cash flows used for operating leases 664 Operating lease liabilities arising from obtaining right-of-use assets 5,155 Weighted-average remaining lease term in years 2.1 Weighted-average discount rate 10.37 % (1) The variable lease costs for the three months ended March 31, 2019 include common area maintenance charges. Future minimum lease payments under the operating lease as of March 31, 2019 are as follows (in thousands): Year Ending December 31, 2019 2,496 2020 2,123 2021 898 2022 167 $ 5,684 Present value adjustment (529 ) Present value of lease liabilities $ 5,155 Rent expense for each of the three months ended March 31, 2019 and 2018 was $0.7 million and $0.6 million, respectively.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Research Agreements The LLS Agreement requires the Company to make certain milestone payments to LLS, that could total up to $25.0 million in the aggregate, upon the receipt of payments by the Company associated with the licensing or transfer of rights to the related compound (or a product) to a third party, upon first regulatory approval of a product in the U.S., or upon the first regulatory approval of a product in Europe or Japan. As of March 31, 2019, and December 31, 2018, no events have occurred that would require payment of the milestones. The Company has several in-license agreements with academic organizations. The Company is obligated to pay annual license maintenance fees of less than $0.1 million per year as well as reimburse certain institutions for costs incurred related to the filing, prosecution and maintenance of patent rights licensed under the agreements. In addition, the Company may be obligated to pay contingent milestone payments of up to a maximum of $15.7 million upon the achievement of certain defined events as well as royalties of low single-digit percentages of sales of licensed products. In certain cases, the maximum payments to the academic organizations are capped. If the Company grants any sublicense rights under the license agreements, the Company has agreed to pay a percentage of sublicense fees received by the Company to the licensors. As of March 31, 2019, and December 31, 2018, no events have occurred that would require payment of the milestones, royalties, or sublicense fees. 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 including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers 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 or officers. 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 such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its financial statements as of March 31, 2019 or December 31, 2018. Legal Proceedings At each reporting date, we evaluate whether or not a potential loss amount or a potential range of losses is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. We expense as incurred the costs related to such legal proceedings. On January 17, 2017, a participant dosed in one of our clinical trials filed a complaint against us in the United States District Court for the District of Arizona, alleging negligence, lack of informed consent, strict products liability and loss of consortium. We filed an answer in March 2017. A dispositive motion is currently pending with the District Court and has yet to be decided. The plaintiff claims damages of $1.5 million. We are working with counsel and our insurer to vigorously defend our position. We believe that we have meritorious defenses, however unfavorable outcome of some amount is reasonably possible.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	14. Net Loss Per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (19,426 ) $ (12,102 ) Net loss attributable to common stockholders $ (19,426 ) $ (12,102 ) Denominator: Weighted average common shares outstanding, basic and diluted 25,804,595 972,981 Net loss per share attributable to common stockholders, basic and diluted $ (0.75 ) $ (12.44 ) The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, 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 Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: March 31, 2019 2018 Convertible preferred shares (as converted to common stock) — 17,663,337 Warrants for the purchase of convertible preferred stock (as converted to common stock) — 34,062 Warrants for the purchase of common stock 95,930 113,541 Options to purchase common stock 4,712,054 2,223,917 4,807,984 20,034,857

Retirement Plan

Retirement Plan	3 Months Ended
	Mar. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Retirement Plan	15. Retirement Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. As currently established, the Company is not required to make contributions to the 401(k) Plan. The Company made matching contributions of $0.1 million for each of the three month periods ended March 31, 2019 and 2018.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Unaudited interim consolidated financial information	Unaudited Interim Consolidated Financial Information The accompanying unaudited condensed consolidated financial statements as of March 31, 2019 and for the three months ended March 31, 2019 and 2018 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim consolidated financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 (the “Annual Report”). The unaudited condensed consolidated financial statements include the accounts of Constellation Pharmaceuticals, Inc. and its wholly owned subsidiary, Constellation Securities Corporation. All intercompany transactions and balances of the subsidiary have been eliminated in consolidation. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of March 31, 2019 and results of operations for the three months ended March 31, 2019 and 2018, and cash flows for the three months ended March 31, 2019 and 2018 have been made. The Company’s results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019.
Concentrations of credit risk and of significant suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains most of its cash, cash equivalents and marketable securities at two accredited financial institutions in amounts that could exceed federally insured limits. Cash equivalents are invested in an institutional money market fund. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Marketable Securities	Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days. The Company classifies its investments with maturities beyond one year as short term based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of marketable securities as available-for-sale. Accordingly, these marketable securities are recorded at fair value and unrealized gains and losses are reported as a component of accumulated other comprehensive loss in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Comprehensive loss	Comprehensive Loss Comprehensive loss consists of net loss and unrealized gain (losses) on available-for-sale marketable securities.
Cash, cash equivalents and restricted cash	Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents consists of all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. As of March 31, 2019, the Company classified $425,000 as restricted cash related to a letter of credit issued as a security deposit in connection with Company's lease of its corporate office facilities (Note 12). Cash, cash equivalents and restricted cash consists of the following: March 31, 2019 December 31, 2018 Cash and cash equivalents $ 52,447 $ 114,592 Restricted cash 425 425 Cash, cash equivalents and restricted cash $ 52,872 $ 115,017
Fair value measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Quoted prices in active markets for identical assets or liabilities. • Level 2 – Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding debt as of March 31, 2019 (see Note 7) approximated fair value (a Level 3 measurement) based on interest rates currently available to the Company.
Revenue recognition	Revenue Recognition On January 1, 2018, the Company adopted the new revenue standard, discussed below under the heading “Recently Adopted Accounting Pronouncements”, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries (“ASC 606”). Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the contract(s) with the customer, (ii) identification of the promised goods or services in the contract and determination of whether the promised goods or services are performance obligations, (iii) measurement of the transaction price, (iv) allocation of the transaction price to the performance obligations, and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that it will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The Company accounts for a contract with a customer that is within the scope of ASC 606 when all of the following criteria are met: (i) the arrangement has been approved by the parties and the parties are committed to perform their respective obligations, (ii) each party’s rights regarding the goods or services to be transferred can be identified, (iii) the payment terms for the goods or services to be transferred can be identified, (iv) the arrangement has commercial substance and (v) collection of substantially all of the consideration to which the Company will be entitled in exchange for the goods or services that will be transferred to the customer is probable. The Company estimates the transaction price based on the amount of consideration the Company expects to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of the potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the transaction price based on which method better predicts the amount of consideration expected to be received. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. For arrangements that include development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such milestones and any related constraint, and if necessary, adjusts the estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue and net income (loss) in the period of adjustment. For sales-based royalties, including milestone payments based on the level of sales, the Company determines whether the sole or predominant item to which the royalties relate is a license. When the license is the sole or predominant item to which the sales-based royalty relates, the Company recognizes revenue at the later of: (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). The Company allocates the transaction price based on the estimated standalone selling price. The Company must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Certain variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated to each performance obligation are consistent with the amounts the Company would expect to receive for each performance obligation. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation in order to determine whether the combined performance obligation is satisfied over time or at a point in time. The Company determines the appropriate method of measuring progress of combined performance obligations satisfied over time for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company receives payments from customers based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Recently adopted accounting pronouncements	Recently adopted accounting pronouncements In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which requires lessees to recognize leases on the balance sheet and disclose key information about leasing arrangements. The Company adopted ASU 2016-02 as of January 1, 2019, using the modified retrospective approach. Prior period amounts have not been adjusted. The main difference between previous GAAP (“Topic 840”) and Topic 842 is the recognition of right-of-use lease assets and lease liabilities by lessees for those leases classified as operating leases under Topic 840. In addition, the Company elected the following practical expedients: • the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed the Company to carry forward the historical lease classification; • the short-term lease practical expedient, which allowed the Company to exclude short-term leases of less than 12 months from recognition in the unaudited consolidated balance sheets; and • the bifurcation of lease and non-lease components practical expedient, which did not require the Company to bifurcate lease and non-lease components for all classes of assets. The adoption of this accounting standard resulted in the recording of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months of $3.1 million and $3.5 million, respectively, as of January 1, 2019. The difference between the operating lease assets and liabilities was recorded as an adjustment to “Other liabilities” on the consolidated and condensed balance sheets, primarily related to deferred rent (lease incentives). The adoption of ASU 2016-02 had no impact on Retained earnings. For additional information regarding how the Company is accounting for leases under Topic 842, refer to Note 12, Leases
Recently issued accounting pronouncements	Recently issued accounting pronouncements In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception . Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents and Restricted Cash	Cash, cash equivalents and restricted cash consists of the following: March 31, 2019 December 31, 2018 Cash and cash equivalents $ 52,447 $ 114,592 Restricted cash 425 425 Cash, cash equivalents and restricted cash $ 52,872 $ 115,017

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following tables present information about the Company’s financial assets and liabilities 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): March 31, 2019 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds included in cash and cash equivalents $ 47,059 $ — $ — $ 47,059 Commercial paper — 5,388 — 5,388 Total $ 47,059 $ 5,388 $ — $ 52,447 Marketable securities: Corporate debt securities $ — $ 10,661 $ — $ 10,661 Commercial paper — $ 34,781 — 34,781 Government securities — 17,902 — 17,902 Total $ — $ 63,344 $ — $ 63,344 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds included in cash and cash equivalents $ 114,592 $ — $ — $ 114,592 $ 114,592 $ — $ — $ 114,592

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Schedule of Marketable Securities and Cash Equivalents	The following table summarizes the Company’s marketable securities and cash equivalents as of March 31, 2019. The Company did not hold any marketable securities as of December 31, 2018. March 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Cash equivalents: Money market funds $ 47,059 $ — $ — $ 47,059 Commercial paper 5,388 — — 5,388 Total cash equivalents $ 52,447 $ — $ — $ 52,447 Marketable securities: Corporate debt securities $ 10,654 $ 7 $ — $ 10,661 Commercial paper 34,781 — — 34,781 Government securities 17,900 2 — 17,902 Total marketable securities $ 63,335 $ 9 $ — $ 63,344 Total cash equivalents and marketable securities $ 115,782 $ 9 $ — $ 115,791

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, 2019 December 31, 2018 Accrued employee compensation and benefits $ 1,569 $ 2,726 Accrued external research and development expense 4,576 5,610 Accrued professional fees 302 255 Other 181 346 $ 6,628 $ 8,937

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Components of Notes Payable	As of March 31, 2019, notes payable consisted of the following: March 31, 2019 Principal amount of term loans $ 20,000 Debt discount current portion — Less: Current portion — Long-term debt, net of current portion 20,000 Debt discount net of current portion (471 ) Long-term debt, net of discount and current portion $ 19,529

Equity (Tables)

Equity (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Schedule of Warrant Activities	As of March 31, 2019, the Company had outstanding warrants to purchase common stock as follows: Issuance Date Term (in years) Exercise Price Number of Common Shares Issuable under Warrant May 23, 2011 10 $ 1.55 61,868 June 28, 2013 10 $ 13.22 11,354 September 30, 2014 10 $ 13.22 22,708 95,930

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Company's Option Activity	The following is a summary of stock option activity for the three months ended March 31, 2019, Weighted- Average Remaining Aggregate Stock Weighted Average Contractual Term Intrinsic Value Options Exercise Price (in years) (in thousands) Outstanding as of December 31, 2018 3,779,403 $ 7.78 8.80 $ 110 Granted 1,115,273 $ 8.90 Exercised (3,839 ) $ 5.58 Forfeited (178,783 ) $ 8.60 Outstanding as of March 31, 2019 4,712,054 $ 8.01 8.87 $ 26,092 Vested and expected to vest at March 31, 2019 4,712,054 $ 8.01 8.87 $ 26,094 Exercisable at March 31, 2019 1,174,589 $ 6.43 7.74 $ 8,367
Schedule of Weighted Average Basis Assumptions Used in Black-Scholes Option-Pricing Model	The Company estimated the fair value of each stock option award using the Black-Scholes option-pricing model based on the following assumptions: Three Months Ended March 31, 2019 2018 Risk-free interest rate 2.49 % 2.71 % Expected volatility 82.13 % 80.66 % Expected dividend yield — — Expected term (in years) 6.06 6.04
Schedule of Stock Based Compensation Expense Related to Statements of Operations and Comprehensive Loss	The Company recorded stock-based compensation expense in the following expense categories of its statements of operations and comprehensive loss (in thousands): Three Months Ended March 31, 2019 2018 Research and development expenses $ 524 $ 211 General and administrative expenses 789 374 Total $ 1,313 $ 585

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Elements of Lease Expense	As of March 31, 2019, assets under operating lease were $4.7 million. The elements of lease expense were as follows (in thousands): For Three Months Ended March 31, 2019 Lease cost: Operating lease cost 695 Variable lease cost (1) 90 Total Lease cost 785 Other information: Operating cash flows used for operating leases 664 Operating lease liabilities arising from obtaining right-of-use assets 5,155 Weighted-average remaining lease term in years 2.1 Weighted-average discount rate 10.37 % (1) The variable lease costs for the three months ended March 31, 2019 include common area maintenance charges.
Schedule of Future Minimum Lease Payments Under the Operating Lease	Future minimum lease payments under the operating lease as of March 31, 2019 are as follows (in thousands):

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Calculations of Basic and Diluted Net Loss per Share	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (19,426 ) $ (12,102 ) Net loss attributable to common stockholders $ (19,426 ) $ (12,102 ) Denominator: Weighted average common shares outstanding, basic and diluted 25,804,595 972,981 Net loss per share attributable to common stockholders, basic and diluted $ (0.75 ) $ (12.44 )
Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss Per Share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: March 31, 2019 2018 Convertible preferred shares (as converted to common stock) — 17,663,337 Warrants for the purchase of convertible preferred stock (as converted to common stock) — 34,062 Warrants for the purchase of common stock 95,930 113,541 Options to purchase common stock 4,712,054 2,223,917 4,807,984 20,034,857

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019USD ($)Tranche	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of tranches in loan agreement | Tranche	4
Line of credit facility, maximum borrowing capacity tranche one	$ 19,500,000
Net losses	19,426,000	$ 12,102,000	$ 59,900,000
Accumulated deficit	253,263,000		$ 233,837,000
Loan Agreement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity tranche four	$ 40,000,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Restricted cash	$ 425		$ 425
Lease liability	5,155	$ 3,500
Right-of-use lease assets - operating	4,700	$ 3,100
Letter of Credit [Member]
Significant Accounting Policies [Line Items]
Restricted cash	$ 425

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 52,447	$ 114,592
Restricted cash	425	425
Cash, cash equivalents and restricted cash	$ 52,872	$ 115,017	$ 71,744	$ 16,646

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash equivalents:
Cash and cash equivalents	$ 52,447
Marketable securities:
Marketable securities	63,344
Cash and cash equivalents	52,447
Total assets		$ 114,592
Commercial Paper [Member]
Cash equivalents:
Cash and cash equivalents	5,388
Marketable securities:
Marketable securities	34,781
Cash and cash equivalents	5,388
Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash and cash equivalents	47,059
Marketable securities:
Cash and cash equivalents	47,059
Total assets		114,592
Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash and cash equivalents	5,388
Marketable securities:
Marketable securities	63,344
Cash and cash equivalents	5,388
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Cash equivalents:
Cash and cash equivalents	5,388
Marketable securities:
Marketable securities	34,781
Cash and cash equivalents	5,388
Money Market Funds [Member]
Cash equivalents:
Cash and cash equivalents	47,059	114,592
Marketable securities:
Cash and cash equivalents	47,059	114,592
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash and cash equivalents	47,059	114,592
Marketable securities:
Cash and cash equivalents	47,059	$ 114,592
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	10,661
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Marketable securities	10,661
Government Securities [Member]
Marketable securities:
Marketable securities	17,902
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Marketable securities	$ 17,902

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value of assets transfers level 1 to level 2	$ 0	$ 0
Fair value of assets transfers level 2 to level 1	0	0
Fair value of liabilities transfers level 1 to level 2	0	0
Fair value of liabilities transfers level 2 to level 1	$ 0	$ 0

Marketable Securities - Schedul

Marketable Securities - Schedule of Marketable Securities and Cash Equivalents (Details) $ in Thousands	Mar. 31, 2019USD ($)
Cash And Cash Equivalents [Line Items]
Amortized Cost	$ 115,782
Unrealized Gains	9
Fair Value	115,791
Total Cash Equivalents [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	52,447
Fair Value	52,447
Total marketable securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	63,335
Unrealized Gains	9
Fair Value	63,344
Corporate Debt Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	10,654
Unrealized Gains	7
Fair Value	10,661
Commercial Paper [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	34,781
Fair Value	34,781
Government Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	17,900
Unrealized Gains	2
Fair Value	17,902
Money Market Funds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	47,059
Fair Value	47,059
Commercial Paper [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	5,388
Fair Value	$ 5,388

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued employee compensation and benefits	$ 1,569	$ 2,726
Accrued external research and development expense	4,576	5,610
Accrued professional fees	302	255
Other	181	346
Accrued expenses and other current liabilities	$ 6,628	$ 8,937

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - LLS Agreement [Member] - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone method revenue recognized	$ 7,300,000	$ 7,300,000
Milestone payments maximum amount	$ 25,000,000

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 20, 2019	Mar. 31, 2019
Debt Instrument [Line Items]
Loan amount		$ 20,000
2016 Loan Agreement [Member]
Debt Instrument [Line Items]
Loan amount		$ 11,800
Annual interest rate		7.60%
Percentage of original principal amount as final payment		5.00%
Loan Agreement [Member]
Debt Instrument [Line Items]
Loan amount		$ 20,000
Term loan principal amount	$ 40,000
Debt instrument, interest rate terms	The term loan bears interest at an annual rate equal to the greater of 8.55% and the prime rate of interest plus 2.55%.
Frequency of periodic payment	The Loan Agreement provides for interest-only payments until April 30, 2021, and repayment of the aggregate outstanding principal balance of the term loan in monthly installments starting on May 1, 2021 and continuing through April 1, 2023 (the “Maturity Date”).
Loan Agreement maturity date	Apr. 1, 2023
Debt instrument, fee description	The Company paid a fee of $255,000 upon closing and are required to pay a fee of 6.35% of the aggregate amount of advances under the Loan Agreement at maturity.
Debt instrument closing fee	$ 255,000
Debt instrument, redemption, description	At its option, the Company may elect to prepay all or a portion of the outstanding advances by paying the entire principal balance (or a portion thereof) and all accrued and unpaid interest thereon plus a prepayment charge equal to the following percentage of the principal amount being prepaid: 2% if an advance is prepaid during the first 12 months following the applicable advance date, 1% if an advance is prepaid after 12 months but prior to 24 months following the applicable advance date, and 0.5% if an advance is prepaid any time after 24 months following the applicable advance date but prior to the Maturity Date.

Debt - Components of Notes Paya

Debt - Components of Notes Payable (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Debt Disclosure [Abstract]
Principal amount of term loans	$ 20,000
Long-term debt, net of current portion	20,000
Debt discount net of current portion	(471)
Long-term debt, net of discount and current portion	$ 19,529

Warrants to Purchase Converti_2

Warrants to Purchase Convertible Preferred Stock - Additional Information (Detail) - shares	Mar. 31, 2019	Jul. 31, 2018
Common Stock [Member]
Class of Warrant or Right Line Items
Number of preferred shares issuable under warrant	95,930	34,062
Series B Preferred Stock [Member]
Class of Warrant or Right Line Items
Number of preferred shares issuable under warrant	375,000

Equity - Additional Information

Equity - Additional Information (Detail) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par value	$ 0.0001	$ 0.0001

Equity - Schedule of Warrant Ac

Equity - Schedule of Warrant Activities (Detail) - Common Stock [Member] - $ / shares	3 Months Ended
	Mar. 31, 2019	Jul. 31, 2018
Class of Warrant or Right [Line Items]
Number of Common Shares Issuable under Warrant	95,930	34,062
Warrant One [Member]
Class of Warrant or Right [Line Items]
Warrant Issue date	May 23, 2011
Warrant Term	10 years
Warrants exercise price	$ 1.55
Number of Common Shares Issuable under Warrant	61,868
Warrant Two [Member]
Class of Warrant or Right [Line Items]
Warrant Issue date	Jun. 28, 2013
Warrant Term	10 years
Warrants exercise price	$ 13.22
Number of Common Shares Issuable under Warrant	11,354
Warrant Three [Member]
Class of Warrant or Right [Line Items]
Warrant Issue date	Sep. 30, 2014
Warrant Term	10 years
Warrants exercise price	$ 13.22
Number of Common Shares Issuable under Warrant	22,708

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended
	Jan. 31, 2019	Jun. 30, 2018	Mar. 31, 2019	Jul. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost related to unvested stock-based awards			$ 20.5
Unvested stock based weighted average period			3 years 2 months 8 days
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted			1,115,273
Weighted average exercise price			$ 8.90
Weighted average grant-date fair value per share			$ 6.33
2008 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting term			4 years
Expiration period			10 years
Number of shares reserved for issuance			4,039,829
Shares remained available for issuance				245,557
2018 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		2,779,544
Shares remained available for issuance			1,919,540
Increase in shares reserved for issuance under the plan	1,032,125
2018 Plan [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock available for issuance		2,216,368
Percentage of shares of common stock available for issuance		4.00%
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		272,504
ESPP [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock available for issuance		545,008
Percentage of shares of common stock available for issuance		1.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Company's Option Activity (Detail) - Employee Stock Option [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Outstanding, Beginning Balance	3,779,403
Number of Shares, Granted	1,115,273
Number of Shares, Exercised	(3,839)
Number of Shares, Forfeited	(178,783)
Number of Shares Outstanding, Ending Balance	4,712,054	3,779,403
Number of Shares, Vested and expected to vest	4,712,054
Number of Shares, Options exercisable	1,174,589
Weighted Average Exercise Price Outstanding, Beginning Balance	$ 7.78
Weighted Average Exercise Price, Granted	8.90
Weighted Average Exercise Price, Exercised	5.58
Weighted Average Exercise Price, Forfeited	8.60
Weighted Average Exercise Price Outstanding, Ending Balance	8.01	$ 7.78
Weighted Average Exercise Price, Vested and expected to vest	8.01
Weighted Average Exercise Price, Options exercisable	$ 6.43
Weighted Average Contractual Term (in years) Outstanding	8 years 10 months 13 days	8 years 9 months 18 days
Weighted Average Contractual Term (in years), Vested and expected to vest	8 years 10 months 13 days
Weighted Average Contractual Term (in years), Options exercisable	7 years 8 months 26 days
Aggregate Intrinsic Value Outstanding, Beginning Balance	$ 26,092	$ 110
Aggregate Intrinsic Value, Vested and expected to vest	26,094
Aggregate Intrinsic Value, Options exercisable	$ 8,367

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Weighted Average Basis Assumptions Used in Black-Scholes Option-Pricing Model (Detail) - Valuation Technique, Option Pricing Model	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.49%	2.71%
Expected volatility	82.13%	80.66%
Expected term (in years)	6 years 21 days	6 years 14 days

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 1,313	$ 585
Research and Development Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	524	211
General and Administrative Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 789	$ 374

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	Feb. 28, 2019ft²	Mar. 31, 2019USD ($)ft²	Mar. 31, 2018USD ($)	Jan. 01, 2019USD ($)
Leases [Abstract]
Office and laboratory space | ft²	11,237	36,309
Lease expiration date	Feb. 28, 2022	Jun. 30, 2020
Operating lease, right-of-use assets		$ 4,700		$ 3,100
Lease liability		5,155		$ 3,500
Rent expenses		$ 700	$ 600

Leases - Elements of Lease Expe

Leases - Elements of Lease Expense (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Lease cost:
Operating lease cost	$ 695
Variable lease cost	90	[1]
Total Lease cost	785
Operating cash flows used for operating leases	664
Operating lease liabilities arising from obtaining right-of-use assets	$ 5,155
Weighted-average remaining lease term in years	2 years 1 month 6 days
Weighted-average discount rate	10.37%
[1]	The variable lease costs for the three months ended March 31, 2019 include common area maintenance charges.

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under the Operating Lease (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2019	$ 2,496
2020	2,123
2021	898
2022	167
Total future minimum lease payments	5,684
Present value adjustment	(529)
Lease liability	$ 5,155	$ 3,500

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019USD ($)
Commitments And Contingencies [Line Items]
License maintenance fees	$ 100,000
Contingent milestone payments	15,700,000
Plaintiff claims damages	1,500,000
LLS Agreement [Member]
Commitments And Contingencies [Line Items]
Milestone payments maximum amount	$ 25,000,000

Net Loss Per Share - Calculatio

Net Loss Per Share - Calculations of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Numerator:
Net loss	$ (19,426)	$ (12,102)	$ (59,900)
Net loss attributable to common stockholders	$ (19,426)	$ (12,102)
Denominator:
Weighted average common shares outstanding, basic and diluted	25,804,595	972,981
Net loss per share attributable to common stockholders, basic and diluted	$ (0.75)	$ (12.44)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	4,807,984	20,034,857
Convertible preferred shares (as converted to common stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities		17,663,337
Warrants for the purchase of convertible preferred stock (as converted to common stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities		34,062
Warrants for the purchase of common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	95,930	113,541
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	4,712,054	2,223,917

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Matching contributions made	$ 0.1	$ 0.1
Defined contribution plan name	401(k) Plan