Verastem (VSTM) 10-K 12 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 07, 2019	Jun. 30, 2018
Document and Entity Information
Entity Registrant Name	Verastem, Inc.
Entity Central Index Key	0001526119
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 503,007,333
Entity Common Stock, Shares Outstanding		73,865,036
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 129,867	$ 82,176
Short-term investments	119,786	4,496
Accounts receivable, net	306
Inventory	327
Prepaid expenses and other current assets	2,973	1,115
Total current assets	253,259	87,787
Property and equipment, net	1,369	861
Intangible assets, net	21,577
Other assets	1,031	1,143
Total assets	277,236	89,791
Current liabilities:
Accounts payable	10,253	9,186
Accrued expenses	21,108	7,942
Current portion of long-term debt	5,716
Total current liabilities	37,077	17,128
Non-current liabilities:
Long-term debt	19,506	14,828
Convertible senior notes	95,231
Other non-current liabilities	1,123	151
Total liabilities	152,937	32,107
Stockholders' equity
Preferred stock, $0.0001 par value; 5,000 shares authorized, no shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively
Common stock, $0.0001 par value; 200,000 and 100,000 shares authorized, 73,806 and 50,801 shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	7	5
Additional paid-in capital	499,741	360,823
Accumulated other comprehensive income (loss)	127	(2)
Accumulated deficit	(375,576)	(303,142)
Total stockholders' equity	124,299	57,684
Total liabilities and stockholders' equity	$ 277,236	$ 89,791

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
CONSOLIDATED BALANCE SHEETS
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
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 (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	100,000,000
Common stock, shares issued	73,806,000	50,801,000
Common stock, shares outstanding	73,806,000	50,801,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Total revenue	$ 26,718
Operating expenses:
Costs of revenues, excluding amortization of acquired intangible assets	165
Research and development	43,648	$ 46,423	$ 19,779
Selling, general and administrative	77,265	21,381	17,223
Amortization of acquired intangible assets	423
Total operating expenses	121,501	67,804	37,002
Loss from operations	(94,783)	(67,804)	(37,002)
Other income	25,556
Interest income	2,603	561	562
Interest expense	(5,810)	(559)
Net loss	$ (72,434)	$ (67,802)	$ (36,440)
Net loss per share—basic	$ (1.12)	$ (1.76)	$ (0.99)
Net loss per share—diluted	$ (1.37)	$ (1.76)	$ (0.99)
Weighted average common shares outstanding used in computing:
Net loss per share—basic (in shares)	64,962	38,422	36,988
Net loss per share—diluted (in shares)	69,321	38,422	36,988
Net loss	$ (72,434)	$ (67,802)	$ (36,440)
Unrealized gain (loss) on available-for-sale securities	129	(31)	(14)
Comprehensive loss	(72,305)	$ (67,833)	$ (36,454)
Product
Revenue
Total revenue	1,718
License
Revenue
Total revenue	$ 25,000

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stockFollow-on offerings	Common stockAt-the-market equity offering program	Common stock	Additional paid-in capitalFollow-on offerings	Additional paid-in capitalAt-the-market equity offering program	Additional paid-in capital	Accumulated other comprehensive (loss) income	Accumulated deficit	Follow-on offerings	At-the-market equity offering program	Total
Balance at Dec. 31, 2015			$ 4			$ 301,305	$ 43	$ (198,883)			$ 102,469
Balance (in shares) at Dec. 31, 2015			36,941,261
Increase (Decrease) in Stockholders' Equity
Net loss								(36,440)			(36,440)
Unrealized gain (loss) on available-for-sale marketable securities							(14)				(14)
Issuance of common stock resulting from exercise of stock options (in shares)			1,605
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings						(5)					(5)
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings (in shares)			49,552
Stock-based compensation expense						6,287					6,287
Balance at Dec. 31, 2016			$ 4			307,587	29	(235,323)			72,297
Balance (in shares) at Dec. 31, 2016			36,992,418
Increase (Decrease) in Stockholders' Equity
Net loss								(67,802)			(67,802)
Unrealized gain (loss) on available-for-sale marketable securities							(31)				(31)
Issuance of common stock, net of issuance costs	$ 1			$ 24,691	$ 23,053				$ 24,692	$ 23,053
Issuance of common stock, net of issuance costs (in shares)	8,422,877	5,036,879
Issuance of common stock resulting from exercise of stock options						442					442
Issuance of common stock resulting from exercise of stock options (in shares)			348,734
Stock-based compensation expense						5,050		(17)			5,033
Balance at Dec. 31, 2017			$ 5			360,823	(2)	(303,142)			$ 57,684
Balance (in shares) at Dec. 31, 2017			50,800,908								50,801,000
Increase (Decrease) in Stockholders' Equity
Net loss								(72,434)			$ (72,434)
Unrealized gain (loss) on available-for-sale marketable securities							129				129
Issuance of common stock, net of issuance costs	$ 1	$ 1		$ 81,188	$ 24,275				$ 81,189	$ 24,276
Issuance of common stock, net of issuance costs (in shares)	16,111,110	6,481,475
Issuance of common stock resulting from exercise of stock options						809					809
Issuance of common stock resulting from exercise of stock options (in shares)			412,851
Stock-based compensation expense						6,671					6,671
Reclassification of derivative liability to equity						25,975					25,975
Balance at Dec. 31, 2018			$ 7			$ 499,741	$ 127	$ (375,576)			$ 124,299
Balance (in shares) at Dec. 31, 2018			73,806,344								73,806,000

CONSOLIDATED STATEMENTS OF ST_2

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Follow-on offerings
Issuance of common stock, issuance costs	$ 361	$ 324
At-the-market equity offering program
Issuance of common stock, issuance costs	$ 0	$ 112

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (72,434)	$ (67,802)	$ (36,440)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	996	556	670
Amortization of acquired intangible asset	423
Stock-based compensation expense	6,671	5,033	6,287
Amortization of deferred financing costs, debt discounts and premiums and discounts on available-for-sale marketable securities	1,814	223	(140)
Change in fair value of conversion option for convertible senior notes	(25,556)
Gain on sale of fixed assets	(79)
Changes in operating assets and liabilities:
Accounts receivable, net	(306)
Inventory	(327)
Prepaid expenses, other current assets and other assets	(1,167)	(943)	(568)
Accounts payable	1,048	5,046	153
Accrued expenses and other liabilities	13,902	577	554
Other long-term liabilities	500
Net cash used in operating activities	(74,515)	(57,310)	(29,484)
Investing activities
Purchases of property and equipment	(1,507)		(39)
Sales of property and equipment	82
Acquisition of intangible asset	(22,000)
Purchases of investments	(125,452)	(7,957)	(82,101)
Maturities of investments	10,500	51,910	119,067
Net cash (used in) provided by investing activities	(138,377)	43,953	36,927
Financing activities
Proceeds from long-term debt, net of issuance costs	9,900	14,811
Deferred debt financing costs		(138)
Proceeds from issuance of convertible senior notes, net of issuance costs	145,297
Proceeds from the exercise of stock options	809	442
Cash used to settle restricted stock liability			(5)
Proceeds from the issuance of common stock, net	105,156	48,069
Net cash provided by (used in) financing activities	261,162	63,184	(5)
Increase in cash, cash equivalents and restricted cash	48,270	49,827	7,438
Cash, cash equivalents and restricted cash at beginning of period	82,338	32,511	25,073
Cash, cash equivalents and restricted cash at end of period	130,608	82,338	$ 32,511
Supplemental disclosure
Cash paid for interest	2,107	295
Supplemental disclosure of non-cash financing activities
Common stock issuance costs included in accounts payable and accrued expenses	$ 15	$ 324

Nature of business

Nature of business	12 Months Ended
	Dec. 31, 2018
Nature of business
Nature of business	1. Nature of business Verastem, Inc. (the Company) is a biopharmaceutical company focused on developing and commercializing medicines to improve the survival and quality of life of cancer patients. O n September 24, 2018, the Company’s first commercial product, COPIKTRA™ (duvelisib), was approved by the U.S. Food and Drug Administration (the FDA) for the treatment of patients with hematologic cancers including chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL) and follicular lymphoma (FL). Both its marketed product, COPIKTRA, and most advanced product candidate, defactinib, utilize a multi-faceted approach designed to treat cancers originating either in the blood or major organ systems. The Company is currently developing its product candidates in both preclinical and clinical studies as potential therapies for certain cancers, including leukemia, lymphoma, lung cancer, ovarian cancer, mesothelioma, and pancreatic cancer. The Company believes that these compounds may be beneficial as therapeutics either as single agents or when used in combination with immuno-oncology agents or other current and emerging standard of care treatments in aggressive cancers that are poorly served by currently available therapies. The Company is subject to the risks associated with other life science companies, including, but not limited to, possible failure of preclinical testing or clinical trials, competitors developing new technological innovations, market acceptance and the successful commercialization of COPIKTRA, or any of the Company’s investigational product candidates following receipt of regulatory approval and protection of proprietary technology. If the Company does not successfully commercialize COPIKTRA or any of its other product candidates, it will be unable to generate product revenue or achieve profitability and may need to raise additional capital. The Company has historical losses from operations and anticipates that it will continue to incur losses for the foreseeable future as it continues the commercialization of COPIKTRA and the research and development of its product candidates. As of December 31, 2018, the Company had cash, cash equivalents and short-term investments of $249.7 million. The Company expects, prior to the consideration of any revenue from future potential sales of COPIKTRA, that its cash, cash equivalents and short-term investments will be sufficient to fund its development plans, U.S. commercial scale-up and other operational activities for at least twelve months from the date of issuance of these consolidated financial statements. The Company expects to finance the future development costs of its clinical product portfolio with its existing cash, cash equivalents and short-term investments, or through strategic financing opportunities that could include, but are not limited to collaboration agreements, future offerings of its equity, or the incurrence of debt. However, there is no guarantee that any of these strategic or financing opportunities will be executed or executed on favorable terms, and some could be dilutive to existing stockholders. If the Company fails to obtain additional future capital, it may be unable to complete its planned preclinical studies and clinical trials and obtain approval of certain investigational product candidates from the FDA or foreign regulatory authorities.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2018
Significant accounting policies
Summary of significant accounting policies	2. Significant accounting policies Basis of presentation The accompanying financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) under the assumption that the Company will continue as a going concern for the next twelve months. Accordingly, they do not include any adjustments that might result from the uncertainty related to the Company’s ability to continue as a going concern. The consolidated financial statements include the accounts of Verastem Securities Company, a wholly-owned subsidiary of the Company. All financial information presented has been consolidated and includes the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation. Use of estimates The preparation of the Company’s financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including estimates related to revenue recognition, including returns, rebates, and other pricing adjustments, accruals and stock‑based compensation expense. The Company bases its estimates on historical experience and other market‑specific or other relevant assumptions that it believes to be reasonable. Actual results could differ from such estimates. Segment and geographic information Operating segments are defined as components of an enterprise about which separate discrete information is available and regularly reviewed by the chief operating decision maker, or decision‑making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment, which is the business of developing and commercializing drugs for the treatment of cancer. All material long-lived assets of the Company reside in the United States. Cash, cash equivalents and restricted cash The Company considers all highly liquid investments with an original or remaining maturity of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of a U.S. Government money market funds and corporate bonds and commercial paper of publicly traded companies. Cash equivalents are reported at fair value. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2018 December 31, 2017 Cash and cash equivalents $ 129,867 $ 82,176 Restricted cash 741 162 Total cash, cash equivalents and restricted cash $ 130,608 $ 82,338 Amounts included in restricted cash as of December 31, 2018 represent cash received pursuant to a funded research and development agreement with the Leukemia and Lymphoma Society (LLS) restricted for future expenditures for specific R&D studies and cash held to collateralize outstanding letters of credit provided as a security deposit for the Company’s office space located in Needham, Massachusetts in the amount of approximately $500,000 and $241,000, respectively. Restricted cash related to LLS funded research and development agreement is included in prepaid and other current assets, while restricted cash for letters of credit are included in other assets on the consolidated balance sheets. Amounts included in restricted cash as of December 31, 2017 represent cash held to collateralize outstanding letters of credit in the amount of $162,000 provided as a security deposit for the Company’s office space located in Needham, Massachusetts and included in other assets on the consolidated balance sheets. Fair value of financial instruments The Company determines the fair value of its financial instruments based upon the fair value hierarchy, which prioritizes valuation inputs based on the observable nature of those inputs. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The hierarchy defines three levels of valuation inputs: Level 1 inputs Quoted prices in active markets for identical assets or liabilities that the Company can access at the measurement date. Level 2 inputs Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. Level 3 inputs Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Items Measured at Fair Value on a Recurring Basis The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis (in thousands): December 31, 2018 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 127,689 $ 60,092 $ 67,597 $ — Short-term investments 119,786 — 119,786 — Total financial assets $ 247,475 $ 60,092 $ 187,383 $ — December 31, 2017 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 80,894 $ 75,478 $ 5,416 $ — Short-term investments 4,496 — 4,496 — Total financial assets $ 85,390 $ 75,478 $ 9,912 $ — The investments and cash equivalents have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market-based approaches and observable market inputs to determine value. These observable market inputs include reportable trades, benchmark yields, credit spreads, broker/dealer quotes, bids, offers, current spot rates and other industry and economic events. The Company validates the prices provided by third party pricing services by reviewing their pricing methods and matrices, obtaining market values from other pricing sources, analyzing pricing data in certain instances and confirming that the relevant markets are active. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by the pricing services as of December 31, 2018 and 2017. During 2018, a derivative liability was initially recorded as a result of the issuance of the 5.00% Convertible Senior Notes due 2048 (the Notes) (see note 11). The Company initially determined fair value of the liability upon issuance, and then again upon the determination that the derivative instrument met the criteria to be reclassified into equity. The fair value measurement of the derivative liability is classified as Level 3 under the fair value hierarchy as it has been valued using unobservable inputs. These inputs include: (1) a simulated share price at the time of conversion of the Notes, (2) assumed timing of conversion of the Notes, and (3) the risk-adjusted discount rate used to present value the probability-weighted cash flows. Significant increases or decreases in any of those inputs in isolation could result in a significantly lower or higher fair value measurement. The fair value of the derivative liability was determined using a binomial lattice model by calculating the fair value of the Notes with the conversion feature as compared to the fair value of the Notes without the conversion feature, with the difference representing the value of the conversion feature, or the derivative liability. The fair value of the Notes with the conversion feature at issuance was assumed to equal the issuance par value of $150.0 million with an implied discount rate of 12.1% which was determined by discounting the cash flows generated by the binomial lattice model back to the issuance par value. The fair value of the Notes without the conversion feature was calculated based on cash payment for the full par value of the Notes and was discounted by the implied discount rate of 12.1%. The fair value of the Notes with and without the conversion feature upon the Company’s shareholders increasing the number of authorized shares of common stock was determined using a similar approach with an implied discount rate of 16.2%, which was determined be evaluating the increase in credit spreads of publicly traded debt over a similar time period. The following table represents a reconciliation of the derivative liability recorded in connection with the issuance of the Notes: January 1, 2018 $ — Fair value recognized upon issuance of Convertible Senior Notes 51,531 Fair value adjustment (25,556) Reclassification to equity (25,975) December 31, 2018 $ — Fair Value of Financial Instruments The fair value of the Company’s long-term debt is determined using a discounted cash flow analysis with current applicable rates for similar instruments as of the consolidated balance sheet dates. The carrying value of the Company’s long-term debt, including the current portion, at December 31, 2018 and 2017, was approximately $25.2 million and $14.8 million, respectively. At December 31, 2018 and 2017, the Company estimates that the fair value of its long-term debt, including the current portion, was approximately $26.9 million and $14.8 million, respectively. The fair value of the Company’s long-term debt was determined using Level 3 inputs. The fair value of the Notes was approximately $108.1 million as of December 18, 2018, which differs from the carrying value of the Notes. The fair value of the Notes is influenced by our stock price and stock price volatility. The fair value of the Notes was determined using Level 3 inputs. Investments Investments and cash equivalents consist of investments in a U.S. Government money market funds, overnight repurchase agreements collateralized by government agency securities or U.S. Treasury securities, corporate bonds and commercial paper of publicly traded companies that are classified as available‑for‑sale pursuant to Accounting Standards Codification (ASC) Topic 320, Investments—Debt and Equity Securities . The Company classifies investments available to fund current operations as current assets on its consolidated balance sheets. Investments are carried at fair value with unrealized gains and losses included as a component of accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity, until such gains and losses are realized. The fair value of these securities is based on quoted prices for identical or similar assets. If a decline in the fair value is considered other‑than‑temporary, based on available evidence, the unrealized loss is transferred from other comprehensive loss to the consolidated statements of operations and comprehensive loss. The Company reviews investments for other‑than‑temporary impairment whenever the fair value of an investment is less than the amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other‑than‑temporary, the Company considers the intent to sell, or whether it is more likely than not that the Company will be required to sell, the investment before recovery of the investment’s amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to year end. Realized gains and losses are determined using the specific identification method and are included in interest income in the consolidated statements of operations and comprehensive loss. There were no realized gains or losses on investments for the years ended December 31, 2018, 2017 or 2016. There were fourteen debt securities and five debt securities in an unrealized loss position as of December 31, 2018 and December 31, 2017, respectively. None of these investments had been in an unrealized loss position for more than 12 months as of December 31, 2018 or December 31, 2017, respectively. The fair value of these securities as of December 31, 2018 and December 31, 2017 was $46.9 million and $9.9 million, respectively, and the aggregate unrealized loss was immaterial. The Company considered the decline in the market value for these securities to be primarily attributable to current economic conditions. As it was not more likely than not that the Company would be required to sell these securities before the recovery of their amortized cost basis, which may be at maturity, the Company did not consider these investments to be other-than-temporarily impaired as of December 31, 2018 and December 31, 2017, respectively. Cash, cash equivalents and investments consist of the following (in thousands): December 31, 2018 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 62,270 $ — $ — $ 62,270 Corporate bonds and commercial paper (due within 90 days) 67,590 8 (1) 67,597 Total cash and cash equivalents $ 129,860 $ 8 $ (1) $ 129,867 Investments: Corporate bonds and commercial paper (due within 1 year) $ 119,666 $ 132 $ (12) $ 119,786 Total investments $ 119,666 $ 132 $ (12) $ 119,786 Total cash, cash equivalents and investments $ 249,526 $ 140 $ (13) $ 249,653 December 31, 2017 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 76,760 $ — $ — $ 76,760 Corporate bonds and commercial paper (due within 90 days) 5,418 $ — $ (2) $ 5,416 Total cash and cash equivalents $ 82,178 $ — $ (2) $ 82,176 Investments: Corporate bonds and commercial paper (due within 1 year) $ 4,496 $ — $ — $ 4,496 Total investments $ 4,496 $ — $ — $ 4,496 Total cash, cash equivalents and investments $ 86,674 $ — $ (2) $ 86,672 Concentrations of credit risk and off‑balance sheet risk Cash and cash equivalents, investments, and trade accounts receivable are financial instruments that potentially subject the Company to concentrations of credit risk. The Company mitigates this risk by maintaining its cash and cash equivalents and investments with high quality, accredited financial institutions. The management of the Company’s investments is not discretionary on the part of these financial institutions. As of December 31, 2018, the Company’s cash, cash equivalents and investments were deposited at two financial institutions and it has no significant off‑balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements. As of December 31, 2018, there were two customers that cumulatively made up more than 50% of the Company’s trade accounts receivable balance. The Company assesses the creditworthiness of all its customers and sets and reassesses customer credit limits to ensure collectability of any trade accounts receivable balances are assured. For the year ended December 31, 2018, two customers, Yakult and CSPC, individually accounted for greater than 10% of the Company’s total revenues. Property and equipment Property and equipment consist of laboratory equipment, office furniture, computer equipment and leasehold improvements. Expenditures for repairs and maintenance are recorded to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. Depreciation and amortization are calculated using the straight‑line method over the following estimated useful lives of the assets: Laboratory equipment 5 years Furniture 5 years Computer equipment 3 years Leasehold improvements Lesser of useful life or life of lease Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is recognized. The Company reviews its long‑lived assets for impairment whenever events or changes in business circumstances indicate that the carrying value of assets may not be recoverable. Recoverability is measured by comparison of the asset’s book value to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No material impairment losses have been recorded through December 31, 2018. Other assets Other assets primarily consist of prepayments made to contract research organizations (CROs). As of December 31, 2018 and 2017, other assets were primarily comprised of approximately $755,000 of prepaid CRO expenses that the Company assumed and paid to Infinity Pharmaceuticals, Inc. (Infinity) pursuant to the license agreement between the Company and Infinity. Research and development costs The Company expenses research and development costs to operations as incurred. Research and development expenses consist of: · employee‑related expenses, including salaries, benefits, travel and stock‑based compensation expense; · external research and development expenses incurred under arrangements with third parties, such as CROs, clinical trial sites, manufacturing organizations and consultants, including the scientific advisory board; · license fees; · facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, depreciation of equipment, and laboratory supplies; and · costs associated with COPIKTRA prior to the Company concluding that regulatory approval is probable and that its net realizable value is recoverable . The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the services have been performed or when the goods have been received rather than when the payment is made. Stock‑based compensation The Company expenses the fair value of employee stock-based awards on a straight-line basis over the requisite service period, which typically is the vesting period. Compensation expense is measured using the fair value of the award at the grant date and is adjusted to reflect actual forfeitures as they occur. Awards subject to performance-based vesting requirements are expensed utilizing an accelerated attribution model if achievement of the performance criteria is determined to be probable. The grant date fair value of employee stock options is estimated using the Black‑Scholes option pricing model that takes into account the fair value of its common stock, the exercise price, the expected life of the option, the expected volatility of its common stock, expected dividends on its common stock, and the risk-free interest rate over the expected life of the option. The Company applies the simplified method described in the Securities and Exchange Commission (SEC) Staff Accounting Bulletin (SAB) Topic 14.D.2 to calculate the expected term as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term for options granted to employees. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post‑vesting termination behavior among its employee population. For annual periods ending on or before December 31, 2017, the computation of expected volatility is based on the historical volatility of five companies, including the Company and a representative group of four public biotechnology and life sciences companies with similar characteristics to the Company, including similar stage of product development and therapeutic focus. As of the first quarter of 2018, the Company had sufficient company-specific historical and implied volatility information. As such, for the annual period ending December 31, 2018, the computation of expected volatility is based only on the historical volatility of the Company’s common stock. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company accounts for forfeitures as they occur. Stock‑based awards issued to non-employees, including directors for non‑board related services, are accounted for based on the fair value of such services received or of the equity instruments issued, whichever is more reliably measured. Stock option awards to non-employees are revalued at each reporting date and upon vesting using the Black‑Scholes option pricing model and are expensed on a straight‑line basis over the vesting period. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (ASC) 606 Revenue from Contracts with Customers . This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. 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 revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five step assessment: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception and once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines which goods and services are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Product Revenue, Net – The Company sells COPIKTRA to a limited number of specialty pharmacies and specialty distributors in the United States. These customers subsequently resell COPIKTRA either directly to patients, or to community hospitals or oncology clinics with in-office dispensaries who in turn distribute COPIKTRA to patients. In addition to distribution agreements with customers, the Company also enters into arrangements with (1) certain government agencies and various private organizations (Third-Party Payers), which may provide for chargebacks or discounts with respect to the purchase of COPIKTRA, and (2) Medicare and Medicaid, which may provide for certain rebates with respect to the purchase of COPIKTRA. The Company recognizes revenue on sales of COPIKTRA when a customer obtains control of the product, which occurs at a point in time (typically upon delivery). Product revenues are recorded at the wholesale acquisition costs, net of applicable reserves for variable consideration. Components of variable consideration include trade discounts and allowances, Third-Party Payer chargebacks and discounts, government rebates, other incentives, such as voluntary co-pay assistance, product returns, and other allowances that are offered within contracts between the Company and customers, payors, and other indirect customers relating to the Company’s sale of COPIKTRA. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable or a current liability. These estimates take into consideration a range of possible outcomes based upon relevant factors such as, customer contract terms, information received from third parties regarding the anticipated payor mix for COPIKTRA, known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled with respect to sales made. The amount of variable consideration which is included in the transaction price may be constrained and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under contracts will not occur in a future period. The Company’s analyses contemplate the application of the constraint in accordance with ASC 606. For the year ended December 31, 2018, the Company determined a material reversal of revenue would not occur in a future period for the estimates detailed below and, therefore, the transaction price was not reduced further. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances: The Company generally provides customers with invoice discounts on sales of COPIKTRA for prompt payment, which are explicitly stated in the Company’s contracts and are recorded as a reduction of revenue in the period the related product revenue is recognized. In addition, the Company compensates its specialty distributor customers for sales order management, data, and distribution services. The Company has determined such services are not distinct from the Company’s sale of COPIKTRA to the specialty distributor customers and, therefore, these payments have also been recorded as a reduction of revenue within the consolidated statements of operations and comprehensive loss through December 31, 2018. Third-Party Payer Chargebacks, Discounts and Fees: The Company executes contracts with Third-Party Payers which allow for eligible purchases of COPIKTRA at prices lower than the wholesale acquisition cost charged to customers who directly purchase the product from the Company. In some cases, customers charge the Company for the difference between what they pay for COPIKTRA and the ultimate selling price to the Third-Party Payers. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and accounts receivable, net. Chargeback amounts are generally determined at the time of resale to the qualified Third-Party Payer by customers, and the Company generally issues credits for such amounts within a few weeks of the customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at the end of each reporting period that the Company expects will be sold to Third-Party Payers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. In addition, the Company compensates certain Third-Party Payers for administrative services, such as account management and data reporting. These administrative service fees have also been recorded as a reduction of product revenue within the consolidated statements of operations and comprehensive loss through December 31, 2018. Government Rebates: The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses on the consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives: Other incentives which the Company offers include voluntary co-pay assistance programs, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payors. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive for product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses on the consolidated balance sheets. Product Returns: Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized. The Company estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company’s limited return policy allows for eligible returns of COPIKTRA for credit under the following circumstances: · Receipt of damaged product; · Shipment errors that were a result of an error by the Company; · Expired product that is returned during the period beginning three months prior to the product’s expiration and ending six months after the expiration date; · Product subject to a recall; and · Product that the Company, at its sole discretion, has specified can be returned for credit. The Company has not received any returns to date and believes that returns of its products will be minimal. If taxes should be collected from customers relating to product sales and remitted to governmental authorities, they will be excluded from product revenue. The Company expenses incremental costs of obtaining a contract when incurred, if the expected amortization period of the asset that the Company would have recognized is one year or less. However, no such costs were incurred during the year ended December 31, 2018. Exclusive Licenses of Intellectual Property - The Company may enter into collaboration and licensing arrangements for research and development, manufacturing, and commercialization activities with collaboration partners for the development and commercialization of its product candidates, which have components within the scope of ASC 606. The arrangements generally contain multiple elements or deliverables, which may include (1) licenses, or options to obtain licenses, to the Company's intellectual property, (2) research and development activities performed for the collaboration partner, (3) participation on joint steering committees, and (4) the manufacturing of commercial, clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon the achievement of significant development events, research and development reimbursements, sales milestones, and royalties on future product sales. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under each of its collaboration and license agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract within the scope of ASC 606; (ii) determinatio

Inventory

Inventory	12 Months Ended
	Dec. 31, 2018
Inventory
Inventory	3. Inventory During the third quarter of 2018, the Company began capitalizing inventory costs for COPIKTRA manufactured in preparation for its launch in the United States based on its evaluation of, among other factors, the status of the COPIKTRA New Drug Application (NDA) in the United States and the ability of its third-party suppliers to successfully manufacture commercial quantities of COPIKTRA, which provided the Company with reasonable assurance that the net realizable value of the inventory would be recoverable. Inventory consists of the following (in thousands): December 31, 2018 December 31, 2017 Raw materials $ — $ — Work in process 63 — Finished goods 264 — Total inventories $ 327 $ — Costs incurred prior to the quarter-ended September 30, 2018 to manufacture COPIKTRA were expensed as operating expenses as incurred.

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2018
Property and equipment, net
Property and equipment, net	4. Property and equipment, net Property and equipment and related accumulated depreciation are as follows (in thousands): December 31, December 31, 2018 2017 Leasehold improvements $ 146 $ 2,104 Laboratory equipment — 908 Furniture and fixtures 1,074 325 Computer equipment 658 279 1,878 3,616 Less: accumulated depreciation (509) (2,755) Total property and equipment, net $ 1,369 $ 861 During the year ended December 31, 2018, an amendment to the Company’s existing office space lease was executed whereby the Company relocated from its previous 15,197 rentable square foot location to an adjacent 27,810 rentable square foot location within the same building. As a result of this amendment, the Company shortened the useful life of the leasehold improvements related to the original location and depreciated this balance through the date which it vacated the original space. Upon vacating the original 15,197 rentable office space, the Company disposed of the leasehold improvements related to this location. No gain or loss from the disposal of leasehold improvements was recognized during the year ended December 31, 2018. The Company recorded approximately $1.0 million, $0.6 million, and $0.7 million in depreciation expense for the years ended December 31, 2018, 2017 and 2016, respectively .

Intangible assets

Intangible assets	12 Months Ended
	Dec. 31, 2018
Intangible assets
Intangible assets	5. Intangible assets The Company’s intangible assets consist of the following (in thousands): December 31, 2018 Estimated useful life Acquired and in-licensed rights $ 22,000 14 years Less: accumulated amortization (423) Total intangible assets, net $ 21,577 Acquired and in-licensed rights as of December 31, 2018, consist of a $22.0 million milestone payment which became payable upon the FDA marketing approval on September 24, 2018 pursuant to the amended and restated license agreement with Infinity. The Company made a milestone payment of $22.0 million to Infinity in November 2018. The Company recorded approximately $0.4 million in amortization expense related to finite-lived intangible assets during the year ended December 31, 2018 using the straight-line methodology. Estimated future amortization expense for finite-lived intangible assets as of December 31, 2018 is approximately $1.6 million per year thereafter.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2018
Accrued expenses
Accrued expenses	6. Accrued expenses Accrued expenses consist of the following (in thousands): December 31, 2018 December 31, 2017 Compensation and related benefits 8,749 2,622 Contract research organization costs 6,682 3,774 Commercialization costs 1,979 131 Interest 1,786 108 Consulting fees 494 448 Professional fees 482 617 Other 936 242 Total accrued expenses $ 21,108 $ 7,942

Long-term debt

Long-term debt	12 Months Ended
	Dec. 31, 2018
Long-term debt
Long-term debt	7. Long-term debt On March 21, 2017 (Closing Date), Verastem, Inc. (the Borrower) entered into a term loan facility of up to $25.0 million with Hercules Capital, Inc. (Hercules). The term loan facility is governed by a loan and security agreement, dated March 21, 2017 (the Original Loan Agreement), which originally provided for up to four separate advances, of which an aggregate of $15.0 million were drawn down during the year ended December 31, 2017. A total of $6.0 million of the proceeds received from the second and third tranches were used to make a milestone payment pursuant to the Company’s license agreement with Infinity, while the remaining proceeds were used for ongoing research and development programs and for general corporate purposes. The Company executed several amendments to the Original Loan Agreement prior to March 6, 2018 which increased the borrowing limit under the Original Loan Agreement from up to $25.0 million to up to $50.0 million (the Term Loan), resulting in $35.0 million of borrowing capacity remaining under the Amended Loan Agreement. The Company drew down an additional $10.0 million in June 2018. The remaining $25.0 million of borrowing capacity may be drawn in minimum increments of $5.0 million in multiple tranches comprised of (i) term loans (each a Term E Loan Advance) in an aggregate principal amount of up to $10.0 million and (ii) subject to Hercules’ sole discretion, term loans (each a Term F Loan Advance) in an aggregate principal amount of up to $25.0 million. The Amended Loan Agreement permits the Borrower to draw Term E Loan Advances subject to (i) the U.S. Food and Drug Administration accepting on or prior to September 30, 2018 the Company’s New Drug Application for duvelisib and (ii) delivery of the Company’s financial and business projections to Hercules in form and substance reasonably acceptable to Hercules. In addition, the Amended Loan Agreement allows the Borrower to draw Term F Loan Advances subject to the prior drawing of all other tranches and Hercules’ sole discretion. The Term Loan will mature on December 1, 2020 (Loan Maturity Date). Each advance accrues interest at a floating per annum rate equal to the greater of either (a) 10.5% or (b) the lesser of (i) 12.75% and (ii) the sum of (x) 10.5% plus (y) (A) the prime rate minus (B) 4.5%. The Term Loan provided for interest-only payments until November 1, 2018, which was extended to May 1, 2019 pursuant to the Amended Loan Agreement upon the Borrower’s receipt of a minimum of $20.0 million cash proceeds from a sale of equity securities in December 2017. Thereafter, amortization payments will be payable monthly in twenty installments of principal and interest (subject to recalculation upon a change in prime rates). Any advance may be prepaid in whole or in part upon seven business days’ prior written notice to Hercules, subject to a prepayment charge of 3.0%, if such advance is prepaid in any of the first twelve (12) months following the Closing Date, 2.0%, if such advance is prepaid after twelve (12) months following the Closing Date but on or prior to twenty-four (24) months following the Closing Date, and 1.0% thereafter. In addition, a final payment equal to 4.5% of the greater of (a) $5.0 million and (b) the total principal amount of the Term Loan extended by Hercules which is due on the Loan Maturity Date, or such earlier date specified in the Amended Loan Agreement. Amounts outstanding during an event of default shall be payable on demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. The Term Loan is secured by a lien on substantially all of the assets of the Borrower, other than intellectual property and contains customary covenants and representations, including a liquidity covenant, financial reporting covenant and limitations on dividends, indebtedness, collateral, investments, distributions, transfers, mergers or acquisitions, taxes, corporate changes, deposit accounts, and subsidiaries. The events of default under the Amended Loan Agreement include, without limitation, and subject to customary grace periods, (1) the Borrower’s failure to make any payments of principal or interest under the Amended Loan Agreement, promissory notes or other loan documents, (2) the Borrower’s breach or default in the performance of any covenant under the Amended Loan Agreement, (3) the Borrower making a false or misleading representation or warranty in any material respect, (4) the Borrower’s insolvency or bankruptcy, (5) certain attachments or judgments on the Borrower’s assets, or (6) the occurrence of any material default under certain agreements or obligations of the Borrower involving indebtedness, or (7) the occurrence of a material adverse effect. If an event of default occurs, Hercules is entitled to take enforcement action, including acceleration of amounts due under the Amended Loan Agreement. The Company assessed all terms and features of the Amended Loan Agreement in order to identify any potential embedded features that would require bifurcation or any beneficial conversion features. As part of this analysis, the Company assessed the economic characteristics and risks of the Amended Loan Agreement, including put and call features. The Company determined that all features of the Amended Loan Agreement were clearly and closely associated with a debt host and did not require bifurcation as a derivative liability, or the fair value of the feature was immaterial to the Company's consolidated financial statements. The Company reassesses the features on a quarterly basis to determine if they require separate accounting. There have been no changes to the Company’s original assessment through December 31, 2018. The future principal payments under the Amended Loan Agreement are as follows as of December 31, 2018 (in thousands): 2019 $ 5,930 2020 19,070 Total principal payments $ 25,000

Product revenue reserves and al

Product revenue reserves and allowances	12 Months Ended
	Dec. 31, 2018
Product revenue reserves and allowances
Product revenue reserves and allowances	8. Product revenue reserves and allowances As of December 31, 2018, the Company’s sole source of product revenue has been from sales of COPIKTRA in the United States, which it began shipping to customers on September 25, 2018. The following table summarizes activity in each of the product revenue allowance and reserve categories for the year ended December 31, 2018 (in thousands): Trade discounts and allowances Third-Party Payer chargebacks, discounts and fees Government rebates and other incentives Returns Total Beginning balance at December 31, 2017 $ — $ — $ — $ — $ — Provision related to sales in the current year 69 120 157 2 348 Adjustments related to prior period sales — — — — — Credits and payments made (40) (32) — — (72) Ending balance at December 31, 2018 $ 29 $ 88 $ 157 $ 2 $ 276 Trade discounts and Third-Party Payer chargebacks and discounts are recorded as a reduction to accounts receivable, net on the consolidated balance sheets. Trade allowances and Third-Party Payer fees, government rebates, other incentives and returns are recorded as a component of accrued expenses on the consolidated balance sheets.

Common stock

Common stock	12 Months Ended
	Dec. 31, 2018
Common stock.
Common stock	9. Common stock As of December 31, 2018 and 2017, the Company had reserved the following shares of common stock for the issuance of common stock for vested restricted stock units, the exercise of stock options, and an outstanding warrant (in thousands): December 31, 2018 2017 Shares reserved under equity compensation plans 15,572 8,264 Shares reserved for inducement grants 6,381 3,616 Shares reserved for Convertible Senior Notes 20,937 — Total shares reserved 42,890 11,880 Each share of common stock is entitled to one vote. The holders of the common stock are also entitled to receive dividends whenever funds are legally available and when declared by the board of directors. At-the-market equity offering programs On March 30, 2017, the Company established an at-the-market equity offering program pursuant to which it was able to offer and sell up to $35.0 million of its common stock at then-current market prices from time to time through Cantor, as sales agent (the 2017 ATM Program). On August 28, 2017, the Company amended its sales agreement with Cantor to increase the maximum aggregate offering price of shares of common stock that can be sold under the 2017 ATM Program to $75.0 million. Through December 31, 2018, the Company sold 11,518,354 shares under the 2017 ATM Program for net proceeds of approximately $47.3 million (after deducting commissions and other offering expenses). Equity offering On May 16, 2018, the Company entered into an underwriting agreement with Cantor relating to the underwritten offering of 7,777,778 shares (the Shares) of the Company’s common stock (the Underwriting Agreement). Cantor agreed to purchase the Shares pursuant to the Underwriting Agreement at a price of $4.31 per share. In addition, the Company granted Cantor an option to purchase, at the public offering price less any underwriting discounts and commissions, an additional 1,166,666 shares of the Company’s common stock, exercisable for 30 days from the date of the prospectus supplement. The option was exercised by Cantor in full on May 23, 2018. The aggregate proceeds from Cantor, net of underwriting discounts and offering costs, were approximately $38.3 million. On June 14, 2018, the Company entered into a purchase agreement with Consonance Capital Master Account L.P. and P Consonance Opportunities Ltd. (collectively, Consonance) relating to the registered offering of 7,166,666 shares of its common stock at a price of $6.00 per share. The aggregate proceeds from Consonance, net of offering costs, were approximately $42.9 million. On December 14, 2017, the Company entered into an underwriting agreement with BTIG, LLC relating to the underwritten offering of 8,422,877 shares of its common stock at a price of $2.97 per share, for aggregate proceeds, net of underwriting discounts and offering costs, of approximately $24.7 million.

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2018
Stock-based compensation
Stock-based compensation	10. Stock‑based compensation Stock‑based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss was as follows (in thousands): Year ended December 31, 2018 2017 2016 Research and development $ 2,043 $ 1,381 $ 1,073 Selling, general and administrative 4,628 3,652 5,145 Total stock-based compensation expense $ 6,671 $ 5,033 $ 6,218 All of the $6.7 million and $5.0 million of stock-based compensation expense recorded during the years ended December 31, 2018 and 2017, respectively, was recorded to additional paid-in capital. Of the $6.2 million of stock-based compensation expense recorded during the year ended December 31, 2016, $6.3 million was recorded to additional paid-in capital and approximately $69,000 was recorded as a decrease in liability classified awards. The Company has awards outstanding under two equity compensation plans, the Amended and Restated 2012 Incentive Plan (the 2012 Plan) and the 2010 Equity Incentive Plan (the 2010 Plan), as well as the inducement award program. Terms of stock award agreements, including vesting requirements, are determined by the board of directors, subject to the provisions of the individual plans. To date, most options granted by the Company vest twenty-five percent (25%) one year from vesting start date and six and a quarter percent (6.25%) for each successive three-month period, thereafter (subject to acceleration of vesting in the event of certain change of control transactions) and are exercisable for a period of ten years from the date of grant. 2012 Incentive Plan The 2012 Plan became effective immediately upon the closing of the Company’s IPO in February 2012. Upon effectiveness of the 2012 Plan, the Company ceased making awards under the 2010 Plan. The 2012 Plan initially allowed the Company to grant awards for up to 3,428,571 shares of common stock, plus the number of shares of common stock available for grant under the 2010 Plan as of the effectiveness of the 2012 Plan (which was an additional 30,101 shares), plus that number of shares of common stock related to awards outstanding under the 2010 Plan which terminate by expiration, forfeiture, cancellation or otherwise. The 2012 Plan included an “evergreen provision” that allowed for an annual increase in the number of shares of common stock available for issuance under the 2012 Plan. The annual increase was added on the first day of each year from 2013 through 2018 and was equal to the lesser of 1,285,714 shares of common stock and 4.0% of the number of shares of common stock outstanding, or a lesser amount as determined by the board of directors. On each of January 1, 2018, January 1, 2017 and January 1, 2016, the number of shares available for issuance under the 2012 Plan increased by 1,285,714 under this provision. On December 18, 2018, the shareholders of the Company approved the Amended and Restated 2012 Incentive Plan which increased the maximum number of shares available for issuance under the 2012 Plan to 16,628,425 and eliminated the evergreen provision. Awards under the 2012 Plan may include the following award types: incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units (RSUs), other stock‑based or cash‑based awards and any combination of the foregoing. As of December 31, 2018, under the 2012 Plan, the Company has granted stock options for 12,099,594 shares of common stock, of which 3,683,349 have been forfeited and 364,958 have been exercised, and restricted stock units for 1,195,918 shares of common stock, of which 219,351 have been forfeited and 759,817 have vested. The exercise price of each option has been equal to the closing price of a share of our common stock on the grant date. Inducement Award Program In December 2014, the Company established an inducement award program (in accordance with Nasdaq Listing Rule 5635(c)(4)) under which it may grant non-statutory stock options to purchase, and RSUs in respect of up to an aggregate of 750,000 shares of common stock to new or prospective employees as inducement to enter into employment with the Company. In December 2016, the Board of Directors authorized and reserved 580,000 additional shares of common stock under this program. In December 2017, the Board of Directors authorized and reserved 2,500,000 additional shares of common stock under this program. In June and December 2018, the Board of Directors authorized and reserved 1,700,000 and 1,250,000 additional shares of common stock under this program, respectively. The program is governed by the terms of the 2012 Plan but shares issued pursuant to the program are not issued under the 2012 Plan. As of December 31, 2018, the Company had granted options for 5,447,841 shares of common stock under the program, of which 721,082 have been forfeited and 323,750 have been exercised, and restricted stock units for 90,000 shares, of which none have been forfeited nor vested. As of December 31, 2018, 1,888,241 remain available for future issuance. Stock Options A summary of the Company’s stock option activity and related information for the year ended December 31, 2018 is as follows: Weighted-average Weighted-average remaining Aggregate exercise price per contractual term intrinsic value Shares share (years) (in thousands) Outstanding at December 31, 2017 8,719,978 $ 5.19 7.9 $ 6,150 Granted 5,539,232 $ 5.33 Exercised (412,851) $ 1.96 Forfeited/cancelled (1,323,492) $ 4.55 Outstanding at December 31, 2018 12,522,867 $ 5.42 7.8 $ 6,909 Vested at December 31, 2018 6,107,429 $ 6.05 6.5 $ 4,469 Vested and expected to vest at December 31, 2018(1) 12,084,867 $ 5.46 7.8 $ 6,811 (1) This represents the number of vested options as of December 31, 2018, plus the number of unvested options expected to vest as of December 31, 2018. The fair value of each stock option was estimated using a Black‑Scholes option‑pricing model with the following assumptions: Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.65 % 2.02 % 1.48 % Volatility 81 % 78 % 75 % Dividend yield — — — Expected term (years) 5.8 The Company recorded stock‑based compensation expense associated with employee stock options of $5.6 million, $4.5 million, and $6.1 million, for the years ended December 31, 2018, 2017, and 2016, respectively. The weighted‑average grant date fair value of options granted in the years ended December 31, 2018, 2017, and 2016 was $3.72, $1.83, and $0.99 per share, respectively. The fair value of options that vested during the years ended December 31, 2018, 2017, and 2016 was $4.7 million, $4.8 million, and $6.9 million, respectively. The aggregate intrinsic value of options exercised (i.e., the difference between the market price at exercise and the price paid by employees to exercise the option) during the years ended December 31, 2018 and 2017 was $1.8 million and $1.1 million, respectively. During the first quarter of 2018, the Company granted stock options to purchase a total of 582,500 shares of common stock to certain executives that vest only upon the achievement of specified performance conditions. The Company determined that two of the performance conditions had been achieved as of December 31, 2018. The Company has recognized approximately $0.7 million of stock-based compensation expense during the year ended December 31, 2018 related to awards that vest upon the achievement of performance conditions. As of December 31, 2018, a total of 438,000 options remain unvested which are related to the future achievement of performance conditions. In June 2016, the Company granted stock options to purchase a total of 500,000 shares of common stock to certain employees that vest only upon the achievement of specified performance conditions. The Company determined that 50% of performance conditions had been achieved during the year ended December 31, 2016. As a result, 250,000 shares vested in October 2016 and the Company recognized stock-based compensation expense related to these awards of approximately $0.2 million for the year ended December 31, 2016. In September 2017, the Company determined that the remaining performance conditions had been achieved and as a result the remaining 250,000 shares vested and the Company recognized stock-based compensation expense of approximately $0.4 million during the year ended December 31, 2017. The increase in stock-based compensation expense recognized for the awards which vested during the year ended December 31, 2017, as compared to the awards which vested during the year ended December 31, 2016, is a result of the revaluation of an award held by a non-employee to fair value on the vesting date. At December 31, 2018, there was $17.5 million of total unrecognized compensation cost related to unvested stock options and the Company expects to recognize this cost over a remaining weighted-average period of 3.7 years. Restricted Stock Units The Company awards RSUs to employees under its 2012 Incentive Plan and Inducement Award Program. Each RSU entitles the holder to receive one share of the Company’s common stock when the RSU vests. The RSUs generally vest in either (i) four substantially equal installments on each of the first four anniversaries of the vesting commencement date, or (ii) 100 percent on the first anniversary of the vesting commencement date, subject to the employee’s continued employment with, or service to, the Company on such vesting date. Compensation expense is recognized on a straight-line basis. A summary of RSU activity during the year ended December 31, 2018 is as follows: Shares Weighted-average grant date fair value per share Outstanding at December 31, 2017 — $ — Granted 376,000 $ 5.28 Vested — $ — Forfeited (69,250) $ 5.45 Outstanding at December 31, 2018 306,750 $ 5.24 The Company recorded stock‑based compensation expense associated with employee RSUs of $0.4 million, less than $0.1 million, and $0.1 million, for the years ended December 31, 2018, 2017, and 2016, respectively. No RSUs were granted during the years ended December 31, 2017 and 2016. The total fair value of restricted stock units vested during the years ended December 31, 2018, 2017, and 2016 was insignificant. At December 31, 2018, there was $1.2 million of total unrecognized compensation cost related to unvested RSUs and the Company expects to recognize this cost over a remaining weighted-average period of 1.87 years.

Convertible Senior Notes

Convertible Senior Notes	12 Months Ended
	Dec. 31, 2018
Convertible Senior Notes
Convertible Senior Notes	11. Convertible Senior Notes On October 17, 2018, the Company closed a registered direct public offering of $150.0 million aggregate principal amount of the Company’s 5.00% Convertible Senior Notes due 2048 (the Notes), for net proceeds of approximately $145.3 million. The Notes are governed by the terms of a base indenture for senior debt securities (the Base Indenture), as supplemented by the first supplemental indenture thereto (the Supplemental Indenture and together with the Base Indenture, the Indenture), each dated October 17, 2018, by and between the Company and Wilmington Trust, National Association, as trustee. The Notes are senior unsecured obligations of the Company and bear interest at a rate of 5.00% per annum, payable semi-annually in arrears on May 1 and November 1 of each year, beginning on May 1, 2019. The Notes will mature on November 1, 2048, unless earlier repurchased, redeemed or converted in accordance with their terms. The Notes are convertible into shares of the Company’s common stock, par value $0.0001 per share, together, if applicable, with cash in lieu of any fractional share, at an initial conversion rate of 139.5771 shares of common stock per $1,000 principal amount of the Notes, which corresponds to an initial conversion price of approximately $7.16 per share of common stock and represents a conversion premium of approximately 15.0% above the last reported sale price of the common stock of $6.23 per share on October 11, 2018. Upon conversion, converting noteholders will be entitled to receive accrued interest on their converted Notes. To the extent the Company has insufficient authorized but unissued shares to settle conversions in shares of common stock, the Company would be required to settle the deficiency in cash. The Company will have the right, exercisable at its option, to cause all Notes then outstanding to be converted automatically if the “Daily VWAP” (as defined in the Indenture) per share of the Company’s common stock equals or exceeds 130% of the conversion price on each of at least 20 VWAP Trading Days (as defined in the Indenture), whether or not consecutive, during any 30 consecutive VWAP Trading Day period commencing on or after the date the Company first issued the Notes. The conversion rate is subject to adjustment from time to time upon the occurrence of certain events, including, but not limited to, the issuance of stock dividends and payment of cash dividends, but will not be adjusted for any accrued and unpaid interest. Prior to November 1, 2022, the Company will not have the right to redeem the Notes. On or after November 1, 2022, the Company may elect to redeem the Notes, in whole or in part, at a cash redemption price equal to the principal amount of the Notes to be redeemed, plus accrued and unpaid interest, if any. Unless the Company has previously called all outstanding Notes for redemption, the Notes will be subject to repurchase by the Company at the holders’ option on each of November 1, 2023, November 1, 2028, November 1, 2033, November 1, 2038 and November 1, 2043 (or, if any such date is not a business day, on the next business day) at a cash repurchase price equal to the principal amount of the Notes to be repurchased, plus accrued and unpaid interest, if any. If a “Fundamental Change” (as defined in the Indenture) occurs at any time, subject to certain conditions, holders may require the Company to purchase all or any portion of their Notes at a purchase price equal to 100% of the principal amount of the Notes to be purchased, plus accrued and unpaid interest. If a “Fundamental Change” occurs on or before November 1, 2022 and a holder elects to convert its Notes in connection with such change, such holder may be entitled to an increase in the conversion rate in certain circumstances as set forth in the Indenture. The Notes are the Company’s senior, unsecured obligations and will be senior in right of payment to the Company’s future indebtedness that is expressly subordinated in right of payment to the Notes; equal in right of payment with the Company’s existing and future indebtedness that is not so subordinated, and effectively subordinated to the Company’s existing and future secured indebtedness, to the extent of the value of the collateral securing such indebtedness. The Notes are structurally subordinated to all existing and future indebtedness and other liabilities, including trade payables, and (to the extent the Company is not a holder thereof) preferred equity, if any, of the Company’s subsidiaries. The Indenture includes customary covenants and set forth certain events of default after which the Notes may be declared immediately due and payable and set forth certain types of bankruptcy or insolvency events of default involving the Company or certain of its subsidiaries after which the Notes become automatically due and payable The Company assessed all terms and features of the Notes in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the Notes, including the conversion, put and call features. Per the terms of the Indenture, upon conversion of the Notes, a portion of the principal may be settled in cash until the date upon which the Company’s stockholders approve an increase in the number of authorized shares of common stock, or the Authorized Share Effective Date, as defined. In consideration of this provision, the Company concluded the conversion feature required bifurcation as a derivative. The fair value of the conversion feature derivative was determined based on the difference between the fair value of the Notes with the conversion option and the fair value of the Notes without the conversion option. The Company determined that the fair value of the derivative upon issuance of the Notes was $51.5 million and recorded this amount as a derivative liability and the offsetting amount as a debt discount as a reduction to the carrying value of the Notes on the closing date, or October 17, 2018. On December 18, 2018, the Authorized Share Effective Date was achieved as the Company’s stockholders approved an increase in the number of authorized shares of Common Stock. Following this approval, no portion of the Notes are settleable in cash upon conversion. As such, the Company determined that the conversion feature no longer met the definition of a derivative following the increase in the number of authorized shares of common stock. As of December 18, 2018, the Company determined the fair value of the conversion feature was $25.9 million. The Company recorded the change in the fair value of the conversion feature for the period from October 17, 2018 to December 18, 2018 of $25.6 million as other income on the consolidated statements of operations and comprehensive loss. As of December 18, 2018, the fair value of the conversion option was reclassified to additional paid-in capital on the consolidated balance sheets as it qualified for a scope exception from derivative accounting. Accordingly, the conversion feature will no longer be measured at fair value on the Company’s financial statements. The Company determined that all other features of the Notes were clearly and closely associated with a debt host and did not require bifurcation as a derivative liability, or the fair value of the feature was immaterial to the Company's consolidated financial statements. The Company reassesses the features on a quarterly basis to determine if they require separate accounting. There have been no changes to the Company’s original assessment through December 31, 2018. The Company determined that the expected life of the Notes was equal to the period through November 1, 2023 as this represents the point at which the Notes are initially subject to repurchase by the Company at the option of the holders. Accordingly, the total debt discount of $56.3 million, inclusive of the fair value of the embedded conversion feature derivative at issuance, is being amortized using the effective interest method through November 1, 2023. For the year ended December 31, 2018, the Company recognized $3.1 million of interest expense related to the Notes.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Net Loss per Share
Net Loss per Share	12. Net Loss per Share ASC 260 “Earnings Per Share” requires the Company to calculate its net loss per share based on basic and diluted net loss per share, as defined. Basic EPS excludes dilution and is computed by dividing net loss by the weighted average number of shares outstanding for the period. For the year ended December 31, 2018, the dilutive effect of the outstanding Notes issued by the Company is reflected in diluted EPS using the if-converted method. For the years ended December 31, 2017 and 2016 periods of net loss, basic and diluted EPS are the same as the assumed exercise of stock options, warrants and restricted stock units are anti-dilutive. The computation of basic and diluted net income (loss) per share attributable to common stockholders consists of the following: Year Ended December 31, 2018 2017 2016 Net loss (72,434) (67,802) (36,440) Less: Other income (25,556) — — Add: Interest expense 3,071 — — Adjusted diluted net loss (94,919) (67,802) (36,440) Weighted average shares outstanding 64,962 38,422 36,988 Add: Dilutive effect of the Notes 4,359 — — Weighted average diluted shares outstanding 69,321 38,422 36,988 Net loss per share - basic (1.12) (1.76) (0.99) Net loss per share - diluted (1.37) (1.76) (0.99) In calculating the effect of the Notes on diluted net loss per share, the change in fair value of the bifurcated derivative of $25.6 million is subtracted while the interest expense of $3.1 million is added to the Company’s net loss. As of December 31, 2018, upon conversion of all outstanding Notes, the Company would be required to issue 20,936,548 shares. Under the “if-converted” method, convertible instruments are assumed to have been converted as of the beginning of the period or when issued, if later. Accordingly, the weighted average number of potentially issuable shares upon conversion of the Notes was determined by weighting the number of shares potentially issuable as of December 31, 2018, 20,936,548 shares, over the total number of days the Notes were outstanding for the period, 76 days, to calculate an additional 4,359,391 shares to be added to the denominator. The following potentially dilutive securities were excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: \ Year Ended December 31, 2018 2017 2016 Outstanding stock options 12,522,867 8,719,978 5,848,470 Outstanding warrants — — 142,857 Outstanding restricted stock units 306,750 — — Total potentially dilutive securities 12,829,617 8,719,978 5,991,327

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2018
Income taxes
Income taxes	13. Income Taxes As of December 31, 2018, the Company had federal and state net operating loss carryforwards of approximately $325.2 million and $325.0 million, respectively, which are available to reduce future taxable income. The Company also had federal and state tax credits of $18.1 million and $2.0 million, respectively, which may be used to offset future tax liabilities. The net operating loss (NOL) and tax credit carryforwards will expire at various dates through 2037. NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three‑year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations follows: December 31, 2018 2017 Income tax benefit using U.S. federal statutory rate 21.00 % 34.00 % State tax benefit, net of federal benefit 6.38 % 5.00 % Research and development tax credits 5.61 % 7.04 % Permanent items (0.65) % (1.24) % Effect of U.S. Tax Cuts and Jobs Act — % (45.19) % Change in the valuation allowance (31.82) % 0.19 % Other (0.52) % 0.20 % — % — % The principal components of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 88,829 $ 64,677 Capitalized research and development 2,545 2,780 Research and development credits 19,725 15,406 Stock-based compensation 3,756 2,560 Other 543 379 Total deferred tax assets 115,398 85,802 Deferred tax liabilities: Derivative liability (13,617) — Total deferred tax liabilities (13,617) — Net deferred tax asset 101,781 85,802 Valuation allowance (101,781) (85,802) Net deferred tax asset $ — $ — The Company has recorded a valuation allowance against its deferred tax assets at December 31, 2018 and 2017 because the Company’s management believes that it is more likely than not that these assets will not be fully realized. The increase in the valuation allowance of approximately $16.0 million in the year ended December 31, 2018 primarily relates to the generation of net operating losses and research and development credits. The Company’s reserves related to taxes are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. From inception and through December 31, 2018, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not conducted a study of research and development (R&D) credit carryforwards. This study may result in an adjustment to the Company’s R&D credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s R&D credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheet or statement of operations if an adjustment were required. The Company would recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. The Company’s uncertain tax positions are related to years that remain subject to examination by relevant tax authorities. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available. During 2017, the Company recorded tax charges for the impact of the Tax Act effects using the current available information and technical guidance on the interpretations of the Tax Act. As permitted by SEC Staff Accounting Bulletin 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, the Company recorded provisional estimates and have subsequently finalized our accounting analysis based on the guidance, interpretations, and data available as of December 31, 2018. Adjustments made in the fourth quarter of 2018 upon finalization of our accounting analysis were not material to our consolidated financial statements.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and contingencies
Commitments and contingencies	14. Commitments and contingencies On April 15, 2014, the Company entered into a lease agreement for approximately 15,197 square feet of office and laboratory space in Needham, Massachusetts. The lease term commenced on April 15, 2014 and was scheduled to expire on September 30, 2019. Effective February 15, 2018, the Company amended its lease agreement to relocate within the facility to another location consisting of 27,810 square feet of office space (the Amended Lease Agreement). The Amended Lease Agreement extends the expiration date of the lease from September 2019 through May 2025. Pursuant to the Amended Lease Agreement, the initial annual base rent amount is approximately $660,000, which increases during the lease term to $1.1 million for the last twelve-month period. The deferred rent obligation is included in accrued expenses (current portion) and other liabilities (noncurrent portion) in the consolidated balance sheets. The Company has also agreed to pay its proportionate share of increases in operating expenses and property taxes for the building in which the leased space is located. In conjunction with the execution of the Amended Lease Agreement, the Company increased its security deposit by increasing its existing letter of credit to approximately $403,000, which was reduced to $241,000 in October 2018. The amount is included in restricted cash on the consolidated balance sheets as of December 31, 2018. The minimum aggregate future lease commitments as of December 31, 2018 are as follows (in thousands): 2019 $ 716 2020 971 2021 1,020 2022 1,041 2023 1,062 Thereafter 1,538 Total $ 6,348 The Company recorded rent expense of approximately $0.8 million, $0.4 million and $0.4 million for the years ended December 31, 2018, 2017 and 2016, respectively. Pursuant to the terms of various agreements, the Company may be required to pay various development, regulatory and commercial milestones. In addition, if any products related to these agreements are approved for sale, the Company may be required to pay significant royalties on future sales. The payment of these amounts, however, is contingent upon the occurrence of various future events, which have a high degree of uncertainty of occurring.

License and collaboration agree

License and collaboration agreements	12 Months Ended
	Dec. 31, 2018
License and collaboration agreements
License and collaboration agreements	15. License and collaboration agreements Infinity In November 2016, the Company entered into an amended and restated license agreement with Infinity, under which it acquired an exclusive worldwide license for the research, development, commercialization, and manufacture of products in oncology indications containing duvelisib. In connection with the license agreement, the Company assumed operational and financial responsibility for certain activities that were part of Infinity’s duvelisib program, including the DUO study for patients with relapsed/refractory CLL, and Infinity maintained a portion of the financial responsibility for the shutdown of certain other clinical studies. The Company is obligated to use diligent efforts to develop and commercialize a product in an oncology indication containing duvelisib. During the term of the license agreement, Infinity has agreed not to research, develop, manufacture or commercialize duvelisib in any other indication in humans or animals. Pursuant to the terms of the license agreement, the Company was required to make the following payments to Infinity in cash or, at the Company’s election, in whole or in part, in shares of the Company’s common stock: (i) $6.0 million upon the completion of the DUO study if the results of the DUO study met certain pre-specified criteria, which was paid in cash by the Company to Infinity in October 2017 and recorded as research and development expense in the consolidated statements of operations and comprehensive loss, and (ii) $22.0 million upon the approval of a New Drug Application in the United States or an application for marketing authorization with a regulatory authority outside of the United States for a product in an oncology indication containing duvelisib, which was paid in cash by the Company to Infinity in November 2018 and recorded as an intangible asset in the consolidated balance sheets. The Company is also obligated to pay Infinity royalties on worldwide net sales of any products in an oncology indication containing duvelisib ranging from the mid-single digits to the high single-digits. The royalties will expire on a product-by-product and country-by-country basis until the latest to occur of (i) the last-to-expire patent right covering the applicable product in the applicable country, (ii) the last-to-expire patent right covering the manufacture of the applicable product in the country of manufacture of such product, (iii) the expiration of non-patent regulatory exclusivity in such country and (iv) ten years following the first commercial sale of a product in a country, provided that if royalties on net sales for a product in the United States are payable solely on the basis of non-patent regulatory exclusivity, the applicable royalty on net sales for such product in the United States will be reduced by 50%. The royalties are also subject to reduction by 50% of certain third-party royalty payments or patent litigation damages or settlements which might be required to be paid by the Company if litigation were to arise, with any such reductions capped at 50% of the amounts otherwise payable during the applicable royalty payment period. In addition to the foregoing, the Company is obligated to pay Infinity an additional royalty of 4% on worldwide net sales of any products in an oncology indication containing duvelisib to cover the reimbursement of research and development costs owed by Infinity to Mundipharma International Corporation Limited (MICL) and Purdue Pharmaceutical Products L.P. (Purdue). Once Infinity has fully reimbursed MICL and Purdue, the royalty obligations will be reduced to 1% of net sales in the United States. These trailing MICL royalties are payable until the later to occur of the last-to-expire of specified patent rights and the expiration of non-patent regulatory exclusivities in a country. Each of the above royalty rates is reduced by 50% on a product-by-product and country-by-country basis if the applicable royalty is payable solely on the basis of non-patent regulatory exclusivity. In addition, the trailing MICL royalties are subject to reduction by 50% of certain third-party royalty payments or patent litigation damages or settlements which might be required to be paid by the Company if litigation were to arise, with any such reductions capped at 50% of the amounts otherwise payable during the applicable royalty payment period. The Company evaluated the license agreement with Infinity under ASC Topic 805 , Business Combinations, and ASU 2017-01 and concluded that as substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar assets, the transaction did not meet the requirements to be accounted for as a business combination and therefore was accounted for as an asset acquisition. All consideration to be paid under the license agreement is contingent in nature and will be recognized when the respective contingency is resolved. During the year ended December 31, 2018, the Company paid Infinity $0.1 million related to the Infinity, MICL, and Purdue royalty payments, which are included in costs of revenue within the consolidated statements of operation. There were no royalties paid to Infinity during the years ended December 31, 2017 and 2016. Pfizer Inc. (Pfizer) On July 11, 2012, the Company entered into a license agreement with Pfizer Inc. (Pfizer), under which Pfizer granted the Company worldwide, exclusive rights to research, develop, manufacture and commercialize products containing certain of Pfizer’s inhibitors of focal adhesion kinase (the FAK Products) for all therapeutic, diagnostic and prophylactic uses in humans. The Company is solely responsible, at its expense, for the clinical development of the FAK Products, which is to be conducted in accordance with an agreed upon development plan. The Company is also responsible for all manufacturing and commercialization activities at its own expense. Pfizer is required to provide the Company with an initial quantity of clinical supply of one of the FAK Products for an agreed upon price. Under the agreement, the Company made a one-time cash payment to Pfizer in the amount of $1.5 million and issued 192,012 shares of its common stock. Pfizer is also eligible to receive up to $2.0 million in developmental milestones and up to an additional $125.0 million based on the successful attainment of regulatory and commercial sales milestones. Pfizer is also eligible to receive high single to mid-double-digit royalties on future net sales of the FAK Products. The Company’s royalty obligations with respect to each FAK Product in each country begin on the date of first commercial sale of the FAK Product in that country, and end on the later of 10 years after the date of first commercial sale of the FAK Product in that country or the date of expiration or abandonment of the last claim contained in any issued patent or patent application licensed by Pfizer to the Company that covers the FAK Product in that country. The Company accounted for the license agreement as the licensing of in process research and development with no alternative future use. Yakult Honsha Co., Ltd. (Yakult) On June 5, 2018, the Company entered into a license and collaboration agreement (the Yakult Agreement) with Yakult, under which the Company granted exclusive rights to Yakult to develop and commercialize products containing duvelisib in Japan for the treatment, prevention, palliation or diagnosis of all oncology indications in humans or animals. Under the terms of the Yakult Agreement, Yakult received an exclusive right to develop and commercialize products containing duvelisib in Japan under mutually agreed upon development and commercialization plans at its own cost and expense. Yakult also received certain limited manufacturing rights in the event that the Company is unable to manufacture or supply sufficient quantities of duvelisib or products containing duvelisib to Yakult during the term of the Yakult Agreement. The Company retained all rights to duvelisib outside of Japan. Yakult paid the Company an upfront, non-refundable payment of $10.0 million in June 2018. The Company is also entitled to receive aggregate payments of up to $90.0 million if certain development, regulatory and commercial milestones are successfully achieved. Yakult is obligated to pay the Company a double-digit royalty on net sales of products containing duvelisib in Japan, subject to reduction in certain circumstances, and to fund certain global development costs related to worldwide clinical trials conducted by the Company in which Yakult has opted to participate (Global Clinical Trials) on a pro-rata basis. Unless earlier terminated by either party, the Yakult Agreement will expire upon the fulfillment of Yakult’s royalty obligations to the Company for the sale of any products containing duvelisib in Japan, which royalty obligations expire, on a product-by-product basis, upon the last to occur of (a) expiration of valid claims covering such product, (b) expiration of regulatory exclusivity for such product or (c) 10 years from first commercial sale of such product. Yakult may terminate the Yakult Agreement in its entirety at any time with 180 days’ written notice. Either party may terminate the Yakult Agreement in its entirety with 60 days’ written notice for the other party’s material breach if such party fails to cure the breach. The Company may terminate the Yakult Agreement if (i) Yakult fails to use commercially reasonable efforts to develop and commercialize products containing duvelisib in Japan or (ii) Yakult challenges any patent licensed by the Company to Yakult under the Yakult Agreement. Either party may terminate the Yakult Agreement in its entirety upon certain insolvency events involving the other party. The Company first assessed the Yakult Agreement under ASC 808 to determine whether the Yakult Agreement (or part of the Yakult Agreement) represents a collaborative arrangement based on the risks and rewards and activities of the parties pursuant to the Yakult Agreement. The Company accounts for collaborative arrangements (or elements within the contract that are deemed part of a collaborative arrangement), which represent a collaborative relationship and not a customer relationship, outside the scope of ASC 606. For a component of the Yakult Agreement, the Company concluded that both the Company and Yakult are exposed to significant risks while developing duvelisib and ultimately would share in the reward upon successful commercialization of duvelisib. The Company then considered each remaining component in the Yakult Agreement to determine if ASC 606 should be applied to those components. Generally, the components in the Yakult Agreement fall under one of two potential research and development activities: (i) the parties’ joint participation in Global Clinical Trials and (ii) the territory-specific development of duvelisib. For the parties’ participation in the Global Clinical Trials, the Company concluded that the research and development activities and payments related to such activities are not within the scope of ASC 606 as Yakult is not a customer of the Company with regards to these activities in the context of the Yakult Agreement. As such, costs incurred to execute the Global Clinical Trials will be recorded as research and development expense and payments received from Yakult related to such will be recorded as a reduction of research and development expense. For Territory-specific activities, the Company concluded that Yakult is a customer with regard to this component in the context of the Yakult Agreement. As such, the Territory-specific component and all related payments are within the scope of ASC 606. The Company determined that there were two material promises associated with the territory-specific activities: (i) an exclusive license to develop and commercialize duvelisib in the territory and (ii) the initial technology transfer. The Company determined that the exclusive license and initial technology transfer were not distinct from another, as the license has limited value without the initial technology. Therefore, the exclusive license and initial technology transfer are combined as a single performance obligation. The Company evaluated the option rights for manufacturing and supply services to determine whether they represent material rights to Yakult and concluded that the options were not issued at a significant and incremental discount and therefore do not represent material rights. As such, they are not performance obligations at the outset of the arrangement. Based on this assessment, the Company concluded one performance obligation exists at the outset of the Yakult Agreement: the exclusive license combined with the initial technology transfer. The Company determined that the upfront payment of $10.0 million constitutes the transaction price as of the outset of the Yakult Agreement. Future potential milestone payments were fully constrained as the risk of significant revenue reversal related to these amounts has not yet been resolved. The achievement of the future potential milestones is not within the Company’s control and is subject to certain research and development success or regulatory approvals and therefore carry significant uncertainty. The Company will reevaluate the likelihood of achieving future milestones at the end of each reporting period. As all performance obligations have been satisfied, if the risk of significant revenue reversal is resolved, any future milestone revenue from the arrangement will be added to the transaction price (and thereby recognized as revenue) in the period the risk is relieved. The Company satisfied the performance obligation upon delivery of the license and initial technology transfer and recognized the upfront payment of $10.0 million as license revenue during year ended December 31, 2018. CSPC Pharmaceutical Group Limited (CSPC) On July 26, 2018, the Company and CSPC entered into an Exclusivity Agreement which granted CSPC the exclusive right to negotiate a licensing agreement with the Company for duvelisib in China. CSPC paid the Company a non-refundable exclusivity fee of $5.0 million in August 2018 (Exclusivity Fee) which was creditable against any payments agreed to under the terms of a potential definitive license agreement . Subsequently, on September 25, 2018, the Company entered into a license and collaboration agreement with CSPC (the CSPC Agreement), under which the Company granted exclusive rights to CSPC to develop and commercialize products containing duvelisib in the People’s Republic of China (China), Hong Kong, Macau and Taiwan (collectively, the CSPC Territory) for the treatment, prevention, palliation or diagnosis of all oncology indications in humans. Under the terms of the CSPC Agreement, CSPC received an exclusive right to develop and commercialize products containing duvelisib in the CSPC Territory under mutually agreed upon development and commercialization plans at its own cost and expense. CSPC also received certain limited manufacturing rights in the event that the Company is unable to manufacture or supply sufficient quantities of duvelisib or products containing duvelisib to CSPC during the term of the CSPC Agreement. The Company retained all rights to duvelisib outside of the CSPC Territory . CSPC paid the Company an aggregate upfront, non-refundable payment of $15.0 million, less the previously paid $5.0 million Exclusivity Fee. The Company is also entitled to receive aggregate payments of up to $160.0 million if certain development, regulatory and commercial milestones are successfully achieved. CSPC is obligated to pay the Company a double-digit royalty on net sales of products containing duvelisib in the CSPC Territory , subject to reduction in certain circumstances, and to fund certain global development costs related to worldwide clinical trials conducted by the Company in which CSPC has opted to participate (Global Clinical Trials) on a pro-rata basis. Unless earlier terminated by either party, the CSPC Agreement will expire upon the fulfillment of CSPC’s royalty obligations to the Company for the sale of any products containing duvelisib in the CSPC Territory , which royalty obligations expire, on a product-by-product basis, upon the last to occur of (a) expiration of valid claims covering such product, (b) expiration of regulatory exclusivity for such product or (c) 10 years from first commercial sale of such product. CSPC may terminate the CSPC Agreement in its entirety at any time with 180 days’ written notice. Either party may terminate the CSPC Agreement in its entirety with 60 days’ written notice for the other party’s material breach if such party fails to cure the breach. The Company may terminate the CSPC Agreement if (i) CSPC fails to use commercially reasonable efforts to develop and commercialize products containing duvelisib in the CSPC Territory or (ii) CSPC challenges any patent licensed by the Company to CSPC under the CSPC Agreement. Either party may terminate the CSPC Agreement in its entirety upon certain insolvency events involving the other party. The Company first assessed the CSPC Agreement under ASC 808 to determine whether the CSPC Agreement (or part of the CSPC Agreement) represents a collaborative arrangement based on the risks and rewards and activities of the parties pursuant to the CSPC Agreement. The Company accounts for collaborative arrangements (or elements within the contract that are deemed part of a collaborative arrangement), which represent a collaborative relationship and not a customer relationship, outside the scope of ASC 606. For a component of the CSPC Agreement, the Company concluded that both the Company and CSPC are exposed to significant risks while developing duvelisib and ultimately would share in the reward upon successful commercialization of duvelisib. The Company then considered each remaining component in the CSPC Agreement to determine if ASC 606 should be applied to those components. Generally, the components in the CSPC Agreement fall under one of two potential research and development activities: (i) the parties’ joint participation in Global Clinical Trials and (ii) the territory-specific development of duvelisib. For the parties’ participation in the Global Clinical Trials, the Company concluded that the research and development activities and payments related to such activities are not within the scope of ASC 606 as CSPC is not a customer of the Company with regards to these activities in the context of the CSPC Agreement. As such, costs incurred to execute the Global Clinical Trials will be recorded as research and development expense and payments received from CSPC related to such will be recorded as a reduction of research and development expense. For CSPC Territory-specific activities, the Company concluded that CSPC is a customer with regard to this component in the context of the CSPC Agreement. As such, the CSPC Territory-specific component and all related payments are within the scope of ASC 606. The Company determined that there were two material promises associated with the territory-specific activities: (i) an exclusive license to develop and commercialize duvelisib in the territory and (ii) the initial technology transfer. The Company determined that the exclusive license and initial technology transfer were not distinct from another, as the license has limited value without the initial technology. Therefore, the exclusive license and initial technology transfer are combined as a single performance obligation. The Company evaluated the option rights for manufacturing and supply services to determine whether they represent material rights to CSPC and concluded that the options were not issued at a significant and incremental discount and therefore do not represent material rights. As such, they are not performance obligations at the outset of the arrangement. Based on this assessment, the Company concluded one performance obligation exists at the outset of the CSPC Agreement: the exclusive license combined with the initial technology transfer. The Company determined that the upfront payment of $15.0 million constitutes the transaction price as of the outset of the CSPC Agreement. Future potential milestone payments were fully constrained as the risk of significant revenue reversal related to these amounts has not yet been resolved. The achievement of the future potential milestones is not within the Company’s control and is subject to certain research and development success or regulatory approvals and therefore carry significant uncertainty. The Company will reevaluate the likelihood of achieving future milestones at the end of each reporting period. As all performance obligations have been satisfied, if the risk of significant revenue reversal is resolved, any future milestone revenue from the arrangement will be added to the transaction price (and thereby recognized as revenue) in the period the risk is relieved. The Company satisfied the performance obligation upon delivery of the license and initial technology transfer and recognized the upfront payment of $15.0 million as license revenue during the year ended December 31, 2018.

Employee benefit plan

Employee benefit plan	12 Months Ended
	Dec. 31, 2018
Employee benefit plan
Employee benefit plan	16. Employee benefit plan In June 2011, the Company adopted a 401(k) retirement and savings plan (the 401(k) Plan) covering all employees. The 401(k) Plan allows employees to make pre‑tax or post‑tax contributions up to the maximum allowable amount set by the Internal Revenue Service. Under the 401(k) Plan, the Company may make discretionary contributions as approved by the board of directors. The Company made contributions to the 401(k) Plan of approximately $0.8 million, $0.3 million, and $0.2 million for the years ended December 31, 2018, 2017, and 2016, respectively.

Quarterly financial information

Quarterly financial information (unaudited, in thousands, except per share data)	12 Months Ended
	Dec. 31, 2018
Quarterly financial information (unaudited, in thousands, except per share data)
Quarterly financial information	17. Quarterly financial information (unaudited, in thousands, except per share data) First Quarter Second Quarter Third Quarter Fourth Quarter Ended Ended Ended Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Revenue: Product revenue, net $ — $ — $ 508 $ 1,210 License revenue — 10,000 15,000 — Total revenue — Operating expenses: Costs of revenues, excluding amortization of acquired intangible assets $ — $ — $ $ 116 Research and development 8,762 Selling, general and administrative 26,199 Amortization of acquired intangible assets — — 392 Total operating expenses Loss from operations Other income — — — 25,556 Interest income 1,306 Interest expense (3,952) Net loss $ $ $ $ Net loss per share —basic $ $ $ $ Net loss per share —diluted $ $ $ $ Weighted-average number of common shares used in net loss per share —basic and diluted Net loss per share —basic (a) (a)(b) Net loss per share —diluted (a) (a)(b) (c) (a) In the first and second quarters of 2018, the Company sold 167,065 and 6,314,410 shares of its common stock under the Company’s at-the-market equity offering program, which resulted in net proceeds of $0.6 million and $23.7 million, respectively. (b) In the second quarter of 2018, the Company closed underwritten offerings in which it sold 8,944,444 shares and 7,166,666 shares of its common stock at a price of $4.31 per share and $6.00 per share, respectively, for aggregate proceeds, net of underwriting discounts and offering costs, of $38.3 million and $42.9 million, respectively (c) In the fourth quarter of 2018, utilizing the “if-converted” method, the Company’s Notes are assumed to have been converted as of the issuance date. Accordingly, the weighted average number of potentially issuable shares upon conversion of the Notes was determined by weighting the number of shares issuable upon conversion at December 31, 2018, or 20,936,548, over the total days outstanding, 76 days, to calculate an additional 17,295,409 shares to be added to the weighted-average number of shares. First Quarter Second Quarter Third Quarter Fourth Quarter Ended Ended Ended Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Operating expenses: Research and development $ $ $ $ Selling, general and administrative Total operating expenses Loss from operations Interest income Interest expense Net loss $ $ $ $ Net loss per share —basic and diluted $ $ $ $ Weighted-average number of common shares used in net loss per share —basic and diluted (d) (d)(e) (d) In the third and fourth quarters of 2017, the Company sold 2,853,753 and 2,183,126 shares of its common stock under the Company’s at-the-market equity offering program, which resulted in net proceeds of $14.1 million and $9.0 million, respectively. (e) In December 2017, the Company closed an underwritten offering in which it sold 8,422,877 shares of its common stock at a price of $2.97 per share, for aggregate proceeds, net of underwriting discounts and offering costs, of $24.7 million.

Subsequent events

Subsequent events	12 Months Ended
	Dec. 31, 2018
Subsequent events.
Subsequent events	18. Subsequent events The Company reviews all activity subsequent to year end but prior to the issuance of the consolidated financial statements for events that could require disclosure or that could impact the carrying value of assets or liabilities as of the consolidated balance sheets date. The Company is not aware of any material subsequent events.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2018
Significant accounting policies
Basis of presentation	Basis of presentation The accompanying financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) under the assumption that the Company will continue as a going concern for the next twelve months. Accordingly, they do not include any adjustments that might result from the uncertainty related to the Company’s ability to continue as a going concern. The consolidated financial statements include the accounts of Verastem Securities Company, a wholly-owned subsidiary of the Company. All financial information presented has been consolidated and includes the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation.
Use of estimates	Use of estimates The preparation of the Company’s financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including estimates related to revenue recognition, including returns, rebates, and other pricing adjustments, accruals and stock‑based compensation expense. The Company bases its estimates on historical experience and other market‑specific or other relevant assumptions that it believes to be reasonable. Actual results could differ from such estimates.
Segment and geographic information	Segment and geographic information Operating segments are defined as components of an enterprise about which separate discrete information is available and regularly reviewed by the chief operating decision maker, or decision‑making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment, which is the business of developing and commercializing drugs for the treatment of cancer. All material long-lived assets of the Company reside in the United States.
Cash, cash equivalents and restricted cash	Cash, cash equivalents and restricted cash The Company considers all highly liquid investments with an original or remaining maturity of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of a U.S. Government money market funds and corporate bonds and commercial paper of publicly traded companies. Cash equivalents are reported at fair value. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2018 December 31, 2017 Cash and cash equivalents $ 129,867 $ 82,176 Restricted cash 741 162 Total cash, cash equivalents and restricted cash $ 130,608 $ 82,338 Amounts included in restricted cash as of December 31, 2018 represent cash received pursuant to a funded research and development agreement with the Leukemia and Lymphoma Society (LLS) restricted for future expenditures for specific R&D studies and cash held to collateralize outstanding letters of credit provided as a security deposit for the Company’s office space located in Needham, Massachusetts in the amount of approximately $500,000 and $241,000, respectively. Restricted cash related to LLS funded research and development agreement is included in prepaid and other current assets, while restricted cash for letters of credit are included in other assets on the consolidated balance sheets. Amounts included in restricted cash as of December 31, 2017 represent cash held to collateralize outstanding letters of credit in the amount of $162,000 provided as a security deposit for the Company’s office space located in Needham, Massachusetts and included in other assets on the consolidated balance sheets.
Fair value of financial instruments	Fair value of financial instruments The Company determines the fair value of its financial instruments based upon the fair value hierarchy, which prioritizes valuation inputs based on the observable nature of those inputs. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The hierarchy defines three levels of valuation inputs: Level 1 inputs Quoted prices in active markets for identical assets or liabilities that the Company can access at the measurement date. Level 2 inputs Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. Level 3 inputs Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Items Measured at Fair Value on a Recurring Basis The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis (in thousands): December 31, 2018 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 127,689 $ 60,092 $ 67,597 $ — Short-term investments 119,786 — 119,786 — Total financial assets $ 247,475 $ 60,092 $ 187,383 $ — December 31, 2017 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 80,894 $ 75,478 $ 5,416 $ — Short-term investments 4,496 — 4,496 — Total financial assets $ 85,390 $ 75,478 $ 9,912 $ — The investments and cash equivalents have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market-based approaches and observable market inputs to determine value. These observable market inputs include reportable trades, benchmark yields, credit spreads, broker/dealer quotes, bids, offers, current spot rates and other industry and economic events. The Company validates the prices provided by third party pricing services by reviewing their pricing methods and matrices, obtaining market values from other pricing sources, analyzing pricing data in certain instances and confirming that the relevant markets are active. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by the pricing services as of December 31, 2018 and 2017. During 2018, a derivative liability was initially recorded as a result of the issuance of the 5.00% Convertible Senior Notes due 2048 (the Notes) (see note 11). The Company initially determined fair value of the liability upon issuance, and then again upon the determination that the derivative instrument met the criteria to be reclassified into equity. The fair value measurement of the derivative liability is classified as Level 3 under the fair value hierarchy as it has been valued using unobservable inputs. These inputs include: (1) a simulated share price at the time of conversion of the Notes, (2) assumed timing of conversion of the Notes, and (3) the risk-adjusted discount rate used to present value the probability-weighted cash flows. Significant increases or decreases in any of those inputs in isolation could result in a significantly lower or higher fair value measurement. The fair value of the derivative liability was determined using a binomial lattice model by calculating the fair value of the Notes with the conversion feature as compared to the fair value of the Notes without the conversion feature, with the difference representing the value of the conversion feature, or the derivative liability. The fair value of the Notes with the conversion feature at issuance was assumed to equal the issuance par value of $150.0 million with an implied discount rate of 12.1% which was determined by discounting the cash flows generated by the binomial lattice model back to the issuance par value. The fair value of the Notes without the conversion feature was calculated based on cash payment for the full par value of the Notes and was discounted by the implied discount rate of 12.1%. The fair value of the Notes with and without the conversion feature upon the Company’s shareholders increasing the number of authorized shares of common stock was determined using a similar approach with an implied discount rate of 16.2%, which was determined be evaluating the increase in credit spreads of publicly traded debt over a similar time period. The following table represents a reconciliation of the derivative liability recorded in connection with the issuance of the Notes: January 1, 2018 $ — Fair value recognized upon issuance of Convertible Senior Notes 51,531 Fair value adjustment (25,556) Reclassification to equity (25,975) December 31, 2018 $ — Fair Value of Financial Instruments The fair value of the Company’s long-term debt is determined using a discounted cash flow analysis with current applicable rates for similar instruments as of the consolidated balance sheet dates. The carrying value of the Company’s long-term debt, including the current portion, at December 31, 2018 and 2017, was approximately $25.2 million and $14.8 million, respectively. At December 31, 2018 and 2017, the Company estimates that the fair value of its long-term debt, including the current portion, was approximately $26.9 million and $14.8 million, respectively. The fair value of the Company’s long-term debt was determined using Level 3 inputs. The fair value of the Notes was approximately $108.1 million as of December 18, 2018, which differs from the carrying value of the Notes. The fair value of the Notes is influenced by our stock price and stock price volatility. The fair value of the Notes was determined using Level 3 inputs.
Investments	Investments Investments and cash equivalents consist of investments in a U.S. Government money market funds, overnight repurchase agreements collateralized by government agency securities or U.S. Treasury securities, corporate bonds and commercial paper of publicly traded companies that are classified as available‑for‑sale pursuant to Accounting Standards Codification (ASC) Topic 320, Investments—Debt and Equity Securities . The Company classifies investments available to fund current operations as current assets on its consolidated balance sheets. Investments are carried at fair value with unrealized gains and losses included as a component of accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity, until such gains and losses are realized. The fair value of these securities is based on quoted prices for identical or similar assets. If a decline in the fair value is considered other‑than‑temporary, based on available evidence, the unrealized loss is transferred from other comprehensive loss to the consolidated statements of operations and comprehensive loss. The Company reviews investments for other‑than‑temporary impairment whenever the fair value of an investment is less than the amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other‑than‑temporary, the Company considers the intent to sell, or whether it is more likely than not that the Company will be required to sell, the investment before recovery of the investment’s amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to year end. Realized gains and losses are determined using the specific identification method and are included in interest income in the consolidated statements of operations and comprehensive loss. There were no realized gains or losses on investments for the years ended December 31, 2018, 2017 or 2016. There were fourteen debt securities and five debt securities in an unrealized loss position as of December 31, 2018 and December 31, 2017, respectively. None of these investments had been in an unrealized loss position for more than 12 months as of December 31, 2018 or December 31, 2017, respectively. The fair value of these securities as of December 31, 2018 and December 31, 2017 was $46.9 million and $9.9 million, respectively, and the aggregate unrealized loss was immaterial. The Company considered the decline in the market value for these securities to be primarily attributable to current economic conditions. As it was not more likely than not that the Company would be required to sell these securities before the recovery of their amortized cost basis, which may be at maturity, the Company did not consider these investments to be other-than-temporarily impaired as of December 31, 2018 and December 31, 2017, respectively. Cash, cash equivalents and investments consist of the following (in thousands): December 31, 2018 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 62,270 $ — $ — $ 62,270 Corporate bonds and commercial paper (due within 90 days) 67,590 8 (1) 67,597 Total cash and cash equivalents $ 129,860 $ 8 $ (1) $ 129,867 Investments: Corporate bonds and commercial paper (due within 1 year) $ 119,666 $ 132 $ (12) $ 119,786 Total investments $ 119,666 $ 132 $ (12) $ 119,786 Total cash, cash equivalents and investments $ 249,526 $ 140 $ (13) $ 249,653 December 31, 2017 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 76,760 $ — $ — $ 76,760 Corporate bonds and commercial paper (due within 90 days) 5,418 $ — $ (2) $ 5,416 Total cash and cash equivalents $ 82,178 $ — $ (2) $ 82,176 Investments: Corporate bonds and commercial paper (due within 1 year) $ 4,496 $ — $ — $ 4,496 Total investments $ 4,496 $ — $ — $ 4,496 Total cash, cash equivalents and investments $ 86,674 $ — $ (2) $ 86,672
Concentrations of credit risk and off-balance sheet risk	Concentrations of credit risk and off‑balance sheet risk Cash and cash equivalents, investments, and trade accounts receivable are financial instruments that potentially subject the Company to concentrations of credit risk. The Company mitigates this risk by maintaining its cash and cash equivalents and investments with high quality, accredited financial institutions. The management of the Company’s investments is not discretionary on the part of these financial institutions. As of December 31, 2018, the Company’s cash, cash equivalents and investments were deposited at two financial institutions and it has no significant off‑balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements. As of December 31, 2018, there were two customers that cumulatively made up more than 50% of the Company’s trade accounts receivable balance. The Company assesses the creditworthiness of all its customers and sets and reassesses customer credit limits to ensure collectability of any trade accounts receivable balances are assured. For the year ended December 31, 2018, two customers, Yakult and CSPC, individually accounted for greater than 10% of the Company’s total revenues.
Property and equipment	Property and equipment Property and equipment consist of laboratory equipment, office furniture, computer equipment and leasehold improvements. Expenditures for repairs and maintenance are recorded to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. Depreciation and amortization are calculated using the straight‑line method over the following estimated useful lives of the assets: Laboratory equipment 5 years Furniture 5 years Computer equipment 3 years Leasehold improvements Lesser of useful life or life of lease Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is recognized. The Company reviews its long‑lived assets for impairment whenever events or changes in business circumstances indicate that the carrying value of assets may not be recoverable. Recoverability is measured by comparison of the asset’s book value to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No material impairment losses have been recorded through December 31, 2018.
Other assets	Other assets Other assets primarily consist of prepayments made to contract research organizations (CROs). As of December 31, 2018 and 2017, other assets were primarily comprised of approximately $755,000 of prepaid CRO expenses that the Company assumed and paid to Infinity Pharmaceuticals, Inc. (Infinity) pursuant to the license agreement between the Company and Infinity.
Research and development costs	Research and development costs The Company expenses research and development costs to operations as incurred. Research and development expenses consist of: · employee‑related expenses, including salaries, benefits, travel and stock‑based compensation expense; · external research and development expenses incurred under arrangements with third parties, such as CROs, clinical trial sites, manufacturing organizations and consultants, including the scientific advisory board; · license fees; · facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, depreciation of equipment, and laboratory supplies; and · costs associated with COPIKTRA prior to the Company concluding that regulatory approval is probable and that its net realizable value is recoverable . The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the services have been performed or when the goods have been received rather than when the payment is made.
Stock-based compensation	Stock‑based compensation The Company expenses the fair value of employee stock-based awards on a straight-line basis over the requisite service period, which typically is the vesting period. Compensation expense is measured using the fair value of the award at the grant date and is adjusted to reflect actual forfeitures as they occur. Awards subject to performance-based vesting requirements are expensed utilizing an accelerated attribution model if achievement of the performance criteria is determined to be probable. The grant date fair value of employee stock options is estimated using the Black‑Scholes option pricing model that takes into account the fair value of its common stock, the exercise price, the expected life of the option, the expected volatility of its common stock, expected dividends on its common stock, and the risk-free interest rate over the expected life of the option. The Company applies the simplified method described in the Securities and Exchange Commission (SEC) Staff Accounting Bulletin (SAB) Topic 14.D.2 to calculate the expected term as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term for options granted to employees. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post‑vesting termination behavior among its employee population. For annual periods ending on or before December 31, 2017, the computation of expected volatility is based on the historical volatility of five companies, including the Company and a representative group of four public biotechnology and life sciences companies with similar characteristics to the Company, including similar stage of product development and therapeutic focus. As of the first quarter of 2018, the Company had sufficient company-specific historical and implied volatility information. As such, for the annual period ending December 31, 2018, the computation of expected volatility is based only on the historical volatility of the Company’s common stock. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company accounts for forfeitures as they occur. Stock‑based awards issued to non-employees, including directors for non‑board related services, are accounted for based on the fair value of such services received or of the equity instruments issued, whichever is more reliably measured. Stock option awards to non-employees are revalued at each reporting date and upon vesting using the Black‑Scholes option pricing model and are expensed on a straight‑line basis over the vesting period.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (ASC) 606 Revenue from Contracts with Customers . This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. 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 revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five step assessment: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception and once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines which goods and services are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Product Revenue, Net – The Company sells COPIKTRA to a limited number of specialty pharmacies and specialty distributors in the United States. These customers subsequently resell COPIKTRA either directly to patients, or to community hospitals or oncology clinics with in-office dispensaries who in turn distribute COPIKTRA to patients. In addition to distribution agreements with customers, the Company also enters into arrangements with (1) certain government agencies and various private organizations (Third-Party Payers), which may provide for chargebacks or discounts with respect to the purchase of COPIKTRA, and (2) Medicare and Medicaid, which may provide for certain rebates with respect to the purchase of COPIKTRA. The Company recognizes revenue on sales of COPIKTRA when a customer obtains control of the product, which occurs at a point in time (typically upon delivery). Product revenues are recorded at the wholesale acquisition costs, net of applicable reserves for variable consideration. Components of variable consideration include trade discounts and allowances, Third-Party Payer chargebacks and discounts, government rebates, other incentives, such as voluntary co-pay assistance, product returns, and other allowances that are offered within contracts between the Company and customers, payors, and other indirect customers relating to the Company’s sale of COPIKTRA. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable or a current liability. These estimates take into consideration a range of possible outcomes based upon relevant factors such as, customer contract terms, information received from third parties regarding the anticipated payor mix for COPIKTRA, known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled with respect to sales made. The amount of variable consideration which is included in the transaction price may be constrained and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under contracts will not occur in a future period. The Company’s analyses contemplate the application of the constraint in accordance with ASC 606. For the year ended December 31, 2018, the Company determined a material reversal of revenue would not occur in a future period for the estimates detailed below and, therefore, the transaction price was not reduced further. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances: The Company generally provides customers with invoice discounts on sales of COPIKTRA for prompt payment, which are explicitly stated in the Company’s contracts and are recorded as a reduction of revenue in the period the related product revenue is recognized. In addition, the Company compensates its specialty distributor customers for sales order management, data, and distribution services. The Company has determined such services are not distinct from the Company’s sale of COPIKTRA to the specialty distributor customers and, therefore, these payments have also been recorded as a reduction of revenue within the consolidated statements of operations and comprehensive loss through December 31, 2018. Third-Party Payer Chargebacks, Discounts and Fees: The Company executes contracts with Third-Party Payers which allow for eligible purchases of COPIKTRA at prices lower than the wholesale acquisition cost charged to customers who directly purchase the product from the Company. In some cases, customers charge the Company for the difference between what they pay for COPIKTRA and the ultimate selling price to the Third-Party Payers. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and accounts receivable, net. Chargeback amounts are generally determined at the time of resale to the qualified Third-Party Payer by customers, and the Company generally issues credits for such amounts within a few weeks of the customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at the end of each reporting period that the Company expects will be sold to Third-Party Payers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. In addition, the Company compensates certain Third-Party Payers for administrative services, such as account management and data reporting. These administrative service fees have also been recorded as a reduction of product revenue within the consolidated statements of operations and comprehensive loss through December 31, 2018. Government Rebates: The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses on the consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives: Other incentives which the Company offers include voluntary co-pay assistance programs, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payors. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive for product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses on the consolidated balance sheets. Product Returns: Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized. The Company estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company’s limited return policy allows for eligible returns of COPIKTRA for credit under the following circumstances: · Receipt of damaged product; · Shipment errors that were a result of an error by the Company; · Expired product that is returned during the period beginning three months prior to the product’s expiration and ending six months after the expiration date; · Product subject to a recall; and · Product that the Company, at its sole discretion, has specified can be returned for credit. The Company has not received any returns to date and believes that returns of its products will be minimal. If taxes should be collected from customers relating to product sales and remitted to governmental authorities, they will be excluded from product revenue. The Company expenses incremental costs of obtaining a contract when incurred, if the expected amortization period of the asset that the Company would have recognized is one year or less. However, no such costs were incurred during the year ended December 31, 2018. Exclusive Licenses of Intellectual Property - The Company may enter into collaboration and licensing arrangements for research and development, manufacturing, and commercialization activities with collaboration partners for the development and commercialization of its product candidates, which have components within the scope of ASC 606. The arrangements generally contain multiple elements or deliverables, which may include (1) licenses, or options to obtain licenses, to the Company's intellectual property, (2) research and development activities performed for the collaboration partner, (3) participation on joint steering committees, and (4) the manufacturing of commercial, clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon the achievement of significant development events, research and development reimbursements, sales milestones, and royalties on future product sales. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under each of its collaboration and license agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract within the scope of ASC 606; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above; and d) the measure of progress in step (v) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. If a license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront 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. In assessing whether a promise or performance obligation is distinct from the other elements, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of its associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining elements, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress of each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, is subject to estimates by management and may change over the course of the arrangement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Customer Options: If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services such as research and development services or manufacturing services, the goods and services underlying the customer options are not considered to be performance obligations at the inception of the arrangement; rather, such goods and services are contingent on exercise of the option, and the associated option fees are not included in the transaction price. The Company evaluates customer options for material rights or options to acquire additional goods or services for free or at a discount. If a customer option is determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the estimated probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments: At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved 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 control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties: For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, 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). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. Collaborative Arrangements: Contracts are considered to be collaborative arrangements when they satisfy the following criteria defined in ASC 808, Collaborative Arrangements : (i) the parties to the contract must actively participate in the joint operating activity and (ii) the joint operating activity must expose the parties to the possibility of significant risk and rewards, based on whether or not the activity is successful. Payments received from or made to a partner that are the result of a collaborative relationship with a partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction or increase to research and development expense, respectively. For a complete discussion of the Company’s accounting for its license and collaboration agreements, see Note 15, License and collaboration agreements .
Accounts Receivable, Net	Accounts Receivable, Net Accounts receivable, net consists of amounts due from customers, net of applicable revenue reserves. Accounts receivable are typically due within 31 days. The Company analyzes accounts that are past due for collectability and provides an allowance for receivables when collection becomes doubtful. Given the nature and limited history of collectability of the Company’s accounts receivable, an allowance for doubtful accounts is not deemed necessary at December 31, 2018.
Inventory	Inventory The Company capitalizes inventories manufactured in preparation for initiating sales of a product candidate when the related product candidate is considered to have a high likelihood of regulatory approval and the related costs are expected to be recoverable through sales of the inventories. In determining whether or not to capitalize such inventories, the Company evaluates, among other factors, information regarding the product candidate’s safety and efficacy, the status of regulatory submissions and communications with regulatory authorities and the outlook for commercial sales, including the existence of current or anticipated competitive drugs and the availability of reimbursement. In addition, the Company evaluates risks associated with manufacturing the product candidate, including the ability of the Company’s third-party suppliers to complete the validation batches and the remaining shelf life of the inventories. Costs associated with manufacturing product candidates prior to satisfying the inventory capitalization criteria are charged to research and development expense as incurred. The Company values its inventories at the lower of cost or estimated net realizable value. The Company determines the cost of its inventories, which includes amounts related to materials and manufacturing overhead, on a first-in, first-out basis. The Company performs an assessment of the recoverability of capitalized inventory during each reporting period, and it writes down any excess and obsolete inventories to their estimated realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded within cost of product revenues. The determination of whether inventory costs will be realizable requires estimates by management. If actual market conditions are less favorable than projected by management, additional write-downs of inventory may be required which would be recorded as a cost of product revenues in the consolidated statements of operations and comprehensive loss. Shipping and handling costs for product shipments are recorded as incurred in cost of product revenues along with costs associated with manufacturing the product, and any inventory write-downs.
Intangible Assets	Intangible Assets The Company records finite-lived intangible assets related to certain capitalized milestone payments related to commercial products at their fair value. These assets are amortized on a straight-line basis over their remaining useful lives, which are estimated based on the shorter of the remaining underlying patent life or the estimated useful life of the underlying product. The Company assesses its finite-lived intangible assets for impairment if indicators are present or changes in circumstance suggest that impairment may exist. Events that could result in an impairment include the receipt of additional clinical or nonclinical data regarding one of the Company’s drug candidates or a potentially competitive drug candidate, changes in the clinical development program for a drug candidate, or new information regarding potential sales for the drug. If impairment indicators are present or changes in circumstance suggest that impairment may exist, the Company performs a recoverability test by comparing the sum of the estimated undiscounted cash flows of each finite-lived intangible asset to its carrying value on the consolidated balance sheets. If the undiscounted cash flows used in the recoverability test are less than the carrying value, the Company would determine the fair value of the finite-lived intangible asset and recognize an impairment loss if the carrying value of the finite-lived intangible asset exceeds its fair value.
Income taxes	Income taxes The Company accounts for income taxes under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Tax benefits are recognized when it is more likely than not that a tax position will be sustained during an audit. Deferred tax assets are reduced by a valuation allowance if current evidence indicates that it is considered more likely than not that these benefits will not be realized.
Net loss per share	Net loss per share Basic net loss per common share is calculated by dividing net loss applicable to common stockholders by the weighted-average number of common shares outstanding during the period. Diluted net loss per common share is calculated by increasing the denominator by the weighted-average number of additional shares that could have been outstanding from securities convertible into common stock, such as stock options, restricted stock units and warrants (using the “treasury stock” method) and Notes (using the “if-converted” method), unless their effect on net loss per share is antidilutive. The effect of computing diluted net loss per common share was antidilutive for any potentially issuable shares of common stock from the conversion of stock options, restricted stock units and warrants and, as such, have been excluded from the calculation. However, under the “if-converted” method, convertible instruments that are-in-the-money, are assumed to have been converted as of the beginning of the period or when issued, if later. Additionally, the effects of any interest expense and changes in fair value of bifurcated derivatives shall be added back to the numerator of the diluted net loss per share calculation. Refer to Note 12 for further details related to the calculation of net loss per share.
Recently Issued and/or Adopted Accounting Standards Updates	Recently Issued Accounting Standards Updates In November 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standard Update (ASU) 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which makes targeted improvements for collaborative arrangements to clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account, adds unit of account guidance in Topic 808 to align with guidance in Topic 606, and clarifies presentation of certain revenues with a collaborative arrangement participant which are not directly related to a third party. ASU 2018-18 is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted. The Company has not elected to early adopt this standard and is currently evaluating the impact the adoption of the standard will have on its consolidated financial statements and related disclosures. In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standard Update (ASU) 2018-15, Intangibles-Goodwill and Other-Internal Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract , which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. ASU 2018-15 is effective for annual and interim periods beginning after December 15, 2019, with early adoption permitted. The Company has not elected to early adopt this standard and is currently evaluating the impact the adoption of the standard will have on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement , which eliminates certain disclosure requirements for fair value measurements for all entities, requires public entities to disclose certain new information and modifies some disclosure requirements. ASU 2018-13 is effective for all entities for annual and interim periods beginning after December 15, 2019. An entity is permitted to early adopt either the entire standard or only the provisions that eliminate or modify requirements. The Company has not elected to early adopt this standard and is currently evaluating the impact the adoption of the standard will have on its consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of Topic 718 to include all share-based payment transactions for acquiring goods and services to be used or consumed in its own operations by issuing share-based payment awards. ASU 2018-07 also clarifies that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract and services from nonemployees. ASU 2018-07 specifies that Topic 718 applies to all share-based payment transactions accounted for under ASC 606. ASU 2018-07 is effective for annual and interim periods beginning after December 15, 2018, with early adoption permitted, but no earlier than the date on which ASC 606 is adopted. The Company has not elected to early adopt this standard and is currently evaluating the impact the adoption of the standard will have on its consolidated financial statements and related disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) , which supersedes the guidance under FASB Accounting Standards Codification (ASC) Topic 840, Leases , resulting in the creation of FASB ASC Topic 842, Leases . ASU 2016-02 requires lessees to recognize in the statement of financial position a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term for both finance and operating leases. The guidance also eliminates the current real estate-specific provisions for all entities. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, which provides entities with relief from the costs of implementing certain aspects of the new leasing standard, ASU 2016-02. Under the amendments in ASU 2018-11, entities may elect not to restate the comparative periods presented when transitioning to ASC 842 (optional transition method) and lessors may elect not to separate lease and non-lease components when certain conditions are met (lessor relief practical expedient). The optional transition method applies to entities that have not yet adopted ASU 2016-02, which is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, with early adoption permitted. The Company has not elected to early adopt this standard and is currently evaluating the impact the adoption of the standard will have on its consolidated financial statements and related disclosures. The Company’s analysis includes, but is not limited to, reviewing existing leases, reviewing other service agreements for embedded leases, establishing policies and procedures, assessing potential disclosures and evaluating the impact of adoption on the Company’s consolidated financial statements. The Company expects to recognize a lease liability and related right-of-use asset on its consolidated balance sheets upon adoption of this standard and expects the impact to its consolidated statements of operations and comprehensive loss will not be material. Recently Adopted Accounting Standards Updates In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting . ASU 2017-09 provides guidance about which changes to the terms or conditions of a share-based award require an entity to apply modification accounting under Topic 718. Specifically, an entity would not apply modification accounting if the fair value, vesting conditions and classification of the awards are the same immediately before and after a modification. ASU 2017-09 was effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this standard prospectively effective January 1, 2018. The adoption of this ASU did not have an effect on the Company’s consolidated financial statements or related disclosures. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash . ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. Amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 was effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this standard effective January 1, 2018. Upon adoption of ASU 2016-18, the Company applied the retrospective transition method for each period presented and included approximately $162,000 of restricted cash in the beginning-of-period and end-of-period cash, cash equivalents and restricted cash balance reflected in the consolidated statements of cash flows for the year ended December 31, 2017. A reconciliation of cash, cash equivalents and restricted cash for each period presented is provided in Note 2 to the consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. ASU 2016-15 adds or clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows. The standard was effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this standard effective January 1, 2018. The adoption of this ASU did not have an effect on the Company’s consolidated financial statements or related disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition . In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients. The Company adopted this new standard on January 1, 2018 using the full retrospective method. There was no change to the Company’s consolidated financial statements as a result of the adoption.

Significant accounting policies

Significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2018
Significant accounting policies
Schedule of reconciliation of cash, cash equivalents and restricted cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2018 December 31, 2017 Cash and cash equivalents $ 129,867 $ 82,176 Restricted cash 741 162 Total cash, cash equivalents and restricted cash $ 130,608 $ 82,338
Schedule of financial instruments measured at fair value on a recurring basis	The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis (in thousands): December 31, 2018 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 127,689 $ 60,092 $ 67,597 $ — Short-term investments 119,786 — 119,786 — Total financial assets $ 247,475 $ 60,092 $ 187,383 $ — December 31, 2017 Description Total Level 1 Level 2 Level 3 Financial assets Cash equivalents $ 80,894 $ 75,478 $ 5,416 $ — Short-term investments 4,496 — 4,496 — Total financial assets $ 85,390 $ 75,478 $ 9,912 $ —
Schedule of derivative liability	January 1, 2018 $ — Fair value recognized upon issuance of Convertible Senior Notes 51,531 Fair value adjustment (25,556) Reclassification to equity (25,975) December 31, 2018 $ —
Schedule of cash, cash equivalents and investments	Cash, cash equivalents and investments consist of the following (in thousands): December 31, 2018 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 62,270 $ — $ — $ 62,270 Corporate bonds and commercial paper (due within 90 days) 67,590 8 (1) 67,597 Total cash and cash equivalents $ 129,860 $ 8 $ (1) $ 129,867 Investments: Corporate bonds and commercial paper (due within 1 year) $ 119,666 $ 132 $ (12) $ 119,786 Total investments $ 119,666 $ 132 $ (12) $ 119,786 Total cash, cash equivalents and investments $ 249,526 $ 140 $ (13) $ 249,653 December 31, 2017 Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value Cash and cash equivalents: Cash and money market accounts $ 76,760 $ — $ — $ 76,760 Corporate bonds and commercial paper (due within 90 days) 5,418 $ — $ (2) $ 5,416 Total cash and cash equivalents $ 82,178 $ — $ (2) $ 82,176 Investments: Corporate bonds and commercial paper (due within 1 year) $ 4,496 $ — $ — $ 4,496 Total investments $ 4,496 $ — $ — $ 4,496 Total cash, cash equivalents and investments $ 86,674 $ — $ (2) $ 86,672
Schedule of estimated useful lives of the assets	Laboratory equipment 5 years Furniture 5 years Computer equipment 3 years Leasehold improvements Lesser of useful life or life of lease

Inventory (Tables)

Inventory (Tables)	12 Months Ended
	Dec. 31, 2018
Inventory
Schedule of inventory	Inventory consists of the following (in thousands): December 31, 2018 December 31, 2017 Raw materials $ — $ — Work in process 63 — Finished goods 264 — Total inventories $ 327 $ —

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2018
Property and equipment, net
Schedule of property equipment and related accumulated depreciation	Property and equipment and related accumulated depreciation are as follows (in thousands): December 31, December 31, 2018 2017 Leasehold improvements $ 146 $ 2,104 Laboratory equipment — 908 Furniture and fixtures 1,074 325 Computer equipment 658 279 1,878 3,616 Less: accumulated depreciation (509) (2,755) Total property and equipment, net $ 1,369 $ 861

Intangible assets (Tables)

Intangible assets (Tables)	12 Months Ended
	Dec. 31, 2018
Intangible assets
Schedule of intangible assets	The Company’s intangible assets consist of the following (in thousands): December 31, 2018 Estimated useful life Acquired and in-licensed rights $ 22,000 14 years Less: accumulated amortization (423) Total intangible assets, net $ 21,577

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Accrued expenses
Schedule of accrued expenses	Accrued expenses consist of the following (in thousands): December 31, 2018 December 31, 2017 Compensation and related benefits 8,749 2,622 Contract research organization costs 6,682 3,774 Commercialization costs 1,979 131 Interest 1,786 108 Consulting fees 494 448 Professional fees 482 617 Other 936 242 Total accrued expenses $ 21,108 $ 7,942

Long-term debt (Tables)

Long-term debt (Tables)	12 Months Ended
	Dec. 31, 2018
Long-term debt
Schedule of future principal payments under the Loan Agreement	The future principal payments under the Amended Loan Agreement are as follows as of December 31, 2018 (in thousands): 2019 $ 5,930 2020 19,070 Total principal payments $ 25,000

Product revenue reserves and _2

Product revenue reserves and allowances (Tables)	12 Months Ended
	Dec. 31, 2018
Product revenue reserves and allowances
Schedule of product revenue allowance and reserve categories	The following table summarizes activity in each of the product revenue allowance and reserve categories for the year ended December 31, 2018 (in thousands): Trade discounts and allowances Third-Party Payer chargebacks, discounts and fees Government rebates and other incentives Returns Total Beginning balance at December 31, 2017 $ — $ — $ — $ — $ — Provision related to sales in the current year 69 120 157 2 348 Adjustments related to prior period sales — — — — — Credits and payments made (40) (32) — — (72) Ending balance at December 31, 2018 $ 29 $ 88 $ 157 $ 2 $ 276

Common stock (Tables)

Common stock (Tables)	12 Months Ended
	Dec. 31, 2018
Common stock.
Schedule of shares of common stock reserved for the issuance of common stock for vested restricted stock units and the exercise of stock options	As of December 31, 2018 and 2017, the Company had reserved the following shares of common stock for the issuance of common stock for vested restricted stock units, the exercise of stock options, and an outstanding warrant (in thousands): December 31, 2018 2017 Shares reserved under equity compensation plans 15,572 8,264 Shares reserved for inducement grants 6,381 3,616 Shares reserved for Convertible Senior Notes 20,937 — Total shares reserved 42,890 11,880

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Stock-based compensation
Schedule of stock-based compensation expense as reflected in the Company's consolidated statements of operations and comprehensive loss	Stock‑based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss was as follows (in thousands): Year ended December 31, 2018 2017 2016 Research and development $ 2,043 $ 1,381 $ 1,073 Selling, general and administrative 4,628 3,652 5,145 Total stock-based compensation expense $ 6,671 $ 5,033 $ 6,218
Summary of stock option activity and related information	Weighted-average Weighted-average remaining Aggregate exercise price per contractual term intrinsic value Shares share (years) (in thousands) Outstanding at December 31, 2017 8,719,978 $ 5.19 7.9 $ 6,150 Granted 5,539,232 $ 5.33 Exercised (412,851) $ 1.96 Forfeited/cancelled (1,323,492) $ 4.55 Outstanding at December 31, 2018 12,522,867 $ 5.42 7.8 $ 6,909 Vested at December 31, 2018 6,107,429 $ 6.05 6.5 $ 4,469 Vested and expected to vest at December 31, 2018(1) 12,084,867 $ 5.46 7.8 $ 6,811 (1) This represents the number of vested options as of December 31, 2018, plus the number of unvested options expected to vest as of December 31, 2018.
Schedule of assumptions used to estimate fair value of each stock option on grant date	Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.65 % 2.02 % 1.48 % Volatility 81 % 78 % 75 % Dividend yield — — — Expected term (years) 5.8
Schedule of restricted stock units	Shares Weighted-average grant date fair value per share Outstanding at December 31, 2017 — $ — Granted 376,000 $ 5.28 Vested — $ — Forfeited (69,250) $ 5.45 Outstanding at December 31, 2018 306,750 $ 5.24

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Net Loss per Share
Schedule of computation of basic and diluted net income (loss) per share	Year Ended December 31, 2018 2017 2016 Net loss (72,434) (67,802) (36,440) Less: Other income (25,556) — — Add: Interest expense 3,071 — — Adjusted diluted net loss (94,919) (67,802) (36,440) Weighted average shares outstanding 64,962 38,422 36,988 Add: Dilutive effect of the Notes 4,359 — — Weighted average diluted shares outstanding 69,321 38,422 36,988 Net loss per share - basic (1.12) (1.76) (0.99) Net loss per share - diluted (1.37) (1.76) (0.99)
Schedule of potentially anti-dilutive securities excluded from calculation of diluted net loss per share	\ Year Ended December 31, 2018 2017 2016 Outstanding stock options 12,522,867 8,719,978 5,848,470 Outstanding warrants — — 142,857 Outstanding restricted stock units 306,750 — — Total potentially dilutive securities 12,829,617 8,719,978 5,991,327

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income taxes
Schedule of reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations	December 31, 2018 2017 Income tax benefit using U.S. federal statutory rate 21.00 % 34.00 % State tax benefit, net of federal benefit 6.38 % 5.00 % Research and development tax credits 5.61 % 7.04 % Permanent items (0.65) % (1.24) % Effect of U.S. Tax Cuts and Jobs Act — % (45.19) % Change in the valuation allowance (31.82) % 0.19 % Other (0.52) % 0.20 % — % — %
Schedule of principal components of deferred tax assets and liabilities	The principal components of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 88,829 $ 64,677 Capitalized research and development 2,545 2,780 Research and development credits 19,725 15,406 Stock-based compensation 3,756 2,560 Other 543 379 Total deferred tax assets 115,398 85,802 Deferred tax liabilities: Derivative liability (13,617) — Total deferred tax liabilities (13,617) — Net deferred tax asset 101,781 85,802 Valuation allowance (101,781) (85,802) Net deferred tax asset $ — $ —

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and contingencies
Schedule of minimum aggregate future lease commitments	The minimum aggregate future lease commitments as of December 31, 2018 are as follows (in thousands): 2019 $ 716 2020 971 2021 1,020 2022 1,041 2023 1,062 Thereafter 1,538 Total $ 6,348

Quarterly financial informati_2

Quarterly financial information (unaudited, in thousands, except per share data) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly financial information (unaudited, in thousands, except per share data)
Schedule of Quarterly Financial Information	Quarterly financial information (unaudited, in thousands, except per share data) First Quarter Second Quarter Third Quarter Fourth Quarter Ended Ended Ended Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Revenue: Product revenue, net $ — $ — $ 508 $ 1,210 License revenue — 10,000 15,000 — Total revenue — Operating expenses: Costs of revenues, excluding amortization of acquired intangible assets $ — $ — $ $ 116 Research and development 8,762 Selling, general and administrative 26,199 Amortization of acquired intangible assets — — 392 Total operating expenses Loss from operations Other income — — — 25,556 Interest income 1,306 Interest expense (3,952) Net loss $ $ $ $ Net loss per share —basic $ $ $ $ Net loss per share —diluted $ $ $ $ Weighted-average number of common shares used in net loss per share —basic and diluted Net loss per share —basic (a) (a)(b) Net loss per share —diluted (a) (a)(b) (c) (a) In the first and second quarters of 2018, the Company sold 167,065 and 6,314,410 shares of its common stock under the Company’s at-the-market equity offering program, which resulted in net proceeds of $0.6 million and $23.7 million, respectively. (b) In the second quarter of 2018, the Company closed underwritten offerings in which it sold 8,944,444 shares and 7,166,666 shares of its common stock at a price of $4.31 per share and $6.00 per share, respectively, for aggregate proceeds, net of underwriting discounts and offering costs, of $38.3 million and $42.9 million, respectively (c) In the fourth quarter of 2018, utilizing the “if-converted” method, the Company’s Notes are assumed to have been converted as of the issuance date. Accordingly, the weighted average number of potentially issuable shares upon conversion of the Notes was determined by weighting the number of shares issuable upon conversion at December 31, 2018, or 20,936,548, over the total days outstanding, 76 days, to calculate an additional 17,295,409 shares to be added to the weighted-average number of shares. First Quarter Second Quarter Third Quarter Fourth Quarter Ended Ended Ended Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Operating expenses: Research and development $ $ $ $ Selling, general and administrative Total operating expenses Loss from operations Interest income Interest expense Net loss $ $ $ $ Net loss per share —basic and diluted $ $ $ $ Weighted-average number of common shares used in net loss per share —basic and diluted (d) (d)(e) (d) In the third and fourth quarters of 2017, the Company sold 2,853,753 and 2,183,126 shares of its common stock under the Company’s at-the-market equity offering program, which resulted in net proceeds of $14.1 million and $9.0 million, respectively. (e) In December 2017, the Company closed an underwritten offering in which it sold 8,422,877 shares of its common stock at a price of $2.97 per share, for aggregate proceeds, net of underwriting discounts and offering costs, of $24.7 million.

Nature of business (Details)

Nature of business (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Nature of business
Cash, cash equivalents, and short-term investments	$ 249.7
Substantial doubt about the ability to continue as a going concern	false

Significant accounting polici_2

Significant accounting policies - Segment and geographic information (Details)	12 Months Ended
	Dec. 31, 2018item
Segment and geographic information
Number of operating segments	1

Significant accounting polici_3

Significant accounting policies - Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 129,867	$ 82,176
Restricted cash	741	162
Cash, cash equivalents and restricted cash	$ 130,608	$ 82,338	$ 32,511	$ 25,073

Significant accounting polici_4

Significant accounting policies - Cash, cash equivalents and restricted cash - Security deposits (Details) - USD ($)	Dec. 31, 2018	Oct. 31, 2018	Feb. 15, 2018	Dec. 31, 2017
Letter of credit | Office and Laboratory Space in Needham, Massachusetts
Operating lease
Security deposit		$ 241,000	$ 403,000
Letter of credit | Office and Laboratory Space in Needham, Massachusetts | Other Assets
Operating lease
Security deposit	$ 241,000			$ 162,000
Leukemia and Lymphoma Society | Prepaid Expenses and Other Current Assets
Operating lease
Security deposit	$ 500,000

Significant accounting polici_5

Significant accounting policies - Fair Value Financial Instruments, Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial assets
Short-term investments	$ 119,786	$ 4,496
Recurring basis
Financial assets
Cash equivalents	127,689	80,894
Short-term investments	119,786	4,496
Total financial assets	247,475	85,390
Recurring basis | Level 1
Financial assets
Cash equivalents	60,092	75,478
Total financial assets	60,092	75,478
Recurring basis | Level 2
Financial assets
Cash equivalents	67,597	5,416
Short-term investments	119,786	4,496
Total financial assets	$ 187,383	$ 9,912

Significant accounting polici_6

Significant accounting policies - Fair Value Financial Instruments, Derivative Liability (Details)	12 Months Ended
	Dec. 31, 2018USD ($)item	Oct. 17, 2018
Reconciliation of derivative liability
Fair value recognized upon issuance of Convertible Senior Notes	$ 51,531
Fair value adjustment	(25,556)
Reclassification to equity	$ (25,975)
5.00% Convertible Senior Notes due 2048
Derivative liability
Interest rate (as a percent)	5.00%	5.00%
Par value of Notes at issuance	$ 150,000,000
5.00% Convertible Senior Notes due 2048 | Discount rate, cash flows
Derivative liability
Discount rate (as a percent) | item	0.121
5.00% Convertible Senior Notes due 2048 | Discount rate, credit spreads
Derivative liability
Discount rate (as a percent) | item	0.162

Significant accounting polici_7

Significant accounting policies - Fair Value Financial Instruments, Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 18, 2018	Dec. 31, 2017
Term loan
Financial liabilities
Debt, carrying value	$ 25.2		$ 14.8
Level 3 | Term loan
Financial liabilities
Debt, fair value	$ 26.9		$ 14.8
Level 3 | 5.00% Convertible Senior Notes due 2048
Financial liabilities
Debt, fair value		$ 108.1

Significant accounting polici_8

Significant accounting policies - Investments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)item	Dec. 31, 2016USD ($)
Investments
Realized gains or losses on investments	$ 0	$ 0	$ 0
Number of investments in unrealized loss position | item	14	5
Number of investments in unrealized loss position for more than 12 months | item	0	0
Unrealized loss on fair value	$ 46,900	$ 9,900
Cash and cash equivalents	129,867	82,176
Total cash, cash equivalents, and investments, gross unrealized gains	140
Gross unrealized losses		(2)
Total cash, cash equivalents, and investments, gross unrealized losses	(13)	(2)
Total cash, cash equivalents, and investments, Fair Value	249,700
Amortized Cost
Investments
Cash and cash equivalents		82,178
Total cash, cash equivalents, and investments	249,526	86,674
Fair Value
Investments
Cash and cash equivalents, Fair value		82,176
Total cash, cash equivalents, and investments, Fair Value	249,653	86,672
Cash and cash equivalents
Investments
Gross unrealized gains	8
Gross unrealized losses	(1)
Cash and cash equivalents | Amortized Cost
Investments
Cash and cash equivalents	129,860
Cash and cash equivalents | Fair Value
Investments
Cash and cash equivalents, Fair value	129,867
Cash and money market accounts | Amortized Cost
Investments
Cash and cash equivalents	62,270	76,760
Cash and money market accounts | Fair Value
Investments
Cash and cash equivalents, Fair value	$ 62,270	$ 76,760
Corporate bonds and commercial paper
Investments
Original maturity period, cash and cash equivalents	90 days	90 days
Gross unrealized gains	$ 8
Gross unrealized losses	(1)	$ (2)
Corporate bonds and commercial paper | Amortized Cost
Investments
Cash and cash equivalents	67,590	5,418
Corporate bonds and commercial paper | Fair Value
Investments
Cash and cash equivalents, Fair value	67,597	5,416
Investments
Investments
Gross unrealized gains	132
Gross unrealized losses	(12)
Investments | Amortized Cost
Investments
Investments	119,666	4,496
Investments | Fair Value
Investments
Short-term investments	$ 119,786	$ 4,496
Corporate bonds and commercial paper
Investments
Maturity period, investments	1 year	1 year
Gross unrealized gains	$ 132
Gross unrealized losses	(12)
Corporate bonds and commercial paper | Amortized Cost
Investments
Due within 1 year	119,666	$ 4,496
Corporate bonds and commercial paper | Fair Value
Investments
Due within 1 year	$ 119,786	$ 4,496

Significant accounting polici_9

Significant accounting policies - Concentration of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2018customeritem
Concentrations of credit risk and off-balance sheet risk
Number of financial institutions in which cash, cash equivalents and investments were deposited | item	2
Trade accounts receivable | Credit Concentration Risk
Concentrations of credit risk and off-balance sheet risk
Number of customer	2
Trade accounts receivable | Credit Concentration Risk | Minimum
Concentrations of credit risk and off-balance sheet risk
Percentage	50.00%
Revenue | Customer Concentration Risk
Concentrations of credit risk and off-balance sheet risk
Number of customer	2
Revenue | Customer Concentration Risk | Minimum
Concentrations of credit risk and off-balance sheet risk
Percentage	10.00%

Significant accounting polic_10

Significant accounting policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)
Share-based compensation
Number of companies expected volatility is based	5
Number of companies with similar characteristics to the Company included in computation	4
Exclusive license agreement | Infinity
Other assets
Prepaid CRO Expenses | $	$ 755,000	$ 755,000
Laboratory equipment
Property and equipment, net
Estimated useful lives	5 years
Furniture and fixtures
Property and equipment, net
Estimated useful lives	5 years
Computer equipment
Property and equipment, net
Estimated useful lives	3 years

Significant accounting polic_11

Significant accounting policies - Revenue Recognition and Accounts Receivable, Net (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Revenue Recognition
Practical expedient, incremental cost of obtaining contract	true
Incremental costs of obtaining a contract	$ 0
Accounts Receivable
Threshold accounts receivable typically due	31 days
Prior to product's expiration
Revenue Recognition
Period for eligible returns of expired product	3 months
After product's expiration date
Revenue Recognition
Period for eligible returns of expired product	6 months

Significant accounting polic_12

Significant accounting policies - Recently Issued or Adopted Accounting Standards Updates (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Recently Adopted Accounting Standards Updates
Restricted cash	$ 741,000	$ 162,000
ASU 2016-18 | Adjustment
Recently Adopted Accounting Standards Updates
Restricted cash		$ 162,000

Inventory (Details)

Inventory (Details) $ in Thousands	Dec. 31, 2018USD ($)
Inventory
Work in process	$ 63
Finished goods	264
Total inventories	$ 327

Property and equipment, net (De

Property and equipment, net (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Feb. 15, 2018ft²	Apr. 15, 2014ft²
Property and equipment
Property and equipment, gross	$ 1,878	$ 3,616
Less: accumulated depreciation	(509)	(2,755)
Property and equipment, net	1,369	861
Depreciation expenses	996	556	$ 670
Office and Laboratory Space in Needham, Massachusetts
Property and equipment
Area of space | ft²				27,810	15,197
Leasehold improvements
Property and equipment
Property and equipment, gross	146	2,104
Leasehold improvements | Office and Laboratory Space in Needham, Massachusetts
Property and equipment
Area of office space vacated | ft²				15,197
Gain or loss from disposal of leasehold improvements	0
Laboratory equipment
Property and equipment
Property and equipment, gross		908
Furniture and fixtures
Property and equipment
Property and equipment, gross	1,074	325
Computer equipment
Property and equipment
Property and equipment, gross	$ 658	$ 279

Intangible assets (Details)

Intangible assets (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 24, 2018
Intangible assets
Less: accumulated amortization		$ (423)		$ (423)
Intangible assets, net		21,577		21,577
Payments for milestone payment				22,000
Amortization of acquired intangible asset		392	$ 31	423
Future amortization per year thereafter		1,600		1,600
Acquired and in-license rights
Intangible assets
Intangible assets, gross		$ 22,000		$ 22,000
Estimated useful life				14 years
Milestone payment payable					$ 22,000
Payments for milestone payment	$ 22,000

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses
Compensation and related benefits	$ 8,749	$ 2,622
Contract research organization costs	6,682	3,774
Commercialization costs	1,979	131
Interest	1,786	108
Consulting fees	494	448
Professional fees	482	617
Other	936	242
Total accrued expenses	$ 21,108	$ 7,942

Long-term debt - Hercules term

Long-term debt - Hercules term loan facility (Details) $ in Thousands	Mar. 05, 2018USD ($)item	Mar. 21, 2017USD ($)item	Jun. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Long-term debt
Amount drawn					$ 9,900	$ 14,811
Exclusive license agreement | Infinity
Long-term debt
Milestone payment				$ 6,000
Term loan
Long-term debt
Maximum borrowing capacity	$ 50,000	$ 25,000
Amount drawn			$ 10,000	$ 15,000
Remaining borrowing capacity	$ 35,000		$ 25,000
Interest rate	10.50%
Minimum cash proceeds to extend interest only payment period	$ 20,000
Amortization payments, installments | item	20
Number of business days prior written notice for prepayment	7 days
Percentage of total principal amount, as final payment	4.50%
Threshold for final debt payment	$ 5,000
Additional interest rate in an event of default	5.00%
Future principal payments
2019					5,930
2020					19,070
Total principal payments					$ 25,000
Term loan | Prime rate
Long-term debt
Basis spread	(4.50%)
Term loan | Minimum
Long-term debt
Interest rate	10.50%
Term loan | Maximum
Long-term debt
Number of separate advances | item		4
Interest rate	12.75%
Term loan | Plan | Minimum
Long-term debt
Amount drawn	$ 5,000
Term loan | Advance prepaid within 12 months following closing date
Long-term debt
Prepayment charge (as a percent)	3.00%
Term loan | Advance prepaid between 12 months and 24 months following closing date
Long-term debt
Prepayment charge (as a percent)	2.00%
Term loan | Advance prepaid after 24 months
Long-term debt
Prepayment charge (as a percent)	1.00%
Term E Loan | Plan | Maximum
Long-term debt
Amount drawn	$ 10,000
Term F Loan | Plan | Maximum
Long-term debt
Amount drawn	$ 25,000

Product revenue reserves and _3

Product revenue reserves and allowances (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Product revenue reserves and allowances
Provision related to sales in the current year	$ 348
Credits and payments made	(72)
Product revenue reserves and allowances at ending	276
Trade discounts and allowances
Product revenue reserves and allowances
Provision related to sales in the current year	69
Credits and payments made	(40)
Product revenue reserves and allowances at ending	29
Third-Party Payer chargebacks, discounts and fees
Product revenue reserves and allowances
Provision related to sales in the current year	120
Credits and payments made	(32)
Product revenue reserves and allowances at ending	88
Government rebates and other incentives
Product revenue reserves and allowances
Provision related to sales in the current year	157
Product revenue reserves and allowances at ending	157
Returns
Product revenue reserves and allowances
Provision related to sales in the current year	2
Product revenue reserves and allowances at ending	$ 2

Common stock (Details)

Common stock (Details) $ in Thousands	Aug. 28, 2017USD ($)	Mar. 30, 2017USD ($)	Jun. 30, 2018USD ($)shares	Mar. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Sep. 30, 2017USD ($)shares	Dec. 31, 2018USD ($)Voteshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2018USD ($)shares
Common stock
Shares reserved for issuance					11,880,000		42,890,000	11,880,000	42,890,000
Number of votes entitled for each share of common stock | Vote							1
Common stock
Net proceeds from issuance of stock | $							$ 105,156	$ 48,069
5.00% Convertible Senior Notes due 2048
Common stock
Shares reserved for issuance							20,937,000		20,937,000
At-the-market equity offering program | Common stock
Common stock
Issuance of common stock, net of issuance costs (in shares)			6,314,410	167,065	2,183,126	2,853,753	6,481,475	5,036,879
Net proceeds from issuance of stock | $			$ 23,700	$ 600	$ 9,000	$ 14,100
Inducement Award Program
Common stock
Shares reserved for issuance					3,616,000		6,381,000	3,616,000	6,381,000
2017 ATM Program | At-the-market equity offering program | Common stock
Common stock
Maximum value of common stock allowed to be sold | $	$ 75,000	$ 35,000
Issuance of common stock, net of issuance costs (in shares)									11,518,354
Net proceeds from issuance of stock | $									$ 47,300
Equity compensation plans
Common stock
Shares reserved for issuance					8,264,000		15,572,000	8,264,000	15,572,000

Common stock - Equity offerings

Common stock - Equity offerings (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 14, 2018	May 23, 2018	May 16, 2018	Dec. 14, 2017	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Common stock
Net proceeds from issuance of stock							$ 105,156	$ 48,069
Cantor | Follow-on offerings | Underwriting Agreement
Common stock
Options are exercisable from date of prospectus supplement			30 days
Common stock | Follow-on offerings
Common stock
Number of shares of common stock issued							16,111,110	8,422,877
Common stock | Follow-on offerings | Underwriting Agreement
Common stock
Number of shares of common stock issued					8,422,877	8,944,444
Share price (in dollars per share)					$ 2.97	$ 4.31		$ 2.97
Net proceeds from issuance of stock					$ 24,700	$ 38,300
Common stock | Follow-on offerings | Purchase Agreement
Common stock
Number of shares of common stock issued						7,166,666
Share price (in dollars per share)						$ 6
Net proceeds from issuance of stock						$ 42,900
Common stock | Cantor | Follow-on offerings | Underwriting Agreement
Common stock
Number of shares of common stock issued		1,166,666
Net proceeds from issuance of stock		$ 38,300
Common stock | Cantor | Follow-on offerings | Underwriting Agreement | Plan
Common stock
Number of shares of common stock issued			7,777,778
Share price (in dollars per share)			$ 4.31
Common stock | Consonance | Follow-on offerings | Purchase Agreement
Common stock
Number of shares of common stock issued	7,166,666
Share price (in dollars per share)	$ 6
Net proceeds from issuance of stock	$ 42,900
Common stock | BTIG, LLC | Follow-on offerings | Underwriting Agreement
Common stock
Number of shares of common stock issued				8,422,877
Share price (in dollars per share)				$ 2.97
Net proceeds from issuance of stock				$ 24,700

Stock-based compensation - Stoc

Stock-based compensation - Stock-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation
Total stock-based compensation expense	$ 6,671,000	$ 5,033,000	$ 6,218,000
Adjustments to additional paid-in capital	6,671,000	5,033,000	6,287,000
Increase (decrease) in stock-based compensation award liability			(69,000)
Research and development
Stock-based compensation
Total stock-based compensation expense	2,043,000	1,381,000	1,073,000
Selling, general and administrative
Stock-based compensation
Total stock-based compensation expense	$ 4,628,000	$ 3,652,000	$ 5,145,000

Stock-based compensation - Equi

Stock-based compensation - Equity Plans (Details)	12 Months Ended
	Dec. 31, 2018item
Stock-based compensation
Number of equity compensation plans	2
Stock options
Stock-based compensation
Expiration term	10 years
Stock options | One year from vesting
Stock-based compensation
Vesting percentage	25.00%
Vesting period	1 year
Stock options | Each successive three-month period, thereafter
Stock-based compensation
Vesting percentage	6.25%
Vesting period	3 months

Stock-based compensation - St_2

Stock-based compensation - Stock Plans and Activity (Details) - shares

Jan. 01, 2018

Jan. 01, 2017

Jan. 01, 2016

Dec. 31, 2018

Jun. 30, 2018

Dec. 31, 2017

Dec. 31, 2016

Feb. 29, 2012

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Dec. 18, 2018

Dec. 31, 2014

Stock options

Stock-based compensation

Granted (in shares)

5,539,232

Exercised (in shares)

412,851

Restricted Stock Units

Stock-based compensation

Granted (in shares)

376,000

0

0

Forfeited (in shares)

69,250

2012 Incentive Plan

Stock-based compensation

Number of shares reserved

3,428,571

16,628,425

Additional number of shares authorized for issuance

1,285,714

1,285,714

1,285,714

30,101

Percentage of outstanding shares of common stock used to compute annual increase in number of shares reserved

4.00%

2012 Incentive Plan | Maximum

Stock-based compensation

Annual increase in number of shares reserved

1,285,714

2012 Incentive Plan | Stock options

Stock-based compensation

Granted (in shares)

12,099,594

Forfeited (in shares)

3,683,349

Exercised (in shares)

364,958

2012 Incentive Plan | Restricted Stock Units

Stock-based compensation

Granted (in shares)

1,195,918

Forfeited (in shares)

219,351

Vested (in shares)

759,817

Inducement Award Program

Stock-based compensation

Number of shares reserved

750,000

Additional number of shares authorized for issuance

1,250,000

1,700,000

2,500,000

580,000

Shares available for future grants

1,888,241

1,888,241

Inducement Award Program | Stock options

Stock-based compensation

Granted (in shares)

5,447,841

Forfeited (in shares)

721,082

Exercised (in shares)

323,750

Inducement Award Program | Restricted Stock Units

Stock-based compensation

Granted (in shares)

90,000

Forfeited (in shares)

0

Vested (in shares)

0

Stock-based compensation - St_3

Stock-based compensation - Stock Options (Details) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended	12 Months Ended
	Sep. 30, 2017shares	Oct. 31, 2016shares	Jun. 30, 2016shares	Mar. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2018USD ($)item$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / shares
Additional disclosures
Stock-based compensation expense | $					$ 6,671	$ 5,033	$ 6,218
Stock options
Shares
Outstanding at the beginning of the period (in shares) | shares				8,719,978	8,719,978
Granted (in shares) | shares					5,539,232
Exercised (in shares) | shares					(412,851)
Forfeited/cancelled (in shares) | shares					(1,323,492)
Outstanding at the end of the period (in shares) | shares					12,522,867	8,719,978
Vested at the end of the period (in shares) | shares					6,107,429
Vested and expected to vest at end of period (in shares) | shares					12,084,867
Weighted-average exercise price per share
Outstanding at the beginning of the period (in dollars per share) | $ / shares				$ 5.19	$ 5.19
Granted (in dollars per share) | $ / shares					5.33
Exercised (in dollars per share) | $ / shares					1.96
Forfeited/cancelled (in dollars per share) | $ / shares					4.55
Outstanding at the end of the period (in dollars per share) | $ / shares					5.42	$ 5.19
Vested at the end of the period (in dollars per share) | $ / shares					6.05
Vested and expected to vest at end of the period (in dollars per share) | $ / shares					$ 5.46
Weighted-average remaining contractual term
Outstanding at the end of the period					7 years 9 months 18 days	7 years 10 months 24 days
Vested at the end of the period					6 years 6 months
Vested and expected to vest at the end of the period					7 years 9 months 18 days
Exercisable at the end of the period					6 years 6 months
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $				$ 6,150	$ 6,150
Outstanding at the end of the period (in dollars) | $					6,909	$ 6,150
Vested at the end of the period (in dollars) | $					4,469
Vested and expected to vest at end of the period (in dollars) | $					6,811
Aggregate intrinsic value of options exercised (in dollars) | $					$ 1,800	$ 1,100
Assumptions used to estimate fair value of each stock-based award on the grant date
Risk-free interest rate (as a percent)					2.65%	2.02%	1.48%
Volatility (as a percent)					81.00%	78.00%	75.00%
Dividend yield (as a percent)					0.00%	0.00%	0.00%
Expected term					5 years 9 months 18 days	5 years 9 months 18 days	5 years 10 months 24 days
Additional disclosures
Stock-based compensation expense | $					$ 5,600	$ 4,500	$ 6,100
Weighted-average grant date fair value (in dollars per share) | $ / shares					$ 3.72	$ 1.83	$ 0.99
Fair value of vested stock options | $					$ 4,700	$ 4,800	$ 6,900
Total unrecognized stock-based compensation expense (in dollars) | $					$ 17,500
Weighted-average recognition period					3 years 8 months 12 days
Stock options vesting based on performance
Shares
Granted (in shares) | shares			500,000	582,500
Vested at the end of the period (in shares) | shares	250,000	250,000
Additional disclosures
Stock-based compensation expense | $					$ 700	$ 400	$ 200
Number of performance conditions achieved | item					2
Number of non-vested stock options | shares					438,000
Percentage of performance conditions achieved							50.00%

Stock-based compensation - Rest

Stock-based compensation - Restricted Stock (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)item$ / sharesshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares
Additional disclosures
Stock-based compensation expense | $	$ 6,671	$ 5,033	$ 6,218
Restricted Stock Units
Stock-based compensation
Number of shares holder to receive when RSU vests | shares	1
Number of installments | item	4
Number of anniversaries | item	4
Vesting percentage	100.00%
Shares
Granted (in shares) | shares	376,000	0	0
Forfeited (in shares) | shares	(69,250)
Unvested at the end of the period (in shares) | shares	306,750
Weighted-average grant date fair value
Granted (in dollars per share) | $ / shares	$ 5.28
Forfeited (in dollars per share) | $ / shares	5.45
Unvested at the end of the period (in dollars per share) | $ / shares	$ 5.24
Additional disclosures
Stock-based compensation expense | $	$ 400		$ 100
Unrecognized stock-based compensation expense | $	$ 1,200
Weighted-average recognition period	1 year 10 months 13 days
Restricted Stock Units | Maximum
Additional disclosures
Stock-based compensation expense | $		$ 100

Convertible Senior Notes (Detai

Convertible Senior Notes (Details) $ / shares in Units, $ in Thousands	Dec. 18, 2018USD ($)	Oct. 17, 2018USD ($)itemD$ / shares	Dec. 31, 2018USD ($)$ / shares	Oct. 11, 2018$ / shares	Dec. 31, 2017$ / shares
Convertible Notes
Net proceeds			$ 145,297
Common stock, par value (in dollars per share) | $ / shares			$ 0.0001		$ 0.0001
Change in fair value of conversion feature			$ 25,556
5.00% Convertible Senior Notes due 2048
Convertible Notes
Aggregate principal amount		$ 150,000
Interest rate (as a percent)		5.00%	5.00%
Net proceeds		$ 145,300
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001
Initial conversion rate		0.1395771
Initial conversion price of Common Stock | $ / shares		$ 7.16
Conversion premium above the last reported sales price of the Common Stock (as a percent)		15.00%
Sale price of the Common Stock | $ / shares				$ 6.23
Percentage of stock price trigger for conversion		130.00%
Trading days | item		20
Consecutive trading days | D		30
Percentage price of principal amount to be purchased plus accrued and unpaid interest		100.00%
Derivative liability		$ 51,500
Debt discount		$ 51,500	$ 56,300
Fair value of conversion feature	$ 25,900
Change in fair value of conversion feature	$ 25,600
Interest expense			$ 3,100

Net Loss per Share - basic and

Net Loss per Share - basic and diluted net income (loss) per share (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 18, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Computation of basic and diluted net income (loss) per share:
Net loss		$ (11,349)	$ (21,668)	$ (18,367)	$ (21,050)	$ (18,235)	$ (23,126)	$ (13,436)	$ (13,005)	$ (72,434)	$ (67,802)	$ (36,440)
Less: Other income		$ (25,556)								(25,556)
Add: Interest expense										3,071
Adjusted diluted net loss										$ (94,919)	$ (67,802)	$ (36,440)
Weighted average shares outstanding										64,962,000	38,422,000	36,988,000
Add: Dilutive effect of the Notes	4,359,391	17,295,409								4,359,000
Weighted average diluted shares outstanding		91,061,000	73,644,000	61,256,000	50,835,000					69,321,000	38,422,000	36,988,000
Net loss per share - basic		$ (0.15)	$ (0.29)	$ (0.30)	$ (0.41)					$ (1.12)	$ (1.76)	$ (0.99)
Net loss per share - diluted		$ (0.37)	$ (0.29)	$ (0.30)	$ (0.41)					$ (1.37)	$ (1.76)	$ (0.99)

Net Loss per Share (Details)

Net Loss per Share (Details) - USD ($) $ in Thousands	Dec. 18, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net loss per share
Change in fair value of the bifurcated derivative			$ 25,600
Interest expense			$ 3,071
Number of shares at issue	20,936,548
Number of shares at issuance		20,936,548	20,936,548
Number of days outstanding to calculate weighted average shares	76 days	76 days
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)			12,829,617	8,719,978	5,991,327
Weighted-average number of shares of adjustments	4,359,391	17,295,409	4,359,000
Stock options
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)			12,522,867	8,719,978	5,848,470
Outstanding warrants
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)					142,857
Restricted Stock Units
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)			306,750

Income taxes - Operating Loss C

Income taxes - Operating Loss Carryforwards (Details) $ in Millions	Dec. 31, 2018USD ($)
Federal
Operating loss carryforwards
Net operating loss carryforwards	$ 325.2
State
Operating loss carryforwards
Net operating loss carryforwards	$ 325

Income taxes - Tax Credits (Det

Income taxes - Tax Credits (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Tax credit carryforwards
Testing period for changes in ownership interest of significant shareholders	3 years
Federal
Tax credit carryforwards
Tax credits	$ 18.1
State
Tax credit carryforwards
Tax credits	$ 2

Income taxes - Reconciliation o

Income taxes - Reconciliation of Income Taxes and Deferred Tax Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of income taxes computed using the U.S. federal statutory rate
Income tax benefit using U.S. federal statutory rate (as a percent)	21.00%	34.00%
State tax benefit, net of federal benefit (as a percent)	6.38%	5.00%
Research and development tax credits (as a percent)	5.61%	7.04%
Permanent items (as a percent)	(0.65%)	(1.24%)
Effect of U.S. Tax Cuts and Jobs Act (as a percent)		(45.19%)
Change in the valuation allowance (as a percent)	(31.82%)	0.19%
Other (as a percent)	(0.52%)	0.20%
Deferred tax assets:
Net operating loss carryforwards	$ 88,829,000	$ 64,677,000
Capitalized research and development	2,545,000	2,780,000
Research and development credits	19,725,000	15,406,000
Stock-based compensation	3,756,000	2,560,000
Other	543,000	379,000
Total deferred tax assets	115,398,000	85,802,000
Deferred tax liabilities:
Derivative liability	(13,617,000)
Total deferred tax liabilities	(13,617,000)
Net deferred tax asset	101,781,000	85,802,000
Valuation allowance	(101,781,000)	(85,802,000)
Additional disclosures
Increase (decrease) in valuation allowance	16,000,000
Unrecognized tax benefits	0
Interest and penalties accrued related to unrecognized tax benefits	0
Liability for uncertain tax positions	$ 0	$ 0	$ 0

Commitments and contingencies_2

Commitments and contingencies (Details) - Office and Laboratory Space in Needham, Massachusetts	Feb. 15, 2018USD ($)ft²	Oct. 31, 2018USD ($)	Apr. 15, 2014ft²
Operating Leased Assets
Area of space | ft²	27,810		15,197
Initial annual base rent	$ 660,000
Base rent for last twelve-month period	$ 1,100,000
Period after which base rent increases	P12M
Letter of credit
Operating Leased Assets
Security deposit	$ 403,000	$ 241,000

Commitments and contingencies -

Commitments and contingencies - Future Lease Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Minimum aggregate future lease commitments
2019	$ 716
2020	971
2021	1,020
2022	1,041
2023	1,062
Thereafter	1,538
Total	6,348
Rent expense	$ 800	$ 400	$ 400

License and collaboration agr_2

License and collaboration agreements - Infinity (Details) - Exclusive license agreement - Infinity - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
License and collaboration agreements
Payments upon study meeting certain pre-specified criteria	$ 6
Payments on approval of new drug application	$ 22
Royalty obligations term	10 years
Percentage of reduction in royalty on net sales	50.00%
Deduction in royalty payments as percentage of amount paid to third party	50.00%
Percentage royalty reductions capped	50.00%
Percentage of additional royalty on worldwide net sales	4.00%
Percentage of net sales for royalty obligations in the United States	1.00%
Percentage of reduction of royalty rate on basis of non-patent regulatory exclusivity	50.00%
Royalty payments		$ 0.1	$ 0	$ 0

License and collaboration agr_3

License and collaboration agreements - Pfizer (Details) - Exclusive license agreement - PFIZER $ in Millions	Jul. 11, 2012USD ($)itemshares
License and collaboration agreements
Number of products required from licensor for an agreed upon price | item	1
One-time cash payment	$ 1.5
Issuance of common stock, net of issuance costs (in shares) | shares	192,012
Royalty obligations term	10 years
Maximum
License and collaboration agreements
Eligible amount payable on developmental milestones to Pfizer	$ 2
Additional payable on successful attainment of regulatory and commercial sales milestones	$ 125

License and collaboration agr_4

License and collaboration agreements - Yakult (Details) $ in Thousands	Jun. 05, 2018item	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
License and collaboration agreements
Revenue recognized			$ 1,210	$ 15,508	$ 10,000	$ 26,718
The Agreement | Yakult
License and collaboration agreements
Proceeds received		$ 10,000
Maximum aggregate payments entitled to receive if all milestones successfully achieved		$ 90,000
Agreement obligations expire from first commercial sale of product (in years)	10 years
Number of days written prior notice required for other party to terminate agreement	180 days
Number of days written notice required to terminate agreement if fails to cure breach	60 days
Number of potential research and development activities | item	2
Number of material promises | item	2
Number of performance obligations at outset of the Agreement | item	1
Revenue recognized						$ 10,000

License and collaboration agr_5

License and collaboration agreements - CSPC (Details) $ in Thousands	Sep. 25, 2018USD ($)item	Aug. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
License and collaboration agreements
Revenue recognized			$ 1,210	$ 15,508	$ 10,000	$ 26,718
Exclusivity Agreement | CSPC
License and collaboration agreements
Proceeds received		$ 5,000
The Agreement | CSPC
License and collaboration agreements
Upfront, non-refundable payment obligation to be received prior to initial Exclusivity Fee	$ 15,000
Maximum aggregate payments entitled to receive if all milestones successfully achieved	$ 160,000
Agreement obligations expire from first commercial sale of product (in years)	10 years
Number of days written prior notice required for other party to terminate agreement	180 days
Number of days written notice required to terminate agreement if fails to cure breach	60 days
Number of potential research and development activities | item	2
Number of material promises | item	2
Number of performance obligations at outset of the Agreement | item	1
Revenue recognized						$ 15,000

Employee benefit plan (Details)

Employee benefit plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee benefit plan
Contributions made under 401(k) plan	$ 0.8	$ 0.3	$ 0.2

Quarterly financial informati_3

Quarterly financial information (unaudited, in thousands, except per share data) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Total revenue	$ 1,210	$ 15,508	$ 10,000						$ 26,718
Operating expenses:
Costs of revenues, excluding amortization of acquired intangible assets	116	49							165
Research and development	8,762	11,571	12,381	$ 10,934	$ 11,253	$ 17,743	$ 9,042	$ 8,385	43,648	$ 46,423	$ 19,779
Selling, general and administrative	26,199	25,426	15,813	9,827	6,799	5,394	4,425	4,763	77,265	21,381	17,223
Amortization of acquired intangible assets	392	31							423
Total operating expenses	35,469	37,077	28,194	20,761	18,052	23,137	13,467	13,148	121,501	67,804	37,002
Loss from operations	(34,259)	(21,569)	(18,194)	(20,761)	(18,052)	(23,137)	(13,467)	(13,148)	(94,783)	(67,804)	(37,002)
Other income	25,556								25,556
Interest income	1,306	763	343	191	145	121	140	155	2,603	561	562
Interest expense	(3,952)	(862)	(516)	(480)	(328)	(110)	(109)	(12)	(5,810)	(559)
Net loss	$ (11,349)	$ (21,668)	$ (18,367)	$ (21,050)	$ (18,235)	$ (23,126)	$ (13,436)	$ (13,005)	$ (72,434)	$ (67,802)	$ (36,440)
Net loss per share—basic	$ (0.15)	$ (0.29)	$ (0.30)	$ (0.41)					$ (1.12)	$ (1.76)	$ (0.99)
Net loss per share—diluted	$ (0.37)	$ (0.29)	$ (0.30)	$ (0.41)					$ (1.37)	$ (1.76)	$ (0.99)
Net loss per share - basic and diluted					$ (0.43)	$ (0.61)	$ (0.36)	$ (0.35)
Weighted average common shares outstanding used in computing:
Net loss per share—basic (in shares)	73,766	73,644	61,256	50,835					64,962	38,422	36,988
Net loss per share—diluted (in shares)	91,061	73,644	61,256	50,835					69,321	38,422	36,988
Weighted-average number of common shares used in net loss per share—basic and diluted					42,027	37,630	36,992	36,992
License
Revenue
Total revenue	$ 1,210	$ 508							$ 25,000
Product
Revenue
Total revenue		$ 15,000	$ 10,000						$ 1,718

Quarterly financial informati_4

Quarterly financial information (unaudited, in thousands, except per share data) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 18, 2018	Dec. 31, 2017	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Net proceeds from issuance of stock								$ 105,156	$ 48,069
Number of shares at issuance			20,936,548					20,936,548
Number of days outstanding to calculate weighted average shares	76 days		76 days
Weighted-average number of shares of adjustments	4,359,391		17,295,409					4,359,000
At-the-market equity offering program | Common stock
Number of shares of common stock issued				6,314,410	167,065	2,183,126	2,853,753	6,481,475	5,036,879
Net proceeds from issuance of stock				$ 23,700	$ 600	$ 9,000	$ 14,100
Follow-on offerings | Common stock
Number of shares of common stock issued								16,111,110	8,422,877
Follow-on offerings | Common stock | Underwriting Agreement
Number of shares of common stock issued		8,422,877		8,944,444
Net proceeds from issuance of stock		$ 24,700		$ 38,300
Share price (in dollars per share)		$ 2.97		$ 4.31		$ 2.97			$ 2.97
Follow-on offerings | Common stock | Purchase Agreement
Number of shares of common stock issued				7,166,666
Net proceeds from issuance of stock				$ 42,900
Share price (in dollars per share)				$ 6