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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2015	Feb. 22, 2016	Jun. 30, 2015
Document and Entity Information
Entity Registrant Name	Verastem, Inc.
Entity Central Index Key	0001526119
Document Type	10-K
Document Period End Date	Dec. 31, 2015
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 Public Float			$ 258,961,368
Entity Common Stock, Shares Outstanding		36,992,418
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 24,870,000	$ 33,901,000
Short-term investments	85,388,000	58,774,000
Prepaid expenses and other current assets	585,000	2,641,000
Total current assets	110,843,000	95,316,000
Property and equipment, net	2,048,000	2,825,000
Restricted cash	203,000	203,000
Other assets		305,000
Total assets	113,094,000	98,649,000
Current liabilities:
Accounts payable	3,942,000	3,216,000
Accrued expenses	6,098,000	5,519,000
Liability classified stock-based compensation awards	69,000	469,000
Total current liabilities	10,109,000	9,204,000
Other liabilities	516,000	677,000
Liability for shares subject to repurchase		2,000
Stockholders' equity
Convertible preferred stock, $0.0001 par value; 5,000 shares authorized, no shares issued and outstanding
Common stock, $0.0001 par value; 100,000 shares authorized, 36,941 and 27,259 shares issued and outstanding at December 31, 2015 and 2014, respectively	4,000	3,000
Additional paid-in capital	301,305,000	229,770,000
Accumulated other comprehensive income	43,000	11,000
Accumulated deficit	(198,883,000)	(141,018,000)
Total stockholders' equity	102,469,000	88,766,000
Total liabilities and stockholders' equity	$ 113,094,000	$ 98,649,000

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares shares in Thousands	Dec. 31, 2015	Dec. 31, 2014
CONDENSED CONSOLIDATED BALANCE SHEETS
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000	5,000
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	100,000	100,000
Common stock, shares issued	36,941	27,259
Common stock, shares outstanding	36,941	27,259

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 40,565	$ 35,448	$ 25,930
General and administrative	17,634	18,159	15,472
Total operating expenses	58,199	53,607	41,402
Loss from operations	(58,199)	(53,607)	(41,402)
Interest income	334	242	200
Net loss	$ (57,865)	$ (53,365)	$ (41,202)
Net loss per share - basic and diluted	$ (1.61)	$ (2.07)	$ (1.82)
Weighted-average number of common shares used in net loss per share-basic and diluted	35,932,000	25,804,000	22,680,000
Net loss	$ (57,865)	$ (53,365)	$ (41,202)
Unrealized gains (losses) on available-for-sale securities	32	(17)	6
Comprehensive loss	$ (57,833)	$ (53,382)	$ (41,196)

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive (loss) income	Accumulated deficit	Total
Balance at Dec. 31, 2012	$ 2	$ 136,893	$ 22	$ (46,451)	$ 90,466
Balance (in shares) at Dec. 31, 2012	20,364,210
Increase (Decrease) in Stockholders' Equity
Net loss				(41,202)	(41,202)
Unrealized gain (loss) on available-for-sale marketable securities			6		6
Issuance of common stock, net of issuance costs of $43 and $235 for 2014 and 2013, respectively	$ 1	59,759			59,760
Issuance of common stock, net of issuance costs of $43 and $235 for 2014 and 2013, respectively	4,255,000
Vesting of restricted stock		9			9
Vesting of restricted stock (in shares)	417,718
Issuance of common stock resulting from exercise of stock options		33			33
Issuance of common stock resulting from exercise of stock options (in shares)	81,795
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings		(1,302)			(1,302)
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings (in shares)	209,727
Stock-based compensation expense		9,676			9,676
Balance at Dec. 31, 2013	$ 3	205,068	28	(87,653)	117,446
Balance (in shares) at Dec. 31, 2013	25,328,450
Increase (Decrease) in Stockholders' Equity
Net loss				(53,365)	(53,365)
Unrealized gain (loss) on available-for-sale marketable securities			(17)		(17)
Issuance of common stock, net of issuance costs of $43 and $235 for 2014 and 2013, respectively		11,646			11,646
Issuance of common stock, net of issuance costs of $43 and $235 for 2014 and 2013, respectively	1,346,676
Vesting of restricted stock		9			9
Vesting of restricted stock (in shares)	321,287
Issuance of common stock resulting from exercise of stock options		20			20
Issuance of common stock resulting from exercise of stock options (in shares)	30,451
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings		(780)			(780)
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings (in shares)	135,008
Shares issued for technology rights		1,447			1,447
Shares issued for technology rights (in shares)	97,500
Stock-based compensation expense		12,360			12,360
Balance at Dec. 31, 2014	$ 3	229,770	11	(141,018)	$ 88,766
Balance (in shares) at Dec. 31, 2014	27,259,372				27,259,000
Increase (Decrease) in Stockholders' Equity
Net loss				(57,865)	$ (57,865)
Unrealized gain (loss) on available-for-sale marketable securities			32		32
Issuance of common stock resulting from follow-on offering	$ 1	50,941			50,942
Issuance of common stock resulting from follow-on offering (in shares)	8,337,500
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $53		10,911			$ 10,911
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $53 (in shares)	1,189,479				1,189,479
Vesting of restricted stock		2			$ 2
Vesting of restricted stock (in shares)	7,995
Issuance of common stock resulting from exercise of stock options		13			13
Issuance of common stock resulting from exercise of stock options (in shares)	33,658
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings		(417)			(417)
Issuance of common stock resulting from vesting of restricted stock units and payment of tax withholdings (in shares)	113,257
Stock-based compensation expense		10,085			10,085
Balance at Dec. 31, 2015	$ 4	$ 301,305	$ 43	$ (198,883)	$ 102,469
Balance (in shares) at Dec. 31, 2015	36,941,261				36,941,000

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY
Issuance of common stock, issuance costs		$ 43	$ 235
Issuance of common stock at market transactions, issuance costs	$ 53

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Cash Provided by (Used in) Operating Activities
Net loss	$ (57,865)	$ (53,365)	$ (41,202)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	754	427	236
Stock-based compensation expense	10,085	12,360	9,676
Amortization of premiums and discounts on available-for-sale marketable securities	264	290
Non-cash expense related to purchase of technology rights		1,197
Loss on disposal of fixed assets	46	4
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	276	27	(382)
Accounts payable	863	287	912
Accrued expenses and other liabilities	418	2,119	3,738
Liability classified stock-based compensation awards	(400)	(248)	717
Net cash used in operating activities	(45,559)	(36,902)	(26,305)
Investing activities
Purchases of property and equipment	(211)	(2,429)	(56)
Purchases of investments	(199,851)	(39,361)	(118,190)
Maturities of investments	173,005	85,047	94,853
Increase in restricted cash		(117)
Net cash (used in) provided by investing activities	(27,057)	43,140	(23,393)
Financing activities
Proceeds from the exercise of stock options	13	20	33
Net proceeds from the issuance of common stock and restricted common stock	63,989	9,534	59,760
Cash used to settle restricted stock liability awards	(417)	(780)	(1,302)
Net cash provided by financing activities	63,585	8,774	58,491
(Decrease) increase in cash and cash equivalents	(9,031)	15,012	8,793
Cash and cash equivalents at beginning of period	33,901	18,889	10,096
Cash and cash equivalents at end of period	$ 24,870	33,901	$ 18,889
Supplemental disclosure of non-cash financing and investing activities
Proceeds from the issuance of common stock included in prepaid expenses and other current assets		2,085
Purchase of property and equipment in accounts payable		$ 196

Nature of business	12 Months Ended
Nature of business	Dec. 31, 2015
Nature of business
Nature of business	1. Nature of business Verastem, Inc. (the “Company”) is a biopharmaceutical company focused on discovering and developing drugs to improve outcomes for patients with cancer. The Company’s operations to date have been limited to organizing and staffing the Company, business planning, raising capital, acquiring and developing its technology, identifying potential product candidates and undertaking preclinical and clinical studies of its most advanced product candidates. The Company is subject to a number of risks similar to other life science companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, inability to obtain marketing approval of product candidates, competitors developing new technological innovations, market acceptance of the Company’s products and protection of proprietary technology. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate product revenue or achieve profitability. As of December 31, 2015, the Company had an accumulated deficit of $198.9 million. The Company expects to continue to incur operating losses in future periods. The Company had cash, cash equivalents and investments of $110.3 million as of December 31, 2015. The Company believes that its existing cash, cash equivalents and investments will be sufficient to fund its current operating plan and capital expenditure requirements for at least the next twelve months.

Significant accounting policies

12 Months Ended

Dec. 31, 2015

Significant accounting policies.

Significant accounting policies

2. Significant accounting policies

Basis of consolidation

Effective December 2012, Verastem Securities Company was incorporated as a wholly owned subsidiary of the Company and 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 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 for evaluation 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 drugs for the treatment of cancer. All material long-lived assets of the Company reside in the United States.

Cash and cash equivalents

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 United States Treasury money market funds, government‑sponsored enterprise securities and corporate bonds and commercial paper of publicly traded companies. Cash equivalents are reported at fair value.

Fair value of financial instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. The fair value hierarchy 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
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

The following table presents information about the Company’s financial assets and liabilities that have been measured at fair value at December 31, 2015 and indicates the fair value hierarchy of the valuation inputs utilized to determine fair value (in thousands).


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	23,036	$	11,464	$	11,572	$	—
Short-term investments		85,388		—		85,388		—
Total financial assets	$	108,424	$	11,464	$	96,960	$	—

Financial liabilities
Liability classified stock-based compensation awards	$	69	$	69	$	—	$	—
Total financial liabilities	$	69	$	69	$	—	$	—

The following table presents information about the Company’s financial assets and liabilities that have been measured at fair value at December 31, 2014 and indicates the fair value hierarchy of the valuation inputs utilized to determine such fair value (in thousands).


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	32,140	$	32,140	$	—	$	—
Short-term investments		58,774		—		58,774		—
Total financial assets	$	90,914	$	32,140	$	58,774	$	—
Financial liabilities
Liability classified stock-based compensation awards	$	469	$	469	$	—	$	—
Total financial liabilities	$	469	$	469	$	—	$	—

Investments

Investments and cash equivalents consist of investments in United States Treasury money market accounts, United States Treasury securities, government‑sponsored enterprise securities and corporate bonds and commercial paper of publicly traded companies that are classified as available‑for‑sale pursuant to Accounting Standards Codification (ASC) 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 classified as long‑term assets on the consolidated balance sheets if (i) the Company has the intent and ability to hold the investments for a period of at least one year and (ii) the contractual maturity date of the investments is greater than one year. 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 (deficit), 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. There were no other‑than‑temporary declines in fair value of short‑term or long‑term investments for the years ended December 31, 2015, 2014 and 2013. 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 recognized for the years ended December 31, 2015, 2014 and 2013.

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. As of December 31, 2015 and 2014, there were no investments with a fair value that was significantly lower than the amortized cost basis or any investments that had been in an unrealized loss position for a significant period.

As of December 31, 2015, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and money market accounts	$	13,298	$	—	$	—	$	13,298
Government-sponsored enterprise securities (original maturities within 90 days)		2,000		—		—		2,000
Corporate bonds and commercial paper (original maturities within 90 days)		9,572		—		—		9,572
Total cash and cash equivalents	$	24,870	$	—	$	—	$	24,870
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	11,932	$	5	$	—	$	11,937
Treasury securities (due within 1 year)		1,005		—		—		1,005
Corporate bonds and commercial paper (due within 1 year)		72,408		57		(19)		72,446
Total investments	$	85,345	$	62	$	(19)	$	85,388
Total cash, cash equivalents, and investments	$	110,215	$	62	$	(19)	$	110,258

As of December 31, 2014, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Total cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	3,700	$	—	$	—	$	3,700
Corporate bonds and commercial paper (due within 1 year)		55,063		18		(7)		55,074
Total investments	$	58,763	$	18	$	(7)	$	58,774
Total cash, cash equivalents, and investments	$	92,664	$	18	$	(7)	$	92,675

Concentrations of credit risk and off‑balance sheet risk

Cash and cash equivalents and investments are financial instruments that potentially subject the Company to concentrations of credit risk. As of December 31, 2015, the Company’s cash, cash equivalents and investments were deposited at two financial institutions. The Company maintains its cash and cash equivalents and investments with high quality, accredited financial institutions and, accordingly, the Company believes it is not exposed to any significant credit risk on these funds. The Company has no significant off‑balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.

Property and equipment

Property and equipment consists 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 is 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, 2015.

Research and development costs

The Company expenses research and development costs to operations as incurred. Research and development expenses consist of costs associated with research activities, including drug discovery efforts and the development of product candidates. 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 service has been performed or when the goods have been received rather than when the payment is made. 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 contract research organizations, or CROs, clinical trial sites, manufacturing organizations and consultants, including the scientific advisory board;

license fees; and

facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, depreciation of equipment, and laboratory supplies.

Stock‑based compensation

The Company expenses the fair value of employee stock options over the requisite service period, which is the vesting period. Compensation expense is measured using the fair value of the award at the grant date, net of estimated forfeitures, and is adjusted annually to reflect actual forfeitures. The grant date fair value of each stock‑based award is estimated using the Black‑Scholes option pricing model and is expensed on a straight‑line basis over the vesting period. 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.

Stock‑based awards issued to nonemployees, 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 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.

Stock‑based compensation awards which allow for greater than the minimum statutory tax withholdings are classified as liabilities. These awards are revalued at each reporting date and are expensed on a straight‑line basis over the vesting period. Upon settlement, the awards are revalued and the amounts are reclassified to additional paid‑in capital. Shares are withheld to cover the tax withholding and amounts paid to settle the tax liability are recorded as a reduction of additional paid‑in capital.

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

Basic and diluted 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, without consideration for common stock equivalents. The Company’s potentially dilutive shares, which include outstanding stock options, restricted stock units, unvested restricted stock and the warrant issued in 2014 are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. All potentially dilutive securities were excluded from the calculation of diluted net loss per share as the securities were anti‑dilutive for all periods presented.

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,
	2015	2014	2013
Outstanding stock options	5,390,130	4,206,440	2,388,062
Outstanding warrants	142,857	142,857	—
Restricted stock units	53,751	293,747	529,850
Unvested restricted stock	—	7,995	329,282
	5,586,738	4,651,039	3,247,194

Recent Accounting Pronouncements

Leases

In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-02, Leases (Topic 842). The ASU requires lessees to put most leases on their balance sheets as a liability for the obligation to make lease payments and as a right-of-use asset, but recognize expenses on the income statements in a manner similar to today’s accounting. The guidance also eliminates the current real estate-specific provisions for all entities. For calendar-year public entities, the guidance becomes effective in 2019 and interim periods within that year. Early adoption is permitted for all entities. The Company has not chosen early adoption for this ASU and is currently evaluating its effect on the Company’s consolidated financial statements.

Income Taxes

In November 2015, the FASB issued ASU 2015-17, Income Taxes: Balance Sheet Classification of Deferred Taxes (Topic 740). ASU 2015-17 simplifies the presentation of deferred income taxes by eliminating the separate classification of deferred income tax assets and liabilities into current and noncurrent amounts in the consolidated balance sheet statement of financial position. The amendments in the update require that all deferred tax assets and liabilities be classified as noncurrent in the consolidated balance sheet. The amendments in this update are effective for annual periods beginning after December 15, 2017, and interim periods therein and may be applied either prospectively or retrospectively to all periods presented. Early adoption is permitted. The Company has early adopted this standard in the fourth quarter of 2015 on a prospective basis, and it did not have an effect on the Company’s consolidated financial statements.

Going Concern

In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements - Going Concern: Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern (Subtopic 205-40). ASU 2014-15 requires management to assess an entity’s ability to continue as a going concern every reporting period, and provide certain disclosures if management has substantial doubt about the entity’s ability to operate as a going concern, or an express statement if not, by incorporating and expanding upon certain principles that are currently in U.S. auditing standards. ASU 2014-15 is effective for the annual period ending after December 15, 2016, and for annual periods and interim periods thereafter. Early application is permitted. The Company has evaluated the impact of the adoption of ASU 2014-15 on its 2015 consolidated financial statements and determined that there is not substantial doubt about the Company’s ability to continue as a going concern for at least one year from the issuance of the 2015 consolidated financial statements.

Revenue Recognition

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606). ASU 2014-09 amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in Accounting Standards Codification Topic 605, Revenue Recognition, and creates a new Topic 606, Revenue from Contracts with Customers. This guidance was originally pronounced to become effective for fiscal years beginning after December 15, 2016, with early adoption not permitted. On July 9, 2015, the FASB decided to defer the effective date of the ASU by one year. As a result, the Company will be required to apply the new revenue standard to annual reporting periods beginning after December 15, 2017, and would be permitted to adopt the ASU early, but not before the original public organization effective date (annual periods beginning after December 15, 2016). Two adoption methods are permitted: retrospectively to all prior reporting periods presented, with certain practical expedients permitted; or retrospectively with the cumulative effect of initially adopting the ASU recognized at the date of initial application. The Company has not yet determined which adoption method it will utilize or the effect, if any, the adoption of this guidance will have on its consolidated financial statements as through December 31, 2015 the Company has yet to record revenue from its operations.

Property and equipment

12 Months Ended

Dec. 31, 2015

Property and equipment

Property and equipment

3. Property and equipment, net

Property and equipment and related accumulated depreciation and amortization are as follows (in thousands):


	December 31,		December 31,
	2015		2014
Leasehold improvements	$	2,104	$	2,096
Laboratory equipment		908		1,040
Furniture and fixtures		325		325
Computer equipment		240		225
		3,577		3,686
Less: accumulated depreciation and amortization		(1,529)		(861)
	$	2,048	$	2,825

Approximate total depreciation and amortization expenses amounted to $754,000, $427,000 and $236,000 for the years ended December 31, 2015, 2014 and 2013, respectively.

Prepaid expenses and other current assets

12 Months Ended

Dec. 31, 2015

Prepaid expenses and other current assets

Prepaid expenses and other current assets

4. Prepaid expenses and other current assets

Prepaid expenses and other current assets consist of the following (in thousands):


	December 31,		December 31,
	2015		2014
Prepaid contract research organization costs	$	404	$	238
Interest receivable		154		113
Receivable from unsettled at-the-market equity transactions		—		2,085
Receivable related to tenant improvement allowance		—		150
Prepaid other		27		55
	$	585	$	2,641

Accrued expenses

12 Months Ended

Dec. 31, 2015

Accrued expenses

Accrued expenses

5. Accrued expenses

Accrued expenses are as follows (in thousands):


	December 31,		December 31,
	2015		2014
Contract research organization costs	$	3,782	$	3,049
Compensation and related benefits		1,802		1,990
Professional fees		260		233
Deferred rent		160		144
Other		94		103
	$	6,098	$	5,519

Common stock

12 Months Ended

Dec. 31, 2015

Common stock.

Common stock

6. Common stock

As of December 31, 2015 and 2014, 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,	December 31,
	2015	2014
Shares reserved under equity compensation plans	5,958	5,083
Shares reserved for inducement grants	750	750
Shares reserved for outstanding warrants	143	143
	6,851	5,976

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 program

In December 2013, 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. In November 2014, the Company commenced sales under this program. During the years ended December 31, 2015 and 2014, the Company sold 1,189,479 and 1,346,676 shares of common stock, respectively, under this program with net proceeds (after deducting commissions and other offering expenses) of $12.9 million and $9.6 million, respectively.

Warrant

In February 2014, in connection with the acquisition of intellectual property rights from Encarta, Inc., the Company issued a warrant to purchase 142,857 shares of common stock exercisable at a price of $17.16 per share that expires 3 years from the issuance date.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB TOPIC 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Paragraph d
-Article 4

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29, 30, 31
-Article 5

Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 17: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Article 4

Reference 18: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

Reference 19: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Article 3

+ Details

Stock-based compensation

12 Months Ended

Dec. 31, 2015

Stock-based compensation

Stock-based compensation

7. 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,
	2015		2014		2013
Research and development	$	2,411	$	3,727	$	3,575
General and administrative		7,273		8,385		6,818
Total stock-based compensation expense	$	9,684	$	12,112	$	10,393

Of the $9.7 million of stock-based compensation expense recorded during the year ended December 31, 2015, $10.1 million was recorded to additional paid-in capital and approximately $400,000 was recorded as a decrease in liability classified awards. Of the $12.1 million of stock-based compensation expense recorded during the year ended December 31, 2014, approximately $12.3 million was recorded to additional paid-in capital and approximately $248,000 was recorded as a decrease in liability classified awards. Of the $10.4 million of stock-based compensation expense recorded during the year ended December 31, 2013, approximately $9.7 million was recorded to additional paid-in capital and approximately $717,000 was recorded as an increase in liability classified awards.

The Company has awards outstanding under two equity compensation plans, the 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 allows 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 includes an “evergreen provision” that allows for an annual increase in the number of shares of common stock available for issuance under the 2012 Plan. The annual increase will be added on the first day of each year beginning in 2013 and each subsequent anniversary until the expiration of the 2012 Plan, equal to the lowest of 1,285,714 shares of common stock, 4.0% of the number of shares of common stock outstanding and an amount determined by the board of directors. On January 1, 2015 and January 1, 2014, the number of shares available for issuance under the 2012 Plan increased by 1,081,045 and 1,026,309, respectively, for shares under this 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, 2015, under the 2012 Plan, the Company has granted stock options for 6,331,493 shares of common stock, of which 1,283,968 have been forfeited and restricted stock units for 909,918 shares of common stock, of which 150,101 have been forfeited. The exercise price of each option has been equal to the closing price of a share of our common stock on the grant date. On January 1, 2016, the number of shares available for issuance under the 2012 Plan increased by 1,285,714.

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 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. The program is governed by the terms of the 2012 Plan but does not fall under the 2012 Plan. As of December 31, 2015 and 2014, the Company granted 210,000 and 0 shares of common stock under the program, of which 75,000 and 0 have been forfeited, respectively.

Restricted Common Stock

In connection with the Company’s formation, the founders purchased an aggregate of 2,857,138 shares of Company’s common stock at fair value on the date of issuance. The shares were issued subject to restricted stock agreements between the Company and each founder, which allow the Company, at its discretion, to repurchase unvested shares if the founder’s relationship with the Company is terminated. Under these agreements, twenty-five percent (25%) of the shares vested immediately and the remaining seventy-five percent (75%) of shares vest ratably in quarterly installments over the subsequent four years.

The Company records stock‑based compensation expense for the common stock subject to repurchase, or restricted common stock grants, based on the grant date fair value for employees and the reporting date and upon vesting fair value for non‑employees. The fair value of the award is considered the intrinsic value as of each measurement date. All of the restricted shares were issued at a purchase price equal to the fair value of the common stock on the date of issuance. The Company recorded stock‑based compensation expense associated with restricted common stock grants of $0, $1.7 million and $3.4 million for the years ended December 31, 2015, 2014 and 2013, respectively.

A summary of the Company’s restricted stock activity and related information is as follows:


		Weighted-average
		grant date
		fair value
	Shares	per share
Unvested at December 31, 2014	7,995	$	0.28
Vested	(7,995)	$	0.28
Unvested at December 31, 2015	—	$	—

No restricted common stock was granted during the years ended December 31, 2015, 2014 and 2013. The total fair value of shares vested during the years ended December 31, 2015, 2014 and 2013 was approximately $59,000, $2.2 million and $3.3 million, respectively. As of December 31, 2015, there was no unrecognized stock‑based compensation expense as all shares of restricted stock had vested.

Restricted Stock Units

A summary of the Company’s restricted stock units (“RSU”) and related information is as follows:


		Weighted-
		average
		grant date
		fair value
	Shares	per share
Unvested at December 31, 2014	293,747	$	10.54
Vested	(164,410)	$	10.70
Forfeited	(75,586)	$	9.85
Unvested at December 31, 2015	53,751	$	11.00

No restricted stock units were granted during the years ended December 31, 2015, 2014 and 2013. The total fair value of restricted stock units vested during the years ended December 31, 2015, 2014 and 2013 was $1.6 million, $2.2 million and $3.4 million, respectively. As of December 31, 2015, there was approximately $104,000 of total unrecognized stock‑based compensation expense related to unvested RSUs granted under the 2012 Plan. The expense is expected to be recognized over a weighted‑average period of 0.1 years.

During 2012, the Company issued a restricted stock unit for 103,306 shares to an employee. The award vests up to 25% per year based on the achievement of stated objectives. The objectives related to 2015 were established on January 8, 2015. In December 2015, the Board of Directors elected not to approve any payment under this award for the year ended December 31, 2015. The Company did not record any stock-based compensation expense for this award during the year ended December 31, 2015. The objectives related to 2014 were established on January 7, 2014 and the objectives were determined to be met on January 8, 2015. The Company recorded $158,000 of stock-based compensation expense for this award during the year ended December 31, 2014 based on the achievement of the stated objectives. The objectives related to 2013 were established on January 15, 2013 and the objectives were determined to be met on December 17, 2013. The Company recorded $294,000 of stock‑based compensation expense for this award during the year ended December 31, 2013 based on the achievement of the stated objectives.

During the first quarter of 2013, the Company amended the terms of certain RSUs related to a total of 697,060 shares of common stock to allow for tax withholdings greater than the minimum required statutory withholding amount. As a result of this change in the terms of the awards, the outstanding RSUs are considered to be liability instruments. As a result of this modification, the Company records a liability for the fair value of the awards as of each reporting date with the change in fair value recorded through the consolidated statements of operations and comprehensive loss. The Company will record stock‑based compensation expense equal to the greater of the original grant date fair value of the awards or the settlement date fair value. During the year ended December 31, 2015, 2014 and 2013, the Company deposited approximately $417,000, $780,000 and $1.3 million, respectively, with tax authorities to settle the tax liability for awards that settled during the respective periods. The liability related to these awards of approximately $69,000, approximately $469,000 and approximately $717,000 is recorded as liability classified stock‑based compensation awards on the consolidated balance sheets as of December 31, 2015, 2014 and 2013, respectively.

Stock Options

A summary of the Company’s stock option activity and related information follows:


				Weighted-average
		Weighted-average		remaining	Aggregate
		exercise price per		contractual term	intrinsic value
	Shares	share		(years)	(in thousands)
Outstanding at December 31, 2014	4,206,440	$	10.38	8.4	$	2,593
Granted	2,424,864	$	5.99
Exercised	(33,638)	$	0.38		$	249
Forfeited	(1,207,536)	$	9.31
Outstanding at December 31, 2015	5,390,130	$	8.71	8.1	$	175
Vested at December 31, 2015	2,753,484	$	9.95	7.3	$	175
Vested and expected to vest at December 31, 2015(1)	5,174,667	$	8.75	8.1	$	175

(1)

This represents the number of vested options as of December 31, 2015, plus the number of unvested options expected to vest as of December 31, 2015, based on the unvested options at December 31, 2015, adjusted for the estimated forfeiture rate.

The fair value of each employee stock option was estimated at the date of grant using a Black‑Scholes option‑pricing model with the following assumptions:


	Year Ended
	December 31,
	2015		2014		2013
Risk-free interest rate	1.75	%	1.99	%	1.14	%
Volatility	73	%	81	%	75	%
Dividend yield	—		—		—
Expected term (years)	6.0		6.0		6.0

The Company uses the simplified method as prescribed by the Securities and Exchange Commission Staff Accounting Bulletin 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. The computation of expected volatility is based on the historical volatility of a representative group of public biotechnology and life sciences companies with similar characteristics to the Company, including early stage of product development and therapeutic focus. The Company also considers its own historical volatility. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. Management assesses expected forfeitures based on the experience of the Company coupled with comparison to data from the representative group of companies and recognizes compensation costs only for those equity awards expected to vest.

The Company recorded stock‑based compensation expense associated with employee stock options of $7.9 million, $7.8 million and $3.6 million for the years ended December 31, 2015, 2014 and 2013, respectively. The weighted‑average grant date fair value of options granted in the years ended December 31, 2015, 2014 and 2013 was $3.80, $8.61 and $6.55 per share, respectively. The fair value of options that vested during the years ended December 31, 2015, 2014 and 2013 was $9.5 million, $6.6 million and $2.9 million, respectively.

At December 31, 2015, there was $10.0 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 2.1 years.

Income taxes

12 Months Ended

Dec. 31, 2015

Income taxes

Income taxes

8. Income Taxes

As of December 31, 2015, the Company had federal and state net operating loss carryforwards of approximately $157.5 million and $155.9 million, respectively, which are available to reduce future taxable income. The Company also had federal and state tax credits of $3.2 million and $1.4 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 2035. Net operating loss 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:


	Year Ended
	December 31,
	2015		2014
Income tax benefit using U.S. federal statutory rate	34.00	%	34.00	%
State tax benefit, net of federal benefit	5.91	%	4.73	%
Research and development tax credits	1.49	%	1.51	%
Permanent items	(3.02)	%	(6.25)	%
Change in the valuation allowance	(41.90)	%	(34.34)	%
Other	3.52	%	0.35	%
	—	%	—	%

The principal components of the Company’s deferred tax assets are as follows (in thousands):


	December 31,
	2015		2014
Deferred tax assets:
Net operating loss carryforwards	$	61,606	$	42,352
Capitalized research and development		1,984		2,154
Research and development credits		4,087		2,880
Stock-based compensation		4,401		726
Other		478		199
Gross deferred tax assets		72,556		48,311
Valuation allowance		(72,556)		(48,311)
Net deferred tax asset	$	—	$	—

The Company has recorded a valuation allowance against its deferred tax assets at December 31, 2015 and 2014 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 $24.2 million, $18.3 million and $14.5 million in the years ended December 31, 2015, 2014 and 2013, respectively, primarily relates to the net loss incurred by the Company.

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, 2015, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not, as yet, 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.

Commitments and contingencies

12 Months Ended

Dec. 31, 2015

Commitments and Contingencies

Commitments and Contingencies

9. Commitments and contingencies

In May 2011, the Company entered into a non‑cancelable operating lease for office and laboratory space, which expired on October 31, 2014. The lease agreement provided for free rent for the first four months of the lease term and included escalating rent payments. Rent expense was recorded on a straight‑line basis over the lease term, accordingly. The Company secured a letter of credit for $86,000 in connection with the lease, which was released upon expiration of the lease.

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 expires on September 30, 2019. The Company began using the leased premises as its corporate headquarters and commenced rent payments effective September 22, 2014. The Company has agreed to pay an initial annual base rent of approximately $493,000, which base rent increases after every twelve-month period during the lease term to approximately $554,000 for the last twelve-month period. The Company is recording rent expense on a straight-line basis, beginning in April 2014. The Company also received a tenant improvement allowance of approximately $684,000 in connection with the lease. The Company has accounted for the allowance as a lease incentive, which is being recorded as a reduction to rent expense over the lease term. Deferred rent and lease incentive obligation are 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. The Company has provided a security deposit in the form of a letter of credit in the amount of approximately $203,000, which may be reduced to approximately $162,000 on April 15, 2016. The amount is included in long term restricted cash on the consolidated balance sheets.

The minimum aggregate future lease commitments are as follows (in thousands):


2016	$	512
2017		527
2018		542
2019		415
2020		—
Thereafter		—
Total	$	1,996

The Company recorded rent expense of approximately $352,000, approximately $541,000 and approximately $366,000 for the years ended December 31, 2015, 2014 and 2013, 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 agreements	12 Months Ended
License agreements	Dec. 31, 2015
License agreements
License agreements	10. License agreements In October 2010, the Company entered into an exclusive license agreement, which was amended and restated in January 2012, with the Whitehead Institute for Biomedical Research (the Licensor) for certain intellectual property. The Company paid the Licensor an upfront license fee and reimbursed patent related fees and costs incurred by the Licensor and affiliates of the Licensor totaling $104,000 in the aggregate and issued 166,664 shares of common stock to the Licensor and entities and individuals affiliated with the Licensor. The fair value of the common stock was determined to be $0.28 per share, and the fair value was determined to be more readily determinable than the fair value of the license. As a result, the fair value of the shares of approximately $46,000 was recorded as research and development expense. Under the terms of the agreement, the Company also agreed to pay annual license maintenance fees, milestone payments, royalties as a percentage of net sales and a percentage of sublicense income the Company receives. Annual license maintenance fees are creditable against royalties earned during the same calendar year and are not material to the financial statements. Milestone payments are triggered upon the achievement of specified development, regulatory and commercialization milestones and are not creditable against royalties. Actual amounts due under the agreement will vary depending on the number of products developed, the type and development path of the products, and other related factors. Milestone payments could total up to $1.6 million. The Company may terminate the agreement at any time with 90 days’ prior written notice. On December 16, 2011, the Company amended and restated an existing non‑exclusive license agreement with the Licensor pursuant to which the Company obtained an exclusive license to certain intellectual property. The Company paid the Licensor an upfront license fee and agreed to make milestone payments of up to $825,000 upon the achievement of specified regulatory and commercialization milestones. In addition, the Company agreed to pay royalties as a percentage of net sales of licensed products. Under the license agreement with Poniard that the Company entered into in November 2011 relating to VS‑4718 and certain other compounds, the Company paid an upfront license fee and agreed to pay Poniard milestone payments upon the achievement of specified development and regulatory milestones. In February 2014, the Company purchased the assets which were the subject of the license agreement with Poniard from Encarta, Inc. (“Encarta”), who had previously purchased these assets in 2013. In consideration for these assets, the Company issued 97,500 shares of common stock, a warrant to purchase 142,857 shares of common stock with an exercise price equal to $17.16 per share and paid $25,000. All existing obligations under the license agreement, including an achieved development milestone and an obligation to issue a warrant, were settled as part of this transaction. The Company incurred $1.2 million of research and development expense in 2014 as a result of this transaction. In connection with the asset purchase agreement, the Company also assumed the rights and obligations under the Scripps License Agreement. Pursuant to the Scripps License Agreement, the Company is obligated to pay Scripps potential product development milestone payments of up to an aggregate of $3,000,000 upon the achievement of specified development and regulatory milestones. In addition, the Company is obligated to pay Scripps low single‑digit royalties as a percentage of net sales of licensed products, subject to adjustments in certain circumstances. The Company’s obligation to pay royalties on net sales is on a country by country basis. On May 11, 2012, the Company acquired from SBio Pte Ltd (SBio), compounds identified as dual inhibitors of PI3K and mTOR, including related patent rights. Under the agreement, the Company paid SBio an upfront fee of $350,000 and has agreed to pay SBio milestone payments of up to an aggregate of approximately $21.0 million upon the achievement of specified development and regulatory milestones. In addition, the Company agreed to pay to SBio tiered, low to mid single digit royalties as a percentage of annual net sales of each product containing an acquired compound as an ingredient. The obligation to pay royalties continues on a product by product and country by country basis until the expiration of all acquired patent rights covering the product in such country. If the Company obtains a license from a third party in order to commercialize an acquired compound contained in a product in a particular country, then the Company may deduct up to 50% of the amount paid to such third party from the royalty payments that Company owes to SBio for such product. The deduction is subject to specified limitations, including that in no event will any such deduction reduce a royalty payment owed to SBio by more than 50% as a result of all such deductions in the aggregate. There were no ongoing clinical trials at the time of the acquisition of the compounds, and the compounds acquired do not have alternative future uses, nor have they reached a stage of technological feasibility. As no processes or activities were acquired, the Company accounted for the transaction as an asset acquisition by recording the $350,000 payment made to SBio to research and development expense for the year ended December 31, 2012. 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 (Products) for all therapeutic, diagnostic and prophylactic uses in humans. The Company is solely responsible, at its expense, for the clinical development of the 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 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 million in developmental milestones and up to an additional $125 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 Products. The Company’s royalty obligations with respect to each Product in each country begin on the date of first commercial sale of the Product in that country, and end on the later of 10 years after the date of first commercial sale of the 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 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. Accordingly, the Company recorded research and development expense for the year ended December 31, 2012 associated with the cash payment of $1.5 million and the fair value of shares of common stock issued to Pfizer of $2.0 million.

Employee benefit plan	12 Months Ended
Employee benefit plan	Dec. 31, 2015
Employee benefit plan
Employee benefit plan	11. 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 IRS. Under the 401(k) Plan, the Company may make discretionary contributions as approved by the board of directors. The Company made approximate contributions to the 401(k) Plan of $295,000, $219,000 and $160,000 for the years ended December 31, 2015, 2014 and 2013, respectively.

Reduction in force	12 Months Ended
Reduction in force	Dec. 31, 2015
Reduction in force
Reduction in force	12. Reduction in force In October 2015, the Company announced a reduction of workforce by approximately 50% to 20 full time employees. All affected employees will receive severance pay and outplacement assistance. As a result of the reduction in force and associated costs, the Company estimates one-time severance and related costs of $1.1 million. Of these one-time severance and related costs, approximately $349,000 was paid through December 31, 2015 and approximately $713,000 will be paid in 2016.

Quarterly financial information (unaudited, in thousands, except per share data)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information (unaudited, in thousands, except per share data)

Quarterly Financial Information (unaudited, in thousands, except per share data)

13. 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,
	2015			2015			2015			2015
Operating expenses:
Research and development	$	10,528		$	11,045		$	11,304		$	7,688
General and administrative		4,714			4,417			4,230			4,273
Total operating expenses		15,242			15,462			15,534			11,961
Loss from operations		(15,242)			(15,462)			(15,534)			(11,961)
Interest income		62			85			89			98
Net loss	$	(15,180)		$	(15,377)		$	(15,445)		$	(11,863)
Net loss per share —basic and diluted	$	(0.46)		$	(0.42)		$	(0.42)		$	(0.32)
Weighted-average number of common shares used in net loss per share —basic and diluted		33,323	(a)(b)		36,522	(b)		36,898	(b)		36,935


	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	Ended		Ended		Ended		Ended
	March 31,		June 30,		September 30,		December 31,
	2014		2014		2014		2014
Operating expenses:
Research and development	$	8,411	$	8,305	$	9,047	$	9,685
General and administrative		4,723		4,782		4,341		4,313
Total operating expenses		13,134		13,087		13,388		13,998
Loss from operations		(13,134)		(13,087)		(13,388)		(13,998)
Interest income		72		65		56		49
Net loss	$	(13,062)	$	(13,022)	$	(13,332)	$	(13,949)
Net loss per share —basic and diluted	$	(0.51)	$	(0.51)	$	(0.52)	$	(0.53)
Weighted-average number of common shares used in net loss per share —basic and diluted		25,478		25,669		25,811		26,248	(c)

(a)

In January 2015, the Company closed a public offering in which it sold 8,337,500 shares of its common stock at a price of $6.50 per share, including 1,087,500 shares issued pursuant to the exercise of the underwriters’ option to purchase additional shares, which resulted in net proceeds of $50.9 million.

(b)

In the first, second and third quarters of 2015, the Company sold 470,309, 690,370 and 28,800 shares of its common stock under the Company’s at-the-market equity offering program, which resulted in net proceeds of $4.4 million, $6.2 million and approximately $250,000, respectively

(c)

In November and December 2014, the Company sold 1,346,676 shares of common stock under the Company’s at-the-market equity offering program which resulted in net proceeds of $11.6 million.

Subsequent events	12 Months Ended
Subsequent events	Dec. 31, 2015
Subsequent Events.
Subsequent Events	14. 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 balance sheet date. The Company is not aware of any material subsequent events.

Significant accounting policies (Policies)

12 Months Ended

Dec. 31, 2015

Significant accounting policies.

Basis of consolidation

Use of estimates

Segment and geographic information

Cash and cash equivalents

Fair value of financial instruments


Level 1 inputs		Quoted prices in active markets for identical assets or liabilities
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


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	23,036	$	11,464	$	11,572	$	—
Short-term investments		85,388		—		85,388		—
Total financial assets	$	108,424	$	11,464	$	96,960	$	—

Financial liabilities
Liability classified stock-based compensation awards	$	69	$	69	$	—	$	—
Total financial liabilities	$	69	$	69	$	—	$	—


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	32,140	$	32,140	$	—	$	—
Short-term investments		58,774		—		58,774		—
Total financial assets	$	90,914	$	32,140	$	58,774	$	—
Financial liabilities
Liability classified stock-based compensation awards	$	469	$	469	$	—	$	—
Total financial liabilities	$	469	$	469	$	—	$	—

Investments

As of December 31, 2015, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and money market accounts	$	13,298	$	—	$	—	$	13,298
Government-sponsored enterprise securities (original maturities within 90 days)		2,000		—		—		2,000
Corporate bonds and commercial paper (original maturities within 90 days)		9,572		—		—		9,572
Total cash and cash equivalents	$	24,870	$	—	$	—	$	24,870
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	11,932	$	5	$	—	$	11,937
Treasury securities (due within 1 year)		1,005		—		—		1,005
Corporate bonds and commercial paper (due within 1 year)		72,408		57		(19)		72,446
Total investments	$	85,345	$	62	$	(19)	$	85,388
Total cash, cash equivalents, and investments	$	110,215	$	62	$	(19)	$	110,258

As of December 31, 2014, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Total cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	3,700	$	—	$	—	$	3,700
Corporate bonds and commercial paper (due within 1 year)		55,063		18		(7)		55,074
Total investments	$	58,763	$	18	$	(7)	$	58,774
Total cash, cash equivalents, and investments	$	92,664	$	18	$	(7)	$	92,675

Concentrations of credit risk and off-balance sheet risk

Concentrations of credit risk and off‑balance sheet risk

Property and equipment


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.

Research and development costs

employee‑related expenses, including salaries, benefits, travel and stock‑based compensation expense;

license fees; and

facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, depreciation of equipment, and laboratory supplies.

Stock-based compensation

Stock‑based compensation

Income taxes

Net loss per share

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,
	2015	2014	2013
Outstanding stock options	5,390,130	4,206,440	2,388,062
Outstanding warrants	142,857	142,857	—
Restricted stock units	53,751	293,747	529,850
Unvested restricted stock	—	7,995	329,282
	5,586,738	4,651,039	3,247,194

Recent Accounting Pronouncements

Leases

Income Taxes

Going Concern

Revenue Recognition

Significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2015

Significant accounting policies.

Schedule of information about financial assets and liabilities measured at fair value


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	23,036	$	11,464	$	11,572	$	—
Short-term investments		85,388		—		85,388		—
Total financial assets	$	108,424	$	11,464	$	96,960	$	—

Financial liabilities
Liability classified stock-based compensation awards	$	69	$	69	$	—	$	—
Total financial liabilities	$	69	$	69	$	—	$	—


			Quoted prices		Significant other		Significant
			in active		observable		unobservable
			markets		inputs		inputs
Description	Total		(Level 1)		(Level 2)		(Level 3)
Financial assets
Cash equivalents	$	32,140	$	32,140	$	—	$	—
Short-term investments		58,774		—		58,774		—
Total financial assets	$	90,914	$	32,140	$	58,774	$	—
Financial liabilities
Liability classified stock-based compensation awards	$	469	$	469	$	—	$	—
Total financial liabilities	$	469	$	469	$	—	$	—

Schedule of cash, cash equivalents and investments

As of December 31, 2015, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and money market accounts	$	13,298	$	—	$	—	$	13,298
Government-sponsored enterprise securities (original maturities within 90 days)		2,000		—		—		2,000
Corporate bonds and commercial paper (original maturities within 90 days)		9,572		—		—		9,572
Total cash and cash equivalents	$	24,870	$	—	$	—	$	24,870
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	11,932	$	5	$	—	$	11,937
Treasury securities (due within 1 year)		1,005		—		—		1,005
Corporate bonds and commercial paper (due within 1 year)		72,408		57		(19)		72,446
Total investments	$	85,345	$	62	$	(19)	$	85,388
Total cash, cash equivalents, and investments	$	110,215	$	62	$	(19)	$	110,258

As of December 31, 2014, cash, cash equivalents and investments included (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Cash and cash equivalents:
Cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Total cash and cash equivalents	$	33,901	$	—	$	—	$	33,901
Investments:
Government-sponsored enterprise securities (due within 1 year)	$	3,700	$	—	$	—	$	3,700
Corporate bonds and commercial paper (due within 1 year)		55,063		18		(7)		55,074
Total investments	$	58,763	$	18	$	(7)	$	58,774
Total cash, cash equivalents, and investments	$	92,664	$	18	$	(7)	$	92,675

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

Schedule of potentially anti-dilutive securities excluded from calculation of diluted net loss per share



	Year Ended December 31,
	2015	2014	2013
Outstanding stock options	5,390,130	4,206,440	2,388,062
Outstanding warrants	142,857	142,857	—
Restricted stock units	53,751	293,747	529,850
Unvested restricted stock	—	7,995	329,282
	5,586,738	4,651,039	3,247,194

Property and equipment (Tables)

12 Months Ended

Dec. 31, 2015

Property and equipment

Schedule of property equipment and related accumulated depreciation

Property and equipment and related accumulated depreciation and amortization are as follows (in thousands):


	December 31,		December 31,
	2015		2014
Leasehold improvements	$	2,104	$	2,096
Laboratory equipment		908		1,040
Furniture and fixtures		325		325
Computer equipment		240		225
		3,577		3,686
Less: accumulated depreciation and amortization		(1,529)		(861)
	$	2,048	$	2,825

Prepaid expenses and other current assets (Tables)

12 Months Ended

Dec. 31, 2015

Prepaid expenses and other current assets

Schedule of prepaid expenses and other current assets

Prepaid expenses and other current assets consist of the following (in thousands):


	December 31,		December 31,
	2015		2014
Prepaid contract research organization costs	$	404	$	238
Interest receivable		154		113
Receivable from unsettled at-the-market equity transactions		—		2,085
Receivable related to tenant improvement allowance		—		150
Prepaid other		27		55
	$	585	$	2,641

Accrued expenses (Tables)

12 Months Ended

Dec. 31, 2015

Accrued expenses

Schedule of accrued expenses

Accrued expenses are as follows (in thousands):


	December 31,		December 31,
	2015		2014
Contract research organization costs	$	3,782	$	3,049
Compensation and related benefits		1,802		1,990
Professional fees		260		233
Deferred rent		160		144
Other		94		103
	$	6,098	$	5,519

Common stock (Tables)

12 Months Ended

Dec. 31, 2015

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


	December 31,	December 31,
	2015	2014
Shares reserved under equity compensation plans	5,958	5,083
Shares reserved for inducement grants	750	750
Shares reserved for outstanding warrants	143	143
	6,851	5,976

Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2015

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,
	2015		2014		2013
Research and development	$	2,411	$	3,727	$	3,575
General and administrative		7,273		8,385		6,818
Total stock-based compensation expense	$	9,684	$	12,112	$	10,393

Summary of unvested restricted stock activity and related information


		Weighted-average
		grant date
		fair value
	Shares	per share
Unvested at December 31, 2014	7,995	$	0.28
Vested	(7,995)	$	0.28
Unvested at December 31, 2015	—	$	—

Summary of restricted stock units (RSUs)


		Weighted-
		average
		grant date
		fair value
	Shares	per share
Unvested at December 31, 2014	293,747	$	10.54
Vested	(164,410)	$	10.70
Forfeited	(75,586)	$	9.85
Unvested at December 31, 2015	53,751	$	11.00

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, 2014	4,206,440	$	10.38	8.4	$	2,593
Granted	2,424,864	$	5.99
Exercised	(33,638)	$	0.38		$	249
Forfeited	(1,207,536)	$	9.31
Outstanding at December 31, 2015	5,390,130	$	8.71	8.1	$	175
Vested at December 31, 2015	2,753,484	$	9.95	7.3	$	175
Vested and expected to vest at December 31, 2015(1)	5,174,667	$	8.75	8.1	$	175

(1)

Schedule of assumptions used to estimate fair value of each stock option on grant date


	Year Ended
	December 31,
	2015		2014		2013
Risk-free interest rate	1.75	%	1.99	%	1.14	%
Volatility	73	%	81	%	75	%
Dividend yield	—		—		—
Expected term (years)	6.0		6.0		6.0

Income taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income taxes

Schedule of reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations


	Year Ended
	December 31,
	2015		2014
Income tax benefit using U.S. federal statutory rate	34.00	%	34.00	%
State tax benefit, net of federal benefit	5.91	%	4.73	%
Research and development tax credits	1.49	%	1.51	%
Permanent items	(3.02)	%	(6.25)	%
Change in the valuation allowance	(41.90)	%	(34.34)	%
Other	3.52	%	0.35	%
	—	%	—	%

Schedule of principal components of deferred tax assets

The principal components of the Company’s deferred tax assets are as follows (in thousands):


	December 31,
	2015		2014
Deferred tax assets:
Net operating loss carryforwards	$	61,606	$	42,352
Capitalized research and development		1,984		2,154
Research and development credits		4,087		2,880
Stock-based compensation		4,401		726
Other		478		199
Gross deferred tax assets		72,556		48,311
Valuation allowance		(72,556)		(48,311)
Net deferred tax asset	$	—	$	—

Commitments and contingencies (Tables)

12 Months Ended

Dec. 31, 2015

Commitments and Contingencies

Schedule of minimum aggregate future lease commitments

The minimum aggregate future lease commitments are as follows (in thousands):


2016	$	512
2017		527
2018		542
2019		415
2020		—
Thereafter		—
Total	$	1,996

Quarterly financial information (unaudited, in thousands, except per share data) (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information (unaudited, in thousands, except per share data)

Schedule of Quarterly Financial Information


	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
	Ended			Ended			Ended			Ended
	March 31,			June 30,			September 30,			December 31,
	2015			2015			2015			2015
Operating expenses:
Research and development	$	10,528		$	11,045		$	11,304		$	7,688
General and administrative		4,714			4,417			4,230			4,273
Total operating expenses		15,242			15,462			15,534			11,961
Loss from operations		(15,242)			(15,462)			(15,534)			(11,961)
Interest income		62			85			89			98
Net loss	$	(15,180)		$	(15,377)		$	(15,445)		$	(11,863)
Net loss per share —basic and diluted	$	(0.46)		$	(0.42)		$	(0.42)		$	(0.32)
Weighted-average number of common shares used in net loss per share —basic and diluted		33,323	(a)(b)		36,522	(b)		36,898	(b)		36,935


	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	Ended		Ended		Ended		Ended
	March 31,		June 30,		September 30,		December 31,
	2014		2014		2014		2014
Operating expenses:
Research and development	$	8,411	$	8,305	$	9,047	$	9,685
General and administrative		4,723		4,782		4,341		4,313
Total operating expenses		13,134		13,087		13,388		13,998
Loss from operations		(13,134)		(13,087)		(13,388)		(13,998)
Interest income		72		65		56		49
Net loss	$	(13,062)	$	(13,022)	$	(13,332)	$	(13,949)
Net loss per share —basic and diluted	$	(0.51)	$	(0.51)	$	(0.52)	$	(0.53)
Weighted-average number of common shares used in net loss per share —basic and diluted		25,478		25,669		25,811		26,248	(c)

(a)

(b)

(c)

In November and December 2014, the Company sold 1,346,676 shares of common stock under the Company’s at-the-market equity offering program which resulted in net proceeds of $11.6 million.

Nature of business (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Nature of business
Deficit accumulated during the development stage	$ (198,883)	$ (141,018)
Cash, cash equivalents and investments	$ 110,258	$ 92,675

- Definition

Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-12.Column C.3,4)
-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611133-123010

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-12C.Column C.7)
-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611225-123010

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
-Subparagraph (SX 210.12-14.Column E.2,3)
-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611322-123010

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 5
-Subparagraph (SX 210.12-13.Column C.6)
-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611282-123010

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column C
-Article 12

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph 3, 4
-Article 12

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 14
-Paragraph 2, 3
-Article 12

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 13
-Paragraph 6
-Article 12

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12C
-Paragraph 7
-Article 12

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 14
-Paragraph Column E
-Article 12

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 13
-Paragraph Column C
-Article 12

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12C
-Paragraph Column C
-Article 12

+ Details

Significant accounting policies - Segment Information and Fair Value Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Financial assets
Cash equivalents	$ 24,870	$ 33,901
Short-term investments	$ 85,388	58,774
Segment and geographic information
Number of operating segments \| item	1
Total
Financial assets
Cash equivalents	$ 23,036	32,140
Short-term investments	85,388	58,774
Total financial assets	108,424	90,914
Financial liabilities
Liability classified stock-based compensation awards	69	469
Total financial liabilities	69	469
Quoted prices in active markets (Level 1)
Financial assets
Cash equivalents	11,464	32,140
Total financial assets	11,464	32,140
Financial liabilities
Liability classified stock-based compensation awards	69	469
Total financial liabilities	69	469
Significant other observable inputs (Level 2)
Financial assets
Cash equivalents	11,572
Short-term investments	85,388	58,774
Total financial assets	$ 96,960	$ 58,774

Significant accounting policies - Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investments
Charges for other-than-temporary declines in fair value of short-term or long-term investments	$ 0	$ 0	$ 0
Realized gains or losses on investments	0	0	0
Investments with a fair value that is significantly lower than the amortized cost basis	0	0
Amortized cost
Cash and cash equivalents, Amortized cost	24,870	33,901	$ 18,889	$ 10,096	$ 33,901
Investments, Amortized Cost	85,345	58,763
Total cash, cash equivalents, and investments, Amortized Cost	110,215	92,664
Investments, Gross unrealized gains
Total cash, cash equivalents, and investments, Gross Unrealized Gains	62	18
Investments, Gross unrealized losses
Total cash, cash equivalents, and investments, Gross Unrealized Losses	(19)	(7)
Fair value
Cash and cash equivalents, Fair value	24,870	33,901
Investments, Fair Value	85,388	58,774
Total cash, cash equivalents, and investments, Fair Value	110,258	92,675
Government-sponsored enterprise securities
Amortized cost
Due within 1 year	11,932	3,700
Investments, Gross unrealized gains
Due within 1 year	5
Fair value
Due within 1 year	11,937	3,700
Treasury securities
Amortized cost
Due within 1 year	1,005
Fair value
Due within 1 year	1,005
Corporate bonds and commercial paper
Amortized cost
Due within 1 year	72,408	55,063
Investments, Gross unrealized gains
Due within 1 year	57	18
Investments, Gross unrealized losses
Due within 1 year	(19)	(7)
Fair value
Due within 1 year	72,446	55,074
Cash and cash equivalents
Amortized cost
Cash and cash equivalents, Amortized cost	13,298	33,901
Fair value
Cash and cash equivalents, Fair value	13,298	$ 33,901
Government-sponsored enterprise securities
Amortized cost
Cash and cash equivalents, Amortized cost	2,000
Fair value
Cash and cash equivalents, Fair value	2,000
Corporate bonds and commercial paper
Amortized cost
Cash and cash equivalents, Amortized cost	9,572
Fair value
Cash and cash equivalents, Fair value	$ 9,572

Significant accounting policies - Concentration of Credit Risk and Property and Equipment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) item
Property and equipment
Impairment losses on long-lived assets \| $	$ 0
Concentrations of credit risk and off-balance sheet risk
Number of financial institutions in which cash, cash equivalents and investments were deposited \| item	2
Significant off-balance sheet concentrations of credit risk	0.00%
Laboratory equipment
Property and equipment
Estimated useful lives	5 years
Furniture and fixtures
Property and equipment
Estimated useful lives	5 years
Computer equipment
Property and equipment
Estimated useful lives	3 years

Significant accounting policies - Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)	5,586,738	4,651,039	3,247,194
Outstanding stock options
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)	5,390,130	4,206,440	2,388,062
Restricted stock units
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)	142,857	142,857
Outstanding warrants
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)	53,751	293,747	529,850
Unvested restricted stock
Net loss per share
Potentially anti-dilutive securities excluded from calculation of diluted net loss per share (in shares)		7,995	329,282

Property and equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property and equipment
Property and equipment, gross	$ 3,577	$ 3,686
Less: accumulated depreciation and amortization	(1,529)	(861)
Property and equipment, net	2,048	2,825
Depreciation and amortization	754	427	$ 236
Leasehold improvements
Property and equipment
Property and equipment, gross	2,104	2,096
Laboratory equipment
Property and equipment
Property and equipment, gross	908	1,040
Furniture and fixtures
Property and equipment
Property and equipment, gross	325	325
Computer equipment
Property and equipment
Property and equipment, gross	$ 240	$ 225

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Prepaid expenses and other current assets
Prepaid contract research organization costs	$ 404	$ 238
Interest receivable	154	113
Receivable from unsettled at-the-market equity transactions		2,085
Receivable related to tenant improvement allowance		150
Prepaid other	27	55
Prepaid expenses and other current assets	$ 585	$ 2,641

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accrued expenses
Contract research organization costs	$ 3,782	$ 3,049
Compensation and related benefits	1,802	1,990
Professional fees	260	233
Deferred rent	160	144
Other	94	103
Accrued expenses	$ 6,098	$ 5,519

Common stock (Details) - USD ($)	1 Months Ended		2 Months Ended	3 Months Ended			12 Months Ended
Common stock (Details) - USD ($)	Jan. 31, 2015	Feb. 28, 2014	Dec. 31, 2014	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common stock
Shares reserved for issuance			5,976,000				6,851,000	5,976,000
Number of votes available for each share of common stock							1
At-the-market equity offering program
Maximum value of common stock allowed to be sold									$ 35,000,000
Issuance of common stock resulting from at-the-market transactions (in shares)			1,346,676	28,800	690,370	470,309	1,189,479
Proceeds from Issuance of Public Offering Net	$ 50,900,000		$ 11,600,000	$ 250,000	$ 6,200,000	$ 4,400,000	$ 12,900,000	$ 9,600,000
Encarta, Inc.
Warrant
Number of shares of common stock in the warrant issued		142,857
Exercise price of shares of common stock in the warrant issued (in dollars per share)		$ 17.16
Expiration period of warrant		3 years
Outstanding warrants
Common stock
Shares reserved for issuance			143,000				143,000	143,000
Inducement Award Program
Common stock
Shares reserved for issuance			750,000				750,000	750,000
Equity compensation plans
Common stock
Shares reserved for issuance			5,083,000				5,958,000	5,083,000

Stock-based compensation - Stock-Based Compensation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Stock-based compensation
Total stock-based compensation expense	$ 9,684	$ 12,112	$ 10,393
Adjustments to Additional Paid in Capital	10,085	12,360	9,676
Increase (decrease) in stock-based compensation award liability	$ (400)	(248)	717
Number of equity compensation plans \| item	2
Vesting percentage in one year from vesting start date	25.00%
Vesting period for twenty five percent from grant date	1 year
Vesting percentage in equal quarterly installment	6.25%
Vesting period for six and a quarter vesting percentage	3 months
Expiration term	10 years
Research and development
Stock-based compensation
Total stock-based compensation expense	$ 2,411	3,727	3,575
General and administrative
Stock-based compensation
Total stock-based compensation expense	7,273	8,385	6,818
Additional paid-in capital
Stock-based compensation
Adjustments to Additional Paid in Capital	$ 10,085	$ 12,360	$ 9,676

Stock-based compensation - Stock Plans and Activity (Details) - USD ($)				1 Months Ended		12 Months Ended
	Jan. 01, 2016	Jan. 01, 2015	Jan. 01, 2014	Feb. 29, 2012	Sep. 30, 2010	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013
Stock-based compensation
Stock-based compensation expense						$ 9,684,000	$ 12,112,000	$ 10,393,000
Additional disclosures
Cash used to settle restricted stock liability awards						417,000	780,000	1,302,000
Liability classified stock-based compensation awards						$ 69,000	469,000	717,000
Outstanding stock options
Stock-based compensation
Shares granted						2,424,864
Shares forfeited						1,207,536
Options exercised (in shares)						(33,638)
Stock-based compensation expense						$ 7,900,000	7,800,000	3,600,000
Additional disclosures
Total unrecognized stock-based compensation expense (in dollars)						$ 10,000,000
Weighted-average recognition period						2 years 1 month 6 days
Unvested restricted stock
Stock-based compensation
Stock-based compensation expense						$ 0	$ 1,700,000	$ 3,400,000
Shares
Unvested at the beginning of the period (in shares)		7,995				7,995
Granted (in shares)						0	0	0
Vested (in shares)						(7,995)
Unvested at the end of the period (in shares)							7,995
Weighted-average grant date fair value
Unvested at the beginning of the period (in dollars per share)		$ 0.280				$ 0.280
Vested (in dollars per share)						0.280
Unvested at the end of the period (in dollars per share)							$ 0.280
Additional disclosures
Fair value of shares vested						$ 59,000	$ 2,200,000	$ 3,300,000
Total unrecognized stock-based compensation expense (in dollars)						$ 0
Unvested restricted stock \| Founders
Stock-based compensation
Shares issued					2,857,138
Unvested restricted stock \| Vesting immediately \| Founders
Stock-based compensation
Vesting percentage						25.00%
Unvested restricted stock \| Vesting ratably in quarterly installments \| Founders
Stock-based compensation
Vesting percentage						75.00%
Vesting period for quarterly installments						4 years
Restricted stock units
Stock-based compensation
Shares issued									103,306
Stock-based compensation expense							$ 158,000	294,000
Shares
Unvested at the beginning of the period (in shares)	53,751	293,747				293,747
Granted (in shares)						0	0
Vested (in shares)						(164,410)
Forfeited (in shares)						(75,586)
Unvested at the end of the period (in shares)						53,751	293,747
Weighted-average grant date fair value
Unvested at the beginning of the period (in dollars per share)	$ 11.00	$ 10.54				$ 10.54
Vested (in dollars per share)						10.70
Forfeited						9.85
Unvested at the end of the period (in dollars per share)						$ 11.00	$ 10.54
Additional disclosures
Fair value of shares vested						$ 1,600,000	$ 2,200,000	3,400,000
Total unrecognized stock-based compensation expense (in dollars)						$ 104,000
Weighted-average recognition period						1 month 6 days
Unvested RSUs with amended terms										697,060
Cash used to settle restricted stock liability awards						$ 417,000	$ 780,000	$ 1,300,000
Restricted stock units \| Vesting immediately
Stock-based compensation
Vesting percentage									25.00%
2012 Plan
Stock-based compensation
Number of shares reserved				3,428,571
Additional number of shares reserved from shares available for future grant under the 2010 plan				30,101
Annual increase in number of shares reserved						1,285,714
Percentage of outstanding shares of common stock used to compute annual increase in number of shares reserved						4.00%
Increase in number of shares reserved	1,285,714	1,081,045	1,026,309
2012 Plan \| Outstanding stock options
Stock-based compensation
Shares granted						6,331,493
Shares forfeited						1,283,968
2012 Plan \| Restricted stock units
Shares
Granted (in shares)						909,918
Forfeited (in shares)						(150,101)
Inducement Award Program
Stock-based compensation
Number of shares reserved							750,000
Shares
Granted (in shares)						210,000	0
Forfeited (in shares)						(75,000)	0

Stock-based compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Additional disclosures
Stock-based compensation expense	$ 9,684	$ 12,112	$ 10,393
Outstanding stock options
Shares
Outstanding at the beginning of the period (in shares)	4,206,440
Granted (in shares)	2,424,864
Exercised (in shares)	(33,638)
Forfeited (in shares)	(1,207,536)
Outstanding at the end of the period (in shares)	5,390,130	4,206,440
Vested at the end of the period (in shares)	2,753,484
Vested and expected to vest at end of period (in shares)	5,174,667
Weighted-average exercise price per share
Outstanding at the beginning of the period (in dollars per share)	$ 10.38
Granted (in dollars per share)	5.99
Exercised (in dollars per share)	0.38
Forfeited (in dollars per share)	9.31
Outstanding at the end of the period (in dollars per share)	8.71	$ 10.38
Vested at the end of the period (in dollars per share)	9.95
Vested and expected to vest at end of the period (in dollars per share)	$ 8.75
Weighted-average remaining contractual term
Outstanding at the beginning of the period	8 years 1 month 6 days	8 years 4 months 24 days
Outstanding at the end of the period	8 years 1 month 6 days	8 years 4 months 24 days
Vested at the end of the period	7 years 3 months 18 days
Vested and expected to vest at the end of the period	8 years 1 month 6 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars)	$ 2,593
Exercised (in dollars)	249
Outstanding at the end of the period (in dollars)	175	$ 2,593
Vested at the end of the period (in dollars)	175
Vested and expected to vest at end of the period (in dollars)	$ 175
Assumptions used to estimate fair value of each stock-based award on the grant date
Risk-free interest rate (as a percent)	1.75%	1.99%	1.14%
Volatility (as a percent)	73.00%	81.00%	75.00%
Expected term	6 years	6 years	6 years
Additional disclosures
Stock-based compensation expense	$ 7,900	$ 7,800	$ 3,600
Weighted-average grant date fair value (in dollars per share)	$ 3.80	$ 8.61	$ 6.55
Fair value of vested stock options	$ 9,500	$ 6,600	$ 2,900
Total unrecognized stock-based compensation expense (in dollars)	$ 10,000
Weighted-average recognition period	2 years 1 month 6 days

Income taxes - Operating Loss Carryforwards (Details) $ in Millions	Dec. 31, 2015 USD ($)
Federal
Operating loss carryforwards
Net operating loss carryforwards	$ 157.5
State
Operating loss carryforwards
Net operating loss carryforwards	$ 155.9

Income taxes - Tax Credits (Details) $ in Millions	12 Months Ended
Income taxes - Tax Credits (Details) $ in Millions	Dec. 31, 2015 USD ($)
Tax credit carryforwards
Testing period for changes in ownership interest of significant shareholders	3 years
Federal
Tax credit carryforwards
Tax credits	$ 3.2
State
Tax credit carryforwards
Tax credits	$ 1.4

Income taxes - Reconciliation of Income Taxes and Deferred Tax Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of income taxes computed using the U.S. federal statutory rate
Income tax benefit using U.S. federal statutory rate (as a percent)	34.00%	34.00%
State tax benefit, net of federal benefit (as a percent)	5.91%	4.73%
Research and development tax credits (as a percent)	1.49%	1.51%
Permanent items (as a percent)	(3.02%)	(6.25%)
Change in the valuation allowance (as a percent)	(41.90%)	(34.34%)
Other (as a percent)	3.52%	0.35%
Deferred tax assets:
Net operating loss carryforwards	$ 61,606,000	$ 42,352,000
Capitalized research and development	1,984,000	2,154,000
Research and development credits	4,087,000	2,880,000
Stock-based compensation	4,401,000	726,000
Other	478,000	199,000
Gross deferred tax assets	72,556,000	48,311,000
Valuation allowance	(72,556,000)	(48,311,000)
Additional disclosures
Increase in valuation allowance	24,200,000	$ 18,300,000	$ 14,500,000
Unrecognized tax benefits	0
Interest and penalties accrued related to unrecognized tax benefits	0
Uncertain tax positions	0
Adjustment for unrecognized income tax benefits	$ 0

Commitments and contingencies (Details)		1 Months Ended	12 Months Ended
Commitments and contingencies (Details)	Apr. 15, 2014 USD ($) ft²	May 31, 2011 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Apr. 15, 2016 USD ($)
Operating Leased Assets
Rent free period		4 months
Letter of credit secured in connection with the lease, included in restricted cash		$ 86,000	$ 203,000	$ 203,000
Minimum aggregate future lease commitments
2016			512,000
2017			527,000
2018			542,000
2019			415,000
Total			1,996,000
Rent expense			$ 352,000	$ 541,000	$ 366,000
Intercontinental Fund III 117 Kendrick Street LLC
Operating Leased Assets
Area of space \| ft²	15,197
Initial annual base rent	$ 493,000
Period after which base rent increases	P12M
Base rent for last twelve-month period	$ 554,000
Tenant improvement allowance	684,000
Letter Of Credit \| Intercontinental Fund III 117 Kendrick Street LLC
Operating Leased Assets
Security deposit	$ 203,000					$ 162,000

License agreements (Details) - USD ($)				1 Months Ended		3 Months Ended								12 Months Ended
License agreements (Details) - USD ($)	Jul. 11, 2012	May 11, 2012	Dec. 16, 2011	Feb. 28, 2014	Oct. 31, 2010	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
License agreements
Fair value of the shares of common stock issued recorded as research and development expense															$ 1,197,000
Research and development expense						$ 7,688,000	$ 11,304,000	$ 11,045,000	$ 10,528,000	$ 9,685,000	$ 9,047,000	$ 8,305,000	$ 8,411,000	$ 40,565,000	35,448,000	$ 25,930,000
Encarta, Inc.
License agreements
Number of shares of common stock in the warrant issued under the license agreement				142,857
Exercise price of shares of common stock in the warrant issued under the license agreement (in dollars per share)				$ 17.16
S*Bio Pte Ltd
License agreements
Research and development expense																	$ 350,000
Exclusive license agreement
License agreements
Research and development expense															$ 1,200,000
Exclusive license agreement \| The Licensor
License agreements
Upfront license fee paid and patent related fees and costs incurred by the Licensor and affiliates reimbursed by the Company					$ 104,000
Common stock issued (in shares)					166,664
Fair value of the shares of common stock (in dollars per share)					$ 0.28
Fair value of the shares of common stock issued recorded as research and development expense					$ 46,000
Number of days' prior written notice required to terminate the agreement					90 days
Exclusive license agreement \| The Licensor \| Maximum
License agreements
Milestone payments upon the achievement of specified development, regulatory and commercialization milestones					$ 1,600,000
Exclusive license agreement \| Encarta, Inc.
License agreements
Common stock issued (in shares)				97,500
Number of shares of common stock in the warrant issued under the license agreement				142,857
Exercise price of shares of common stock in the warrant issued under the license agreement (in dollars per share)				$ 17.16
Exclusive license agreement \| Scripps
License agreements
Milestone payments upon the achievement of specified development and regulatory milestones				$ 3,000,000
Exclusive license agreement \| Pfizer
License agreements
Common stock issued (in shares)	192,012
Fair value of the shares of common stock issued recorded as research and development expense																	2,000,000
Research and development expense																	$ 1,500,000
One-time cash payment	$ 1,500,000
Royalty obligations term	10 years
Exclusive license agreement \| Pfizer \| Maximum
License agreements
Eligible amount receivable on developmental milestones by Pfizer	$ 2,000,000
Additional payable on successful attainment of regulatory and commercial sales milestones	$ 125,000,000
Asset purchase agreement \| Encarta, Inc.
License agreements
Cash payment made under the agreement				$ 25,000
Amendment and restatement of an existing non-exclusive license agreement \| The Licensor \| Maximum
License agreements
Milestone payments triggered upon the achievement of specified regulatory and commercialization milestones			$ 825,000
Dual inhibitors of PI3K and mTOR, including related patent right agreement \| S*Bio Pte Ltd
License agreements
Upfront license fee paid and patent related fees and costs incurred by the Licensor and affiliates reimbursed by the Company		$ 350,000
Dual inhibitors of PI3K and mTOR, including related patent right agreement \| S*Bio Pte Ltd \| Maximum
License agreements
Milestone payments upon the achievement of specified development and regulatory milestones		$ 21,000,000
Deduction in royalty payments as percentage of amount paid to third party														50.00%

Employee benefit plan (Details) - USD ($)	12 Months Ended
Employee benefit plan (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee benefit plan
Contributions made under 401(k) plan	$ 295,000	$ 219,000	$ 160,000

Reduction in force (Details) - Employee Severance	1 Months Ended	12 Months Ended
Reduction in force (Details) - Employee Severance	Oct. 31, 2015 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Costs
Reduction of workforce	50.00%
Number of full time employees \| item	20
Estimated one- time severance and related costs	$ 1,100,000
Severance cost paid			$ 349,000
Scenario, Forecast
Restructuring Costs
Severance and related costs		$ 713,000

Quarterly financial information (unaudited, in thousands, except per share data) (Details) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development			$ 7,688,000	$ 11,304,000	$ 11,045,000	$ 10,528,000	$ 9,685,000	$ 9,047,000	$ 8,305,000	$ 8,411,000	$ 40,565,000	$ 35,448,000	$ 25,930,000
General and administrative			4,273,000	4,230,000	4,417,000	4,714,000	4,313,000	4,341,000	4,782,000	4,723,000	17,634,000	18,159,000	15,472,000
Total operating expenses			11,961,000	15,534,000	15,462,000	15,242,000	13,998,000	13,388,000	13,087,000	13,134,000	58,199,000	53,607,000	41,402,000
Loss from operations			(11,961,000)	(15,534,000)	(15,462,000)	(15,242,000)	(13,998,000)	(13,388,000)	(13,087,000)	(13,134,000)	(58,199,000)	(53,607,000)	(41,402,000)
Interest income			98,000	89,000	85,000	62,000	49,000	56,000	65,000	72,000	334,000	242,000	200,000
Net loss			$ (11,863,000)	$ (15,445,000)	$ (15,377,000)	$ (15,180,000)	$ (13,949,000)	$ (13,332,000)	$ (13,022,000)	$ (13,062,000)	$ (57,865,000)	$ (53,365,000)	$ (41,202,000)
Net loss per share - basic and diluted			$ (0.32)	$ (0.42)	$ (0.42)	$ (0.46)	$ (0.53)	$ (0.52)	$ (0.51)	$ (0.51)	$ (1.61)	$ (2.07)	$ (1.82)
Weighted-average number of common shares used in net loss per share—basic and diluted			36,935,000	36,898,000	36,522,000	33,323,000	26,248,000	25,811,000	25,669,000	25,478,000	35,932,000	25,804,000	22,680,000
Issuance of common stock resulting from follow-on offering (in shares)	8,337,500
Issuance of common stock resulting from at-the-market transactions (in shares)		1,346,676		28,800	690,370	470,309					1,189,479
Share price (in dollars per share)	$ 6.50
Shares of common stock sold under underwriters' over-allotment option	1,087,500
Net proceeds from issuance of common stock in public offering	$ 50,900,000	$ 11,600,000		$ 250,000	$ 6,200,000	$ 4,400,000					$ 12,900,000	$ 9,600,000