Larimar Therapeutics (LRMR) 10-K 15 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Mar. 07, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	ZFGN
Entity Registrant Name	Zafgen, Inc.
Entity Central Index Key	1,374,690
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		27,268,943
Entity Public Float			$ 478.8

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 35,595	$ 58,103
Marketable securities	149,484	57,359
Tax incentive receivable	1,323	391
Prepaid expenses and other current assets	1,708	1,345
Total current assets	188,110	117,198
Property and equipment, net	902	79
Other assets	94	242
Total assets	189,106	117,519
Current liabilities:
Accounts payable	7,495	2,348
Accrued expenses	6,112	3,172
Notes payable, current	2,936	1,381
Total current liabilities	16,543	6,901
Notes payable, net of discount, long-term	3,453	6,177
Total liabilities	$ 19,996	$ 13,078
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock; $0.001 par value per share; 5,000,000 shares authorized at December 31, 2015 and 2014; no shares issued and outstanding at December 31, 2015 and 2014
Common stock, $0.001 par value per share; 115,000,000 shares authorized at December 31, 2015 and 2014; 27,242,503 and 22,879,160 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 27	$ 23
Additional paid-in capital	348,961	209,838
Accumulated deficit	(179,671)	(105,385)
Accumulated other comprehensive loss	(207)	(35)
Total stockholders' equity	169,110	104,441
Total liabilities and stockholders' equity	$ 189,106	$ 117,519

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Preferred stock, par value per share	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 0.001	$ 0.001
Common stock, shares authorized	115,000,000	115,000,000
Common stock, shares issued	27,242,503	22,879,160
Common stock, shares outstanding	27,242,503	22,879,160

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0
Operating expenses:
Research and development	54,618	27,391	9,561
General and administrative	19,195	8,141	4,219
Total operating expenses	73,813	35,532	13,780
Loss from operations	(73,813)	(35,532)	(13,780)
Other income (expense):
Interest income	438	28
Interest expense	(806)	(870)
Foreign currency transaction gains (losses), net	(105)	(104)	(247)
Total other income (expense), net	(473)	(946)	(247)
Net loss	(74,286)	(36,478)	(14,027)
Accretion of redeemable convertible preferred stock to redemption value		(92)	(213)
Net loss attributable to common stockholders	$ (74,286)	$ (36,570)	$ (14,240)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.78)	$ (3)	$ (19.53)
Weighted average common shares outstanding, basic and diluted	26,756,079	12,189,155	729,001
Comprehensive loss:
Net loss	$ (74,286)	$ (36,478)	$ (14,027)
Other comprehensive loss:
Unrealized loss on marketable securities	(172)	(35)
Total other comprehensive loss	(172)	(35)
Total comprehensive loss	$ (74,458)	$ (36,513)	$ (14,027)

Consolidated Statements of Chan

Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock [Member]	Redeemable Convertible Preferred Stock [Member]Series D Preferred Stock [Member]	Redeemable Convertible Preferred Stock [Member]Series E Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2012	$ (54,729)				$ 1	$ 150	$ (54,880)
Issuance of redeemable convertible preferred stock, value			$ 5,955	$ 34,844
Beginning balance, shares at Dec. 31, 2012					736,026
Temporary equity, beginning balance at Dec. 31, 2012		$ 62,785
Temporary equity, beginning balance, shares at Dec. 31, 2012		73,991,017
Repurchase and retirement of common stock, at cost					(6,635)
Issuance of common stock, shares			4,381,914	16,110,473
Accretion of redeemable convertible preferred stock to redemption value	(213)	$ 213				(213)
Stock-based compensation expense	395					395
Net loss	(14,027)						(14,027)
Ending balance at Dec. 31, 2013	(68,574)				$ 1	332	(68,907)
Ending balance, shares at Dec. 31, 2013					729,391
Temporary equity, ending balance at Dec. 31, 2013		$ 103,797
Temporary equity, ending balance, shares at Dec. 31, 2013		94,483,404
Issuance of redeemable convertible preferred stock, value				$ 442
Issuance of common stock, shares				204,101	6,900,000
Issuance of common stock	100,164				$ 7	100,157
Conversion of redeemable convertible preferred stock to common stock, value	104,331	$ (104,331)			$ 15	104,316
Conversion of redeemable convertible preferred stock to common stock, shares		(94,687,505)			15,077,621
Accretion of redeemable convertible preferred stock to redemption value	(92)	$ 92				(92)
Stock-based compensation expense	1,553					1,553
Issuance of common stock upon exercise of stock options, value	3					3
Issuance of common stock upon exercise of stock options, shares					398
Issuance of common stock in lieu of milestone payment	3,569					3,569
Issuance of common stock in lieu of milestone payment, shares					171,750
Unrealized loss on marketable securities	(35)							$ (35)
Net loss	(36,478)						(36,478)
Ending balance at Dec. 31, 2014	104,441				$ 23	209,838	(105,385)	(35)
Ending balance, shares at Dec. 31, 2014					22,879,160
Temporary equity, ending balance, shares at Dec. 31, 2014		0
Issuance of common stock, shares					3,942,200
Issuance of restricted stock, shares					2,902
Issuance of common stock	129,571				$ 4	129,567
Issuance of restricted stock	0	$ 0			$ 0	0	0	0
Issuance of common stock upon exercise of stock options and employee stock purchase plan, shares					418,241
Issuance of common stock upon exercise of stock options and employee stock purchase plan	974					974
Stock-based compensation expense	$ 8,582					8,582
Issuance of common stock upon exercise of stock options, shares	410,701
Unrealized loss on marketable securities	$ (172)							(172)
Net loss	(74,286)						(74,286)
Ending balance at Dec. 31, 2015	$ 169,110				$ 27	$ 348,961	$ (179,671)	$ (207)
Ending balance, shares at Dec. 31, 2015					27,242,503
Temporary equity, ending balance at Dec. 31, 2015		$ 0
Temporary equity, ending balance, shares at Dec. 31, 2015		0

Consolidated Statements of Cha6

Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Issuance of redeemable convertible preferred stock, issuance costs	$ 2,312	$ 196
Series D Preferred Stock [Member]
Issuance of redeemable convertible preferred stock, issuance costs		1
Series E Preferred Stock [Member]
Issuance of redeemable convertible preferred stock, issuance costs	$ 1	$ 156

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (74,286)	$ (36,478)	$ (14,027)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	8,582	1,553	395
Non-cash interest expense	62	46
Depreciation expense	70	16	12
Common stock issued in lieu of milestone payment		3,569
Unrealized foreign currency transaction losses	82	93	250
Premium on marketable securities, net	(2,036)	(225)
Amortization of premium on marketable securities	1,243	31
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(363)	(1,121)	165
Tax incentive receivable	(1,014)	1,133	(1,237)
Accounts payable	4,914	815	237
Accrued expenses	3,192	2,327	(799)
Net cash used in operating activities	(59,554)	(28,241)	(15,004)
Cash flows from investing activities:
Proceeds from maturities of marketable securities	195,987
Purchases of marketable securities	(287,491)	(57,200)
Purchases of property and equipment	(595)	(58)	(17)
Deposits for leased property	(19)	(57)
Net cash used in investing activities	(92,118)	(57,315)	(17)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		442	40,799
Proceeds from issuance of notes payable, net of issuance costs		7,386
Payments of debt offering costs		(49)
Repayments of notes payable	(1,381)
Proceeds from exercise of common stock options and employee stock purchase plan	974	3
Proceeds from public offerings, net of commissions and underwriting discounts	130,044	102,672
Payments of public offering costs	(473)	(2,312)	(196)
Net cash provided by financing activities	129,164	108,142	40,603
Net (decrease) increase in cash and cash equivalents	(22,508)	22,586	25,582
Cash and cash equivalents at beginning of period	58,103	35,517	9,935
Cash and cash equivalents at end of period	35,595	58,103	35,517
Supplemental disclosure of non-cash investing and financing activities:
Accretion of redeemable convertible preferred stock to redemption values		92	213
Deferred offering costs included in accounts payable and accrued expenses		148	$ 547
Property and equipment included in accounts payable	298
Conversion of redeemable preferred stock to common stock		104,331
Conversion of milestone liabilities to common stock		3,569
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 584	$ 632

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Zafgen, Inc. (the “Company”) was incorporated on November 22, 2005 under the laws of the State of Delaware. The Company is a biopharmaceutical company dedicated to significantly improving the health and well-being of patients affected by obesity and complex metabolic disorders. Zafgen is focused on developing novel therapeutics that treat the underlying biological mechanisms through the methionine aminopeptidase 2 (“MetAP2”) pathway. Beloranib, the Company’s lead product candidate, is a novel, first-in-class, twice-weekly subcutaneous injection being developed for the treatment of multiple indications, including severe obesity in two rare diseases, Prader-Willi syndrome and obesity caused by hypothalamic injury, including craniopharyngioma-associated obesity. Zafgen is also developing ZGN-839, a liver-targeted MetAP2 inhibitor, for the treatment of nonalcoholic steatohepatitis and abdominal obesity, as well as other second-generation MetAP2 inhibitors for severe obesity. Since its inception, the Company has devoted substantially all of its efforts to research and development, recruiting management, acquiring operating assets and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. The Company’s product candidates are all in the development stage. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries Zafgen Securities Corporation, Zafgen Australia Pty Limited, and Zafgen Animal Health, LLC. All significant intercompany balances and transactions have been eliminated. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). On June 24, 2014, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 6,900,000 shares at a price of $16.00 per share. The Company received net proceeds from the IPO of approximately $102,672 based upon the price of $16.00 per share and after deducting underwriting discounts and commissions paid by the Company. The Company also incurred offering costs of $2,508 related to the IPO. On January 28, 2015, the Company completed a follow-on offering of its common stock, which resulted in the sale of 3,942,200 shares at a price of $35.00 per share. The Company received net proceeds from the follow-on offering of $130,044 based upon the price of $35.00 per share after deducting underwriting discounts and commissions. The Company also incurred offering costs of $473 related to the follow-on offering.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of common stock prior to the IPO and stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of ninety days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market funds, U.S. government securities, corporate bonds, and commercial paper, are stated at fair value. Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents and marketable securities. The Company has all cash and cash equivalents and marketable securities balances at two accredited financial institutions in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Marketable securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. Fair value is determined based on quoted market prices. Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering or as a reduction to the carrying value of preferred stock issued. As of December 31, 2014, the Company recorded $148 of deferred offering costs, included in other assets in the accompanying consolidated balance sheet, in contemplation of the Company’s follow-on offering of its common stock, which closed in January 2015. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over a five-year estimated useful life for equipment, furniture and fixtures and office equipment. Leasehold improvements are amortized over the shorter of the asset life or the term of the lease agreement. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets. Research and Development Costs Research and development costs are expensed as incurred. Included in research and development expenses are wages, stock-based compensation and benefits of employees, third-party license fees and milestones and other operational costs related to the Company’s research and development activities, including facility-related expenses and external costs of outside vendors engaged to conduct pre-clinical studies, manufacturing activities, and clinical trials. The Company records research and development expenses net of any research and development tax incentives the Company is entitled to receive from government authorities. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain. Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors at the fair value on the date of the grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions. For stock-based awards granted to consultants and nonemployees, compensation expense is recognized over the period during which services are rendered by such consultants and nonemployees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on advancing novel therapeutics for patients suffering from severe obesity and obesity-related disorders. No revenue has been generated since inception, and all tangible assets are held in the United States. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2015 and 2014, the Company’s only element of other comprehensive loss was unrealized loss on marketable securities. For the year ended December 31, 2013, there was no difference between net loss and comprehensive loss. Net Income (Loss) Per Share Upon the closing of the Company’s IPO in June 2014, all of the Company’s outstanding redeemable convertible preferred shares were converted into shares of common stock. Prior to this conversion, the Company followed the two-class method when computing net income (loss) per share as the Company had issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred shares contractually entitled the holders of such shares to participate in dividends, but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, the two-class method did not apply for periods in which the Company reported a net loss or a net loss attributable to common stockholders resulting from dividends or accretion related to its redeemable convertible preferred shares. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common shares, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per common share attributable to common stockholders is the same as basic net loss per common share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is antidilutive. Recently Issued and Adopted Accounting Pronouncements In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes. In February 2016, the FASB issued ASU No. 2016-02, Leases

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities Fair Value Measurements The following tables present information about the Company’s financial assets that have been measured at fair value at December 31, 2015 and 2014, and indicate the fair value of the hierarchy of the valuation inputs utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair value determined by Level 2 inputs utilize observable inputs other than Level 1 prices, such as quoted prices, for similar assets or liabilities, quoted market prices in markets that are not active or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities. Fair values determined by Level 3 inputs are unobservable data points for the asset or liability, and includes situations where there is little, if any, market activity for the asset or liability. The following tables summarize the Company’s cash equivalents and marketable securities as of December 31, 2015 and 2014: December 31, 2015 Total Quoted Significant Significant Cash equivalents: Money market funds $ 13,231 $ 13,231 $ — $ — U.S. government securities 4,999 — 4,999 — Commercial paper 4,697 — 4,697 — Corporate bonds 3,000 — 3,000 — Total cash equivalents 25,927 13,231 12,696 — Marketable securities: Corporate bonds 125,516 — 125,516 — Commercial paper 19,468 — 19,468 — U.S. government securities 4,500 — 4,500 — Total marketable securities 149,484 — 149,484 — Total cash equivalents and marketable securities $ 175,411 $ 13,231 $ 162,180 $ — December 31, 2014 Total Quoted Significant Significant Cash equivalents: Money market funds $ 14,742 $ 14,742 $ — $ — U.S. government securities 27,767 — 27,767 — Total cash equivalents 42,509 14,742 27,767 — Marketable securities: U.S. government securities 34,191 — 34,191 — Corporate bonds 22,168 — 22,168 — Commercial paper 1,000 — 1,000 — Total marketable securities 57,359 — 57,359 — Total cash equivalents and marketable securities $ 99,868 $ 14,742 $ 85,126 $ — The carrying amounts reflected in the consolidated balance sheets for tax incentive receivable, accounts payable, and accrued expenses approximate fair value due to their short-term maturities. The carrying value of the Company’s outstanding notes payable approximates fair value (a level 2 fair value measurement), reflecting discount rates currently available to the Company. Marketable Securities The following table summarizes the Company’s marketable securities as of December 31, 2015 and 2014: December 31, 2015 Amortized Gross Gross Fair Value Assets: Corporate bonds (due within 1 year) $ 112,825 $ 1 $ (166 ) $ 112,660 Corporate bonds (due after 1 year through 2 years) 12,884 3 (31 ) 12,856 Commercial paper (due within 1 year) 19,478 — (10 ) 19,468 U.S. government securities (due within 1 year) 4,500 — — 4,500 $ 149,687 $ 4 $ (207 ) $ 149,484 December 31, 2014 Amortized Cost Gross Gross Fair Value Assets: U.S. government securities (due within 1 year) $ 34,200 $ — $ (9 ) $ 34,191 Corporate bonds (due within 1 year) 18,716 — (18 ) 18,698 Corporate bonds (due after 1 year through 2 years) 3,478 — (8 ) 3,470 Commercial paper (due within 1 year) 1,000 — — 1,000 $ 57,394 $ — $ (35 ) $ 57,359

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Property and Equipment, net	4. Property and Equipment, net Property and equipment, net consisted of the following as of December 31, 2015 and 2014: December 31, Useful Life 2015 2014 Office equipment 5 years $ 359 $ 79 Furniture and fixtures 5 years 186 50 Equipment 5 years 71 — Leasehold improvements * 406 — 1,022 129 Less: Accumulated depreciation (120 ) (50 ) $ 902 $ 79 * shorter of asset life or lease term Depreciation expense was $70, $16 and $12 for the years ended December 31, 2015, 2014 and 2013, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2015
Payables and Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following as of December 31, 2015 and 2014: December 31, 2015 2014 Accrued payroll and related expenses $ 1,802 $ 1,239 Accrued research and development expenses 3,727 1,180 Accrued professional fees 422 585 Accrued other 161 168 $ 6,112 $ 3,172

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Notes Payable	6. Notes Payable On March 31, 2014, the Company entered into a loan and security agreement with Oxford Finance LLC and Midcap Financial (the “Credit Facility”). The Credit Facility provided for initial borrowings of $7,500 under a term loan (“Term Loan A”) and additional borrowings of up to $12,500 under other term loans, for a maximum of $20,000. On March 31, 2014, the Company received proceeds of $7,500 from the issuance of promissory notes under the Term Loan A. Of the additional $12,500 amount that was available, $7,500 (“Term Loan B”) was available to be drawn down until September 30, 2014 and $5,000 (“Term Loan C”) was available to be drawn down for a 30-day period upon the completion of the Company’s IPO that occurred in June 2014. The Company elected not to draw down Term Loan B or Term Loan C and these amounts are no longer available to the Company. All promissory notes issued under the Credit Facility are collateralized by substantially all of the Company’s personal property, other than its intellectual property. Upon entering into this Credit Facility, the Company was obligated to make monthly, interest-only payments on any term loans funded under the Credit Facility until December 1, 2014 and, thereafter, to pay 36 consecutive, equal monthly installments of principal and interest from January 1, 2015 through December 1, 2017. As per the terms of the agreement, in June 2014, upon the completion of the Company’s IPO, the term of monthly, interest-only payments were extended until June 1, 2015. Outstanding term loans under the Credit Facility bear interest at an annual rate of 8.1%. In addition, a final payment equal to 6.0% of any amounts drawn under the Credit Facility is due upon the earlier of the maturity date, acceleration of the term loans or prepayment of all or part of the term loans. The Company accrues the final payment amount due relating to Term Loan A of $450, to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date. Term Loan A was recorded in the balance sheet net of debt discount of $114 that was related to fees assessed by the lender at the time of borrowing. The debt discount is being accreted to the principal amount of the debt. In addition, deferred financing costs of $49 are being amortized to interest expense using the effective-interest method over the same term. For the years ended December 31, 2015 and 2014, the company recorded additional interest expense of $62 and $50, respectively, related to the accretion of debt discount and amortization of deferred financing costs. The effective annual interest rate of the outstanding debt under the Credit Facility is approximately 10.8%. The Company was obligated to pay a separate fee upon any IPO; a sale of substantially all of the Company’s assets; or a merger, reorganization or sale of the Company’s voting equity securities where existing voting stockholders hold less than 50% of voting equity securities after such transaction. There are no financial covenants associated with the debt facility; however, there are negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering or granting a security interest in its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and certain other business transactions. The Credit Facility also includes events of default, the occurrence and continuation of any of which provides the lenders the right to exercise remedies against the Company and the collateral securing the loans under the Credit Facility, including cash in the amount of the outstanding balance. These events of default include, among other things, failure to pay any amounts due under the Credit Facility, insolvency, the occurrence of a material adverse event, the occurrence of any default under certain other indebtedness and a final judgment against the Company in an amount greater than $250. Notes payable consist of the following: December 31, 2015 2014 Notes payable $ 6,119 $ 7,500 Less: current portion (2,936 ) (1,381 ) Notes payable, net of current portion 3,183 6,119 Debt discount, net of accretion (36 ) (79 ) Accretion related to final payment 306 137 Notes payable, net of discount, long term $ 3,453 $ 6,177 Estimated future principal payments due under the Term Loan A are as follows: Years Ending December 31, 2016 $ 2,936 2017 3,183 Total $ 6,119 During the years ended December 31, 2015 and 2014, the Company recognized $806 and $870 of interest expense related to the Credit Facility.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Stockholders' Equity	7. Stockholders’ Equity On June 5, 2014, the Company effected a 1-for-6.28 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of redeemable convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in these consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the redeemable convertible preferred stock conversion ratios. On June 24, 2014, the Company completed an IPO of its common stock, which resulted in the sale of 6,900,000 shares at a price of $16.00 per share. The Company received net proceeds from the IPO of $102,672 based upon the price of $16.00 per share and after deducting underwriting discounts and commissions paid by the Company. The Company also incurred offering costs of $2,508 related to the IPO. Upon closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock were converted into 15,077,621 shares of common stock. On January 28, 2015, the Company completed a follow-on offering of its common stock, which resulted in the sale of 3,942,200 shares at a price of $35.00 per share. The Company received net proceeds from the follow-on offering of $130,044 based upon the price of $35.00 per share after deducting underwriting discounts and commissions. The Company also incurred offering costs of $473 related to the follow-on offering. As of December 31, 2015 and 2014, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 5,000,000 shares of $0.001 par value preferred stock. The rights, preferences, restrictions, qualifications and limitations of such stock are to be determined by the Company’s board of directors. As of December 31, 2015 and 2014, the Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 115,000,000 shares of $0.001 par value common stock.

Stock-Based Awards

Stock-Based Awards	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Awards	8. Stock-Based Awards Stock Option Plans The Company’s Amended and Restated 2006 Stock Option Plan (the “2006 Plan”) provided for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2006 Plan was administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or a committee of the board of directors if so delegated, except that the exercise price per share of stock options could not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option could not be greater than ten years. The total number of shares of common stock that could have been issued under the 2006 Plan was 1,889,150 shares. Upon closing of the Company’s IPO, 168,221 shares reserved and not then subject to outstanding options were transferred to the 2014 Stock Option and Incentive Plan, and no further awards will be made under the 2006 Plan. On June 5, 2014, the Company’s stockholders approved the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”), which became effective upon the completion of the IPO of the Company’s shares of common stock in June 2014. The 2014 Stock Option Plan provides for the grant of stock options, stock appreciation rights, restricted stock awards, restricted stock units, unrestricted stock awards, performance-share awards, cash-based awards and dividend equivalent rights. The number of shares initially reserved for issuance under the 2014 Stock Option Plan is 2,168,221 shares of common stock and may be increased by the number of shares under the 2006 Plan that are not needed to fulfill the Company’s obligations for awards issued under the 2006 Plan as a result of forfeiture, expiration, cancellation, termination or net issuances of awards thereunder. The number of shares of common stock that may be issued under the 2014 Stock Option Plan is also subject to increase on the first day of each fiscal year by the lesser of (i) 4% of the Company’s outstanding shares of common stock as of that date, or (ii) an amount determined by the board of directors. As of December 31, 2015, 1,625,761 shares are available for grant under the 2014 Stock Option Plan, including 915,166 shares automatically added to the 2014 Stock Option Plan on January 1, 2015 as a result of a provision in the 2014 Stock Option Plan. The Company generally grants stock-based awards with service conditions only (“service-based” awards). As required by the 2006 Plan and 2014 Stock Option Plan, the exercise price for stock options granted is not to be less than the fair value of common shares as of the date of grant. Prior to the IPO, the value of common stock was determined by the board of directors by taking into consideration its most recently available valuation of common shares performed by management and the board of directors as well as additional factors which might have changed since the date of the most recent contemporaneous valuation through the date of grant. During the year ended December 31, 2015, the Company granted stock options for the purchase of 992,505 shares of common stock, of which options for the purchase of 988,505 shares were granted to employees and directors and options for the purchase of 4,000 shares were granted to a consultant. During the year ended December 31, 2014, the Company granted stock options for the purchase of 698,642 shares of common stock, of which options for the purchase of 697,050 shares were granted to employees and options for the purchase of 1,592 shares were granted to a consultant. 2014 Employee Stock Purchase Plan On June 5, 2014, the Company’s stockholders approved the 2014 Employee Stock Purchase Plan (the “ESPP”). A total of 265,000 shares of common stock were reserved for issuance under this plan. The ESPP became effective upon the completion of the IPO of the Company’s shares of common stock. The first offering period commenced on September 1, 2014 and ended on December 31, 2014. During 2015 there were two offering periods, January 1, 2015 through June 30, 2015 and July 1, 2015 through December 31, 2015. The per share purchase price for offerings is equal to the lesser of 85% of the closing market price of the Company’s common stock on the first day or last day of the offering period. The Company issued 7,540 shares during the year ended December 31, 2015; this was the first year the Company issued shares from the ESPP. As of December 31, 2015 and 2014, there are 257,460 and 265,000 shares, respectively, of common stock available for issuance to participating employees under the ESPP. The weighted-average fair value of stock purchase rights granted as part of the ESPP was $11.31 per share for the year ended December 31, 2015. The fair value was estimated using the Black-Scholes option-pricing model. During the year ended December 31, 2015, the Company used a weighted-average stock-price volatility of approximately 66%, expected option life assumption of six months and a risk-free interest rate of 0.09%. Stock Option Valuation The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to its IPO in June 2014, the Company was a private company and lacks company-specific historical and implied volatility information. Therefore, the Company estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to nonemployees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors are as follows, presented on a weighted average basis: 2015 2014 2013 Risk-free interest rate 1.75 % 1.90 % 1.12 % Expected term (in years) 6.25 6.25 6.25 Expected volatility 87 % 90 % 85 % Expected dividend yield 0 % 0 % 0 % The following table summarizes the Company’s stock option activity since December 31, 2014: Shares Issuable Weighted Average Aggregate Intrinsic Value (In years) Outstanding as of December 31, 2014 2,008,574 $ 6.65 Granted 992,505 $ 39.37 Exercised (410,701 ) $ 1.95 Forfeited (35,268 ) $ 32.41 Outstanding as of December 31, 2015 2,555,110 $ 19.76 7.9 $ 3,918 Options vested and expected to vest as of December 31, 2015 2,555,110 $ 19.76 7.9 $ 3,918 Options exercisable as of December 31, 2015 995,324 $ 5.60 6.5 $ 3,243 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised was $13,949 and $195 during the years ended December 31, 2015 and 2014, respectively. The Company received cash proceeds from the exercise of stock options of $799 and $3 during the years ended December 31, 2015 and 2014, respectively. The weighted average grant-date fair value of stock options granted to employees and directors during the years ended December 31, 2015, 2014 and 2013 was $28.97, $11.73 and $1.83 per share, respectively. As of December 31, 2015 and 2014, there were outstanding unvested service-based stock options held by nonemployees for the purchase of 9,363 and 13,205 shares of common stock, respectively. Additionally as of December 31, 2014, there were outstanding unvested performance-based stock options held by nonemployees for the purchase of 796 shares of common stock. Restricted Stock Awards The 2006 Plan provides for the award of restricted stock awards. The Company has granted restricted stock awards with time-based vesting conditions. Unvested shares of restricted stock awards may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The Company values restricted stock awards on the grant-date using the grant-date market price of the Company’s common stock. There were no restricted stock awards that vested in the years ended December 31, 2015 and 2014. The aggregate intrinsic value of restricted stock awards that vested during the year ended December 31, 2013 was $38. The aggregate intrinsic value of restricted stock awards is calculated as the fair value of the Company’s common stock on the date it vests and is released. The Company did not grant any restricted stock awards during the years ended December 31, 2015, 2014 or 2013. As of December 31, 2015, 2014 and 2013, there were no unvested restricted stock awards subject to repurchase. Restricted Stock Units The 2014 Stock Option Plan provides for the award of restricted stock units. The Company has granted restricted stock units with time-based vesting conditions. The restrictions lapse according to the time based vesting conditions of each award. The Company values restricted stock units on the grant-date using the grant-date market price of the Company’s common stock. The aggregate intrinsic value of restricted stock units that vested during the year ended December 31, 2015 was $115. There were no restricted stock units that vested in the years ended December 31, 2014 or 2013. The aggregate intrinsic value of restricted stock units is calculated as the fair value of the Company’s common stock on the date it vests and is released. During the year ended December 31, 2015, the Company granted 4,769 restricted stock units, all of which vested during 2015, at a weighted average grant-date fair value of $34.46. The Company did not grant any restricted stock units during the years ended December 31, 2014 or 2013. As of December 31, 2015, 2014 and 2013, there were no unvested restricted stock units outstanding. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories within its consolidated statements of operations: Year Ended December 31, 2015 2014 2013 Research and development $ 2,930 $ 490 $ 176 General and administrative 5,652 1,063 219 $ 8,582 $ 1,553 $ 395 As of December 31, 2015, the Company had an aggregate of $27,515 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.6 years.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Loss Per Share	9. Net Loss Per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2015 2014 2013 Basic and diluted net loss per share attributable to common stockholders: Numerator: Net loss $ (74,286 ) $ (36,478 ) $ (14,027 ) Accretion of redeemable convertible preferred stock to redemption value — (92 ) (213 ) Net loss attributable to common stockholders $ (74,286 ) $ (36,570 ) $ (14,240 ) Denominator: Weighted average common shares outstanding, basic and diluted 26,756,079 12,189,155 729,001 Net loss per share attributable to common stockholders, basic and diluted $ (2.78 ) $ (3.00 ) $ (19.53 ) The Company excluded the following common stock equivalents, outstanding as of December 31, 2015, 2014 and 2013, from the computation of diluted net loss per share attributable to common stockholders for the years ended December 31, 2015, 2014 and 2013 because they had an anti-dilutive impact due to the net loss attributable to common stockholders incurred for the periods: As of December 31, 2015 2014 2013 Options to purchase common stock 2,555,110 2,008,574 1,310,330

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Leases The Company has a lease for office space in Boston, Massachusetts, effective as of July 28, 2014, with a term expiring July 31, 2017 and an option to extend the lease for three additional years. In March 2015, the Company entered into an operating lease for additional office space in Boston, Massachusetts, effective as of April 15, 2015, with a term expiring on July 31, 2017, and two options to extend the lease for three additional years each. In October 2015, the Company entered into an operating lease for office space in San Diego, California, effective as of October 1, 2015, with a term expiring on September 30, 2019, and an option to extend the lease for five additional years. Future minimum lease payments for its operating leases as of December 31, 2015 were as follows: Years Ending December 31, 2016 $ 432 2017 292 2018 106 2019 82 $ 912 During the years ended December 31, 2015, 2014 and 2013, the Company recognized $326, $160 and $105, respectively, of rental expense related to office space. Intellectual Property Licenses The Company has acquired exclusive rights to develop patented compounds and related know-how for beloranib under two licensing agreements with two third parties in the course of its research and development activities. The licensing rights obligate the Company to make payments to the licensors for license fees, milestones, license maintenance fees and royalties. The Company is also responsible for patent prosecution costs. Related to these license agreements, the Company recorded research and development expenses in its consolidated statements of operations as follows: Year Ended December 31, 2015 2014 2013 License, milestone, and license maintenance fees $ — $ 7,019 $ — $ — $ 7,019 $ — As of December 31, 2015, the Company is obligated to make additional milestone payments of up to $12,250 upon reaching certain pre-commercialization milestones, such as clinical trials and government approvals (including the U.S. Food and Drug Administration (“FDA”), approval of a New Drug Application (“NDA”), and up to $12,500 upon reaching certain product commercialization milestones. Under one of the license agreements, the Company is also obligated to pay up to $1,250 with respect to each subsequent licensed product, if any, that is a new chemical entity. In addition, the Company will owe single-digit royalties on sales of commercial products developed using these licensed technologies, if any. During the year ended December 31, 2014, the Company recorded expenses of $7,019 relating to milestones achieved in September 2014 upon the initiation of a first Phase 3 clinical trial including the cost of the issuance of common stock valued at $3,569. There were no milestones achieved during the years ended December 31, 2015 or 2013. The Company is also obligated to pay to the licensors a percentage of fees received if and when the Company sublicenses the technology. As of December 31, 2015, the Company has not yet developed a commercial product using the licensed technologies and it has not entered into any sublicense agreements for the technologies. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners, and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2015. Legal Proceedings The Company accrues a liability for legal contingencies when it believes that it is both probable that a liability has been incurred and that the Company can reasonably estimate the amount of the loss. The Company reviews these accruals and adjusts them to reflect ongoing negotiations, settlements, rulings, advice of legal counsel and other relevant information. To the extent new information is obtained and the views on the probable outcomes of claims, suits, assessments, investigations or legal proceedings change, changes in the Company’s accrued liabilities would be recorded in the period in which such determination is made. In addition, in accordance with the relevant authoritative guidance, for any matters in which the likelihood of material loss is at least reasonably possible, the Company will provide disclosure of the possible loss or range of loss. If a reasonable estimate cannot be made, however, the Company will provide disclosure to that effect. The Company expenses legal costs as they are incurred. On October 21, 2015, a purported stockholder of the Company filed a putative class action lawsuit in the U.S. District Court for the District of Massachusetts, against the Company and Thomas E. Hughes, captioned Aviad Bessler v. Zafgen, Inc. and Thomas E. Hughes, No. 1:15-cv-13618. An amended complaint was filed on February 22, 2016. The amended complaint alleges violations of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and SEC Rule 10b-5 based on allegedly false and misleading statements and omissions regarding the Company’s clinical trials for its drug beloranib. The lawsuit seeks, among other things, unspecified compensatory damages in connection with the Company’s allegedly inflated stock price between June 19, 2014 and October 16, 2015, as a result of those allegedly false and misleading statements, as well as punitive damages, interest, attorneys’ fees and costs. The Company has not yet filed a responsive pleading. Since an estimate of the possible loss or range of loss cannot be made at this time, no accruals have been recorded as of December 31, 2015. The Company may periodically become subject to other legal proceedings and claims arising in connection with ongoing business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. Other than the above action, the Company is not aware of any other material claims as of December 31, 2015.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes During the years ended December 31, 2015, 2014 and 2013, the Company recorded no income tax benefits for the net operating losses incurred in each year due to its uncertainty of realizing a benefit from those items. The domestic and foreign components of loss before income taxes are as follows: 2015 2014 2013 Domestic $ (72,147 ) $ (35,818 ) $ (12,325 ) Foreign (2,139 ) (660 ) (1,702 ) Loss before income taxes $ (74,286 ) $ (36,478 ) $ (14,027 ) A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2015 2014 2013 Federal statutory income tax rate (34.0 %) (34.0 %) (34.0 %) Federal and state research and development tax credit (3.9 ) (1.9 ) (3.7 ) State taxes, net of federal benefit (3.9 ) (4.0 ) (3.9 ) Orphan drug tax credit (3.5 ) (3.2 ) (3.1 ) Stock compensation expense 0.9 0.6 0.6 Nondeductible Australia research and development expenses 1.0 0.6 4.1 Other items 0.1 2.4 — Change in deferred tax asset valuation allowance 43.3 39.5 40.0 Effective income tax rate 0.0 % 0.0 % 0.0 % Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Current deferred tax assets: Accrued expenses $ — $ 431 Other temporary differences — 6 Total current deferred tax assets — 437 Noncurrent deferred tax assets: Capitalized research and development expenses 46,568 29,048 Net operating loss carryforwards 11,186 6,328 Tax credit carryforwards 11,570 4,928 Capitalized legal expenses 1,773 1,229 Stock-based compensation 2,780 428 Accrued expenses 648 — Other temporary differences 16 — Total noncurrent deferred tax assets 74,541 41,961 Total gross deferred tax assets 74,541 42,398 Valuation allowance (74,541 ) (42,398 ) Net deferred tax assets $ — $ — In 2015, the Company identified a misstatement related to the research and development credits that the Company had previously calculated and claimed on its tax returns. The Company previously had been assuming that all prior research and development credits would result in a higher credit whereas the Company has now determined that certain actual costs only qualify for a reduced credit. The net impact of the misstatement was to overstate gross deferred tax assets by $3.5 million and overstate the valuation allowance by $3.5 million with no impact to net deferred tax assets or the provision for income taxes. In addition, the misstatement had an impact of 2.5% and 5.4% to the 2014 and 2013 rate reconciliation, respectively, for the federal and state R&D tax credits that each had an offsetting adjustment to the change in valuation allowance. The misstatement was determined to be immaterial and had no effect on the Company’s consolidated balance sheets, statements of operations, statements of changes in stockholders’ equity (deficit) or statements cash flows for any period presented. As a result, the 2014 amounts in the deferred tax table and the 2014 and 2013 amounts in the rate reconciliation table presented herein have been revised to correct for this misstatement. In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2015, 2014 and 2013 related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and tax credit carryforwards and were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance as of beginning of year $ 42,398 $ 27,831 $ 22,229 Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision 32,143 14,567 5,602 Valuation allowance as of end of year $ 74,541 $ 42,398 $ 27,831 As of December 31, 2015, the Company had net operating loss carryforwards for federal and state income tax purposes of $42,493 and $31,249, respectively, which begin to expire in 2026 and 2030, respectively. Included within these net operating loss carryforwards are amounts of $12,812 and $10,642, respectively, that were attributable to stock option exercises, which will be recorded as an increase in additional paid-in capital once they are realized in accordance with accounting for stock-based compensation awards. As of December 31, 2015, the Company also had available tax credit carryforwards for federal and state income tax purposes of $10,555 and $1,537, respectively, which begin to expire in 2026 and 2021, respectively. Utilization of the net operating loss carryforwards and tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or tax credit carryforwards before utilization. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position. As of December 31, 2015 and 2014, the Company’s gross deferred tax asset balance of $74,541 and $42,398, respectively, was comprised principally of net operating loss carryforwards, capitalized research and development expenses and tax credit carryforwards. During the years ended December 31, 2015, 2014 and 2013, gross deferred tax assets increased due to additional net operating loss carryforwards, research and development tax credits generated and additional research and development expenses capitalized for tax purposes. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2015 and 2014. Management reevaluates the positive and negative evidence at each reporting period. The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2015 or 2014. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are still open under statute from 2011 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision. The Company considers only the direct effects of windfall tax deductions.

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Retirement Plan	12. Retirement Plan The Company has a Savings Incentive Match Plan for employees. Under the terms of the plan, the Company contributes 2% of an employee’s annual base salary, up to a maximum of the annual Internal Revenue Service compensation limits, for all full-time employees. During the years ended December 31, 2015, 2014 and 2013, the Company recognized $144, $61 and $26, respectively, of expense related to its contributions to the plan.

Australia Research and Developm

Australia Research and Development Tax Incentive	12 Months Ended
	Dec. 31, 2015
Research and Development [Abstract]
Australia Research and Development Tax Incentive	13. Australia Research and Development Tax Incentive The Company’s wholly owned subsidiary, Zafgen Australia Pty Limited, which conducts core research and development activities on behalf of the Company, is eligible to receive a 45% refundable tax incentive for qualified research and development activities. For the years ended December 31, 2015, 2014 and 2013, $1,357, $424 and $1,237, respectively, was recorded as a reduction to research and development expenses in the consolidated statements of operations. These amounts represented 45% of the Company’s qualified research and development spending in Australia. The refund is denominated in Australian dollars and, therefore, the related receivable is re-measured into U.S. dollars as of each reporting date. For the years ended December 31, 2015, 2014 and 2013, the Company recorded in its consolidated statements of operations unrealized foreign currency exchange losses of $82, $104 and $250, respectively, related to this tax incentive receivable. As of December 31, 2015 and 2014, the Company’s tax incentive receivable from the Australian government was $1,323 and $391, respectively.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	14. Quarterly Financial Data (Unaudited) The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, 2015 September 30, December 31, (in thousands, except per share data) Revenue $ — $ — $ — $ — Operating expenses 13,240 17,610 19,717 23,246 Net loss attributable to common stockholders (13,472 ) (17,756 ) (19,884 ) (23,174 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.53 ) $ (0.66 ) $ (0.73 ) $ (0.85 ) Weighted average common shares outstanding,basic and diluted 25,615,282 27,011,960 27,138,667 27,238,079 Three Months Ended March 31, June 30, 2014 September 30, December 31, (in thousands, except per share data) Revenue $ — $ — $ — $ — Operating expenses 4,521 5,986 14,361 10,664 Net loss attributable to common stockholders (4,507 ) (6,443 ) (14,689 ) (10,931 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.18 ) $ (2.96 ) $ (0.65 ) $ (0.48 ) Weighted average common shares outstanding,basic and diluted 729,391 2,178,465 22,707,012 22,783,817

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of common stock prior to the IPO and stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Cash Equivalents	Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of ninety days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market funds, U.S. government securities, corporate bonds, and commercial paper, are stated at fair value.
Concentration of Credit Risk and of Significant Suppliers	Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents and marketable securities. The Company has all cash and cash equivalents and marketable securities balances at two accredited financial institutions in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Marketable securities	Marketable securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. Fair value is determined based on quoted market prices.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering or as a reduction to the carrying value of preferred stock issued. As of December 31, 2014, the Company recorded $148 of deferred offering costs, included in other assets in the accompanying consolidated balance sheet, in contemplation of the Company’s follow-on offering of its common stock, which closed in January 2015.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over a five-year estimated useful life for equipment, furniture and fixtures and office equipment. Leasehold improvements are amortized over the shorter of the asset life or the term of the lease agreement. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Included in research and development expenses are wages, stock-based compensation and benefits of employees, third-party license fees and milestones and other operational costs related to the Company’s research and development activities, including facility-related expenses and external costs of outside vendors engaged to conduct pre-clinical studies, manufacturing activities, and clinical trials. The Company records research and development expenses net of any research and development tax incentives the Company is entitled to receive from government authorities.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors at the fair value on the date of the grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions. For stock-based awards granted to consultants and nonemployees, compensation expense is recognized over the period during which services are rendered by such consultants and nonemployees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Segment Data	Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on advancing novel therapeutics for patients suffering from severe obesity and obesity-related disorders. No revenue has been generated since inception, and all tangible assets are held in the United States.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2015 and 2014, the Company’s only element of other comprehensive loss was unrealized loss on marketable securities. For the year ended December 31, 2013, there was no difference between net loss and comprehensive loss.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Upon the closing of the Company’s IPO in June 2014, all of the Company’s outstanding redeemable convertible preferred shares were converted into shares of common stock. Prior to this conversion, the Company followed the two-class method when computing net income (loss) per share as the Company had issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred shares contractually entitled the holders of such shares to participate in dividends, but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, the two-class method did not apply for periods in which the Company reported a net loss or a net loss attributable to common stockholders resulting from dividends or accretion related to its redeemable convertible preferred shares. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common shares, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per common share attributable to common stockholders is the same as basic net loss per common share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is antidilutive.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes. In February 2016, the FASB issued ASU No. 2016-02, Leases

Fair Value Measurements and M23

Fair Value Measurements and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Summary of Cash Equivalents and Marketable Securities	The following tables summarize the Company’s cash equivalents and marketable securities as of December 31, 2015 and 2014: December 31, 2015 Total Quoted Significant Significant Cash equivalents: Money market funds $ 13,231 $ 13,231 $ — $ — U.S. government securities 4,999 — 4,999 — Commercial paper 4,697 — 4,697 — Corporate bonds 3,000 — 3,000 — Total cash equivalents 25,927 13,231 12,696 — Marketable securities: Corporate bonds 125,516 — 125,516 — Commercial paper 19,468 — 19,468 — U.S. government securities 4,500 — 4,500 — Total marketable securities 149,484 — 149,484 — Total cash equivalents and marketable securities $ 175,411 $ 13,231 $ 162,180 $ — December 31, 2014 Total Quoted Significant Significant Cash equivalents: Money market funds $ 14,742 $ 14,742 $ — $ — U.S. government securities 27,767 — 27,767 — Total cash equivalents 42,509 14,742 27,767 — Marketable securities: U.S. government securities 34,191 — 34,191 — Corporate bonds 22,168 — 22,168 — Commercial paper 1,000 — 1,000 — Total marketable securities 57,359 — 57,359 — Total cash equivalents and marketable securities $ 99,868 $ 14,742 $ 85,126 $ —
Summary of Marketable Securities	The following table summarizes the Company’s marketable securities as of December 31, 2015 and 2014: December 31, 2015 Amortized Gross Gross Fair Value Assets: Corporate bonds (due within 1 year) $ 112,825 $ 1 $ (166 ) $ 112,660 Corporate bonds (due after 1 year through 2 years) 12,884 3 (31 ) 12,856 Commercial paper (due within 1 year) 19,478 — (10 ) 19,468 U.S. government securities (due within 1 year) 4,500 — — 4,500 $ 149,687 $ 4 $ (207 ) $ 149,484 December 31, 2014 Amortized Cost Gross Gross Fair Value Assets: U.S. government securities (due within 1 year) $ 34,200 $ — $ (9 ) $ 34,191 Corporate bonds (due within 1 year) 18,716 — (18 ) 18,698 Corporate bonds (due after 1 year through 2 years) 3,478 — (8 ) 3,470 Commercial paper (due within 1 year) 1,000 — — 1,000 $ 57,394 $ — $ (35 ) $ 57,359

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Schedule of Property Plant and Equipment Net	Property and equipment, net consisted of the following as of December 31, 2015 and 2014: December 31, Useful Life 2015 2014 Office equipment 5 years $ 359 $ 79 Furniture and fixtures 5 years 186 50 Equipment 5 years 71 — Leasehold improvements * 406 — 1,022 129 Less: Accumulated depreciation (120 ) (50 ) $ 902 $ 79 * shorter of asset life or lease term

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following as of December 31, 2015 and 2014: December 31, 2015 2014 Accrued payroll and related expenses $ 1,802 $ 1,239 Accrued research and development expenses 3,727 1,180 Accrued professional fees 422 585 Accrued other 161 168 $ 6,112 $ 3,172

Notes Payable (Tables)

Notes Payable (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Schedule of Notes Payable	Notes payable consist of the following: December 31, 2015 2014 Notes payable $ 6,119 $ 7,500 Less: current portion (2,936 ) (1,381 ) Notes payable, net of current portion 3,183 6,119 Debt discount, net of accretion (36 ) (79 ) Accretion related to final payment 306 137 Notes payable, net of discount, long term $ 3,453 $ 6,177
Schedule of Estimated Future Principal Payments	Estimated future principal payments due under the Term Loan A are as follows: Years Ending December 31, 2016 $ 2,936 2017 3,183 Total $ 6,119

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Assumptions used to Determine Fair Value of Stock Options Granted	The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors are as follows, presented on a weighted average basis: 2015 2014 2013 Risk-free interest rate 1.75 % 1.90 % 1.12 % Expected term (in years) 6.25 6.25 6.25 Expected volatility 87 % 90 % 85 % Expected dividend yield 0 % 0 % 0 %
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2014: Shares Issuable Weighted Average Aggregate Intrinsic Value (In years) Outstanding as of December 31, 2014 2,008,574 $ 6.65 Granted 992,505 $ 39.37 Exercised (410,701 ) $ 1.95 Forfeited (35,268 ) $ 32.41 Outstanding as of December 31, 2015 2,555,110 $ 19.76 7.9 $ 3,918 Options vested and expected to vest as of December 31, 2015 2,555,110 $ 19.76 7.9 $ 3,918 Options exercisable as of December 31, 2015 995,324 $ 5.60 6.5 $ 3,243
Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories within its consolidated statements of operations: Year Ended December 31, 2015 2014 2013 Research and development $ 2,930 $ 490 $ 176 General and administrative 5,652 1,063 219 $ 8,582 $ 1,553 $ 395

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Computation of Basic and Diluted Earnings Per Share	Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2015 2014 2013 Basic and diluted net loss per share attributable to common stockholders: Numerator: Net loss $ (74,286 ) $ (36,478 ) $ (14,027 ) Accretion of redeemable convertible preferred stock to redemption value — (92 ) (213 ) Net loss attributable to common stockholders $ (74,286 ) $ (36,570 ) $ (14,240 ) Denominator: Weighted average common shares outstanding, basic and diluted 26,756,079 12,189,155 729,001 Net loss per share attributable to common stockholders, basic and diluted $ (2.78 ) $ (3.00 ) $ (19.53 )
Summary of Common Stock Equivalents Outstanding	The Company excluded the following common stock equivalents, outstanding as of December 31, 2015, 2014 and 2013, from the computation of diluted net loss per share attributable to common stockholders for the years ended December 31, 2015, 2014 and 2013 because they had an anti-dilutive impact due to the net loss attributable to common stockholders incurred for the periods: As of December 31, 2015 2014 2013 Options to purchase common stock 2,555,110 2,008,574 1,310,330

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments for Operating Leases	Future minimum lease payments for its operating leases as of December 31, 2015 were as follows: Years Ending December 31, 2016 $ 432 2017 292 2018 106 2019 82 $ 912
Research and Development Expenses Related to License Agreements	Related to these license agreements, the Company recorded research and development expenses in its consolidated statements of operations as follows: Year Ended December 31, 2015 2014 2013 License, milestone, and license maintenance fees $ — $ 7,019 $ — $ — $ 7,019 $ —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of Domestic and Foreign Components of Loss Before Income Taxes	The domestic and foreign components of loss before income taxes are as follows: 2015 2014 2013 Domestic $ (72,147 ) $ (35,818 ) $ (12,325 ) Foreign (2,139 ) (660 ) (1,702 ) Loss before income taxes $ (74,286 ) $ (36,478 ) $ (14,027 )
Reconciliation of Federal Statutory Income Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2015 2014 2013 Federal statutory income tax rate (34.0 %) (34.0 %) (34.0 %) Federal and state research and development tax credit (3.9 ) (1.9 ) (3.7 ) State taxes, net of federal benefit (3.9 ) (4.0 ) (3.9 ) Orphan drug tax credit (3.5 ) (3.2 ) (3.1 ) Stock compensation expense 0.9 0.6 0.6 Nondeductible Australia research and development expenses 1.0 0.6 4.1 Other items 0.1 2.4 — Change in deferred tax asset valuation allowance 43.3 39.5 40.0 Effective income tax rate 0.0 % 0.0 % 0.0 %
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Current deferred tax assets: Accrued expenses $ — $ 431 Other temporary differences — 6 Total current deferred tax assets — 437 Noncurrent deferred tax assets: Capitalized research and development expenses 46,568 29,048 Net operating loss carryforwards 11,186 6,328 Tax credit carryforwards 11,570 4,928 Capitalized legal expenses 1,773 1,229 Stock-based compensation 2,780 428 Accrued expenses 648 — Other temporary differences 16 — Total noncurrent deferred tax assets 74,541 41,961 Total gross deferred tax assets 74,541 42,398 Valuation allowance (74,541 ) (42,398 ) Net deferred tax assets $ — $ —
Summary Changes in the Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2015, 2014 and 2013 related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and tax credit carryforwards and were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance as of beginning of year $ 42,398 $ 27,831 $ 22,229 Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision 32,143 14,567 5,602 Valuation allowance as of end of year $ 74,541 $ 42,398 $ 27,831

Quarterly Financial Data (Una31

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of Unaudited Consolidated Financial Statements	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, 2015 September 30, December 31, (in thousands, except per share data) Revenue $ — $ — $ — $ — Operating expenses 13,240 17,610 19,717 23,246 Net loss attributable to common stockholders (13,472 ) (17,756 ) (19,884 ) (23,174 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.53 ) $ (0.66 ) $ (0.73 ) $ (0.85 ) Weighted average common shares outstanding,basic and diluted 25,615,282 27,011,960 27,138,667 27,238,079 Three Months Ended March 31, June 30, 2014 September 30, December 31, (in thousands, except per share data) Revenue $ — $ — $ — $ — Operating expenses 4,521 5,986 14,361 10,664 Net loss attributable to common stockholders (4,507 ) (6,443 ) (14,689 ) (10,931 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.18 ) $ (2.96 ) $ (0.65 ) $ (0.48 ) Weighted average common shares outstanding,basic and diluted 729,391 2,178,465 22,707,012 22,783,817

Nature of the Business and Ba32

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 28, 2015	Jun. 24, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date			Nov. 22, 2005
Proceeds from public offerings, net of commissions and underwriting discounts		$ 102,672	$ 130,044	$ 102,672
Offering costs		$ 2,508
Underwriting discounts, commissions and offering costs incurred			$ 473	$ 2,312	$ 196
IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock issued		6,900,000
Common stock issued price per share		$ 16
Follow-on Offering [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock issued	3,942,200
Common stock issued price per share	$ 35
Proceeds from offering, net of commissions and underwriting discounts	$ 130,044
Underwriting discounts, commissions and offering costs incurred	$ 473

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Deferred offering costs		$ 148,000
Estimated useful life	5 years
Impairment losses on long-lived assets	$ 0
Percentage of likelihood to be realized upon ultimate settlement, description	50.00%

Fair Value Measurements and M34

Fair Value Measurements and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 25,927	$ 42,509
Marketable securities	149,484	57,359
Total cash equivalents and marketable securities	175,411	99,868
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	13,231	14,742
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,999	27,767
Marketable securities	4,500	34,191
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,000
Marketable securities	125,516	22,168
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,697
Marketable securities	19,468	1,000
Quoted Prices in Active Markets, (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	13,231	14,742
Total cash equivalents and marketable securities	13,231	14,742
Quoted Prices in Active Markets, (Level 1) [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	13,231	14,742
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	12,696	27,767
Marketable securities	149,484	57,359
Total cash equivalents and marketable securities	162,180	85,126
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,999	27,767
Marketable securities	4,500	34,191
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,000
Marketable securities	125,516	22,168
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,697
Marketable securities	$ 19,468	$ 1,000

Fair Value Measurements and M35

Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 149,687	$ 57,394
Gross Unrealized Gains	4
Gross Unrealized Losses	(207)	(35)
Fair Value	149,484	57,359
U.S. Government Securities (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,500	34,200
Gross Unrealized Losses		(9)
Fair Value	4,500	34,191
Corporate Bonds (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,825	18,716
Gross Unrealized Gains	1
Gross Unrealized Losses	(166)	(18)
Fair Value	112,660	18,698
Corporate Bonds (Due After 1 Year Through 2 Years) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,884	3,478
Gross Unrealized Gains	3
Gross Unrealized Losses	(31)	(8)
Fair Value	12,856	3,470
Commercial Paper (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,478	1,000
Gross Unrealized Losses	(10)
Fair Value	$ 19,468	$ 1,000

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property Plant and Equipment Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, Useful Life	5 years
Property and equipment, gross	$ 1,022	$ 129
Less: Accumulated depreciation	(120)	(50)
Property and equipment, net	$ 902	79
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful Life	5 years
Property and equipment, gross	$ 359	79
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful Life	5 years
Property and equipment, gross	$ 186	$ 50
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful Life	5 years
Property and equipment, gross	$ 71
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 406

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property Plant And Equipment Capitalized Interest Costs [Abstract]
Depreciation expense	$ 70	$ 16	$ 12

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued payroll and related expenses	$ 1,802	$ 1,239
Accrued research and development expenses	3,727	1,180
Accrued professional fees	422	585
Accrued other	161	168
Accrued expenses	$ 6,112	$ 3,172

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($)	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Debt Instrument [Line Items]
Credit facility initiation date		Mar. 31, 2014
Maximum line of credit	$ 20,000,000
Line of credit facility description		The Company was obligated to make monthly, interest-only payments on any term loans funded under the Credit Facility until December 1, 2014 and, thereafter, to pay 36 consecutive, equal monthly installments of principal and interest from January 1, 2015 through December 1, 2017. As per the terms of the agreement, in June 2014, upon the completion of the Company’s IPO, the term of monthly, interest-only payments were extended until June 1, 2015.
Credit facility maturity date		Dec. 1, 2017
Line of credit facility annual interest rate		8.10%
Line of credit facility additional fee percentage		6.00%
Interest expense		$ 806,000	$ 870,000
Covenants under credit facility		0
Credit facility default amount		$ 250,000
Credit Facility [Member]
Debt Instrument [Line Items]
Sale of voting equity threshold		50.00%
Secured Debt [Member]
Debt Instrument [Line Items]
Interest expense on credit facility		$ 806,000	870,000
Term Loan A [Member]
Debt Instrument [Line Items]
Current line of credit	7,500,000
Proceeds from issuance of promissory notes	7,500,000
Charges to interest expense of debt outstanding		$ 450,000
Term Loan A [Member] | Credit Facility [Member]
Debt Instrument [Line Items]
Net of debt discount	114,000
Deferred financing costs	49,000
Effective annual interest rate		10.80%
Term Loan A [Member] | Credit Facility [Member] | Debt Discount and Finance Costs [Member]
Debt Instrument [Line Items]
Interest expense		$ 62,000	$ 50,000
Term Loan B and C [Member]
Debt Instrument [Line Items]
Current line of credit	12,500,000
Additional line of credit	$ 12,500,000
Term Loan B [Member]
Debt Instrument [Line Items]
Additional line of credit				$ 7,500,000
Term Loan C [Member] | Qualified IPO [Member]
Debt Instrument [Line Items]
Additional line of credit				$ 5,000,000

Notes Payable - Schedule of Not

Notes Payable - Schedule of Notes Payable (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Notes payable	$ 6,119	$ 7,500
Less: current portion	(2,936)	(1,381)
Notes payable, net of current portion	3,183	6,119
Notes payable, net of current portion	3,183	6,119
Notes payable, net of discount, long term	3,453	6,177
Secured Debt [Member] | Credit Facility [Member]
Debt Instrument [Line Items]
Debt discount, net of accretion	(36)	(79)
Accretion related to final payment	$ 306	$ 137

Notes Payable - Schedule of Est

Notes Payable - Schedule of Estimated Future Principal Payments (Detail) - Term Loan A [Member] $ in Thousands	Dec. 31, 2015USD ($)
Line of Credit Facility [Line Items]
2,016	$ 2,936
2,017	3,183
Total	$ 6,119

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands	Jan. 28, 2015USD ($)$ / sharesshares	Jun. 24, 2014USD ($)$ / sharesshares	Jun. 05, 2014	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares	Dec. 31, 2013USD ($)
Proceeds from public offerings, net of commissions and underwriting discounts | $		$ 102,672		$ 130,044	$ 102,672
Offering costs | $		$ 2,508
Payments of public offering costs | $				$ 473	$ 2,312	$ 196
Preferred stock, shares authorized				5,000,000	5,000,000
Preferred stock, par value | $ / shares				$ 0.001	$ 0.001
Common stock, shares authorized				115,000,000	115,000,000
Common stock, par value | $ / shares				$ 0.001	$ 0.001
Common Stock [Member]
Reverse stock split, description				Company effected a 1-for-6.28 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of redeemable convertible preferred stock.
Convertible preferred stock, conversion ratio			6.28
Common stock issued				3,942,200	6,900,000
Follow-on Offering [Member]
Common stock issued	3,942,200
Common stock issued price per share | $ / shares	$ 35
Proceeds from offering, net of commissions and underwriting discounts | $	$ 130,044
Payments of public offering costs | $	$ 473
IPO [Member]
Common stock issued		6,900,000
Common stock issued price per share | $ / shares		$ 16
IPO [Member] | Common Stock [Member]
Redeemable convertible preferred stock converted into common stock shares				15,077,621

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($)	Jan. 01, 2015	Sep. 01, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 05, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation, stock options granted			992,505	698,642
Weighted average grant-date fair value of stock options granted			$ 28.97	$ 11.73	$ 1.83
Expected option term			6 years 3 months	6 years 3 months	6 years 3 months
Risk-free interest rate			1.75%	1.90%	1.12%
Aggregate intrinsic value of stock options exercised			$ 13,949,000	$ 195,000
Proceeds from exercise of common stock options			799,000	$ 3,000
Unrecognized stock-based compensation cost			$ 27,515
Stock-based compensation recognized weighted average period			2 years 7 months 6 days
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation, stock options granted			988,505	697,050
Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation, stock options granted			4,000	1,592
Nonemployee Service Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of outstanding unvested shares held by nonemployees			9,363	13,205
Nonemployee Performance Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of outstanding unvested shares held by nonemployees				796
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of restricted stock awards vested during the period					$ 38,000
Number of restricted stock granted			0	0	0
Number of restricted stock units vested			0	0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of restricted stock awards vested during the period			$ 115,000
Number of restricted stock granted			4,769	0	0
Number of restricted stock units vested			4,769
Weighted average grant-date fair value of restricted stock units granted			$ 34.46
Number of restricted stock units unvested			0	0	0
2006 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock authorized under stock option plan			1,889,150
Shares of common stock reserved for issuance			168,221
2006 Stock Option Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options purchase price of common stock, percent of fair market value			100.00%
2006 Stock Option Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum term of stock option			10 years
2014 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance			2,168,221
Percentage of outstanding shares of common stock			4.00%
Common stock available for issuance			1,625,761
Shares added to plan	915,166
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance						265,000
Common stock available for issuance			257,460	265,000
Commencement date of first offering period		Sep. 1, 2014
End date of first offering period		Dec. 31, 2014
Commencement date of second offering period			Jan. 1, 2015
End date of second offering period			Jun. 30, 2015
Commencement date of third offering period			Jul. 1, 2015
End date of third offering period			Dec. 31, 2015
Stock-based awards , shares			7,540
Weighted average grant-date fair value of stock options granted			$ 11.31
Stock options, method used			Black-Scholes option-pricing model
Weighted-average stock-price volatility			66.00%
Expected option term			6 months
Risk-free interest rate			0.09%
2014 Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount from market price of the company's common stock offering		85.00%

Stock-Based Awards - Assumption

Stock-Based Awards - Assumptions used to Determine Fair Value of Stock Options Granted (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rate	1.75%	1.90%	1.12%
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Expected volatility	87.00%	90.00%	85.00%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares issuable under options, beginning balance	2,008,574
Shares issuable under options, granted	992,505	698,642
Shares issuable under options, exercised	(410,701)
Shares issuable under options, forfeited	(35,268)
Shares issuable under options, ending balance	2,555,110	2,008,574
Shares issuable under options, options vested and expected to vest	2,555,110
Shares issuable under options, options exercisable	995,324
Weighted-average exercise price, beginning balance	$ 6.65
Weighted-average exercise price, granted	39.37
Weighted-average exercise price, exercised	1.95
Weighted-average exercise price, forfeited	32.41
Weighted-average exercise price, ending balance	19.76	$ 6.65
Weighted-average exercise price, options vested and expected to vest	19.76
Weighted-average exercise price, options exercisable	$ 5.60
Average remaining contractual life	7 years 10 months 24 days
Average remaining contractual life, options vested and expected to vest	7 years 10 months 24 days
Average remaining contractual life, options exercisable	6 years 6 months
Aggregate intrinsic value, ending balance	$ 3,918
Aggregate intrinsic value, options vested and expected to vest	3,918
Aggregate intrinsic value, options exercisable	$ 3,243

Stock-Based Awards - Summary 46

Stock-Based Awards - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 8,582	$ 1,553	$ 395
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	2,930	490	176
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 5,652	$ 1,063	$ 219

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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
Numerator:
Net loss									$ (74,286)	$ (36,478)	$ (14,027)
Accretion of redeemable convertible preferred stock to redemption value										(92)	(213)
Net loss attributable to common stockholders	$ (23,174)	$ (19,884)	$ (17,756)	$ (13,472)	$ (10,931)	$ (14,689)	$ (6,443)	$ (4,507)	$ (74,286)	$ (36,570)	$ (14,240)
Denominator:
Weighted average common shares outstanding, basic and diluted	27,238,079	27,138,667	27,011,960	25,615,282	22,783,817	22,707,012	2,178,465	729,391	26,756,079	12,189,155	729,001
Net loss per share attributable to common stockholders, basic and diluted	$ (0.85)	$ (0.73)	$ (0.66)	$ (0.53)	$ (0.48)	$ (0.65)	$ (2.96)	$ (6.18)	$ (2.78)	$ (3)	$ (19.53)

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Common Stock Equivalents Outstanding (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from computation of earnings per share	2,555,110	2,008,574	1,310,330

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Oct. 01, 2015	Mar. 31, 2015Option	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Commitment And Contingencies [Line Items]
Operating lease, rental expense			$ 326,000	$ 160,000	$ 105,000
Issuance of common stock in lieu of milestone payment				3,569,000
Development Based Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments due			0	$ 7,019,000	$ 0
Subsequent Licensed Product [Member]
Commitment And Contingencies [Line Items]
Licensing fees per product maximum			$ 1,250,000
Boston Office Space [Member]
Commitment And Contingencies [Line Items]
Lease expiration date			Jul. 31, 2017
Lease extension period			3 years
Additional Boston Office Space [Member]
Commitment And Contingencies [Line Items]
Lease expiration date		Jul. 31, 2017
Lease extension period		3 years
Number of options to extend lease | Option		2
San Diego Office Space [Member]
Commitment And Contingencies [Line Items]
Lease expiration date	Sep. 30, 2019
Lease extension period	5 years
Maximum [Member] | Pre Commercialization Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments			$ 12,250,000
Maximum [Member] | Product Commercialization Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments			$ 12,500,000

Commitments and Contingencies50

Commitments and Contingencies - Schedule of Future Minimum Lease Payments for Operating Leases (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,016	$ 432
2,017	292
2,018	106
2,019	82
Total	$ 912

Commitments and Contingencies51

Commitments and Contingencies - Research and Development Expenses Related to License Agreements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments And Contingencies [Line Items]
License, milestone, and license maintenance fees	$ 54,618	$ 27,391	$ 9,561
License, milestone, and license maintenance fees, Total	$ 54,618	27,391	$ 9,561
License Agreements [Member]
Commitments And Contingencies [Line Items]
License, milestone, and license maintenance fees		7,019
License, milestone, and license maintenance fees, Total		$ 7,019

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes Disclosure [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
Change in federal and state research and development tax credit due to misstatement	3.90%	1.90%	3.70%
Operating loss carryforwards, federal	$ 10,555,000
Operating loss carryforwards, state	1,537,000
Uncertain tax position	0
Gross deferred tax assets	74,541,000	$ 42,398,000
Unrecognized tax benefits	0	$ 0
Restatement Adjustment [Member]
Income Taxes Disclosure [Line Items]
Change in gross deferred tax assets due to misstatement	3,500,000
Change in valuation allowance due to misstatement	3,500,000
Change in federal and state research and development tax credit due to misstatement		2.50%	5.40%
Domestic Tax Authority [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 42,493,000
Net operating loss carryforwards, beginning of expiration period	2,026
Research and development tax credit carryforwards, beginning of expiration period	2,026
Domestic Tax Authority [Member] | Options to Purchase Common Stock [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 12,812,000
State and Local Jurisdiction [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 31,249,000
Net operating loss carryforwards, beginning of expiration period	2,030
Research and development tax credit carryforwards, beginning of expiration period	2,021
State and Local Jurisdiction [Member] | Options to Purchase Common Stock [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 10,642,000

Income Taxes - Schedule of Dome

Income Taxes - Schedule of Domestic and Foreign Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Domestic	$ (72,147)	$ (35,818)	$ (12,325)
Foreign	(2,139)	(660)	(1,702)
Loss before income taxes	$ (74,286)	$ (36,478)	$ (14,027)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
Federal and state research and development tax credit	(3.90%)	(1.90%)	(3.70%)
State taxes, net of federal benefit	(3.90%)	(4.00%)	(3.90%)
Orphan drug tax credit	(3.50%)	(3.20%)	(3.10%)
Stock compensation expense	0.90%	0.60%	0.60%
Nondeductible Australia research and development expenses	1.00%	0.60%	4.10%
Other items	0.10%	2.40%
Change in deferred tax asset valuation allowance	43.30%	39.50%	40.00%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current deferred tax assets:
Accrued expenses		$ 431
Other temporary differences		6
Total current deferred tax assets		437
Noncurrent deferred tax assets:
Capitalized research and development expenses	$ 46,568	29,048
Net operating loss carryforwards	11,186	6,328
Tax credit carryforwards	11,570	4,928
Capitalized legal expenses	1,773	1,229
Stock-based compensation	2,780	428
Accrued expenses	648
Other temporary differences	16
Total noncurrent deferred tax assets	74,541	41,961
Total gross deferred tax assets	74,541	42,398
Valuation allowance	(74,541)	(42,398)	$ (27,831)	$ (22,229)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ 42,398	$ 27,831	$ 22,229
Decreases recorded as benefit to income tax provision	0	0	0
Increases recorded to income tax provision	32,143	14,567	5,602
Valuation allowance as of end of year	$ 74,541	$ 42,398	$ 27,831

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Compensation and Retirement Disclosure [Abstract]
Percentage of employer's contribution on employee's base salary	2.00%
Contribution expense	$ 144	$ 61	$ 26

Australia Research and Develo58

Australia Research and Development Tax Incentive - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Unrealized foreign currency transaction losses	$ (82)	$ (93)	$ (250)
Research Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Percentage of refundable tax incentive	45.00%
Reduction to research and development expenses	$ 1,357	424	1,237
Unrealized foreign currency transaction losses	$ (82)	(104)	$ (250)
Australia [Member] | Research Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Percentage of reduction in research and development costs	45.00%
Tax incentive receivable	$ 1,323	$ 391

Quarterly Financial Data - Sche

Quarterly Financial Data - Schedule of Unaudited Consolidated Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses	23,246	19,717	17,610	13,240	10,664	14,361	5,986	4,521	73,813	35,532	13,780
Net loss attributable to common stockholders	$ (23,174)	$ (19,884)	$ (17,756)	$ (13,472)	$ (10,931)	$ (14,689)	$ (6,443)	$ (4,507)	$ (74,286)	$ (36,570)	$ (14,240)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.85)	$ (0.73)	$ (0.66)	$ (0.53)	$ (0.48)	$ (0.65)	$ (2.96)	$ (6.18)	$ (2.78)	$ (3)	$ (19.53)
Weighted average common shares outstanding, basic and diluted	27,238,079	27,138,667	27,011,960	25,615,282	22,783,817	22,707,012	2,178,465	729,391	26,756,079	12,189,155	729,001