Flexion Therapeutics (FLXN) 10-K 10 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Mar. 02, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	FLXN
Entity Registrant Name	Flexion Therapeutics Inc
Entity Central Index Key	1,419,600
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		31,731,824
Entity Public Float			$ 341,322,482

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 30,915,374	$ 62,944,044
Marketable securities	174,688,468	48,302,507
Accounts receivable	0	95,285
Prepaid expenses and other current assets	3,789,732	760,872
Total current assets	209,393,574	112,102,708
Property and equipment, net	11,663,559	7,442,477
Long-term investments	4,725,041	7,357,423
Other assets		155,904
Restricted cash	480,000	80,000
Total assets	226,262,174	127,138,512
Current liabilities:
Accounts payable	2,160,869	3,692,414
Accrued expenses and other current liabilities	6,245,233	4,366,560
Current portion of long-term debt	9,133,991
Total current liabilities	17,540,093	8,058,974
Long-term debt	21,398,792	15,002,039
Other long-term liabilities	291,333	91,055
Total liabilities	39,230,218	23,152,068
Commitments and contingencies
Preferred Stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2016 and December 31, 2015 and 0 shares issued and outstanding at December 31, 2016 and December 31, 2015
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized; 31,667,469 and 21,570,395 shares issued and outstanding, at December 31, 2016 and December 31, 2015, respectively	31,667	21,570
Additional paid-in capital	398,757,135	243,853,799
Accumulated other comprehensive income	(70,613)	(96,651)
Accumulated deficit	(211,686,233)	(139,792,274)
Total stockholders’ equity	187,031,956	103,986,444
Total liabilities and stockholders’ equity	$ 226,262,174	$ 127,138,512

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	31,667,469	21,570,395
Common stock, shares outstanding	31,667,469	21,570,395

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating expenses:
Research and development	$ 41,313,942	$ 32,691,445	$ 17,923,348
General and administrative	28,466,350	13,371,631	9,063,926
Total operating expenses	69,780,292	46,063,076	26,987,274
Loss from operations	(69,780,292)	(46,063,076)	(26,987,274)
Other income (expense):
Interest income	1,520,659	1,246,133	478,715
Interest expense	(1,747,186)	(570,990)	(401,370)
Other income (expense), net	(1,887,140)	(927,518)	(403,735)
Total other income (expense)	(2,113,667)	(252,375)	(326,390)
Net loss	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Net loss per common share, basic and diluted	$ (2.84)	$ (2.15)	$ (1.97)
Weighted average common shares outstanding, basic and diluted	25,296,527	21,497,119	13,893,961
Other comprehensive gain (loss) :
Unrealized gains (losses) from available-for-sale securities, net of tax of $0	$ 26,038	$ (91,411)	$ (5,212)
Total other comprehensive income (loss)	26,038	(91,411)	(5,212)
Comprehensive loss	$ (71,867,921)	$ (46,406,862)	$ (27,318,876)

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Unrealized gains from available-for-sale securities, tax	$ 0	$ 0	$ 0

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	Total	Series A and B Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2013	$ (64,703,890)	$ 74,806,213	$ 794	$ 1,458,503	$ (28)	$ (66,163,159)
Balance (in shares) at Dec. 31, 2013		72,780,250	794,090
Conversion of Series A and Series B Convertible Preferred Stock	74,806,213	$ (74,806,213)	$ 8,952	74,797,261
Conversion of Series A and Series B Convertible Preferred Stock (in shares)		(72,780,250)	8,952,057
Issuance of Common Stock net of issuance costs	159,322,738		$ 11,546	159,311,192
Issuance of Common Stock net of issuance costs (in shares)			11,546,000
Exercise of stock options	$ 304,496		$ 141	304,355
Exercise of stock options (in shares)	141		141,141
Employee Stock Purchase Plan	$ 80,631		$ 7	80,624
Employee Stock Purchase Plan (in shares)			6,770
Stock-based compensation expense	2,450,579			2,450,579
Net loss	(27,313,664)					(27,313,664)
Other comprehensive income (loss)	(5,212)				(5,212)
Balance at Dec. 31, 2014	144,941,891		$ 21,440	238,402,514	(5,240)	(93,476,823)
Balance (in shares) at Dec. 31, 2014			21,440,058
Exercise of stock options	$ 592,787		$ 109	592,678
Exercise of stock options (in shares)	109,441		109,441
Employee Stock Purchase Plan	$ 275,949		$ 21	275,928
Employee Stock Purchase Plan (in shares)			20,896
Stock-based compensation expense	4,582,679			4,582,679
Net loss	(46,315,451)					(46,315,451)
Other comprehensive income (loss)	(91,411)				(91,411)
Balance at Dec. 31, 2015	103,986,444		$ 21,570	243,853,799	(96,651)	(139,792,274)
Balance (in shares) at Dec. 31, 2015			21,570,395
Issuance of Common Stock net of issuance costs	147,501,165		$ 10,040	147,491,125
Issuance of Common Stock net of issuance costs (in shares)			10,040,000
Exercise of stock options	$ 166,816		$ 30	166,786
Exercise of stock options (in shares)	30,195		30,194
Employee Stock Purchase Plan	$ 475,648		$ 27	475,621
Employee Stock Purchase Plan (in shares)			26,880
Stock-based compensation expense	6,769,804			6,769,804
Net loss	(71,893,959)					(71,893,959)
Other comprehensive income (loss)	26,038				26,038
Balance at Dec. 31, 2016	$ 187,031,956		$ 31,667	$ 398,757,135	$ (70,613)	$ (211,686,233)
Balance (in shares) at Dec. 31, 2016			31,667,469

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	1,150,961	238,153	120,341
Stock-based compensation expense	6,769,804	4,582,679	2,450,579
Other non-cash charges	36,607	41,103	16,500
Amortization of premium (discount) on marketable	728,552	871,215	366,231
Loss on disposal of fixed assets	2,283,344	149,983
Premium paid on securities purchased	(542,567)
Changes in operating assets and liabilities:
Accounts receivable	95,285
Prepaid expenses, other current and long-term assets	(2,872,955)	(526,247)	(320,352)
Accounts payable	(992,739)	1,581,164	518,032
Accrued expenses and other current and long-term liabilities	2,765,899	428,769	1,017,412
Net cash used in operating activities	(62,471,768)	(38,948,632)	(23,144,921)
Cash flows from investing activities
Purchases of property and equipment	(8,439,577)	(5,197,326)	(802,481)
Change in restricted cash	(400,000)	48,000
Purchases of marketable securities	(196,060,561)	(145,797,556)	(79,383,553)
Sale and redemption of marketable securities	72,147,035	137,702,156	30,735,000
Net cash used in investing activities	(132,753,103)	(13,244,726)	(49,451,034)
Cash flows from financing activities
Payment of debt issuance costs	(42,034)	(107,741)
Payments on debt		(3,500,000)	(1,500,000)
Proceeds from borrowings under term loan	15,000,000	15,003,533
Payments of public offering costs	(293,368)	(224,648)	(1,517,484)
Proceeds from the issuance of common stock	147,889,139		162,137,580
Proceeds from the exercise of stock options	166,816	592,787	304,496
Proceeds from Employee Stock Purchase Plan	475,648	275,949	80,631
Net cash provided by financing activities	163,196,201	12,039,880	159,505,223
Net increase (decrease) in cash and cash equivalents	(32,028,670)	(40,153,478)	86,909,268
Cash and cash equivalents at beginning of period	62,944,044	103,097,522	16,188,254
Cash and cash equivalents at end of period	30,915,374	62,944,044	103,097,522
Supplemental disclosures of cash flow information:
Cash paid for interest	1,296,875	572,340	364,889
Supplemental disclosures of non-cash financing activities:
Public offering costs included in accounts payable or accrued	94,606		224,648
Conversion of convertible preferred stock into common stock			74,806,213
Purchases of property and equipment in accounts payable and accrued expenses	$ 622,005	$ 1,575,907	$ 52,011

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Flexion Therapeutics, Inc. (“Flexion” or the “Company”) was incorporated under the laws of the state of Delaware on November 5, 2007. Flexion is a specialty pharmaceutical company focused on the development and commercialization of novel, local therapies for the treatment of patients with musculoskeletal conditions, beginning with osteoarthritis, a type of degenerative arthritis (“OA”). Flexion’s lead product candidates addresses the OA pain treatment spectrum, from moderate to severe knee pain, and provides the Company with multiple opportunities to achieve its goal of commercializing novel, patient-focused therapies. The Company is subject to risks and uncertainties common to early-stage companies in the biopharmaceutical industry, including, but not limited to, new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, and ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance reporting capabilities. The Company’s product candidates are all in the development stage. There can be no assurance that development efforts, including clinical trials, will be successful. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. The Company has incurred recurring losses and negative cash flows from operations. As of December 31, 2016 and 2015, the Company had cash, cash equivalents, marketable securities, and long-term investments of ,

Financing Transactions_Common S

Financing Transactions/Common Stock	12 Months Ended
	Dec. 31, 2016
Common Stock [Member]
Financing Transactions/Common Stock	12. Common Stock Upon inception of the Company on November 5, 2007, the Company authorized 10,000,000 shares of common stock and issued 109 shares to the founders. In 2009, the Company amended its Certificate of Incorporation and authorized an additional 69,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 79,000,000. In 2012, the Company amended its Certificate of Incorporation and authorized an additional 15,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 94,000,000. In 2014, the Company amended its Certificate of Incorporation and authorized an additional 6,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 100,000,000. On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,750,000 shares of common stock at a price to the public of $13.00 per share, including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding. On December 17, 2014, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,796,000 shares of the Company’s common stock at a price to the public of $17.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. On June 7, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,900,000 shares of the Company’s common stock at a price to the public of $14.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. On November 15, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 4,140,000 shares of the Company’s common stock at a price to the public of $18.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to the preferential dividend rights of any holders of Preferred Stock. As of December 31, 2016, no dividends have been declared.
IPO [Member]
Financing Transactions/Common Stock	2. Financing Transactions On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,750,000 shares of common stock at a price to the public of $13.00 per share, including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the IPO of $67.2 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a 1-for-8.13 reverse stock split of the Company’s common stock and a proportional adjustment to the existing conversion ratios for each series of Convertible Preferred Stock, effective January 27, 2014. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding. On December 17, 2014, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,796,000 shares of the Company’s common stock at a price to the public of $17.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $92.2 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. On June 8, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,900,000 shares of the Company’s common stock at a price to the public of $14.00 per share including shares sold pursuant to the partial exercise of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $77.4 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. On November 15, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 4,140,000 shares of the Company’s common stock at a price to the public of $18.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $70.1 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. The Company’s total issued common stock as of December 31, 2016 was 31,667,469 shares.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management. Consolidation The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions. In addition, Flexion Therapeutics, Inc. is registered to do business in the United Kingdom through its branch office located in Swindon, United Kingdom. U.S. Government Grant The Company performs research and development for a U.S. Government agency under a cost reimbursable grant for clinical development of Zilretta. The related costs incurred under the grant are included in research and development expense in the statement of operations. Reimbursements were recorded as an offset to R&D expense when invoices for allowable costs were prepared and submitted to the Department of Defense. Due to challenges of enrolling military personnel with post-traumatic knee OA, we discontinued the Phase 2 trial and terminated the grant. Payments under cost reimbursable grants with agencies of the U.S. Government are provisional payments subject to adjustment upon audit by the U.S. government. Accounts Receivable Accounts receivable represents allowable costs under the Company’s U.S. Government agency grant for which the Company has not yet received reimbursement. The Company invoices the government on a quarterly basis for reimbursable costs under the grant. Reimbursable costs that have not been invoiced on the last day of the quarter are recorded as unbilled accounts receivable. As of December 31, 2016 and 2015, there were unbilled accounts receivable of $0 and $95,285, respectively. Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper. Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income. Restricted Cash The Company purchased a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank. In 2016, the Company placed $400,000 in an account at a commercial bank to further collateralize the credit card account. These balances are classified as long-term restricted cash as of December 31, 2016 and 2015. In addition the Company posted a letter of credit to the lessor of the Company’s Burlington facility as a security deposit pursuant to the lease agreement. As of December 31, 2016 and 2015, there was $50,000 as long-term restricted cash related to the aforementioned lease agreement. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Property and equipment includes construction-in-progress, which is not yet in service, and is estimated to have a useful life of 7 years once placed into service. Foreign Currencies The Company maintains a bank account designated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations within other income and expenses, and were not significant in any period. Impairment of Long-Lived Assets The Company reviews its long-lived assets, including property and equipment, for impairment 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. Debt Issuance Costs, net As of December 31, 2016 and 2015, the carrying value of debt issuance costs was $100,760 and $124,041, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $36,607, $41,103 and $16,500 respectively, of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the years ended December 31, 2016, 2015 and 2014. 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’ deficit as a reduction of additional paid-in capital generated as a result of the offering. If the equity financing is no longer considered probable of being consummated, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. The Company completed its initial public offering in February 2014 and recorded deferred offering costs of $1,623,540 as a reduction to additional paid-in capital. The Company did not record any deferred offering costs during the year ended December 31, 2016 or 2015. Research and Development Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred. 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 at the fair value at the date of 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 non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees 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 the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified. The Company recognizes compensation expense only for 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. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods. Concentration of Credit Risk and Significant Suppliers Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and corporate bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. 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 completely dependent on third-party manufacturers and product suppliers for research and potential commercial activities. In particular, the Company relies and expects to continue to rely on a limited number of manufacturers and relies on them to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials, and if Zilretta is approved, for commercial supply. These programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients. Comprehensive Loss Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities. Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. 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 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 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 financial statements. The amount of 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. Fair Value Measurements Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 — Quoted market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities. • Level 2 — Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, and its term loan (Note 9). The estimated fair value of the Company’s financial instruments approximates their carrying values. Net Loss Per Share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net 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 on their respective rights to receive dividends as if all income for the period had been distributed. The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends, but do not contractually require the holders of such shares to participate in the losses of the Company. Accordingly, in periods in which the Company reports a net loss or a net loss attributable to common stockholders resulting from preferred stock dividends, net losses are not allocated to participating securities. In periods of net loss, the Company does not increase its net loss attributable to common stockholders by accreting dividends on preferred stock, as the dividends are not cumulative under the terms of the preferred stock. As of December 31, 2016, there were no shares of preferred stock issued and outstanding. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including outstanding stock options and unvested restricted common stock. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common stock. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014. Segment Data The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a specialty pharmaceutical company focused on the development and commercialization of novel, local therapies. No revenue has been generated since inception, and all assets are held in the United States. Recently Issued and Adopted Accounting Pronouncements In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued Accounting Standards Update 2015-14, Revenue from Contracts with Customers: Deferral of the Effective Date. This latest standard defers the effective date of revenue standard ASU 2014-09 by one year and permits early adoption on a limited basis. Since the Company has not generated revenue to date, this guidance will only impact future periods, if any, when revenue is earned. This update will replace existing revenue recognition guidance under GAAP when it becomes effective for the Company beginning January 1, 2018, with early adoption permitted in the first quarter of 2017. The updated standard will permit the use of either the retrospective or cumulative effect transition method. The Company is adopting this guidance as of January 1, 2017 and is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s future financial statements. In August 2014, the FASB issued accounting guidance for the disclosure of uncertainties related to an entity’s ability to continue as a going concern. The new standard requires management to perform an assessment at interim and annual periods as to the entity’s ability to continue as a going concern and provides specific disclosure guidance. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2016 and early adoption is permitted. The Company has included disclosures regarding its ability to continue as a going concern in the Company’s annual financial statements dated December 31, 2016. In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases In March 2016, the FASB released ASU 2016-09, which amends ASC Topic 718, Compensation-Stock Compensation, to require changes to several areas of employee share-based payment accounting in an effort to simplify share-based reporting. The update revises requirements in the following areas: minimum statutory withholding, accounting for income taxes, forfeitures, and intrinsic value accounting for private entities. For public companies, the new rules will become effective for annual reporting periods beginning after December 15, 2016, and interim reporting periods within such annual period. The Company adopted this guidance beginning on January 1, 2017. Upon adoption, the Company will no longer record stock compensation expense net of forfeitures. In August 2016, the FASB issued ASU 2016-15, Statement of cash flows (Topic 230), to increase the consistency of presentation in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. ASU 2016-15 will become effective for fiscal years, and the interim periods within those years, beginning after December 15, 2017. The Company is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements.

Fair Value of Financial Assets

Fair Value of Financial Assets	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	4. Fair Value of Financial Assets The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2016 and 2015 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 9,830,127 $ — $ 9,830,127 Marketable securities — 179,413,509 — 179,413,509 $ — $ 189,243,636 $ — $ 189,243,636 Fair Value Measurements as of December 31, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 61,533,797 $ — $ 61,533,797 Marketable securities — 55,659,930 — 55,659,930 $ — $ 117,193,727 $ — $ 117,193,727 As of December 31, 2016 and 2015, the Company’s cash equivalents that are invested in money market funds are valued based on Level 2 inputs. The Company measures the fair value of marketable securities using Level 2 inputs and primarily relies on quoted prices in active markets for similar marketable securities. During the years ended December 31, 2016 and 2015, there were no transfers between Level 1, Level 2 and Level 3. Amortization and accretion of discounts and premiums are recorded in other income. As outlined in Note 9, the 2013 term loan with MidCap Financial SBIC, LP (“2013 term loan”) and 2015 term loan with MidCap Financial Trust (“2015 term loan”), outstanding under the Company’s credit and security agreements, are reported at their carrying value in the accompanying consolidated balance sheet. The Company determined the fair value of the term loans using an income approach which utilizes a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk. The term loans were valued using Level 2 inputs as of December 31, 2016 and December 31, 2015. The result of the calculation yielded a fair value that approximates carrying value.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Marketable Securities	5. Marketable Securities As of December 31, 2016 and 2015, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2016 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial Paper $ 7,769,334 $ — $ — $ 7,769,334 U.S.Government obligations $ 75,523,761 $ 5,029 $ (11,860 ) $ 75,516,930 Corporate Bonds $ 96,192,720 $ 811 $ (66,286 ) $ 96,127,245 $ 179,485,815 $ 5,840 $ (78,146 ) $ 179,413,509 December 31, 2015 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds 55,756,581 3,783 (100,434 ) 55,659,930 $ 55,756,581 $ 3,783 $ (100,434 ) $ 55,659,930 At December 31, 2016, marketable securities consisted of $174,688,468 of investments that mature within twelve months and $4,725,041 of investments that mature within fifteen months. At December 31, 2015, marketable securities consisted of $48,302,507 of investments that mature within twelve months and $7,357,423 of investments that mature within fifteen.

Prepaid Expenses, Other Current

Prepaid Expenses, Other Current Assets, and Other Assets	12 Months Ended
	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses, Other Current Assets, and Other Assets	6. Prepaid Expenses, Other Current Assets, and Other Assets Prepaid expenses and other current assets and other assets consisted of the following as of December 31, 2016 and 2015: December 31, 2016 2015 Prepaid expenses $ 1,085,198 $ 135,002 Security Deposits 2,099,029 182,009 Interest receivable on marketable securities 604,794 441,766 Employee advances 711 2,095 Total prepaid expenses and other current assets $ 3,789,732 $ 760,872 On December 1, 2016, Flexion paid a refundable NDA fee in the amount of $2,038,100.00 to the FDA. The Company evaluated each of the published criteria to qualify for a waiver and concluded all criteria were met and thus, obtaining a refund of the fee was probable. As of December 31, 2016 the NDA fee was classified as a deposit in other current assets.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Property and Equipment, Net	7. Property and Equipment, Net Property and equipment, net, as of December 31, 2016 and 2015 consisted of the following: December 31, (In thousands) 2016 2015 Computer and office equipment $ 572,901 $ 393,148 Manufacturing equipment 10,098,577 2,533,737 Furniture and fixtures 402,301 290,577 Software 433,838 341,906 Leasehold improvements 278,288 239,456 Construction—in progress 1,253,738 4,133,893 13,039,643 7,932,717 Less: Accumulated depreciation (1,376,084 ) (490,240 ) Total property and equipment, net $ 11,663,559 $ 7,442,477 Depreciation expense for the years ended December 31, 2016, 2015 and 2014, was $1,150,961, $238,153, and $120,341, respectively. During the years ended December 31, 2016 and 2015, $2,670,413 and $166,335 of property and equipment was disposed of, resulting in a loss of $2,283,343 and $149,983, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	8. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities at December 31, 2016 and 2015 consisted of the following: December 31, (In thousands) 2016 2015 Research and Development $ 1,604,810 $ 2,059,800 Payroll and other employee-related expenses 3,393,201 1,648,447 Professional services fees 926,550 503,453 Interest expense 161,458 80,729 Other 159,214 74,131 Total accrued expenses and other current liabilities $ 6,245,233 $ 4,366,560

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-term Debt	9. Long-term Debt On January 3, 2013, the Company entered into a credit and security agreement with MidCap Financial SBIC, LP (“MidCap”) under which it immediately borrowed $5,000,000 as a term loan (“2013 term loan”). The term loan accrued interest monthly at an interest rate of 8% per annum and had a term of 45 months. As the term loan had a 15-month interest-only period, the term loan principal balance, along with any accrued interest, was to be paid in 30 equal monthly installments beginning April 1, 2014 and ending September 1, 2016. In addition to these principal payments, the Company was required to make a payment of $175,000 to the lender on September 1, 2016, which was being accreted to the carrying value of the debt using the effective interest rate method. On March 31, 2015, the Company paid MidCap $3,236,019, representing the outstanding principal of the debt along with accrued interest as of that date, the $175,000 final payment, a prepayment fee of $30,000 and associated legal expenses to satisfy the Company’s obligation under the credit and security agreement. In connection with the credit and security agreement dated January 3, 2013, the Company incurred total debt issuance costs of $61,876. The Company was amortizing these debt issuance costs over the estimated term of the debt using the straight-line method. On March 31, 2015, the Company paid off the debt, including $28,875 of debt issuance costs incurred. Prior to the debt repayment, the term loan outstanding under the Company’s credit and security agreement with MidCap was reported at its carrying value in the accompanying consolidated balance sheets. The Company determined the fair value of the term loan using an income approach, utilizing a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk. The term loan was valued using Level 2 inputs as of December 31, 2014. The result of the calculation yielded a fair value that approximated carrying value. On August 4, 2015, the Company entered into a credit and security agreement with MidCap Financial Trust, as administrative agent, MidCap Financial Funding XIII Trust and Silicon Valley Bank, as lenders, (the “Lenders”), to borrow up to $30,000,000 in term loans, (“2015 term loan”). The Company concurrently borrowed $15,000,000 under an initial term loan. The remaining $15,000,000 under the facility may be drawn down in the form of a second term loan at the Company’s option through September 2016, subject to the Company’s receipt of positive Phase 3 Zilretta clinical trial data meeting the trial’s primary endpoint which is sufficient to file a New Drug Application (NDA) for Zilretta, as well as other customary conditions for funding. The Company granted the Lenders a security interest in substantially all of its personal property, rights and assets, other than intellectual property, to secure the payment of all amounts owed under the credit facility. The Company also agreed not to encumber any of its intellectual property without the Lenders’ prior written consent. The Company must maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5 percent for so long as the Company maintains any cash or cash equivalents in non-secured bank accounts. The credit and security agreement also contains certain representations, warranties, and covenants of the Company, as well as a material adverse event clause. As of December 31, 2016, the Company was compliant with all covenants and there were no material adverse events. On July 22, 2016, the Company borrowed the remaining $15,000,000 under the credit and security agreement, in the form of a second term loan after receiving positive Phase 3 Zilretta clinical trial data meeting the trial’s primary endpoint and which is sufficient to file an NDA for Zilretta. The second term loan is subject to the same credit terms as the initial term loan under the facility. Borrowings under the credit facility accrue interest monthly at a fixed interest rate of 6.25% per annum. Following an interest-only period of 19 months, principal will be due in 36 equal monthly installments commencing March 1, 2017 and ending February 1, 2020 (the “maturity date”). Upon the maturity date, the Company will be obligated to pay a final payment equal to 9% of the total principal amounts borrowed under the facility. The final payment amount is being accreted to the carrying value of the debt using the effective interest rate method. As of December 31, 2016, the carrying value of the term loan was $30,532,783 of which $9,133,991 is classified as current portion of long-term debt and $21,398,792 as long-term debt on the consolidated balance sheets as of December 31, 2016. In connection with entering into a credit and security agreement dated August 4, 2015, the Company incurred debt acquisition costs of $100,760. The Company is amortizing these debt issuance costs over the estimated term of the debt using the straight-line method, which approximates the effective interest method. Total amortization expense of the debt issuance costs arising from the credit and security agreement dated August 4, 2015 was $36,607 and $12,228 for the year ended December 31, 2016 and 2015. Total amortization expense of the debt issuance costs associated with the credit and security agreement dated January 3, 2013 was $16,500 for the year ended December 31, 2014. As of December 31, 2016, annual payments due under the Company’s 2015 term loan are as follows Year Aggregate Minimum Payments 2017 10,035,734 2018 11,082,176 2019 10,448,495 2020 4,383,475 Total $ 35,949,880 Less interest (2,717,097 ) Less final payment (2,700,000 ) Total $ 30,532,783

Preferred and Convertible Prefe

Preferred and Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2016
Preferred and Convertible Preferred Stock	11. Preferred Stock On February 17, 2014, the Company filed an amended and restated Certificate of Incorporation (the “Restated Certificate”) in connection with the closing of the Company’s initial public offering. As of December 31, 2016, under the Restated Certificate, the Company is authorized to issue 10,000,000 shares of preferred stock with a par value of $0.001 per share.
Convertible Preferred Stock [Member]
Preferred and Convertible Preferred Stock	10. Convertible Preferred Stock As of December 31, 2013, the Company’s Certificate of Incorporation, as amended, authorized the Company to issue 73,780,250 shares of preferred stock with a par value of $0.001 per share. The Company issued Series A and Series B Convertible Preferred Stock (collectively, the “Convertible Preferred Stock”). Of the 73,780,250 authorized shares, 72,780,250 shares were issued; 55,043,464 of the issued shares were designated as Series A Convertible Preferred Stock (“Series A Preferred Stock”) and 18,736,786 were designated as Series B Convertible Preferred Stock (“Series B Preferred Stock”). On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a 1-for-8.13 reverse stock split of the Company’s common stock and a proportional adjustment to the existing conversion ratios for each series of Convertible Preferred Stock. On January 27, 2014, the Company and holders of a majority of the Company’s Series A Convertible Preferred Stock and Series B Convertible Preferred Stock entered into a Conversion, Amendment and Waiver Agreement. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding. The following is a summary of the Company’s Convertible Preferred Stock as of December 31, 2013: December 31, 2013 Preferred Shares Authorized Preferred Shares Issued and Outstanding Carrying Value Liquidation Preference Common Stock Issuable Upon Conversion Series A Preferred Stock 55,043,464 55,043,464 $ 54,917,735 $ 55,043,464 6,770,411 Series B Preferred Stock 18,736,786 17,736,786 19,888,478 20,000,000 2,181,646 73,780,250 72,780,250 $ 74,806,213 $ 75,043,464 8,952,057 The holders of the preferred stock had the following rights and preferences: Voting Rights The holders of preferred stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote. Each preferred stockholder was entitled to the number of votes equal to the number of shares of common stock into which each preferred share is convertible at the time of such vote. Liquidation and Conversion The Company does not currently have any holders of securities with either liquidation preferences or conversion rights as the Company’s has no shares of preferred stock issued or outstanding at December 31, 2016. Series A and Series B preferred stock issued prior to the closing of the Company’s IPO on February 18, 2014 were converted into shares of common stock on a 0.123001-for-1 basis.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Operating Leases In May 2013, the Company entered into a lease for office space in Burlington, Massachusetts. The lease is for a 42-month term with minimum monthly lease payments beginning at $17,588 per month and escalating over the lease term. The Company provided a letter of credit to the lessor in the amount of $98,000 as a security deposit pursuant to the lease agreement to secure its obligations under the lease. During 2015, this letter of credit was reduced to $50,000 pursuant to the original lease agreement. In July 2015 the Company entered into a first amendment to its existing lease for approximately 4,700 square feet of additional office space (the “Additional Space”) in Burlington, Massachusetts, as well as approximately 6,700 square feet of temporary space to be leased prior to the delivery of the Additional Space (which is anticipated to be delivered on May 1, 2016). The amendment extended the term of the original lease through October 31, 2019, contemporaneous with the Additional Space, and also provided the Company with an option to lease an additional 5,400 square feet of office space (the “Option Space”). On September 30, 2015, the Company exercised its option for the Option Space. In addition, the Company has the option to extend the term of a portion or the entire lease space for one additional three-year period. The Company may terminate the amendment for convenience with nine months’ notice upon the occurrence of certain events connected to its clinical stage programs. In addition to the base rent, the Company is also responsible for its share of operating expenses and real estate taxes. On September 21, 2016 the Company entered into a second amendment to its existing lease for approximately 6,748 additional square feet of rented space located in Burlington, Massachusetts. The lease began October 1, 2016 and expires on October 31, 2017. During October 2016, the Company’s lease payment for this additional space was $18,300 in incremental rent. Beginning in November 2016, through October 2017, the Company’s lease payments for the additional space will increase to $19,000 per month in incremental rental cash outflow. The Company incurred rent expense of $668,350, $373,202, and $242,946 for the years ended December 31, 2016, 2015 and 2014, respectively. Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments 2017 924,806 2018 758,299 2019 647,106 Total $ 2,330,211 Manufacturing and Supply Agreement with Patheon U.K. Limited In July 2015, the Company and Patheon U.K. Limited (“Patheon”) entered into a Manufacturing and Supply Agreement (the “Manufacturing Agreement”) and Technical Transfer and Service Agreement (the “Technical Transfer Agreement”) for the manufacture of Zilretta, the Company’s lead program, which is an intra-articular (IA), extended-release steroid for the treatment of osteoarthritis. Patheon has agreed in the Technical Transfer Agreement to undertake certain transfer activities and construction services needed to prepare Patheon’s United Kingdom facility for the commercial manufacture of Zilretta in dedicated manufacturing suites. The Company will provide Patheon with certain equipment and materials necessary to manufacture Zilretta and it will pay Patheon a monthly fee for such activities and reimburse Patheon for certain material, equipment and miscellaneous expenses and additional services. The initial term of the Manufacturing Agreement is 10 years from approval by the U.S. Food and Drug Administration, or FDA, of the Patheon manufacturing suites for Zilretta. The Company will pay a monthly base fee to Patheon for the operation of the manufacturing suites and a per product fee for each vial based upon a forecast of commercial demand. The Company will also reimburse Patheon for purchases of materials and equipment made on its behalf, certain nominal expenses and additional services. The Manufacturing Agreement will remain in full effect unless and until it expires or is terminated. Upon termination of the Manufacturing Agreement (other than termination by Flexion in the event that Patheon does not meet the construction and manufacturing milestones or for a breach by Patheon), Flexion will be obligated to pay for the costs incurred by Patheon associated with the removal of our manufacturing equipment and for Patheon’s termination costs up to a capped amount. Future minimum payments under the Company’s agreed obligations are as follows: Year Ending December 31, 2017 5,529,600 2018 6,266,880 2019 7,372,800 2020 and thereafter 44,236,800 Total $ 63,406,080 Evonik Supply Agreement In November 2016, the Company entered into a Supply Agreement with Evonik Corporation (“Evonik”) for the purchase of PLGA which is used in the manufacturing of potential clinical and commercial supply of Zilretta. Pursuant to the Supply Agreement, Flexion is obligated to submit rolling monthly forecasts to Evonik for PLGA supply, a portion of which will constitute binding orders. In addition, Flexion agreed to certain minimum purchase requirements, which decrease over time, and which do not apply (i) during periods in which Evonik is in material breach of the Supply Agreement or is unable to perform its obligations due to a force majeure event, (ii) with respect to orders that Evonik is unable to supply in excess of binding orders, (iii) for orders Evonik is unable to timely deliver or does not deliver conforming product and provides a credit for such order, or (iv) during an uncured material quality failure by Evonik. Flexion agreed to purchase PLGA batches at a specified price per gram in U.S. dollars, subject to adjustment from time to time, including due to changes in price indices and in the event the initial term of the Supply Agreement is extended. The total term of the agreement is five years. Upon termination of the Supply Agreement (other than termination due to the bankruptcy of either Evonik or Flexion) Flexion is obligated to pay the costs associated with the binding supply forecast provided to Evonik. The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties. Southwest Research Institute License Agreement On July 25, 2014, the Company entered into an exclusive worldwide license agreement with Southwest Research Institute (“SwRI”) with respect to the use of SwRI’s proprietary microsphere manufacturing technologies for certain steroids formulated with PLGA, including Zilretta. Under the agreement, the paid an upfront fee of $120,000 to SwRI. In February 2017, Flexion executed an agreement with SwRI to transfer manufacturing equipment to SwRI in consideration for SwRI deeming the additional milestone payment to have been fully paid by Flexion.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	14. Stock-Based Compensation 2013 Equity Incentive Plan On January 27, 2014, the Company’s stockholders approved the 2013 Equity Incentive Plan (the “2013 Plan”), which became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock was priced for its initial public offering. Prior to the effective date of the 2013 Plan, the Company granted stock-based awards pursuant to the 2009 Stock Incentive Plan (the “2009 Plan), which had similar features to the 2013 Plan. The 2013 Plan provides for the grant of incentive stock options (“ISOs”), non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance-based stock awards, and other forms of equity compensation. Initially, the maximum number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2013 Plan was 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting). Additionally, the number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and continuing through and including January 1, 2023, by 4% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2013 Plan is 4,684,989 shares. As of December 31, 2016, there were 508,882 options outstanding under the 2009 Plan. The Company currently grants stock-based awards pursuant to the 2013 Plan. As of December 31, 2016, 1,287,381 shares were available for future issuance under the 2013 Plan. Stock option vesting typically occurs over four years for employees and directors and is at the discretion of the board of directors. Options granted have a maximum term of up to 10 years. As of December 31, 2016, there were 2,759,593 options outstanding under the 2013 Plan. Stock Options During the years ended December 31, 2016, 2015 and 2014, the Company granted stock options for the purchase of 1,816,575, 657,250, and 727,575 shares of common stock, respectively, to certain employees and directors. The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months, except for annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months and whose annual grants vest in equal monthly installments during the 12-month period following the grant date, pursuant to the Company’s Non-Employee Director Compensation Policy. Awards typically expire after 10 years. Stock options for the purchase of 403,382 shares of common stock were granted in 2012; of these, 264,944 were granted with performance-based vesting conditions to certain executives. The options were to vest in the event of a corporate transaction with the amount to vest contingent upon the transaction. The grant date fair value of these options was $236,940. In September 2012, performance-based options for the purchase of 18,450 shares of common stock were forfeited. No expense was recognized related to these options for the year ended December 31, 2012 as the performance conditions were not considered probable of achievement at December 31, 2012. On July 16, 2013, in connection with the Company’s proposed initial public offering, the board of directors exercised its election to change the vesting conditions of these stock options from performance-based vesting to time-based vesting. As a result, these stock options now vest over a four-year period commencing effective August 29, 2012. The change in the vesting conditions was accounted for as a modification of these stock options. The modification resulted in an aggregate increase in the fair value of the options of $2,185,729, of which $481,729 was recorded as stock-based compensation expense on the modification date, July 16, 2013, and $1,704,000 was unrecognized stock-based compensation expense, which was recognized over the remaining vesting terms of the options through August 2016. 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. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of its publicly-traded 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 non-employees 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 relevant data used to determine the value of the stock option grants for the years ended December 31, 2016, 2015 and 2014 is as follows: December 31, 2016 2015 2014 Risk-free interest rates 0.74-1.75% 1.49-1.92% 1.54-2.04% Expected dividend yield 0.00% 0.00% 0.00% Expected term (in years) 5.6 6.0 6.0 Expected volatility 67.3-99.9% 76.4-83.9% 61.9-68.0% The following table summarizes stock option activity for the year ended December 31, 2016: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2015 1,657 $ 14.28 Granted 1,816 15.43 Exercised (30 ) 5.53 Cancelled (175 ) 17.22 Outstanding as of December 31, 2016 3,268 $ 14.84 Options vested and expected to vest at December 31, 2016 2,275 $ 14.47 Options exercisable at December 31, 2016 1,174 $ 12.31 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. A total of 30,195, 109,441, and, 141 options were exercised during the years ended December 31, 2016, 2015 and 2014, respectively. The aggregate intrinsic value of stock options exercised was $236,889, $1,584,657, and $1,476,861 for the years ended December 31, 2016, 2015 and 2014, respectively. At December 31, 2016, 2015 and 2014 the Company had options for the purchase of 3,268,475, 1,657,225, and 1,289,082 shares of common stock outstanding, with a weighted average remaining contractual term of 7.8, 7.9, and 8.1 years, respectively, and with a weighted average exercise price of $14.84, $14.28, and $10.26 per share, respectively. At December 31, 2016, 2015 and 2014 there were options for the purchase of 1,173,671, 728,621, and 450,109 shares of common stock exercisable under these stock option awards, with a weighted average remaining contractual life of 6.7, 6.9, 6.7 years , respectively, and an aggregate intrinsic value of $8,745,505, $7,714,057, and 7,489,976, respectively. The weighted average grant date fair value of options granted during the years ended December 31, 2016, 2015 and 2014 was $11.50, $15.08, and $9.99, respectively. Restricted Common Stock The Company’s 2013 Equity Incentive Plan provides for the award of restricted stock. As of December 31, 2016 and 2015, there were no shares related to restricted stock awards that were unvested and subject to repurchase. As of December 31, 2016 and 2015, there were no shares related to restricted stock awards that were unvested and subject to repurchase. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options, restricted stock and shares purchased under the employee stock purchase plan for the years ended December 31, 2016, 2015 and 2014 as follows: Year Ended December 31, (In thousands) 2016 2015 2014 Research and development $ 2,340,847 $ 1,412,153 $ 684,511 General and administrative 4,428,957 3,170,526 1,766,068 $ 6,769,804 $ 4,582,679 $ 2,450,579 As of December 31, 2016, unrecognized stock-based compensation expense for stock options outstanding was $15,932,348 which is expected to be recognized over a weighted average period of 3.04 years. Restricted Stock Units On January 4, 2016, the Company granted restricted common stock units with performance-based vesting conditions to certain executives. The restricted stock units vest, and the underlying shares of common stock become deliverable, in the event the Company receives approval from the U.S. Food and Drug Administration of a new drug application for Zilretta (the “Milestone” Employee Stock Purchase Plan On January 27, 2014, the Company’s stockholders approved the Employee Stock Purchase Plan. A total of 209,102 shares of common stock were reserved for issuance under this plan. The Employee Stock Purchase Plan became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock was priced for its initial public offering. During the year ended December 31, 2016 and 2015, 26,880 and 20,896 shares, respectively, were purchased by employees under the plan. Additionally, the number of shares of common stock reserved for issuance under the Employee Stock Purchase Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and continuing through and including January 1, 2023, by 1% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Loss Per Share	15. Net Loss Per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2016, 2015 and 2014: Year ended December 31, (In thousands) 2016 2015 2014 Numerator: Net loss $ (71,893,959 ) $ (46,315,451 ) $ (27,313,664 ) Net loss: $ (71,893,959 ) $ (46,315,451 ) $ (27,313,664 ) Denominator: Weighted average common shares outstanding, basic and diluted 25,296,527 21,497,119 13,893,961 Net loss per share, basic and diluted $ (2.84 ) $ (2.15 ) $ (1.97 ) Stock options for the purchase of 2,532,675, 1,655,394, and 1,185,253 weighted average shares of common stock were excluded from the computation of diluted net loss per share attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014, respectively, because those options had an anti-dilutive impact due to the net loss attributable to common stockholders incurred for those periods.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The Company has generated losses since inception. Accordingly, there is no tax provision or benefit for the years ended December 31, 2016, and 2015, respectively other than $14,267 related to federal tax on the Company's Security Corporation. 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, 2016 2015 2014 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State taxes, net of federal benefit 5.0 4.9 4.7 Federal and state research and development tax credits 2.7 4.7 3.5 Change in deferred tax asset valuation allowance (40.0 ) (45.4 ) (37.8 ) Other (1.7 ) 1.8 (4.4 ) Effective income tax rate — % — % — % The Company’s net deferred tax assets consisted of the following: December 31, 2016 2015 Net operating loss carryforwards $ 36,880,381 $ 24,226,949 Research and development tax credit carryforwards 7,077,546 5,368,524 Accruals and other temporary differences 4,896,980 2,339,444 Capitalized research and development expenses, net 34,579,391 22,838,452 Total deferred tax assets 83,434,298 54,773,369 Valuation allowance (83,434,298 ) (54,773,369 ) Net deferred tax asset $ — $ — As of December 31, 2016, the Company had federal and state net operating loss carryforwards of approximately $94,659,000 and $88,945,478, respectively, which begin to expire in 2029 for federal purposes and in 2030 for state purposes. In addition, the Company had federal and state research and development tax credit carryforwards of approximately $5,202,362 and $2,841,187, respectively, available to reduce future tax liabilities, which begin to expire in 2029 for federal purposes and 2025 for state purposes. Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and capitalized research and development expenses. Management has considered the Company’s history of cumulative net losses incurred since inception, as well as its lack of commercialization of any products or generation of any revenue from product sales since inception, and determined that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. As a result, a full valuation allowance has been established at December 31, 2016 and 2015. Section 382 of the Internal Revenue Code of 1986, as amended (“Section 382”), contains rules that limit the ability of a company that undergoes an ownership change to utilize its net operating losses (“NOLs”) and tax credits existing as of the date of such ownership change. Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company’s stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company. During the quarter ended June 30, 2014, the Company completed a Section 382 study through February 11, 2014. The results of this study showed that as of February 11, 2014, one historical ownership change within the meaning of section 382 had occurred in 2009. In addition, the company further updated its existing section 382 study through June 8, 2015. The results of this study showed that as of June 8, 2015, one additional historical ownership change within the meaning of section 382 took place. The company believes that none of its NOLs will expire as a result of these identified changes in ownership, assuming sufficient future taxable income and no future limitations. Subsequent ownership changes as defined by section 382 may potentially limit the amount of net operating loss carry forward that could be utilized annually to offset future taxable income. The company has generated losses since inception and therefore has recorded no income tax benefits for those losses due to its uncertainty of realizing a benefit from those losses. During the quarter ended September 30, 2016, the Company completed a Section 382 study through June 30, 2016. The results of this study showed that one historical ownership change within the meaning of Section 382 had occurred on June 8, 2016 in connection with the Company’s 2016 offering. Although a Section 382 change in ownership has occurred, it is not currently anticipated that a portion of the Company’s NOLs will expire unutilized as a result of this Section 382 limitation. Any subsequent ownership changes as defined by Section 382 may potentially limit the amount of our NOL carryforwards that could be utilized annually to offset any future taxable income. The Company has generated losses since inception and therefore has recorded no income tax benefits for those losses due to its uncertainty of realizing a benefit from those losses. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2016 and 2015 were as follows: Year Ended December 31, 2016 2015 2014 Valuation allowance as of beginning of year $ (54,773,369 ) $ (33,825,873 ) $ (23,493,416 ) Decreases recorded as benefit to income tax provision 4,771,490 2,857,886 2,020,565 Increases recorded to income tax provision (33,432,419 ) (23,805,382 ) (12,353,022 ) Valuation allowance as of end of year $ (83,434,298 ) $ (54,773,369 ) $ (33,825,873 ) In each reporting period, the Company considers whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. No liabilities for unrecognized tax benefits were recorded as of December 31, 2016 and 2015. 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 tax examinations. The Company’s tax years are still open under statute from 2013 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 resolution of tax matters is not expected to have a material effect on the Company’s consolidated financial statements.

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	17. 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, 2016 June 30, 2016 September 30, 2016 December 31, 2016 Operating expenses $ 16,673 $ 14,120 $ 17,435 $ 21,552 Net loss (16,815 ) (14,185 ) (17,782 ) (23,112 ) Net loss per common share—basic and diluted $ (0.78 ) $ (0.63 ) $ (0.65 ) $ (0.79 ) Weighted average common shares—basic and diluted 21,570 22,666 27,524 29,347 Three Months Ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 Operating expenses $ 9,015 $ 12,544 $ 11,026 $ 13,479 Net loss (9,174 ) (12,437 ) (11,136 ) (13,569 ) Net loss per common share—basic and diluted $ (0.43 ) $ (0.58 ) $ (0.52 ) $ (0.63 ) Weighted average common shares—basic and diluted 21,451 21,475 21,507 21,554

Summary of Significant Accoun23

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management.
Consolidation	Consolidation The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions. In addition, Flexion Therapeutics, Inc. is registered to do business in the United Kingdom through its branch office located in Swindon, United Kingdom.
U.S. Government Grant	U.S. Government Grant The Company performs research and development for a U.S. Government agency under a cost reimbursable grant for clinical development of Zilretta. The related costs incurred under the grant are included in research and development expense in the statement of operations. Reimbursements were recorded as an offset to R&D expense when invoices for allowable costs were prepared and submitted to the Department of Defense. Due to challenges of enrolling military personnel with post-traumatic knee OA, we discontinued the Phase 2 trial and terminated the grant. Payments under cost reimbursable grants with agencies of the U.S. Government are provisional payments subject to adjustment upon audit by the U.S. government.
Accounts Receivable	Accounts Receivable Accounts receivable represents allowable costs under the Company’s U.S. Government agency grant for which the Company has not yet received reimbursement. The Company invoices the government on a quarterly basis for reimbursable costs under the grant. Reimbursable costs that have not been invoiced on the last day of the quarter are recorded as unbilled accounts receivable. As of December 31, 2016 and 2015, there were unbilled accounts receivable of $0 and $95,285, respectively.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper.
Marketable Securities	Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income.
Restricted Cash	Restricted Cash The Company purchased a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank. In 2016, the Company placed $400,000 in an account at a commercial bank to further collateralize the credit card account. These balances are classified as long-term restricted cash as of December 31, 2016 and 2015. In addition the Company posted a letter of credit to the lessor of the Company’s Burlington facility as a security deposit pursuant to the lease agreement. As of December 31, 2016 and 2015, there was $50,000 as long-term restricted cash related to the aforementioned lease agreement.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Property and equipment includes construction-in-progress, which is not yet in service, and is estimated to have a useful life of 7 years once placed into service.
Foreign Currencies	Foreign Currencies The Company maintains a bank account designated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations within other income and expenses, and were not significant in any period.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews its long-lived assets, including property and equipment, for impairment 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.
Debt Issuance Costs, net	Debt Issuance Costs, net As of December 31, 2016 and 2015, the carrying value of debt issuance costs was $100,760 and $124,041, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $36,607, $41,103 and $16,500 respectively, of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the years ended December 31, 2016, 2015 and 2014.
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’ deficit as a reduction of additional paid-in capital generated as a result of the offering. If the equity financing is no longer considered probable of being consummated, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. The Company completed its initial public offering in February 2014 and recorded deferred offering costs of $1,623,540 as a reduction to additional paid-in capital. The Company did not record any deferred offering costs during the year ended December 31, 2016 or 2015.
Research and Development	Research and Development Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.
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 at the fair value at the date of 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 non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees 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 the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified. The Company recognizes compensation expense only for 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. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.
Concentration of Credit Risk and Significant Suppliers	Concentration of Credit Risk and Significant Suppliers Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and corporate bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. 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 completely dependent on third-party manufacturers and product suppliers for research and potential commercial activities. In particular, the Company relies and expects to continue to rely on a limited number of manufacturers and relies on them to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials, and if Zilretta is approved, for commercial supply. These programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients.
Comprehensive Loss	Comprehensive Loss Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. 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 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 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 financial statements. The amount of 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.
Fair Value Measurements	Fair Value Measurements Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 — Quoted market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities. • Level 2 — Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, and its term loan (Note 9). The estimated fair value of the Company’s financial instruments approximates their carrying values.
Net Loss Per Share	Net Loss Per Share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net 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 on their respective rights to receive dividends as if all income for the period had been distributed. The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends, but do not contractually require the holders of such shares to participate in the losses of the Company. Accordingly, in periods in which the Company reports a net loss or a net loss attributable to common stockholders resulting from preferred stock dividends, net losses are not allocated to participating securities. In periods of net loss, the Company does not increase its net loss attributable to common stockholders by accreting dividends on preferred stock, as the dividends are not cumulative under the terms of the preferred stock. As of December 31, 2016, there were no shares of preferred stock issued and outstanding. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including outstanding stock options and unvested restricted common stock. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common stock. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014.
Segment Data	Segment Data The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a specialty pharmaceutical company focused on the development and commercialization of novel, local therapies. No revenue has been generated since inception, and all assets are held in the United States.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued Accounting Standards Update 2015-14, Revenue from Contracts with Customers: Deferral of the Effective Date. This latest standard defers the effective date of revenue standard ASU 2014-09 by one year and permits early adoption on a limited basis. Since the Company has not generated revenue to date, this guidance will only impact future periods, if any, when revenue is earned. This update will replace existing revenue recognition guidance under GAAP when it becomes effective for the Company beginning January 1, 2018, with early adoption permitted in the first quarter of 2017. The updated standard will permit the use of either the retrospective or cumulative effect transition method. The Company is adopting this guidance as of January 1, 2017 and is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s future financial statements. In August 2014, the FASB issued accounting guidance for the disclosure of uncertainties related to an entity’s ability to continue as a going concern. The new standard requires management to perform an assessment at interim and annual periods as to the entity’s ability to continue as a going concern and provides specific disclosure guidance. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2016 and early adoption is permitted. The Company has included disclosures regarding its ability to continue as a going concern in the Company’s annual financial statements dated December 31, 2016. In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases In March 2016, the FASB released ASU 2016-09, which amends ASC Topic 718, Compensation-Stock Compensation, to require changes to several areas of employee share-based payment accounting in an effort to simplify share-based reporting. The update revises requirements in the following areas: minimum statutory withholding, accounting for income taxes, forfeitures, and intrinsic value accounting for private entities. For public companies, the new rules will become effective for annual reporting periods beginning after December 15, 2016, and interim reporting periods within such annual period. The Company adopted this guidance beginning on January 1, 2017. Upon adoption, the Company will no longer record stock compensation expense net of forfeitures. In August 2016, the FASB issued ASU 2016-15, Statement of cash flows (Topic 230), to increase the consistency of presentation in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. ASU 2016-15 will become effective for fiscal years, and the interim periods within those years, beginning after December 15, 2017. The Company is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements.

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Property Plant and Equipment Estimated Useful Lives	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2016 and 2015 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 9,830,127 $ — $ 9,830,127 Marketable securities — 179,413,509 — 179,413,509 $ — $ 189,243,636 $ — $ 189,243,636 Fair Value Measurements as of December 31, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 61,533,797 $ — $ 61,533,797 Marketable securities — 55,659,930 — 55,659,930 $ — $ 117,193,727 $ — $ 117,193,727

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of December 31, 2016 and 2015, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2016 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial Paper $ 7,769,334 $ — $ — $ 7,769,334 U.S.Government obligations $ 75,523,761 $ 5,029 $ (11,860 ) $ 75,516,930 Corporate Bonds $ 96,192,720 $ 811 $ (66,286 ) $ 96,127,245 $ 179,485,815 $ 5,840 $ (78,146 ) $ 179,413,509 December 31, 2015 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds 55,756,581 3,783 (100,434 ) 55,659,930 $ 55,756,581 $ 3,783 $ (100,434 ) $ 55,659,930

Prepaid Expenses, Other Curre27

Prepaid Expenses, Other Current Assets, and Other Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets and Other Assets	Prepaid expenses and other current assets and other assets consisted of the following as of December 31, 2016 and 2015: December 31, 2016 2015 Prepaid expenses $ 1,085,198 $ 135,002 Security Deposits 2,099,029 182,009 Interest receivable on marketable securities 604,794 441,766 Employee advances 711 2,095 Total prepaid expenses and other current assets $ 3,789,732 $ 760,872

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment, net, as of December 31, 2016 and 2015 consisted of the following: December 31, (In thousands) 2016 2015 Computer and office equipment $ 572,901 $ 393,148 Manufacturing equipment 10,098,577 2,533,737 Furniture and fixtures 402,301 290,577 Software 433,838 341,906 Leasehold improvements 278,288 239,456 Construction—in progress 1,253,738 4,133,893 13,039,643 7,932,717 Less: Accumulated depreciation (1,376,084 ) (490,240 ) Total property and equipment, net $ 11,663,559 $ 7,442,477

Accrued Expenses and Other Cu29

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities at December 31, 2016 and 2015 consisted of the following: December 31, (In thousands) 2016 2015 Research and Development $ 1,604,810 $ 2,059,800 Payroll and other employee-related expenses 3,393,201 1,648,447 Professional services fees 926,550 503,453 Interest expense 161,458 80,729 Other 159,214 74,131 Total accrued expenses and other current liabilities $ 6,245,233 $ 4,366,560

Long-term Debt (Tables)

Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Schedule of Annual Payments Due Under Term Loan	As of December 31, 2016, annual payments due under the Company’s 2015 term loan are as follows Year Aggregate Minimum Payments 2017 10,035,734 2018 11,082,176 2019 10,448,495 2020 4,383,475 Total $ 35,949,880 Less interest (2,717,097 ) Less final payment (2,700,000 ) Total $ 30,532,783

Preferred and Convertible Pre31

Preferred and Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Summary of Convertible Preferred Stock	The following is a summary of the Company’s Convertible Preferred Stock as of December 31, 2013: December 31, 2013 Preferred Shares Authorized Preferred Shares Issued and Outstanding Carrying Value Liquidation Preference Common Stock Issuable Upon Conversion Series A Preferred Stock 55,043,464 55,043,464 $ 54,917,735 $ 55,043,464 6,770,411 Series B Preferred Stock 18,736,786 17,736,786 19,888,478 20,000,000 2,181,646 73,780,250 72,780,250 $ 74,806,213 $ 75,043,464 8,952,057

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments under Operating Leases	Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments 2017 924,806 2018 758,299 2019 647,106 Total $ 2,330,211
Schedule of Future Minimum Payments under Contractual Obligations	Future minimum payments under the Company’s agreed obligations are as follows: Year Ending December 31, 2017 5,529,600 2018 6,266,880 2019 7,372,800 2020 and thereafter 44,236,800 Total $ 63,406,080

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants	The relevant data used to determine the value of the stock option grants for the years ended December 31, 2016, 2015 and 2014 is as follows: December 31, 2016 2015 2014 Risk-free interest rates 0.74-1.75% 1.49-1.92% 1.54-2.04% Expected dividend yield 0.00% 0.00% 0.00% Expected term (in years) 5.6 6.0 6.0 Expected volatility 67.3-99.9% 76.4-83.9% 61.9-68.0%
Summary of Stock Option Activity	The following table summarizes stock option activity for the year ended December 31, 2016: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2015 1,657 $ 14.28 Granted 1,816 15.43 Exercised (30 ) 5.53 Cancelled (175 ) 17.22 Outstanding as of December 31, 2016 3,268 $ 14.84 Options vested and expected to vest at December 31, 2016 2,275 $ 14.47 Options exercisable at December 31, 2016 1,174 $ 12.31
Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan	The Company recorded stock-based compensation expense related to stock options, restricted stock and shares purchased under the employee stock purchase plan for the years ended December 31, 2016, 2015 and 2014 as follows: Year Ended December 31, (In thousands) 2016 2015 2014 Research and development $ 2,340,847 $ 1,412,153 $ 684,511 General and administrative 4,428,957 3,170,526 1,766,068 $ 6,769,804 $ 4,582,679 $ 2,450,579

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2016, 2015 and 2014: Year ended December 31, (In thousands) 2016 2015 2014 Numerator: Net loss $ (71,893,959 ) $ (46,315,451 ) $ (27,313,664 ) Net loss: $ (71,893,959 ) $ (46,315,451 ) $ (27,313,664 ) Denominator: Weighted average common shares outstanding, basic and diluted 25,296,527 21,497,119 13,893,961 Net loss per share, basic and diluted $ (2.84 ) $ (2.15 ) $ (1.97 )

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective 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, 2016 2015 2014 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State taxes, net of federal benefit 5.0 4.9 4.7 Federal and state research and development tax credits 2.7 4.7 3.5 Change in deferred tax asset valuation allowance (40.0 ) (45.4 ) (37.8 ) Other (1.7 ) 1.8 (4.4 ) Effective income tax rate — % — % — %
Net Deferred Tax Assets	The Company’s net deferred tax assets consisted of the following: December 31, 2016 2015 Net operating loss carryforwards $ 36,880,381 $ 24,226,949 Research and development tax credit carryforwards 7,077,546 5,368,524 Accruals and other temporary differences 4,896,980 2,339,444 Capitalized research and development expenses, net 34,579,391 22,838,452 Total deferred tax assets 83,434,298 54,773,369 Valuation allowance (83,434,298 ) (54,773,369 ) Net deferred tax asset $ — $ —
Changes in Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2016 and 2015 were as follows: Year Ended December 31, 2016 2015 2014 Valuation allowance as of beginning of year $ (54,773,369 ) $ (33,825,873 ) $ (23,493,416 ) Decreases recorded as benefit to income tax provision 4,771,490 2,857,886 2,020,565 Increases recorded to income tax provision (33,432,419 ) (23,805,382 ) (12,353,022 ) Valuation allowance as of end of year $ (83,434,298 ) $ (54,773,369 ) $ (33,825,873 )

Quarterly Financial Data (una36

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Recurring Adjustments Necessary for Fair Statement	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, 2016 June 30, 2016 September 30, 2016 December 31, 2016 Operating expenses $ 16,673 $ 14,120 $ 17,435 $ 21,552 Net loss (16,815 ) (14,185 ) (17,782 ) (23,112 ) Net loss per common share—basic and diluted $ (0.78 ) $ (0.63 ) $ (0.65 ) $ (0.79 ) Weighted average common shares—basic and diluted 21,570 22,666 27,524 29,347 Three Months Ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 Operating expenses $ 9,015 $ 12,544 $ 11,026 $ 13,479 Net loss (9,174 ) (12,437 ) (11,136 ) (13,569 ) Net loss per common share—basic and diluted $ (0.43 ) $ (0.58 ) $ (0.52 ) $ (0.63 ) Weighted average common shares—basic and diluted 21,451 21,475 21,507 21,554

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007
Cash and cash equivalents and marketable securities	$ 210,328,883	$ 118,603,974

Financing Transactions - Additi

Financing Transactions - Additional Information (Detail)	Nov. 15, 2016USD ($)$ / sharesshares	Jun. 08, 2016USD ($)$ / sharesshares	Jun. 07, 2016$ / sharesshares	Dec. 17, 2014USD ($)$ / sharesshares	Feb. 18, 2014USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2014USD ($)shares	Dec. 31, 2015shares
Class of Stock [Line Items]
Net proceeds from follow-on public offering | $						$ 147,889,139	$ 162,137,580
Reverse stock split conversion ratio description						1-for-8.13 reverse stock split
Common stock, shares issued						31,667,469		21,570,395
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued						10,040,000	11,546,000
Common stock issued upon conversion of redeemable preferred stock					8,952,057
Reverse stock split conversion ratio						0.123
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued					5,750,000
Sale of common stock price per share | $ / shares					$ 13
Net proceeds from follow-on public offering | $					$ 67,200,000
Reverse stock split conversion ratio description						1-for-8.13 reverse stock split
IPO [Member] | Common Stock [Member]
Class of Stock [Line Items]
Reverse stock split conversion ratio					0.123
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Common stock issued upon conversion of redeemable preferred stock					8,952,057
Follow On Public Offering [Member]
Class of Stock [Line Items]
Common stock, shares issued	4,140,000	5,900,000	5,900,000	5,796,000
Sale of common stock price per share | $ / shares	$ 18	$ 14	$ 14	$ 17
Net proceeds from follow-on public offering | $	$ 70,100,000	$ 77,400,000		$ 92,200,000

Summary of Significant Accoun39

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 28, 2014
Summary Of Significant Accounting Policies [Line Items]
Unbilled accounts receivable	$ 0	$ 95,285
Long-term restricted cash	480,000	80,000
Carrying value of debt issuance costs reported in prepaid expenses and other current assets	100,760	124,041
Amortization of debt issuance costs recognized as interest expense	36,607	41,103	$ 16,500
Deferred offering costs	$ 0	$ 0		$ 1,623,540
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Construction-in Progress [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	7 years
Letter of Credit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 50,000	$ 50,000
Certificates of Deposit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	30,000	30,000
Commercial Bank [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 400,000	$ 400,000

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2016
Computers, Office Equipment, and Minor Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years

Fair Value of Financial Asset41

Fair Value of Financial Assets - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 174,688,468	$ 48,302,507
Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,830,127	61,533,797
Marketable securities	179,413,509	55,659,930
Assets, Total	189,243,636	117,193,727
Level 2 [Member] | Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,830,127	61,533,797
Marketable securities	179,413,509	55,659,930
Assets, Total	$ 189,243,636	$ 117,193,727

Fair Value of Financial Asset42

Fair Value of Financial Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0

Marketable Securities - Summary

Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 179,485,815	$ 55,756,581
Gross Unrealized Gains	5,840	3,783
Gross Unrealized Losses	(78,146)	(100,434)
Fair Value	179,413,509	55,659,930
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96,192,720	55,756,581
Gross Unrealized Gains	811	3,783
Gross Unrealized Losses	(66,286)	(100,434)
Fair Value	96,127,245	$ 55,659,930
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,523,761
Gross Unrealized Gains	5,029
Gross Unrealized Losses	(11,860)
Fair Value	75,516,930
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,769,334
Fair Value	$ 7,769,334

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 174,688,468	$ 48,302,507
Marketable securities, noncurrent	$ 4,725,041	$ 7,357,423

Prepaid Expenses, Other Curre45

Prepaid Expenses, Other Current Assets, and Other Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,085,198	$ 135,002
Security Deposits	2,099,029	182,009
Interest receivable on marketable securities	604,794	441,766
Employee advances	711	2,095
Total prepaid expenses and other current assets	$ 3,789,732	$ 760,872	$ 28,875

Prepaid Expenses, Other Curre46

Prepaid Expenses, Other Current Assets, and Other Assets - Additional Information (Detail)	Dec. 01, 2016USD ($)
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Refundable NDA fee paid to FDA	$ 2,038,100

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 13,039,643	$ 7,932,717
Less: Accumulated depreciation	(1,376,084)	(490,240)
Total property and equipment, net	11,663,559	7,442,477
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	572,901	393,148
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	10,098,577	2,533,737
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	402,301	290,577
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	278,288	239,456
Construction-in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	1,253,738	4,133,893
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 433,838	$ 341,906

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Depreciation	$ 1,150,961	$ 238,153	$ 120,341
Property and equipment disposals	2,670,413	166,335
Gain (Loss) on sale of property and equipment	$ (2,283,343)	$ (149,983)

Accrued Expenses and Other Cu49

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Research and Development	$ 1,604,810	$ 2,059,800
Payroll and other employee-related expenses	3,393,201	1,648,447
Professional services fees	926,550	503,453
Interest expense	161,458	80,729
Other	159,214	74,131
Total accrued expenses and other current liabilities	$ 6,245,233	$ 4,366,560

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	Jul. 22, 2016	Aug. 04, 2015	Jan. 03, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2015
Debt Instrument [Line Items]
Debt issuance costs			$ 61,876
Prepaid expenses and other current assets				$ 3,789,732	$ 760,872		$ 28,875
Carrying value of term loan				30,532,783
Current portion of long-term debt				9,133,991
Long-term debt				21,398,792	15,002,039
Amortization expense for debt issuance costs				36,607	41,103	$ 16,500
Secured Debt [Member] | Mid Cap [Member]
Debt Instrument [Line Items]
Capitalized debt acquisition costs				100,760
Amortization expense for debt issuance costs				$ 36,607	$ 12,228	$ 16,500
Mid Cap [Member] | 2013 Term Loan [Member]
Debt Instrument [Line Items]
Term loan, borrowings			$ 5,000,000
Debt instrument interest rate			8.00%
Term loan, Interest payment period			45 months
Term loan, payment description				15-month interest-only period, the term loan principal balance, along with any accrued interest, was to be paid in 30 equal monthly installments
Term loan, first periodic payment date			Apr. 1, 2014
Term loan, last periodic payment date			Sep. 1, 2016
Final principal payments made to lender							175,000
Debt instrument principal payment							3,236,019
Debt instrument prepayment fee							$ 30,000
Mid Cap [Member] | 2013 Term Loan [Member] | Interest-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period			15 months
Mid Cap [Member] | 2013 Term Loan [Member] | Principal-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period			30 months
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.25%
Term loan, payment description				Interest-only period of 19 months, principal will be due in 36 equal monthly installments
Term loan, first periodic payment date		Mar. 1, 2017
Term loan, last periodic payment date		Feb. 1, 2020
Term loan, maximum borrowings		$ 30,000,000
Debt instrument description				The Company must maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5 percent
Debt instrument percentage of cash or cash equivalents addition to principal balance		5.00%
Interest on final payment		9.00%
Carrying value of term loan				$ 30,532,783
Current portion of long-term debt				9,133,991
Long-term debt				$ 21,398,792
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Initial Term Loan [Member]
Debt Instrument [Line Items]
Term loan, currently borrowed		$ 15,000,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Second Term Loan [Member]
Debt Instrument [Line Items]
Term loan, remaining borrowing		$ 15,000,000
Term loan remaining amount borrowed	$ 15,000,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Interest-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period		19 months
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Principal-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period		36 months

Long Term Debt - Schedule of An

Long Term Debt - Schedule of Annual Payments Due Under Term Loan (Detail)	Dec. 31, 2016USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,017	$ 10,035,734
2,018	11,082,176
2,019	10,448,495
2,020	4,383,475
Total	35,949,880
Less interest	(2,717,097)
Less final payment	(2,700,000)
Total	$ 30,532,783

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional information (Detail)	Feb. 18, 2014shares	Dec. 31, 2016shares	Dec. 31, 2015shares	Dec. 31, 2013$ / sharesshares
Temporary Equity [Line Items]
Reverse stock split conversion ratio description		1-for-8.13 reverse stock split
Preferred stock, shares issued		0	0
Preferred stock, shares outstanding		0	0
IPO [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio description		1-for-8.13 reverse stock split
Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized				73,780,250
Preferred stock, par value | $ / shares				$ 0.001
Preferred Stock, Shares issued				72,780,250
Preferred stock conversion basis description		0.123001-for-1 basis
Common Stock [Member]
Temporary Equity [Line Items]
Common stock issued upon conversion of redeemable preferred stock	8,952,057
Reverse stock split conversion ratio		0.123
Common Stock [Member] | IPO [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio	0.123
Series A Convertible Preferred Stock [Member] | Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized				55,043,464
Preferred Stock, Shares issued				55,043,464
Series B Convertible Preferred Stock [Member] | Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized				18,736,786
Preferred Stock, Shares issued				17,736,786
Redeemable Convertible Preferred Stock [Member] | IPO [Member]
Temporary Equity [Line Items]
Common stock issued upon conversion of redeemable preferred stock	8,952,057

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Detail) - Convertible Preferred Stock [Member]	12 Months Ended
	Dec. 31, 2013USD ($)shares
Temporary Equity [Line Items]
Preferred Shares Authorized	73,780,250
Preferred Shares Issued	72,780,250
Preferred Shares Outstanding	72,780,250
Carrying Value | $	$ 74,806,213
Liquidation Preference | $	$ 75,043,464
Common Stock Issuable Upon Conversion	8,952,057
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	55,043,464
Preferred Shares Issued	55,043,464
Preferred Shares Outstanding	55,043,464
Carrying Value | $	$ 54,917,735
Liquidation Preference | $	$ 55,043,464
Common Stock Issuable Upon Conversion	6,770,411
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	18,736,786
Preferred Shares Issued	17,736,786
Preferred Shares Outstanding	17,736,786
Carrying Value | $	$ 19,888,478
Liquidation Preference | $	$ 20,000,000
Common Stock Issuable Upon Conversion	2,181,646

Preferred Stock - Additional In

Preferred Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001

Common Stock - Additional infor

Common Stock - Additional information (Detail)	Nov. 15, 2016$ / sharesshares	Jun. 08, 2016$ / sharesshares	Jun. 07, 2016$ / sharesshares	Dec. 31, 2014Vote / shares$ / sharesshares	Dec. 17, 2014$ / sharesshares	Feb. 18, 2014$ / sharesshares	Dec. 31, 2012$ / sharesshares	Dec. 31, 2009$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2014$ / sharesshares	Dec. 31, 2015$ / sharesshares	Nov. 05, 2007shares
Class of Stock [Line Items]
Common stock, share authorized				100,000,000			94,000,000	79,000,000	100,000,000	100,000,000	100,000,000	10,000,000
Common stock, shares issued									31,667,469		21,570,395
Additional common stock share authorized				6,000,000			15,000,000	69,000,000
Common stock, par value | $ / shares				$ 0.001			$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Number of vote per common stock | Vote / shares				1
Dividend declared | $									$ 0
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued									10,040,000	11,546,000
Common stock issued upon conversion of redeemable preferred stock						8,952,057
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued						5,750,000
Sale of common stock price per share | $ / shares						$ 13
Follow On Public Offering [Member]
Class of Stock [Line Items]
Common stock, shares issued	4,140,000	5,900,000	5,900,000		5,796,000
Sale of common stock price per share | $ / shares	$ 18	$ 14	$ 14		$ 17
Founders [Member]
Class of Stock [Line Items]
Common stock, shares issued												109

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Sep. 21, 2016ft²	Jul. 13, 2015ft²	Nov. 30, 2016USD ($)Term	Oct. 31, 2016USD ($)	Jul. 31, 2015	May 31, 2013USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Commitment And Contingencies [Line Items]
Operating lease term						42 months
Minimum monthly lease payments						$ 17,588
Security deposit paid to lessor							$ 2,099,029	$ 182,009
Rent expense							668,350	373,202	$ 242,946
Zilretta [Member] | Southwest Research Institute (SwRI) [Member] | Upon FDA First Approval [Member]
Commitment And Contingencies [Line Items]
License agreement, upfront fee paid							$ 120,000
Evonik Corporation [Member]
Commitment And Contingencies [Line Items]
Supply agreement period			5 years
Supply agreement renewal term			2 years
Number of renewal terms | Term			2
Supply agreement description							The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties.
Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]
Commitment And Contingencies [Line Items]
Manufacturing agreement period					10 years
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Additional leased office space | ft²	6,748	4,700
Office space leased on temporary agreement | ft²		6,700
Anticipated lease commencement date of extended office space		May 1, 2016
Lease termination date	Oct. 31, 2017	Oct. 31, 2019
Extended leased office space | ft²		5,400
Extended lease agreement period		3 years
Operating lease early termination notice period		9 months
Lease commencement date	Oct. 1, 2016
Lease payment for additional space in incremental rent per month			$ 19,000	$ 18,300
Letter of Credit [Member]
Commitment And Contingencies [Line Items]
Security deposit paid to lessor						$ 98,000
Reduction in letter of credit								$ 50,000

Commitments and Contingencies57

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail)	Dec. 31, 2016USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,017	$ 924,806
2,018	758,299
2,019	647,106
Total	$ 2,330,211

Commitments and Contingencies58

Commitments and Contingencies - Schedule of Future Minimum Payments under Contractual Obligations (Detail) - Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]	Dec. 31, 2016USD ($)
Property Subject to or Available for Operating Lease [Line Items]
2,017	$ 5,529,600
2,018	6,266,880
2,019	7,372,800
2020 and thereafter	44,236,800
Total	$ 63,406,080

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jan. 04, 2016	Jul. 16, 2013	Sep. 30, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012	Jan. 27, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding				3,268,475	1,657,225	1,289,082
Stock options granted				1,816,000
Stock-based compensation expense				$ 6,769,804	$ 4,582,679	$ 2,450,579
Unrecognized stock-based compensation expense				$ 15,932,348
Number of options exercised				30,195	109,441	141
Aggregate intrinsic value of stock options exercised				$ 236,889	$ 1,584,657	$ 1,476,861
Weighted average remaining contractual term				7 years 9 months 18 days	7 years 10 months 24 days	8 years 1 month 6 days
Weighted average exercise price				$ 14.84	$ 14.28	$ 10.26
Options exercisable, shares				1,173,671	728,621	450,109
Weighted average remaining contractual term				6 years 8 months 12 days	6 years 10 months 24 days	6 years 8 months 12 days
Aggregate intrinsic value, stock options exercisable				$ 8,745,505	$ 7,714,057	$ 7,489,976
Weighted average grant date fair value of options granted				$ 11.50	$ 15.08	$ 9.99
Unrecognized compensation costs, weighted-average recognition periods				3 years 15 days
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period				4 years
Share based award, expiration period				10 years
Stock options granted				1,816,575	657,250	727,575	403,382
Awards vesting right				The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months
Employee Stock Option [Member] | Non Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting right				Annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months
Employee Stock Option [Member] | Vesting after One Year and Monthly Thereafter for the Next 36 Months [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of share based award, vested				25.00%
Employee Stock Option [Member] | Vesting after One Year and Monthly Thereafter for the Next 24 Months [Member] | Non Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of share based award, vested				25.00%
Performance-Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period				4 years
Stock options granted							264,944
Grant date fair value of options							$ 236,940
Stock options forfeited			18,450
Stock-based compensation expense		$ 481,729					$ 0
Aggregate increase in fair value of options		2,185,729
Unrecognized stock-based compensation expense		$ 1,704,000
Stock-based compensation, modification date				Jul. 16, 2013
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards, shares unvested and subject to repurchase				0	0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, weighted-average recognition periods	2 years
Aggregate number of restricted common stock units granted under performance-based vesting	189,300
Grant date fair value of restricted common stock units under performance-based vesting	$ 3,997,371
2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of aggregate number of shares common stock that may be issued				Initially, the maximum number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2013 Plan was 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting).
Share based compensation, shares issued in period				1,230,012
Percentage of outstanding shares of common stock				4.00%
Options to purchase common stock, outstanding				2,759,593
Number of shares available for future issuance				1,287,381
2009 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding				508,882
Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of outstanding shares of common stock				1.00%
Number of shares available for future issuance								209,102
Number of shares purchased by employees				26,880	20,896
Maximum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based award, expiration period				10 years
Maximum [Member] | 2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, authorized				2,337,616
Number of options exercised				4,684,989

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rates, Minimum	0.74%	1.49%	1.54%
Risk-free interest rates, Maximum	1.75%	1.92%	2.04%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	5 years 7 months 6 days	6 years	6 years
Expected volatility, Minimum	67.30%	76.40%	61.90%
Expected volatility, Maximum	99.90%	83.90%	68.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Issuable Under Options, Beginning balance	1,657,225	1,289,082
Shares Issuable Under Options, Granted	1,816,000
Shares Issuable Under Options, Exercised	(30,195)	(109,441)	(141)
Shares Issuable Under Options, Cancelled	(175,000)
Shares Issuable Under Options, Ending balance	3,268,475	1,657,225	1,289,082
Shares Issuable Under Options, Options vested and expected to vest at December 31, 2016	2,275,000
Shares Issuable Under Options, Options exercisable at December 31, 2016	1,173,671	728,621	450,109
Weighted Average Exercise Price, Beginning balance	$ 14.28	$ 10.26
Weighted Average Exercise Price, Granted	15.43
Weighted Average Exercise Price, Exercised	5.53
Weighted Average Exercise Price, Cancelled	17.22
Weighted Average Exercise Price, Ending balance	14.84	$ 14.28	$ 10.26
Weighted Average Exercise Price, Options vested and expected to vest	14.47
Weighted Average Exercise Price, Options exercisable	$ 12.31

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 6,769,804	$ 4,582,679	$ 2,450,579
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	2,340,847	1,412,153	684,511
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4,428,957	$ 3,170,526	$ 1,766,068

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (13,569)	$ (11,136)	$ (12,437)	$ (9,174)	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Net loss:	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (13,569)	$ (11,136)	$ (12,437)	$ (9,174)	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Denominator:
Weighted average common shares outstanding, basic and diluted	29,347	27,524	22,666	21,570	21,554	21,507	21,475	21,451	25,296,527	21,497,119	13,893,961
Net loss per share, basic and diluted	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (0.63)	$ (0.52)	$ (0.58)	$ (0.43)	$ (2.84)	$ (2.15)	$ (1.97)

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share attributable to common stockholders	2,532,675	1,655,394	1,185,253

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Tax provision or benefit	$ 0	$ 0
Federal income tax benefit	$ 14,267	14,267
Limit of utilization of net operating losses (NOLs) and tax credits description	Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company’s stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company.
Liabilities for unrecognized tax benefits	$ 0	$ 0
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 94,659,000
Operating loss carryforwards expiration year	2,029
Federal [Member] | Research and Development Tax Credit Carryforwards [Member]
Income Tax [Line Items]
Tax credit carryforwards	$ 5,202,362
Tax credit expiration year	2,029
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 88,945,478
Operating loss carryforwards expiration year	2,030
State and Local Jurisdiction [Member] | Research and Development Tax Credit Carryforwards [Member]
Income Tax [Line Items]
Tax credit carryforwards	$ 2,841,187
Tax credit expiration year	2,025

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	5.00%	4.90%	4.70%
Federal and state research and development tax credits	2.70%	4.70%	3.50%
Change in deferred tax asset valuation allowance	(40.00%)	(45.40%)	(37.80%)
Other	(1.70%)	1.80%	(4.40%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 36,880,381	$ 24,226,949
Research and development tax credit carryforwards	7,077,546	5,368,524
Accruals and other temporary differences	4,896,980	2,339,444
Capitalized research and development expenses, net	34,579,391	22,838,452
Total deferred tax assets	83,434,298	54,773,369
Valuation allowance	(83,434,298)	(54,773,369)	$ (33,825,873)	$ (23,493,416)
Net deferred tax asset	$ 0	$ 0

Income Taxes - Changes in Valua

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes Disclosure [Line Items]
Valuation allowance as of beginning of year	$ (54,773,369)	$ (33,825,873)	$ (23,493,416)
Valuation allowance as of end of year	(83,434,298)	(54,773,369)	(33,825,873)
Decreases Recorded as Benefit to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increases) recorded as benefit to income tax provision	4,771,490	2,857,886	2,020,565
Increases Recorded to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increases) recorded as benefit to income tax provision	$ (33,432,419)	$ (23,805,382)	$ (12,353,022)

Quarterly Financial Data (una69

Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 21,552	$ 17,435	$ 14,120	$ 16,673	$ 13,479	$ 11,026	$ 12,544	$ 9,015	$ 69,780,292	$ 46,063,076	$ 26,987,274
Net loss	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (13,569)	$ (11,136)	$ (12,437)	$ (9,174)	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Net loss per common share—basic and diluted	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (0.63)	$ (0.52)	$ (0.58)	$ (0.43)	$ (2.84)	$ (2.15)	$ (1.97)
Weighted average common shares—basic and diluted	29,347	27,524	22,666	21,570	21,554	21,507	21,475	21,451	25,296,527	21,497,119	13,893,961