Flexion Therapeutics (FLXN) 10-K 11 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 02, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	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		21,570,395
Entity Public Float			$ 392,188,000

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 62,944,044	$ 103,097,522
Marketable securities	48,302,507	48,527,156
Accounts receivable	95,285
Prepaid expenses and other current assets	760,872	485,814
Total current assets	112,102,708	152,110,492
Property and equipment, net	7,442,477	1,109,391
Long-term investments	7,357,423
Other assets	155,904
Restricted cash	80,000	128,000
Total assets	127,138,512	153,347,883
Current liabilities:
Current portion of long-term debt		1,983,500
Accounts payable	3,692,414	1,584,822
Accrued expenses and other current liabilities	4,366,560	3,213,704
Total current liabilities	8,058,974	6,782,026
Long-term debt	15,002,039	1,580,958
Other long-term liabilities	91,055	43,008
Total liabilities	$ 23,152,068	$ 8,405,992
Commitments and contingencies:
Preferred Stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2015 and 2014, 0 shares issued and outstanding at December 31, 2015 and 2014
Stockholders' equity (deficit):
Common stock, $0.001 par value; 100,000,000 shares authorized at December 31, 2015 and 2014; 21,570,395 and 21,440,058 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 21,570	$ 21,440
Additional paid-in capital	243,853,799	238,402,514
Accumulated other comprehensive income	(96,651)	(5,240)
Accumulated deficit	(139,792,274)	(93,476,823)
Total stockholders' equity	103,986,444	144,941,891
Total liabilities and stockholders' equity	$ 127,138,512	$ 153,347,883

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
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	21,570,395	21,440,058
Common stock, shares outstanding	21,570,395	21,440,058

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0
Operating expenses:
Research and development	32,691,445	17,923,348	11,060,912
General and administrative	13,371,631	9,063,926	6,704,297
Total operating expenses	46,063,076	26,987,274	17,765,209
Loss from operations	(46,063,076)	(26,987,274)	(17,765,209)
Other income (expense):
Interest income	1,246,133	478,715	233,999
Interest expense	(570,990)	(401,370)	(448,889)
Other income (expense), net	(927,518)	(403,735)	(206,625)
Total other income (expense)	(252,375)	(326,390)	(421,515)
Net loss	$ (46,315,451)	$ (27,313,664)	$ (18,186,724)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.15)	$ (1.97)	$ (23.02)
Weighted average common shares outstanding, basic and diluted	21,497,119	13,893,961	790,038
Other comprehensive (loss):
Unrealized (losses) gains from available-for-sale securities, net of tax of $0	$ (91,411)	$ (5,212)	$ (2,478)
Total other comprehensive (loss)	(91,411)	(5,212)	(2,478)
Comprehensive loss	$ (46,406,862)	$ (27,318,876)	$ (18,189,202)

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Other comprehensive (loss) income, 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]	Deficit Accumulated During the Development Stage [Member]
Balance at Dec. 31, 2012	$ (47,523,126)	$ 74,806,213	$ 789	$ 450,070	$ 2,450	$ (47,976,435)
Balance (in shares) at Dec. 31, 2012		72,780,250	789,222
Exercise of stock options	$ 12,271		$ 5	12,266
Exercise of stock options (in shares)	4,868		4,868
Stock-based compensation expense	$ 996,167			996,167
Net loss	(18,186,724)					(18,186,724)
Other comprehensive income	(2,478)				(2,478)
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
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		141,141
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	(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
Conversion of Series A and Series B Convertible Preferred Stock (in shares)		72,780,250	8,952,057
Conversion of Series A and Series B Convertible Preferred Stock (in shares)		(72,780,250)	(8,952,057)
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	(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

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities
Net loss	$ (46,315,451)	$ (27,313,664)	$ (18,186,724)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	238,153	120,341	79,808
Stock-based compensation expense	4,582,679	2,450,579	996,167
Amortization of premium (discount) on marketable securities	871,215	366,231	151,138
Loss on disposal of property and equipment	149,983	0	14,111
Other non-cash charges	41,103	16,500	63,167
Changes in operating assets and liabilities:
Prepaid expenses and other current and long-term assets	(526,247)	(320,352)	343,896
Accounts payable	1,581,164	518,032	475,821
Accrued expenses and other current and long-term liabilities	428,769	1,017,412	(124,328)
Net cash used in operating activities	(38,948,632)	(23,144,921)	(16,186,944)
Cash flows from investing activities
Purchases of property and equipment	(5,197,326)	(802,481)	(405,315)
Change in restricted cash	48,000		(98,000)
Purchases of marketable securities	(145,797,556)	(79,383,553)	(15,015,663)
Redemption of marketable securities	137,702,156	30,735,000	31,160,000
Net cash provided by (used in) investing activities	(13,244,726)	(49,451,034)	15,641,022
Cash flows from financing activities
Proceeds from borrowings under term loan	15,003,533		5,000,000
Payments of debt issuance costs	(107,741)		(40,715)
Payments on debt	(3,500,000)	(1,500,000)
Payment of public offering costs	(224,648)	(1,517,484)	(1,072,710)
Proceeds from the issuance of common stock		162,137,580
Proceeds from the exercise of stock options	592,787	304,496	12,271
Proceeds from Employee Stock Purchase Plan	275,949	80,631
Net cash provided by financing activities	12,039,880	159,505,223	3,898,846
Net increase (decrease) in cash and cash equivalents	(40,153,478)	86,909,268	3,352,924
Cash and cash equivalents at beginning of period	103,097,522	16,188,254	12,835,330
Cash and cash equivalents at end of period	62,944,044	103,097,522	16,188,254
Supplemental disclosures of cash flow information:
Cash paid for interest	572,340	364,889	367,778
Supplemental disclosures of non-cash financing activities:
Deferred initial public offering costs included in accounts payable or accrued expenses			550,830
Public offering costs included in accounts payable or accrued expenses		224,648
Conversion of convertible preferred stock into common stock		74,806,213
Purchases of property, plant and equipment included in accounts payable and accrued expenses	1,575,907	52,011
Series A Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series A Convertible Preferred Stock, net of issuance costs	0	0	0
Series B Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series A Convertible Preferred Stock, net of issuance costs	$ 0	$ 0	$ 0

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2015
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”) and post-operative pain. Flexion’s portfolio of product candidates addresses the OA pain treatment spectrum, from moderate to severe 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, 2015 and 2014, the Company had cash, cash equivalents, marketable securities, and long-term investments of $118,603,974 and $151,624,678, respectively. Management believes that current cash, cash equivalents and marketable securities on hand at December 31, 2015 should be sufficient to fund operations for at least the next twelve months. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations and to fund increased research and development costs in order to seek approval for commercialization of its product candidates. The Company may not be able to obtain financing on acceptable terms, or at all. The Company’s failure to raise capital as and when needed would have a negative impact on its financial condition and its ability to pursue its business strategies. This capital is necessary for the Company to perform the research and development activities required to develop the Company’s product candidates in order to generate future revenue streams.

Financing Transactions_Common S

Financing Transactions/Common Stock	12 Months Ended
	Dec. 31, 2015
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. 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, 2015, 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. The Company’s total issued common stock as of December 31, 2015 was 21,570,395 shares.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
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. The Company is reimbursed and offsets research and development expenses in the statement of operations when invoices for allowable costs are prepared and submitted to the U.S. Government agency. 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. When the final determination of the allowable cost for any year has been made, research and development expenses may be adjusted accordingly. The grant also provides the U.S. government agency the ability to terminate the grant for various reasons, including if the Company fails to meet its obligations as set forth in the grant. 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, 2015 and 2014, there were unbilled accounts receivable of $95,285 and $0, 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 that was classified as long-term restricted cash as of December 31, 2015 and 2014. In addition, the Company posted a letter of credit to the lessor of the Company’s Burlington facility in the amount of $98,000 as a security deposit pursuant to the lease agreement in the year ended December 31, 2014. The remaining amount, $50,000 and $98,000, remained as long-term restricted cash at December 31, 2015 and 2014, respectively. 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 Computers and office equipment 3 Computer software 7 Manufacturing equipment 7 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. 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 represent legal costs related to the Company’s Credit and Security Agreement (Note 9). In April 2015, the FASB issued ASU 2015-03, Interest – Imputation of Interest As of December 31, 2015 and 2014, the carrying value of debt issuance costs was $124,041 and $28,875, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $41,103, $16,500 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, 2015, 2014 and 2013. 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, 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, 2015, 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, 2015, 2014 and 2013. 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, 2015, 2014 and 2013. 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 Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers: Deferral of the Effective Date 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 is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements. In April 2015, the FASB issued ASU 2015-03, Interest—Imputation of Interest In May 2015, the FASB issued Accounting Standards Update 2015-07, Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent) In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases

Fair Value of Financial Assets

Fair Value of Financial Assets	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 and indicate the level of the fair value hierarchy utilized to determine such fair value: 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 Fair Value Measurements as of December 31, 2014 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 101,687,995 $ — $ 101,687,995 Marketable securities — 48,527,156 — 48,527,156 $ — $ 150,215,151 $ — $ 150,215,151 As of December 31, 2015 and 2014, 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, 2015 and 2014, 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, 2015 and December 31, 2014. The result of the calculation yielded a fair value that approximates carrying value.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	5. Marketable Securities As of December 31, 2015 and 2014, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2015 Amortized Gross Unrealized Gross Unrealized Fair Value Corporate bonds 55,756,581 3,783 (100,434 ) 55,659,930 $ 55,756,581 $ 3,783 $ (100,434 ) $ 55,659,930 December 31, 2014 Amortized Gross Unrealized Gross Unrealized Fair Value Commercial paper $ 8,991,820 $ 7,570 $ — $ 8,999,390 U.S. Government obligations 28,300,921 181 (5,101 ) 28,296,001 Corporate bonds 11,239,655 2 (7,892 ) 11,231,765 $ 48,532,396 $ 7,753 $ (12,993 ) $ 48,527,156 At December 31, 2014, marketable securities consisted of investments that mature within twelve 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 months.

Prepaid Expenses, Other Current

Prepaid Expenses, Other Current Assets, and Other Assets	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014: December 31, 2015 2014 Prepaid expenses $ 135,002 $ 269,199 Security Deposits 182,009 — Interest receivable on marketable securities 441,766 216,615 Employee advances 2,095 — Total prepaid expenses and other current assets $ 760,872 $ 485,814

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	7. Property and Equipment, Net Property and equipment, net, as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Computer and office equipment $ 393,148 $ 229,980 Manufacturing equipment 2,533,737 153,140 Furniture and fixtures 290,577 181,366 Software 341,906 77,454 Leasehold improvements 239,456 134,573 Construction—in Progress 4,133,893 601,317 $ 7,932,717 $ 1,377,830 Less: Accumulated depreciation $ (490,240 ) $ (268,439 ) Total property and equipment, net $ 7,442,477 $ 1,109,391 Depreciation expense for the years ended December 31, 2015, 2014 and 2013, was $238,153, $120,341, and $79,808, respectively. During the years ended December 31, 2015 and 2014, $166,335 and $113,502 of property and equipment was disposed of, resulting in a loss of $149,983 and $0, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 consisted of the following: December 31, 2015 2014 Clinical research $ 552,325 $ 1,035,510 Contract manufacturing services 1,443,600 294,900 Payroll and other employee-related expenses 1,648,447 1,172,978 Preclinical services — 119,500 Regulatory Services 63,875 — Consultant fees and expenses 70,000 26,900 Professional services fees 433,453 439,874 Interest expense 80,729 24,111 Other 74,131 99,931 Total accrued expenses and other current liabilities $ 4,366,560 $ 3,213,704

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2015
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 8.0% 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 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, 2015, the Company was compliant with all covenants and there were no material adverse events. 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, 2015, the carrying value of the term loan was $15,126,080, the entire balance of which is classified as long-term debt on the consolidated balance sheets as of December 31, 2015. In connection with entering into a credit and security agreement dated August 4, 2015, the Company incurred debt acquisition costs of $136,446. The Company elected the early adoption of ASU 2015-03, Interest – Imputation of Interest 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 $12,228 for the year ended December 31, 2015. Total amortization expense of the debt issuance costs associated with the credit and security agreement dated January 3, 2013 was $16,500 for each of the years ended December 31, 2014 and 2013. As of December 31, 2015, annual payments due under the Company’s 2015 term loan are as follows (in thousands): Year Aggregate 2016 $ 953,125 2017 5,017,868 2018 5,541,088 2019 5,224,248 2020 2,190,060 Total $ 18,926,389 Less interest (2,574,350 ) Less final payment (1,350,000 ) Long-term debt $ 15,002,039

Preferred and Convertible Prefe

Preferred and Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2015
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, 2015, 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”). In December 2012, the Company issued 17,736,786 shares of Series B Preferred Stock at a purchase price of $1.1276 per share (“Series B Original Issue Price”). The Series B Preferred Stock issuance resulted in net proceeds of $19,888,478. Included in the Series B Preferred Stock issuance were contingently issuable warrants for the purchase of 218,160 shares of common stock at $0.01 per share. These warrants were contingently issuable if the Company did not initiate and enroll the first patient in a multi-dose Phase 2b clinical trial with planned enrollment of at least 100 patients for the Company’s product candidate identified as FX005 by February 15, 2014. Additionally, the obligation to issue the warrants would terminate on the earlier to occur of (i) the consummation of a registered initial public offering, which is defined as a firmly underwritten public offering pursuant to an effective registration statement under the Securities Act of 1933, as amended, covering the offer and sale of common stock for the account of the Company and (ii) the occurrence of a liquidation event or the consummation of a deemed liquidation event. The Company determined that the contingently issuable common stock warrants were equity in nature. As such, the Company allocated the Series B Preferred Stock proceeds to both the Series B Preferred Stock and the contingently issuable common stock warrants based on relative fair values at the issuance date. Given the low probability assessed by the Company of not satisfying the contingency and having to issue the warrants, the amount ascribed to the warrants was immaterial and the full amount of the proceeds was allocated to the 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. This agreement amended the milestone date associated with the contingently issuable common stock warrants from February 15, 2014 to February 21, 2014. 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. As the Company’s IPO was completed prior to February 21, 2014 these warrants expired. The following is a summary of the Company’s Convertible Preferred Stock as of December 31, 2013: December 31, 2013 Preferred Preferred Carrying Liquidation Common 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. Dividends The holders of both Series A and B Preferred Stock were entitled to receive, out of funds legally available, non-cumulative dividends at an annual rate of 8.0%, when and if declared by the board of directors. Holders of Series B Preferred Stock receive such dividends in preference to any dividend on Series A Preferred Stock or common stock. After payment of any amounts payable to holders of Series B Preferred Stock, the holders of Series A Preferred Stock were entitled to receive such dividends in preference to any dividend on common stock. No dividends have been declared or paid from inception to December 31, 2015. Liquidation Prior to the conversion of the Company’s Series A and Series B preferred stock on February 18, 2014, in connection with the closing of the Company’s IPO, the following liquidation preferences and conversion rates were in effect. The Company does not currently have any holders of securities with either liquidation preferences or conversion rights. Upon any liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary (each, a “Liquidation Event”), before any distribution or payment would have been made to the holders of any common stock, the Company would have made payment to the holders of preferred stock as follows: The holders of the Series B Preferred Stock, in preference to the holders of the Series A Preferred Stock and common stock, were entitled to be paid out of the assets of the Company legally available for distribution for each share of Series B Preferred Stock held by them, an amount per share equal to the applicable Series B Original Issue Price plus all declared and unpaid dividends on such shares of Series B Preferred Stock, if any (the “Series B Liquidation Preference”). After the payment of the full liquidation preference of the Series B Preferred Stock, the holders of Series A Preferred Stock, in preference to holders of common stock, would have been entitled to be paid out of the assets of the Company legally available for distribution for each share of Series A Preferred Stock held by them, an amount per share of Series A Preferred Stock equal to the Series A Original Issue Price plus all declared and unpaid dividends on the Series A Preferred Stock, if any (the “Series A Liquidation Preference”). After payment in full of the Series B Liquidation Preference and the Series A Liquidation Preference as set forth above, the remaining assets of the Company legally available for distribution, if any, would have been distributed ratably to the holders of the common stock. Shares of Series A and B Preferred Stock would not have been entitled to be converted into shares of common stock in order to participate in any distribution, or series of distributions, as shares of common stock, without first foregoing participation in the distribution, or series of distributions, as shares of Series A and B Preferred Stock. Conversion Each share of Series A and B Preferred Stock was convertible at any time at the option of the stockholder into fully paid and non-assessable shares of common stock. The number of shares of common stock to which a holder of any series of Series A and B Preferred Stock was entitled upon conversion was the product obtained by multiplying the Conversion Rate then in effect for such series of preferred stock by the number of shares of such series of preferred stock being converted. The conversion rate in effect at any time for conversion of the preferred stock (the “Conversion Rate”) was (i) for the Series A Preferred Stock, the quotient obtained by dividing the Series A Original Issue Price by the Conversion Price of the Series A Preferred Stock and (ii) for the Series B Preferred Stock, the quotient obtained by dividing the Series B Original Issue Price by the Conversion Price of the Series B Preferred Stock. The Conversion Price of Series A Preferred Stock was $8.13 as of December 31, 2013, and the Conversion Price of Series B Preferred Stock was $9.166575 as of December 31, 2013. As of December 31, 2013, all outstanding shares of Series A Preferred Stock and Series B Preferred Stock were convertible into common stock on a 0.123001-for-1 basis. The Conversion Price of each series of preferred stock was subject to adjustment for stock splits, stock dividends and other recapitalizations in accordance with the Company’s Certificate of Incorporation, as amended. In addition to the above optional conversion feature, the Series A and B Preferred Stock included a mandatory conversion feature whereby upon a qualifying initial public offering, all outstanding shares of Series A and B Preferred Stock would automatically be converted into shares of common stock at the then effective conversion rate. A qualifying initial public offering was an initial public offering in which (i) the per share price was at least $27.48 (as adjusted for any stock splits, dividends, combinations, recapitalizations and the like after the filing date), and (ii) the gross cash proceeds to the Company (before underwriting discounts, commissions and fees) were at least $40,000,000. All shares that were required to be surrendered per the provisions above were deemed to have been retired and canceled and may not be reissued as shares of preferred stock.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
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. The total cash obligation for the base rent from inception through the lease termination date for the office space committed to under the original lease, and the amendment including the Option Space, is approximately $2,900,000. In addition to the base rent, the Company is also responsible for its share of operating expenses and real estate taxes. The Company incurred rent expense of $373,202, $242,946, and $227,381 for the years ended December 31, 2015, 2014 and 2013, respectively. Future minimum lease payments under the Company’s lease obligations are as follows: Year Ending December 31, 2016 $ 527,231 2017 736,425 2018 758,299 2019 647,106 Total $ 2,669,061 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), sustained-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 Company estimates that the aggregate monthly base fees and reimbursement costs for equipment will be approximately 100 million British Pounds (GBP) over the entire term of the Manufacturing Agreement. 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, 2016 $ 2,890,598 2017 6,670,620 2018 7,560,036 2019 8,894,160 2020 and thereafter 53,364,960 Total $ 79,380,374 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, if the Company’s NDA for Zilretta is approved by the FDA for the treatment of knee OA (the “Milestone Event”), $120,000 is payable to SwRI; provided, however, that if the Milestone Event has not been achieved by December 31, 2019 and until the Milestone Event has been achieved, $20,000 will be payable to SwRI on January 1, 2020, $40,000 on January 1, 2021, and $60,000 on January 1, 2022. The time-based payments to SwRI are fully creditable against the payment due to SwRI upon achievement of the Milestone Event such that the aggregate amount owed to SwRI shall not exceed $120,000. AstraZeneca License Agreements On September 3, 2010, the Company entered into an exclusive license agreement with AstraZeneca for FX007. The agreement grants the Company an exclusive, royalty-bearing right and license to the patent rights detailed in the agreement. Per the terms of the license agreement, the Company was required to pay a nonrefundable fee of $1,000,000: $500,000 within thirty days of the execution of the agreement and $500,000 six months after the execution of the agreement. The Company recorded $1,000,000 as research and development expense during 2010. The agreement includes terms for potential future milestone payments including up to an aggregate of $21 million upon the achievement of certain regulatory and development milestones for a first licensed product for OA indications, or up to an aggregate of $15 million upon the achievement of certain regulatory and development milestones for a first-licensed product for non-OA indications. Upon commercialization of a product that results from the technology licensed under the agreement, the Company will owe AstraZeneca tiered royalty payments on net sales based on a percentage ranging from low single digits to low double digits, depending on the volume of sales of the applicable product, as well as up to $75 million in additional payments based on the achievement of certain sales milestones. There were no payments made or expenses recorded under this agreement in 2015, 2014 or 2013. On June 12, 2009, the Company entered into an exclusive license agreement with AstraZeneca for FX005. The agreement grants the Company an exclusive, royalty-bearing right and license to the patent rights detailed in the agreement. Per the terms of the license agreement, the Company paid a nonrefundable fee of $1,000,000 upon execution of the agreement. The Company recorded the $1,000,000 payment as research and development expense during 2009. The agreement includes terms for potential future milestone payments including up to an aggregate of $17 million upon the achievement of certain regulatory and development milestones for a first licensed product for OA indications, or up to an aggregate of $11 million upon the achievement of certain regulatory and development milestones for a first-licensed product for non-OA indications. Upon commercialization of a product that results from the technology licensed under the agreement, the Company will owe AstraZeneca tiered royalty payments on net sales based on a percentage ranging from low to high single digits, depending on the volume of sales of the applicable product, as well as up to $45 million in additional payments based on the achievement of certain sales milestones. There were no payments made or expenses recorded under this agreement in 2015, 2014 or 2013.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based 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, 2015, there were 530,680 options outstanding under the 2009 Plan. The Company currently grants stock-based awards pursuant to the 2013 Plan. As of December 31, 2015, 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, 2015, there were 1,126,575 options outstanding under the 2013 Plan. Stock Options During the years ended December 31, 2015, 2014 and 2013, the Company granted stock options for the purchase of 657,250, 727,575, and 201,721 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 is expected to be 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, 2015, 2014 and 2013 is as follows: December 31, 2015 2014 2013 Risk-free interest rates 1.49-1.92 % 1.54-2.04 % 1.00 % Expected dividend yield 0.00 % 0.00 % 0.00 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 76.4-83.9 % 61.9-68.0 % 71 % The following table summarizes stock option activity for the year ended December 31, 2015: Shares Weighted Outstanding as of December 31, 2014 1,289,082 $ 10.26 Granted 657,250 21.84 Exercised (109,441 ) 5.44 Canceled or forfeited (179,636 ) 18.42 Outstanding as of December 31, 2015 1,657,255 $ 14.28 Options vested and expected to vest at December 31, 2015 1,541,800 13.92 Options exercisable at December 31, 2015 728,621 8.81 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 109,441, 141,141, and 4,868 options were exercised during the years ended December 31, 2015, 2014 and 2013, respectively. The aggregate intrinsic value of stock options exercised was $1,584,657, $1,476,861, and $45,125 for the years ended December 31, 2015, 2014 and 2013, respectively. At December 31, 2015, 2014 and 2013 the Company had options for the purchase of 1,657,255, 1,289,082, and 834,983 shares of common stock outstanding, with a weighted average remaining contractual term of 7.9, 8.1, and 8.5 years, respectively, and with a weighted average exercise price of $14.28, $10.26, and $2.99 per share, respectively. At December 31, 2015, 2014 and 2013 there were options for the purchase of 728,621, 450,109, and 386,334 shares of common stock exercisable under these stock option awards, with a weighted average remaining contractual life of 6.9, 6.7, 6.9 years , respectively, and an aggregate intrinsic value of $7,714,057, $7,489,976, $4,071,356, respectively. The weighted average grant date fair value of options granted during the years ended December 31, 2015, 2014 and 2013 was $15.08, $9.99, and $6.99, respectively. Restricted Common Stock The Company’s 2009 and 2013 Plans provide for the award of restricted stock. The Company has granted restricted common stock with time-based vesting conditions. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting. During the years ended December 31, 2015, 2014, and 2013 the Company did not issue any shares of restricted common stock. As of December 31, 2015 and 2014, 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 and restricted stock for the years ended December 31, 2015, 2014 and 2013 as follows: Year Ended December 31, 2015 2014 2013 Research and development $ 1,412,153 $ 684,511 $ 347,470 General and administrative 3,170,526 1,766,068 648,697 $ 4,582,679 $ 2,450,579 $ 996,167 As of December 31, 2015, unrecognized stock-based compensation expense for stock options outstanding was $8,629,518, which is expected to be recognized over a weighted average period of 2.6 years. 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, 2015 and 2014, 20,896 and 6,770 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, 2015
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, 2015, 2014 and 2013: Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (46,315,451 ) $ (27,313,664 ) $ (18,186,724 ) Accretion of dividends on convertible preferred stock — — — Net income attributable to participating securities — — — Net loss attributable to common stockholders: $ (46,315,451 ) $ (27,313,664 ) $ (18,186,724 ) Denominator: Weighted average common shares outstanding, basic and diluted 21,497,119 13,893,961 790,038 Net loss per share attributable to common stockholders, basic and diluted $ (2.15 ) $ (1.97 ) $ (23.02 ) Stock options for the purchase of 1,655,394, 1,185,253, and 648,591 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, 2015, 2014 and 2013, respectively, because those options had an anti-dilutive impact due to the net loss attributable to common stockholders incurred for those periods. In addition, 0 weighted average shares of unvested restricted common stock were excluded from the computation of basic and diluted net loss per share attributable to common stockholders for the years ended December 31, 2015, 2014 and 2013, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The Company has generated losses since inception. The Company has recorded no income tax benefits for those losses during the years ended December 31, 2015 and 2014, respectively, due to its uncertainty of realizing a benefit from those losses. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2015 2014 2013 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State taxes, net of federal benefit 4.9 4.7 4.8 Federal and state research and development tax credits 4.7 3.5 3.7 Change in deferred tax asset valuation allowance (45.4 ) (37.8 ) (38.1 ) Other 1.8 (4.4 ) (4.4 ) Effective income tax rate — % — % — % The Company’s net deferred tax assets consisted of the following: December 31, 2015 2014 Net operating loss carryforwards $ 24,226,949 $ 17,728,075 Research and development tax credit carryforwards 5,368,524 3,185,552 Capitalized research and development expenses, net 22,838,452 12,780,675 Accruals and other temporary differences 2,339,444 131,571 Total deferred tax assets 54,773,369 33,825,873 Valuation allowance (54,773,369 ) (33,825,873 ) Net deferred tax asset $ — $ — As of December 31, 2015, the Company had federal and state net operating loss carryforwards of approximately $62,423,730 and $56,872,736, 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 $3,593,673 and $2,689,168, 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, 2015 and 2014. 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. As a result of this Section 382 limitation, approximately $0.3 million of NOLs will expire unutilized. In addition, the Company completed another Section 382 study through December 31, 2014. The results of this study showed that the Company experienced an ownership change in 2014 as part of the follow-on offering, however, none of the NOLs will expire due to the Section 382 limitation associated with the ownership change, 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 carryforwards 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. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2015 and 2014 were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance as of beginning of year $ (33,825,873 ) $ (23,493,416 ) $ (16,548,999 ) Decreases recorded as benefit to income tax provision 2,857,886 2,020,565 1,726,638 Increases recorded to income tax provision (23,805,382 ) (12,353,022 ) (8,671,055 ) Valuation allowance as of end of year $ (54,773,369 ) $ (33,825,873 ) $ (23,493,416 ) 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, 2015 and 2014. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company’s tax years are still open under statute from 2012 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, 2015
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, June 30, September 30, December 31, Operating expenses $ 9,014,943 $ 12,543,819 $ 11,025,625 $ 13,478,689 Net loss (9,173,523 ) (12,436,594 ) (11,136,412 ) (13,568,922 ) Net loss per common share—basic and diluted $ (0.43 ) $ (0.58 ) $ (0.52 ) $ (0.63 ) Weighted average common shares—basic and diluted 21,451,317 21,474,763 21,506,721 21,554,310 Three Months Ended March 31, June 30, September 30, December 31, Operating expenses $ 6,434,760 $ 5,848,825 $ 6,962,319 $ 7,741,370 Net loss (6,542,244 ) (5,930,602 ) (7,035,607 ) (7,805,211 ) Net loss per common share—basic and diluted $ (0.86 ) $ (0.38 ) $ (0.45 ) $ (0.47 ) Weighted average common shares—basic and diluted 7,632,786 15,619,151 15,624,963 16,523,385

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent Events Restricted Common 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 ”

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
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. The Company is reimbursed and offsets research and development expenses in the statement of operations when invoices for allowable costs are prepared and submitted to the U.S. Government agency. 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. When the final determination of the allowable cost for any year has been made, research and development expenses may be adjusted accordingly. The grant also provides the U.S. government agency the ability to terminate the grant for various reasons, including if the Company fails to meet its obligations as set forth in the grant.
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, 2015 and 2014, there were unbilled accounts receivable of $95,285 and $0, 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 that was classified as long-term restricted cash as of December 31, 2015 and 2014. In addition, the Company posted a letter of credit to the lessor of the Company’s Burlington facility in the amount of $98,000 as a security deposit pursuant to the lease agreement in the year ended December 31, 2014. The remaining amount, $50,000 and $98,000, remained as long-term restricted cash at December 31, 2015 and 2014, respectively.
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 Computers and office equipment 3 Computer software 7 Manufacturing equipment 7 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.
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 Debt issuance costs, net represent legal costs related to the Company’s Credit and Security Agreement (Note 9). In April 2015, the FASB issued ASU 2015-03, Interest – Imputation of Interest As of December 31, 2015 and 2014, the carrying value of debt issuance costs was $124,041 and $28,875, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $41,103, $16,500 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, 2015, 2014 and 2013.
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, 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, 2015, 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, 2015, 2014 and 2013. 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, 2015, 2014 and 2013.
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 Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers: Deferral of the Effective Date 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 is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements. In April 2015, the FASB issued ASU 2015-03, Interest—Imputation of Interest In May 2015, the FASB issued Accounting Standards Update 2015-07, Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent) In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Property Plant and Equipment Estimated Useful Lives	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 Computers and office equipment 3 Computer software 7 Manufacturing equipment 7 Furniture and fixtures 5

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 and indicate the level of the fair value hierarchy utilized to determine such fair value: 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 Fair Value Measurements as of December 31, 2014 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 101,687,995 $ — $ 101,687,995 Marketable securities — 48,527,156 — 48,527,156 $ — $ 150,215,151 $ — $ 150,215,151

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of December 31, 2015 and 2014, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2015 Amortized Gross Unrealized Gross Unrealized Fair Value Corporate bonds 55,756,581 3,783 (100,434 ) 55,659,930 $ 55,756,581 $ 3,783 $ (100,434 ) $ 55,659,930 December 31, 2014 Amortized Gross Unrealized Gross Unrealized Fair Value Commercial paper $ 8,991,820 $ 7,570 $ — $ 8,999,390 U.S. Government obligations 28,300,921 181 (5,101 ) 28,296,001 Corporate bonds 11,239,655 2 (7,892 ) 11,231,765 $ 48,532,396 $ 7,753 $ (12,993 ) $ 48,527,156

Prepaid Expenses, Other Curre28

Prepaid Expenses, Other Current Assets, and Other Assets (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014: December 31, 2015 2014 Prepaid expenses $ 135,002 $ 269,199 Security Deposits 182,009 — Interest receivable on marketable securities 441,766 216,615 Employee advances 2,095 — Total prepaid expenses and other current assets $ 760,872 $ 485,814

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Components of Property and Equipment	Property and equipment, net, as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Computer and office equipment $ 393,148 $ 229,980 Manufacturing equipment 2,533,737 153,140 Furniture and fixtures 290,577 181,366 Software 341,906 77,454 Leasehold improvements 239,456 134,573 Construction—in Progress 4,133,893 601,317 $ 7,932,717 $ 1,377,830 Less: Accumulated depreciation $ (490,240 ) $ (268,439 ) Total property and equipment, net $ 7,442,477 $ 1,109,391

Accrued Expenses and Other Cu30

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities at December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Clinical research $ 552,325 $ 1,035,510 Contract manufacturing services 1,443,600 294,900 Payroll and other employee-related expenses 1,648,447 1,172,978 Preclinical services — 119,500 Regulatory Services 63,875 — Consultant fees and expenses 70,000 26,900 Professional services fees 433,453 439,874 Interest expense 80,729 24,111 Other 74,131 99,931 Total accrued expenses and other current liabilities $ 4,366,560 $ 3,213,704

Long-term Debt (Tables)

Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Schedule of Annual Payments Due Under Term Loan	As of December 31, 2015, annual payments due under the Company’s 2015 term loan are as follows (in thousands): Year Aggregate 2016 $ 953,125 2017 5,017,868 2018 5,541,088 2019 5,224,248 2020 2,190,060 Total $ 18,926,389 Less interest (2,574,350 ) Less final payment (1,350,000 ) Long-term debt $ 15,002,039

Preferred and Convertible Pre32

Preferred and Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2015
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 Preferred Carrying Liquidation Common 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, 2015
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 Ending December 31, 2016 $ 527,231 2017 736,425 2018 758,299 2019 647,106 Total $ 2,669,061
Schedule of Future Minimum Payments under Contractual Obligations	Future minimum payments under the Company’s agreed obligations are as follows: Year Ending December 31, 2016 $ 2,890,598 2017 6,670,620 2018 7,560,036 2019 8,894,160 2020 and thereafter 53,364,960 Total $ 79,380,374

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based 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, 2015, 2014 and 2013 is as follows: December 31, 2015 2014 2013 Risk-free interest rates 1.49-1.92 % 1.54-2.04 % 1.00 % Expected dividend yield 0.00 % 0.00 % 0.00 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 76.4-83.9 % 61.9-68.0 % 71 %
Summary of Stock Option Activity	The following table summarizes stock option activity for the year ended December 31, 2015: Shares Weighted Outstanding as of December 31, 2014 1,289,082 $ 10.26 Granted 657,250 21.84 Exercised (109,441 ) 5.44 Canceled or forfeited (179,636 ) 18.42 Outstanding as of December 31, 2015 1,657,255 $ 14.28 Options vested and expected to vest at December 31, 2015 1,541,800 13.92 Options exercisable at December 31, 2015 728,621 8.81
Stock-Based Compensation Expense Related to Stock Options	The Company recorded stock-based compensation expense related to stock options and restricted stock for the years ended December 31, 2015, 2014 and 2013 as follows: Year Ended December 31, 2015 2014 2013 Research and development $ 1,412,153 $ 684,511 $ 347,470 General and administrative 3,170,526 1,766,068 648,697 $ 4,582,679 $ 2,450,579 $ 996,167

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015, 2014 and 2013: Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (46,315,451 ) $ (27,313,664 ) $ (18,186,724 ) Accretion of dividends on convertible preferred stock — — — Net income attributable to participating securities — — — Net loss attributable to common stockholders: $ (46,315,451 ) $ (27,313,664 ) $ (18,186,724 ) Denominator: Weighted average common shares outstanding, basic and diluted 21,497,119 13,893,961 790,038 Net loss per share attributable to common stockholders, basic and diluted $ (2.15 ) $ (1.97 ) $ (23.02 )

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 2013 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State taxes, net of federal benefit 4.9 4.7 4.8 Federal and state research and development tax credits 4.7 3.5 3.7 Change in deferred tax asset valuation allowance (45.4 ) (37.8 ) (38.1 ) Other 1.8 (4.4 ) (4.4 ) Effective income tax rate — % — % — %
Net Deferred Tax Assets	The Company’s net deferred tax assets consisted of the following: December 31, 2015 2014 Net operating loss carryforwards $ 24,226,949 $ 17,728,075 Research and development tax credit carryforwards 5,368,524 3,185,552 Capitalized research and development expenses, net 22,838,452 12,780,675 Accruals and other temporary differences 2,339,444 131,571 Total deferred tax assets 54,773,369 33,825,873 Valuation allowance (54,773,369 ) (33,825,873 ) 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, 2015 and 2014 were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance as of beginning of year $ (33,825,873 ) $ (23,493,416 ) $ (16,548,999 ) Decreases recorded as benefit to income tax provision 2,857,886 2,020,565 1,726,638 Increases recorded to income tax provision (23,805,382 ) (12,353,022 ) (8,671,055 ) Valuation allowance as of end of year $ (54,773,369 ) $ (33,825,873 ) $ (23,493,416 )

Quarterly Financial Data (una37

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
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, June 30, September 30, December 31, Operating expenses $ 9,014,943 $ 12,543,819 $ 11,025,625 $ 13,478,689 Net loss (9,173,523 ) (12,436,594 ) (11,136,412 ) (13,568,922 ) Net loss per common share—basic and diluted $ (0.43 ) $ (0.58 ) $ (0.52 ) $ (0.63 ) Weighted average common shares—basic and diluted 21,451,317 21,474,763 21,506,721 21,554,310 Three Months Ended March 31, June 30, September 30, December 31, Operating expenses $ 6,434,760 $ 5,848,825 $ 6,962,319 $ 7,741,370 Net loss (6,542,244 ) (5,930,602 ) (7,035,607 ) (7,805,211 ) Net loss per common share—basic and diluted $ (0.86 ) $ (0.38 ) $ (0.45 ) $ (0.47 ) Weighted average common shares—basic and diluted 7,632,786 15,619,151 15,624,963 16,523,385

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007
Cash and cash equivalents and marketable securities	$ 118,603,974	$ 151,624,678

Financing Transactions - Additi

Financing Transactions - Additional Information (Detail) $ / shares in Units, $ in Millions	Dec. 17, 2014USD ($)$ / sharesshares	Feb. 18, 2014USD ($)$ / sharesshares	Nov. 05, 2007shares	Dec. 31, 2015shares	Dec. 31, 2014shares
Class of Stock [Line Items]
Sale of common stock price per share | $ / shares	$ 17	$ 13
Net proceeds from initial public offering | $	$ 92.2	$ 67.2
Reverse stock split conversion ratio				1-for-8.13 reverse stock split
Common stock, shares outstanding				21,570,395	21,440,058
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued			109		11,546,000
Reverse stock split conversion ratio				0.123
Common stock issued upon conversion of redeemable preferred stock					8,952,057
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued	5,796,000	5,750,000
Reverse stock split conversion ratio				1-for-8.13 reverse stock split
Common stock, shares outstanding				21,570,395
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Common stock issued upon conversion of redeemable preferred stock		8,952,057
IPO [Member] | Common Stock [Member]
Class of Stock [Line Items]
Reverse stock split conversion ratio					0.123

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 28, 2014
Summary Of Significant Accounting Policies [Line Items]
Unbilled accounts receivable	$ 95,285	$ 0
Long-term restricted cash	80,000	$ 128,000
Deferred offering costs	$ 0			$ 1,623,540
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Debt issuance costs	$ 136,446
Debt issuance costs reclassified	$ 28,875
Construction-in Progress [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	7 years
Mid Cap [Member] | Secured Debt [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying value of debt issuance costs reported in prepaid expenses and other current assets	$ 124,041	$ 28,875
Amortization of debt issuance costs recognized as interest expense	41,103	16,500	$ 16,500
Certificates of Deposit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	30,000	30,000
Letter of Credit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 50,000	$ 98,000	$ 98,000

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2015
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	3 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	7 years
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	7 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	5 years

Fair Value of Financial Asset42

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 48,302,507	$ 48,527,156
Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	61,533,797	101,687,995
Marketable securities	55,659,930	48,527,156
Assets, Total	117,193,727	150,215,151
Level 2 [Member] | Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	61,533,797	101,687,995
Marketable securities	55,659,930	48,527,156
Assets, Total	$ 117,193,727	$ 150,215,151

Fair Value of Financial Asset43

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 55,756,581	$ 48,532,396
Gross Unrealized Gains	3,783	7,753
Gross Unrealized Losses	(100,434)	(12,993)
Fair Value	55,659,930	48,527,156
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		8,991,820
Gross Unrealized Gains		7,570
Fair Value		8,999,390
U.S Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		28,300,921
Gross Unrealized Gains		181
Gross Unrealized Losses		(5,101)
Fair Value		28,296,001
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	55,756,581	11,239,655
Gross Unrealized Gains	3,783	2
Gross Unrealized Losses	(100,434)	(7,892)
Fair Value	$ 55,659,930	$ 11,231,765

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Marketable securities	$ 48,302,507	$ 48,527,156
Marketable securities, noncurrent	$ 7,357,423

Prepaid Expenses, Other Curre46

Prepaid Expenses, Other Current Assets, and Other Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) - USD ($)	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 135,002		$ 269,199
Security Deposits	182,009
Interest receivable on marketable securities	441,766		216,615
Employee advances	2,095
Total prepaid expenses and other current assets	$ 760,872	$ 28,875	$ 485,814

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 7,932,717	$ 1,377,830
Less: Accumulated depreciation	(490,240)	(268,439)
Total property and equipment, net	7,442,477	1,109,391
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	393,148	229,980
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	2,533,737	153,140
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	290,577	181,366
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	239,456	134,573
Construction-in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	4,133,893	601,317
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 341,906	$ 77,454

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Abstract]
Depreciation	$ 238,153	$ 120,341	$ 79,808
Property and equipment disposals	166,335	113,502
Gain (Loss) on sale of property and equipment	$ (149,983)	$ 0	$ (14,111)

Accrued Expenses and Other Cu49

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Clinical research	$ 552,325	$ 1,035,510
Contract manufacturing services	1,443,600	294,900
Payroll and other employee-related expenses	1,648,447	1,172,978
Preclinical services		119,500
Regulatory Services	63,875
Consultant fees and expenses	70,000	26,900
Professional services fees	433,453	439,874
Interest expense	80,729	24,111
Other	74,131	99,931
Total accrued expenses and other current liabilities	$ 4,366,560	$ 3,213,704

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	Aug. 04, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 03, 2013
Debt Instrument [Line Items]
Debt issuance costs						$ 61,876
Prepaid expenses and other current assets		$ 28,875	$ 760,872	$ 485,814
Carrying value of term loan			15,002,039,000
Debt issuance costs reclassified			28,875
Secured Debt [Member] | Mid Cap [Member]
Debt Instrument [Line Items]
Capitalized debt acquisition costs			136,446
Amortization expense for debt issuance costs			41,103	$ 16,500	$ 16,500
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
Interest on final payment	9.00%
Carrying value of term loan			$ 15,126,080
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
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
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

Long Term Debt - Schedule of An

Long Term Debt - Schedule of Annual Payments Due Under Term Loan (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,016	$ 953,125
2,017	5,017,868
2,018	5,541,088
2,019	5,224,248
2,020	2,190,060
Total	18,926,389
Less interest	(2,574,350)
Less final payment	(1,350,000)
Long-term debt	$ 15,002,039

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional information (Detail)	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)$ / sharesshares	Dec. 31, 2013USD ($)$ / sharesshares	Dec. 31, 2012USD ($)$ / sharesshares	Feb. 18, 2014shares
Temporary Equity [Line Items]
Reverse stock split conversion ratio	1-for-8.13 reverse stock split
Dividend at annual rate	8.00%
Dividends declared or paid | $	$ 0
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
IPO [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio	1-for-8.13 reverse stock split
IPO [Member] | Qualifying Initial Public Offering That Triggers Mandatory Conversion [Member] | Minimum [Member]
Temporary Equity [Line Items]
Initial public offering , per share price | $ / shares		$ 27.48
Gross cash proceed from initial public offering | $		$ 40,000,000
IPO [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio		0.123
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Net proceed from issuance of convertible preferred stock | $	$ 0	$ 0	$ 0
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
Conversion price of preferred stock | $ / shares			$ 8.13
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Net proceed from issuance of convertible preferred stock | $	$ 0	$ 0	$ 0
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
Preferred stock issued during period				17,736,786
Initial public offering , per share price | $ / shares				$ 1.1276
Net proceed from issuance of convertible preferred stock | $				$ 19,888,478
Warrants issuable				218,160
Warrants issuable, per share | $ / shares				$ 0.01
Conversion price of preferred stock | $ / shares			$ 9.166575
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, 2015	Dec. 31, 2014
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)	Dec. 17, 2014$ / sharesshares	Feb. 18, 2014$ / sharesshares	Nov. 05, 2007shares	Dec. 31, 2014Vote / shares$ / sharesshares	Dec. 31, 2012$ / sharesshares	Dec. 31, 2009$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares
Class of Stock [Line Items]
Common stock, share authorized			10,000,000	100,000,000	94,000,000	79,000,000	100,000,000
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
Sale of common stock price per share | $ / shares	$ 17	$ 13
Number of vote per common stock | Vote / shares				1
Dividend declared | $							$ 0
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued	5,796,000	5,750,000
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Common stock issued upon conversion of redeemable preferred stock		8,952,057
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued			109	11,546,000
Common stock issued upon conversion of redeemable preferred stock				8,952,057

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) £ in Millions	Jul. 13, 2015USD ($)ft²	Jun. 12, 2009USD ($)	Jul. 31, 2015GBP (£)	May. 31, 2013USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Sep. 03, 2010USD ($)
Commitment And Contingencies [Line Items]
Minimum monthly lease payments				$ 17,588
Security deposit paid to lessor					$ 182,009
Operating lease term				42 months
Rent expense					373,202	$ 242,946	$ 227,381
Research and development expense					32,691,445	17,923,348	11,060,912
Zilretta [Member] | South West Research Institute (SWRI) [Member] | Upon FDA First Approval [Member]
Commitment And Contingencies [Line Items]
License agreement, fee payable					120,000
Zilretta [Member] | South West Research Institute (SWRI) [Member] | FDA First Approval Not Received by December 31, 2019 [Member]
Commitment And Contingencies [Line Items]
License agreement, fee payable on January 1, 2020					20,000
License agreement, fee payable on January 1, 2021					40,000
License agreement, fee payable on January 1, 2022					60,000
AstraZeneca [Member]
Commitment And Contingencies [Line Items]
License agreement, fee payable										$ 1,000,000
Research and development expense								$ 1,000,000	$ 1,000,000
License agreement expense					0	$ 0	$ 0
Licensing agreement fees paid		$ 1,000,000
AstraZeneca [Member] | Within Thirty Days of the Execution of the Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, fee payable										500,000
AstraZeneca [Member] | Six Months After the Execution of the Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, fee payable										500,000
AstraZeneca [Member] | Achievement of Certain Sales Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payment		45,000,000								75,000,000
AstraZeneca [Member] | OA Indications [Member]
Commitment And Contingencies [Line Items]
Milestone payment		17,000,000								21,000,000
AstraZeneca [Member] | Non - OA Indications [Member]
Commitment And Contingencies [Line Items]
Milestone payment		$ 11,000,000								$ 15,000,000
Letter of Credit [Member]
Commitment And Contingencies [Line Items]
Security deposit paid to lessor				$ 98,000
Reduction in letter of credit					50,000
Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]
Commitment And Contingencies [Line Items]
Manufacturing agreement period			10 years
Monthly base fee and reimbursement costs | £			£ 100
License agreement, fee payable					79,380,374
License agreement, fee payable on January 1, 2020					$ 8,894,160
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Additional leased office space | ft²	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, 2019
Extended leased office space | ft²	5,400
Extended lease agreement period	3 years
Operating lease early termination notice period	9 months
Base rent for original, amended office space	$ 2,900,000

Commitments and Contingencies57

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail)	Dec. 31, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,016	$ 527,231
2,017	736,425
2,018	758,299
2,019	647,106
Total	$ 2,669,061

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, 2015USD ($)
Property Subject to or Available for Operating Lease [Line Items]
2,016	$ 2,890,598
2,017	6,670,620
2,018	7,560,036
2,019	8,894,160
2020 and thereafter	53,364,960
Total	$ 79,380,374

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jul. 16, 2013	Sep. 30, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 27, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding			1,657,255	1,289,082	834,983
Stock options granted			657,250
Stock-based compensation expense			$ 4,582,679	$ 2,450,579	$ 996,167
Unrecognized stock-based compensation expense			$ 8,629,518
Number of options exercised			109,441	141,141	4,868
Aggregate intrinsic value of stock options exercised			$ 1,584,657	$ 1,476,861	$ 45,125
Weighted average remaining contractual term			7 years 10 months 24 days	8 years 1 month 6 days	8 years 6 months
Weighted average exercise price			$ 14.28	$ 10.26	$ 2.99
Options exercisable, shares			728,621	450,109	386,334
Weighted average remaining contractual term			6 years 10 months 24 days	6 years 8 months 12 days	6 years 10 months 24 days
Aggregate intrinsic value, stock options exercisable			$ 7,714,057	$ 7,489,976	$ 4,071,356
Weighted average grant date fair value of options granted			$ 21.84	$ 9.99	$ 6.99
Unrecognized compensation costs, weighted-average recognition periods			2 years 7 months 6 days
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%
2009 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding			530,680
2013 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding			1,126,575
Number of shares available for future issuance			1,287,381
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			20,896	6,770
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, issued			0	0
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			657,250	727,575	201,721	403,382
Percentage of share based award, vested			25.00%
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
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
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
Maximum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based award, expiration period			10 years

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, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rates			1.00%
Risk-free interest rates, Minimum	1.49%	1.54%
Risk-free interest rates, Maximum	1.92%	2.04%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years	6 years	6 years
Expected volatility			71.00%
Expected volatility, Minimum	76.40%	61.90%
Expected volatility, Maximum	83.90%	68.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares Issuable Under Options, Beginning balance	1,289,082	834,983
Shares Issuable Under Options, Granted	657,250
Shares Issuable Under Options, Exercised	(109,441)	(141,141)	(4,868)
Shares Issuable Under Options, Canceled or forfeited	(179,636)
Shares Issuable Under Options, Ending balance	1,657,255	1,289,082	834,983
Shares Issuable Under Options, Options vested and expected to vest at September 30, 2015	1,541,800
Shares Issuable Under Options, Options exercisable at September 30, 2015	728,621	450,109	386,334
Weighted Average Exercise Price, Beginning balance	$ 10.26	$ 2.99
Weighted Average Exercise Price, Granted	21.84	9.99	$ 6.99
Weighted Average Exercise Price, Exercised	5.44
Weighted Average Exercise Price, Canceled or forfeited	18.42
Weighted Average Exercise Price, Ending balance	$ 14.28	$ 10.26	$ 2.99

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4,582,679	$ 2,450,579	$ 996,167
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,412,153	684,511	347,470
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 3,170,526	$ 1,766,068	$ 648,697

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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (13,568,922)	$ (11,136,412)	$ (12,436,594)	$ (9,173,523)	$ (7,805,211)	$ (7,035,607)	$ (5,930,602)	$ (6,542,244)	$ (46,315,451)	$ (27,313,664)	$ (18,186,724)
Accretion of dividends on convertible preferred stock									0	0	0
Net income attributable to participating securities									0	0	0
Net loss attributable to common stockholders:									$ (46,315,451)	$ (27,313,664)	$ (18,186,724)
Denominator:
Weighted average common shares outstanding, basic and diluted	21,554,310	21,506,721	21,474,763	21,451,317	16,523,385	15,624,963	15,619,151	7,632,786	21,497,119	13,893,961	790,038
Net loss per share attributable to common stockholders, basic and diluted	$ (0.63)	$ (0.52)	$ (0.58)	$ (0.43)	$ (0.47)	$ (0.45)	$ (0.38)	$ (0.86)	$ (2.15)	$ (1.97)	$ (23.02)

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	1,655,394	1,185,253	648,591
Restricted Stock [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	0	0	0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 0	$ 0
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.
Net operating losses (NOLs) expire unutilized	$ 300,000
Liabilities for unrecognized tax benefits	0	$ 0
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 62,423,730
Operating loss carryforwards expiration year	2,029
Federal [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 3,593,673
Tax credit expiration year	2,029
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 56,872,736
Operating loss carryforwards expiration year	2,030
State and Local Jurisdiction [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 2,689,168
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, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	4.90%	4.70%	4.80%
Federal and state research and development tax credits	4.70%	3.50%	3.70%
Change in deferred tax asset valuation allowance	45.40%	(37.80%)	(38.10%)
Other	1.80%	(4.40%)	(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, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 24,226,949	$ 17,728,075
Research and development tax credit carryforwards	5,368,524	3,185,552
Capitalized research and development expenses, net	22,838,452	12,780,675
Accruals and other temporary differences	2,339,444	131,571
Total deferred tax assets	54,773,369	33,825,873
Valuation allowance	(54,773,369)	(33,825,873)	$ (23,493,416)	$ (16,548,999)
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, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes Disclosure [Line Items]
Valuation allowance as of beginning of year	$ (33,825,873)	$ (23,493,416)	$ (16,548,999)
Valuation allowance as of end of year	(54,773,369)	(33,825,873)	(23,493,416)
Decreases Recorded as Benefit to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increase) recorded as benefit to income tax provision	2,857,886	2,020,565	1,726,638
Increases Recorded to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increase) recorded as benefit to income tax provision	$ (23,805,382)	$ (12,353,022)	$ (8,671,055)

Quarterly Financial Data (una69

Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 13,478,689	$ 11,025,625	$ 12,543,819	$ 9,014,943	$ 7,741,370	$ 6,962,319	$ 5,848,825	$ 6,434,760	$ 46,063,076	$ 26,987,274	$ 17,765,209
Net loss	$ (13,568,922)	$ (11,136,412)	$ (12,436,594)	$ (9,173,523)	$ (7,805,211)	$ (7,035,607)	$ (5,930,602)	$ (6,542,244)	$ (46,315,451)	$ (27,313,664)	$ (18,186,724)
Net loss per common share-basic and diluted	$ (0.63)	$ (0.52)	$ (0.58)	$ (0.43)	$ (0.47)	$ (0.45)	$ (0.38)	$ (0.86)	$ (2.15)	$ (1.97)	$ (23.02)
Weighted average common shares-basic and diluted	21,554,310	21,506,721	21,474,763	21,451,317	16,523,385	15,624,963	15,619,151	7,632,786	21,497,119	13,893,961	790,038

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Restricted Stock Units (RSUs) [Member] - Subsequent Event [Member]	Jan. 04, 2016USD ($)shares
Subsequent Event [Line Items]
Aggregate number of restricted common stock units granted under performance-based vesting | shares	189,300
Grant date fair value of restricted common stock units under performance-based vesting | $	$ 3,445,000