Flexion Therapeutics (FLXN) 10-Q 9 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 02, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
Trading Symbol	FLXN
Entity Registrant Name	Flexion Therapeutics Inc
Entity Central Index Key	1,419,600
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		21,539,396

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 73,507,121	$ 103,097,522
Marketable securities	58,658,480	48,527,156
Accounts receivable	46,443
Prepaid expenses and other current assets	802,635	485,814
Total current assets	133,014,679	152,110,492
Property and equipment, net	4,153,389	1,109,391
Restricted cash	80,000	128,000
Total assets	137,248,068	153,347,883
Current liabilities:
Accounts payable	1,917,540	1,584,822
Accrued expenses and other current liabilities	4,668,377	3,213,704
Current portion of long-term debt		1,983,500
Total current liabilities	6,585,917	6,782,026
Long-term debt	14,942,401	1,580,958
Other long-term liabilities	13,459	43,008
Total liabilities	$ 21,541,777	$ 8,405,992
Commitments and contingencies
Preferred Stock, $0.001 par value; 10,000,000 shares authorized at September 30, 2015 and December 31, 2014 and 0 shares issued and outstanding at September 30, 2015 and December 31, 2014
Stockholders' equity:
Common stock, $0.001 par value; 100,000,000 shares authorized; 21,539,396 and 21,440,058 shares issued and outstanding, at September 30, 2015 and December 31, 2014, respectively	$ 21,539	$ 21,440
Additional paid-in capital	241,905,674	238,402,514
Accumulated other comprehensive income	2,430	(5,240)
Accumulated deficit	(126,223,352)	(93,476,823)
Total stockholders' equity	115,706,291	144,941,891
Total liabilities and stockholders' equity	$ 137,248,068	$ 153,347,883

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 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	21,539,396	21,440,058
Common stock, shares outstanding	21,539,396	21,440,058

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	7,828,927	4,658,293	23,724,055	12,423,733
General and administrative	3,196,698	2,304,026	8,860,332	6,822,171
Total operating expenses	11,025,625	6,962,319	32,584,387	19,245,904
Loss from operations	(11,025,625)	(6,962,319)	(32,584,387)	(19,245,904)
Other income (expense):
Interest income	273,679	153,122	882,001	318,524
Interest expense	(202,399)	(96,926)	(405,867)	(314,630)
Other income (expense), net	(182,067)	(129,484)	(638,276)	(266,443)
Total other income (expense)	(110,787)	(73,288)	(162,142)	(262,549)
Net loss	$ (11,136,412)	$ (7,035,607)	$ (32,746,529)	$ (19,508,453)
Net loss per share basic and diluted	$ (0.52)	$ (0.45)	$ (1.52)	$ (1.50)
Weighted average common shares outstanding, basic and diluted	21,506,721	15,624,963	21,477,830	13,007,892
Other comprehensive (loss) income:
Unrealized gains from available-for-sale securities, net of tax of $0	$ 8,879	$ (2,468)	$ 7,670	$ 782
Total other comprehensive (loss) income	8,879	(2,468)	7,670	782
Comprehensive loss	$ (11,127,533)	$ (7,038,075)	$ (32,738,859)	$ (19,507,671)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Other comprehensive (loss) income, Tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (32,746,529)	$ (19,508,453)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	130,369	84,943
Stock-based compensation expense	3,099,092	1,752,234
Amortization of premium (discount) on marketable securities	637,208	226,834
Other non-cash charges	31,177	12,374
Changes in operating assets and liabilities:
Accounts receivable	(46,443)
Prepaid expenses, other current and long-term assets	(242,642)	(350,534)
Accounts payable	388,905	296,331
Accrued expenses and other current and long-term liabilities	830,030	671,891
Other	(44,458)
Net cash used in operating activities	(27,963,291)	(16,814,380)
Cash flows from investing activities
Purchases of property and equipment	(2,415,380)	(326,571)
Change in restricted cash	24,000
Purchases of marketable securities	(106,465,361)	(72,359,552)
Sale and redemption of marketable securities	95,704,499	20,160,000
Net cash used in investing activities	(13,152,242)	(52,526,123)
Cash flows from financing activities
Payment of public offering costs	(224,648)	(1,282,785)
Payments on debt	(3,500,000)	(1,000,001)
Payment of debt issuance costs	(107,741)
Proceeds from the issuance of common stock		69,517,500
Proceeds from the issuance of notes payable	15,003,533
Proceeds from the exercise of stock options	216,454	282,545
Proceeds from Employee Stock Purchase Plan	137,534
Net cash provided by financing activities	11,525,132	67,517,259
Net decrease in cash and cash equivalents	(29,590,401)	(1,823,244)
Cash and cash equivalents at beginning of period	103,097,522	16,188,254
Cash and cash equivalents at end of period	73,507,121	14,365,010
Supplemental disclosures of cash flow information:
Cash paid for interest	369,719	287,370
Supplemental disclosures of non-cash financing activities:
Conversion of convertible preferred stock into common stock		74,806,213
Purchases of property and equipment in accounts payable and accrued expenses	$ 810,998	$ 52,011

Overview and Nature of the Busi

Overview and Nature of the Business	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview and Nature of the Business	1. Overview and 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, injectable pain therapies. The Company is targeting anti-inflammatory and analgesic therapies for the treatment of patients with musculoskeletal conditions, beginning with osteoarthritis, a type of degenerative arthritis (“OA”) and post-operative pain. Flexion’s broad and diversified 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 pain therapies. The Company is subject to risks and uncertainties common to 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements as of September 30, 2015, and for the three and nine months ended September 30, 2015 and 2014, 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 consolidated financial information including the accounts of the Company and its wholly-owned subsidiary after elimination of all significant intercompany accounts and transactions. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all adjustments which are necessary for a fair statement of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K filed with the SEC on March 24, 2015. The information presented in the condensed consolidated financial statements and related notes as of September 30, 2015, and for the three and nine months ended September 30, 2015 and 2014, is unaudited. The December 31, 2014 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the three and nine months ended September 30, 2015 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2015, or any future period. The accompanying condensed 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 September 30, 2015 and December 31, 2014, the Company had cash and cash equivalents and marketable securities of $132,165,601 and $151,624,678, respectively. Management believes that current cash, cash equivalents and marketable securities on hand at September 30, 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’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 as 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. In April 2015, the FASB issued ASU 2015-03, Interest—Imputation of Interest Consolidation The accompanying condensed consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the three and nine months ended September 30, 2015 and the year ended December 31, 2014, the year Flexion Securities Corporation, Inc. was established. 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 FX006. The related costs incurred under the grant are included in research and development expense in the statements 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 costs 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 September 30, 2015 there were no unbilled accounts receivable. 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 condensed 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. 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, software and office equipment 3 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 improvements 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. Construction-in-progress, which represents direct costs related to the construction of manufacturing equipment, is not depreciated until the asset is ready for its intended use.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s assets that are measured at fair value on a recurring basis as of September 30, 2015 and December 31, 2014 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of September 30, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 58,760,286 $ — $ 58,760,286 Marketable securities — 58,658,480 — 58,658,480 $ — $ 117,418,766 $ — $ 117,418,766 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 September 30, 2015 and December 31, 2014, the Company’s cash equivalents and marketable securities that were invested primarily in U.S. treasury bills, corporate bonds, money market funds, commercial paper and U.S. Government agency holdings were valued based primarily 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 nine months ended September 30, 2015 and year ended December 31, 2014, there were no transfers between Level 1, Level 2 and Level 3. The carrying values of accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these balances. 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 balance sheet. The Company determined the fair value of the term loans using an income approach, that 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 September 30, 2015 and December 31, 2014. The result of the calculations yielded fair values that approximate carrying value.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities As of September 30, 2015 and December 31, 2014, the fair value of available-for-sale marketable securities by type of security was as follows: September 30, 2015 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Corporate bonds $ 44,916,443 $ 8,301 $ (11,341 ) $ 44,913,403 Commercial paper 13,744,467 610 — 13,745,077 $ 58,660,910 $ 8,911 $ (11,341 ) $ 58,658,480 December 31, 2014 Amortized Cost 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 September 30, 2015 and December 31, 2014, marketable securities consisted of investments that mature within twelve months.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment as of September 30, 2015 and December 31, 2014 consisted of the following: September 30, December 31, Computer and office equipment $ 350,345 $ 229,980 Manufacturing equipment 153,140 153,140 Furniture and fixtures 236,810 181,366 Software 274,142 77,454 Leasehold improvements 239,456 134,573 Construction—in progress 3,298,304 601,317 4,552,197 1,377,830 Less: Accumulated depreciation (398,808 ) (268,439 ) Total property and equipment, net $ 4,153,389 $ 1,109,391 Depreciation expense for the three months ended September 30, 2015 and 2014 was $47,945 and $31,897, respectively. Depreciation expense for the nine months ended September 30, 2015 and 2014 was $130,369 and $84,943, respectively. During the nine months ended September 30, 2015 and 2014, there were no disposals of property and equipment. Construction-in progress is primarily comprised of amounts related to the construction of new manufacturing equipment for use by our contract manufacturers.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2015
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: September 30, December 31, Clinical research $ 1,145,600 $ 1,035,510 Contract manufacturing services 1,349,875 294,900 Payroll and other employee-related expenses 1,392,377 1,172,978 Preclinical services 103,000 119,500 Consultant fees and expenses 203,750 26,900 Professional services fees 329,568 439,874 Interest expense 78,125 24,111 Other 66,082 99,931 Total accrued expenses and other current liabilities $ 4,668,377 $ 3,213,704

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation Stock Option Valuation The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. Prior to the IPO, the Company was a private company and therefore lacked company-specific historical and implied volatility information. Therefore, the Company 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 nine months ended September 30, 2015 and 2014 are as follows: Nine months ended September 30, 2015 2014 Risk-free interest rates 1.49-1.92 % 1.54-2.04 % Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.0 6.0 Expected volatility 76.4-81.4 % 61.9-67.9 % The following table summarizes stock option activity for the nine months ended September 30, 2015: Shares Issuable Under Options Weighted Average Outstanding as of December 31, 2014 1,289,082 $ 10.26 Granted 556,550 22.81 Exercised (88,371 ) 3.03 Canceled (179,636 ) 18.42 Outstanding as of September 30, 2015 1,577,625 $ 14.15 Options vested and expected to vest at September 30, 2015 1,363,360 Options exercisable at September 30, 2015 666,984 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 88,371 options were exercised during the nine months ended September 30, 2015. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2015 was $1,530,551. At September 30, 2015 and 2014, the Company had options for the purchase of 1,577,625 and 1,238,973 shares of common stock outstanding, respectively, with a weighted average remaining contractual term of 8.1, and 8.3 years, respectively, and with a weighted average exercise price of $10.32 and $9.78 per share, respectively. The weighted average grant date fair value of options granted during the nine months ended September 30, 2015 and 2014 was $15.67 and $10.15, respectively. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options for the three and nine months ended September 30, 2015 and 2014 as follows: Three months ended September 30, Nine months ended 2015 2014 2015 2014 Research and development $ 331,551 $ 170,668 $ 940,698 $ 466,465 General and administrative 789,432 504,141 2,158,394 1,285,769 $ 1,120,983 $ 674,809 $ 3,099,092 $ 1,752,234 As of September 30, 2015, unrecognized stock-based compensation expense for stock options outstanding was $11,499,534, which is expected to be recognized over a weighted average period of 2.7 years. As of September 30, 2014, unrecognized stock-based compensation expense for stock options outstanding was $7,822,984, which was expected to be recognized over a weighted average period of 3.0 years.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Net Loss Per Share	8. Net Loss Per Share Basic and diluted net loss per share was calculated as follows for the three and nine months ended September 30, 2015 and 2014: For the three months ended For the nine months ended 2015 2014 2015 2014 Numerator: Net loss $ (11,136,412 ) $ (7,035,607 ) $ (32,746,529 ) $ (19,508,453 ) Net loss: $ (11,136,412 ) $ (7,035,607 ) $ (32,746,529 ) $ (19,508,453 ) Denominator: Weighted average common shares outstanding, basic and diluted 21,506,721 15,624,963 21,477,830 13,007,892 Net loss per share, basic and diluted $ (0.52 ) $ (0.45 ) $ (1.52 ) $ (1.50 ) Stock options for the purchase of 1,715,837 and 1,234,199 weighted average shares of common stock were excluded from the computation of diluted net loss per share for the three months ended September 30, 2015 and 2014, respectively, and 1,658,292 and 1,130,000 weighted average shares of common stock were excluded from the computation of diluted net loss per share for the nine months ended September 30, 2015 and 2014, respectively. These options were excluded from the computations because the options had an anti-dilutive impact due to the net loss incurred for those periods.

Long-term Debt

Long-term Debt	9 Months Ended
	Sep. 30, 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 amount was 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. 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 balance sheet. 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 FX006 clinical trial data meeting the trial’s primary endpoint which is sufficient to file a New Drug Application (NDA) for FX006, 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. 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 September 30, 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 September 30, 2015, the carrying value of the term loan was $14,942,401 the entire balance of which is classified as long-term debt on the balance sheet as of September 30, 2015. In connection with the credit and security agreement, the Company incurred debt issuance costs totaling $112,309. These costs will be amortized over the estimated term of the debt using the straight-line method which approximates the effective interest method. As of September 30, 2015, annual payments due under the Company’s 2015 term loan are as follows (in thousands): Year Aggregate 2015 (remaining three months) $ 236,979 2016 953,125 2017 5,017,868 2018 5,541,088 2019 5,224,248 2020 840,061 Total $ 17,813,369

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Leases On July 13, 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 extends the term of the original lease through October 31, 2019, contemporaneous with the Additional Space, and also provides 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. As of September 30, 2015, annual aggregate minimum payments due under the Company’s lease obligations are as follows (in thousands): Year Aggregate 2015 (remaining three months) $ 127,999 2016 521,364 2017 736,425 2018 758,299 2019 647,106 Total $ 2,791,193 Manufacturing and Supply Agreement with Patheon U.K. Limited On July 31, 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 FX006, 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 technical transfer activities and construction services needed to prepare Patheon’s Swindon, United Kingdom facility for the commercial manufacture of FX006 in dedicated manufacturing suites. The Company will provide Patheon with certain equipment and materials necessary to manufacture FX006 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 FX006. 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.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements as of September 30, 2015, and for the three and nine months ended September 30, 2015 and 2014, 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 consolidated financial information including the accounts of the Company and its wholly-owned subsidiary after elimination of all significant intercompany accounts and transactions. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all adjustments which are necessary for a fair statement of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K filed with the SEC on March 24, 2015. The information presented in the condensed consolidated financial statements and related notes as of September 30, 2015, and for the three and nine months ended September 30, 2015 and 2014, is unaudited. The December 31, 2014 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the three and nine months ended September 30, 2015 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2015, or any future period. The accompanying condensed 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 September 30, 2015 and December 31, 2014, the Company had cash and cash equivalents and marketable securities of $132,165,601 and $151,624,678, respectively. Management believes that current cash, cash equivalents and marketable securities on hand at September 30, 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’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 as 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. In April 2015, the FASB issued ASU 2015-03, Interest—Imputation of Interest
Consolidation	Consolidation The accompanying condensed consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the three and nine months ended September 30, 2015 and the year ended December 31, 2014, the year Flexion Securities Corporation, Inc. was established.
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 FX006. The related costs incurred under the grant are included in research and development expense in the statements 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 costs 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 September 30, 2015 there were no unbilled accounts receivable.
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 condensed 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.
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, software and office equipment 3 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 improvements 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. Construction-in-progress, which represents direct costs related to the construction of manufacturing equipment, is not depreciated until the asset is ready for its intended use.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 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, software and office equipment 3 Manufacturing equipment 7 Furniture and fixtures 5

Fair Value of Financial Asset19

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 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 that are measured at fair value on a recurring basis as of September 30, 2015 and December 31, 2014 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of September 30, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 58,760,286 $ — $ 58,760,286 Marketable securities — 58,658,480 — 58,658,480 $ — $ 117,418,766 $ — $ 117,418,766 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)	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of September 30, 2015 and December 31, 2014, the fair value of available-for-sale marketable securities by type of security was as follows: September 30, 2015 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Corporate bonds $ 44,916,443 $ 8,301 $ (11,341 ) $ 44,913,403 Commercial paper 13,744,467 610 — 13,745,077 $ 58,660,910 $ 8,911 $ (11,341 ) $ 58,658,480 December 31, 2014 Amortized Cost 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

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Components of Property and Equipment	Property and equipment as of September 30, 2015 and December 31, 2014 consisted of the following: September 30, December 31, Computer and office equipment $ 350,345 $ 229,980 Manufacturing equipment 153,140 153,140 Furniture and fixtures 236,810 181,366 Software 274,142 77,454 Leasehold improvements 239,456 134,573 Construction—in progress 3,298,304 601,317 4,552,197 1,377,830 Less: Accumulated depreciation (398,808 ) (268,439 ) Total property and equipment, net $ 4,153,389 $ 1,109,391

Accrued Expenses and Other Cu22

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, December 31, Clinical research $ 1,145,600 $ 1,035,510 Contract manufacturing services 1,349,875 294,900 Payroll and other employee-related expenses 1,392,377 1,172,978 Preclinical services 103,000 119,500 Consultant fees and expenses 203,750 26,900 Professional services fees 329,568 439,874 Interest expense 78,125 24,111 Other 66,082 99,931 Total accrued expenses and other current liabilities $ 4,668,377 $ 3,213,704

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2015 and 2014 are as follows: Nine months ended September 30, 2015 2014 Risk-free interest rates 1.49-1.92 % 1.54-2.04 % Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.0 6.0 Expected volatility 76.4-81.4 % 61.9-67.9 %
Summary of Stock Option Activity	The following table summarizes stock option activity for the nine months ended September 30, 2015: Shares Issuable Under Options Weighted Average Outstanding as of December 31, 2014 1,289,082 $ 10.26 Granted 556,550 22.81 Exercised (88,371 ) 3.03 Canceled (179,636 ) 18.42 Outstanding as of September 30, 2015 1,577,625 $ 14.15 Options vested and expected to vest at September 30, 2015 1,363,360 Options exercisable at September 30, 2015 666,984
Stock-Based Compensation Expense Related to Stock Options	The Company recorded stock-based compensation expense related to stock options for the three and nine months ended September 30, 2015 and 2014 as follows: Three months ended September 30, Nine months ended 2015 2014 2015 2014 Research and development $ 331,551 $ 170,668 $ 940,698 $ 466,465 General and administrative 789,432 504,141 2,158,394 1,285,769 $ 1,120,983 $ 674,809 $ 3,099,092 $ 1,752,234

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share	Basic and diluted net loss per share was calculated as follows for the three and nine months ended September 30, 2015 and 2014: For the three months ended For the nine months ended 2015 2014 2015 2014 Numerator: Net loss $ (11,136,412 ) $ (7,035,607 ) $ (32,746,529 ) $ (19,508,453 ) Net loss: $ (11,136,412 ) $ (7,035,607 ) $ (32,746,529 ) $ (19,508,453 ) Denominator: Weighted average common shares outstanding, basic and diluted 21,506,721 15,624,963 21,477,830 13,007,892 Net loss per share, basic and diluted $ (0.52 ) $ (0.45 ) $ (1.52 ) $ (1.50 )

Long-term Debt (Tables)

Long-term Debt (Tables)	9 Months Ended
	Sep. 30, 2015
Debt Disclosure [Abstract]
Schedule of Annual Payments Due Under Term Loan	As of September 30, 2015, annual payments due under the Company’s 2015 term loan are as follows (in thousands): Year Aggregate 2015 (remaining three months) $ 236,979 2016 953,125 2017 5,017,868 2018 5,541,088 2019 5,224,248 2020 840,061 Total $ 17,813,369

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Annual Aggregate Minimum Payments Due under Lease Obligations	As of September 30, 2015, annual aggregate minimum payments due under the Company’s lease obligations are as follows (in thousands): Year Aggregate 2015 (remaining three months) $ 127,999 2016 521,364 2017 736,425 2018 758,299 2019 647,106 Total $ 2,791,193

Overview and Nature of the Bu27

Overview and Nature of the Business - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash and cash equivalents and marketable securities	$ 132,165,601	$ 132,165,601	$ 151,624,678
Debt issuance costs		112,309
Debt issuance costs reclassified	28,875
Unbilled accounts receivable	$ 0	$ 0

Summary of Significant Accoun29

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	9 Months Ended
	Sep. 30, 2015
Computers, Software and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	3 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 Asset30

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 58,658,480	$ 48,527,156
Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	58,760,286	101,687,995
Marketable securities	58,658,480	48,527,156
Assets, Total	117,418,766	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	58,760,286	101,687,995
Marketable securities	58,658,480	48,527,156
Assets, Total	$ 117,418,766	$ 150,215,151

Fair Value of Financial Asset31

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 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 ($)	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 58,660,910	$ 48,532,396
Gross Unrealized Gains	8,911	7,753
Gross Unrealized Losses	(11,341)	(12,993)
Fair Value	58,658,480	48,527,156
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,744,467	8,991,820
Gross Unrealized Gains	610	7,570
Fair Value	13,745,077	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	44,916,443	11,239,655
Gross Unrealized Gains	8,301	2
Gross Unrealized Losses	(11,341)	(7,892)
Fair Value	$ 44,913,403	$ 11,231,765

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 4,552,197	$ 1,377,830
Less: Accumulated depreciation	(398,808)	(268,439)
Total property and equipment, net	4,153,389	1,109,391
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	350,345	229,980
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	153,140	153,140
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	236,810	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	3,298,304	601,317
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 274,142	$ 77,454

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Property, Plant and Equipment [Abstract]
Depreciation	$ 47,945	$ 31,897	$ 130,369	$ 84,943
Property and equipment disposals			$ 0	$ 0

Accrued Expenses and Other Cu35

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Clinical research	$ 1,145,600	$ 1,035,510
Contract manufacturing services	1,349,875	294,900
Payroll and other employee-related expenses	1,392,377	1,172,978
Preclinical services	103,000	119,500
Consultant fees and expenses	203,750	26,900
Professional services fees	329,568	439,874
Interest expense	78,125	24,111
Other	66,082	99,931
Total accrued expenses and other current liabilities	$ 4,668,377	$ 3,213,704

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
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%
Expected term (in years)	6 years	6 years
Expected volatility, Minimum	76.40%	61.90%
Expected volatility, Maximum	81.40%	67.90%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares Issuable Under Options, Beginning balance	1,289,082
Shares Issuable Under Options, Granted	556,550
Shares Issuable Under Options, Exercised	(88,371)
Shares Issuable Under Options, Canceled	(179,636)
Shares Issuable Under Options, Ending balance	1,577,625	1,238,973
Shares Issuable Under Options, Options vested and expected to vest at September 30, 2015	1,363,360
Shares Issuable Under Options, Options exercisable at September 30, 2015	666,984
Weighted Average Exercise Price, Beginning balance	$ 10.26
Weighted Average Exercise Price, Granted	22.81	$ 10.15
Weighted Average Exercise Price, Exercised	3.03
Weighted Average Exercise Price, Canceled	18.42
Weighted Average Exercise Price, Ending balance	$ 14.15	$ 9.78

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options exercised	88,371
Aggregate intrinsic value of stock options exercised	$ 1,530,551
Options to purchase common stock, outstanding	1,577,625	1,238,973	1,289,082
Weighted average remaining contractual term	8 years 1 month 6 days	8 years 3 months 18 days
Weighted average exercise price	$ 14.15	$ 9.78	$ 10.26
Weighted average grant date fair value of options granted	$ 22.81	$ 10.15
Unrecognized stock-based compensation expense	$ 11,499,534	$ 7,822,984
Unrecognized compensation costs, weighted-average recognition periods	2 years 8 months 12 days	3 years

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,120,983	$ 674,809	$ 3,099,092	$ 1,752,234
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	331,551	170,668	940,698	466,465
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 789,432	$ 504,141	$ 2,158,394	$ 1,285,769

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Basic and Diluted Net Loss per Share (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net loss	$ (11,136,412)	$ (7,035,607)	$ (32,746,529)	$ (19,508,453)
Net loss:	$ (11,136,412)	$ (7,035,607)	$ (32,746,529)	$ (19,508,453)
Denominator:
Weighted average common shares outstanding, basic and diluted	21,506,721	15,624,963	21,477,830	13,007,892
Net loss per share, basic and diluted	$ (0.52)	$ (0.45)	$ (1.52)	$ (1.50)

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	1,715,837	1,234,199	1,658,292	1,130,000

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	Aug. 04, 2015	Mar. 31, 2015	Sep. 30, 2015	Jan. 03, 2013
Debt Instrument [Line Items]
Debt issuance costs incurred			$ 112,309
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			14,942,401
Debt issuance costs incurred			$ 112,309
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	Sep. 30, 2015USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2015 (remaining three months)	$ 236,979
2,016	953,125
2,017	5,017,868
2,018	5,541,088
2,019	5,224,248
2,020	840,061
Total	$ 17,813,369

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) £ in Millions	Jul. 13, 2015USD ($)ft²	Jul. 31, 2015GBP (£)
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
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Additional leased office space	4,700
Office space leased on temporary agreement	6,700
Anticipated lease commencement date of extended office space	May 1, 2016
Lease termination date	Oct. 31, 2019
Extended leased office space	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 Contingencies45

Commitments and Contingencies - Annual Aggregate Minimum Payments Due under Lease Obligations (Detail) $ in Thousands	Sep. 30, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
2015 (remaining three months)	$ 127,999
2,016	521,364
2,017	736,425
2,018	758,299
2,019	647,106
Total	$ 2,791,193