Flexion Therapeutics (FLXN) 10-Q 8 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 01, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,636,890

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 147,304	$ 127,789
Marketable securities	224,338	264,589
Accounts receivable, net	2,397	410
Inventories	2,525	1,799
Prepaid expenses and other current assets	4,023	3,403
Total current assets	380,587	397,990
Property and equipment, net	10,964	11,189
Long-term investments	4,938	31,538
Restricted cash	600	600
Total assets	397,089	441,317
Current liabilities:
Accounts payable	3,420	6,222
Accrued expenses and other current liabilities	11,060	14,383
Current portion of long-term debt	9,967	9,967
Total current liabilities	24,447	30,572
Long-term debt, net	10,612	12,936
2024 convertible notes, net	138,983	137,107
Other long-term liabilities	440	428
Total liabilities	174,482	181,043
Commitments and contingencies
Preferred stock, $0.001 par value; 10,000,000 shares authorized at March 31, 2018 and December 31, 2017 and 0 shares issued and outstanding at March 31, 2018 and December 31, 2017	0	0
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized; 37,632,589 and 37,610,897 shares issued and outstanding, at March 31, 2018 and December 31, 2017, respectively	38	38
Additional paid-in capital	613,881	609,810
Accumulated other comprehensive loss	(576)	(407)
Accumulated deficit	(390,736)	(349,167)
Total stockholders’ equity	222,607	260,274
Total liabilities and stockholders’ equity	$ 397,089	$ 441,317

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
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	37,632,589	37,610,897
Common stock, shares outstanding	37,632,589	37,610,897

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue
Product revenue, net	$ 2,194
Operating expenses:
Cost of sales	2,698
Research and development	11,551	$ 10,756
Selling, general and administrative	26,899	13,026
Total operating expenses	41,148	23,782
Loss from operations	(38,954)	(23,782)
Other income (expense):
Interest income	1,161	557
Interest expense	(3,919)	(632)
Other income (expense)	143	(22)
Total other income (expense)	(2,615)	(97)
Net loss	$ (41,569)	$ (23,879)
Net loss per common share, basic and diluted	$ (1.10)	$ (0.75)
Weighted average common shares outstanding, basic and diluted	37,620	31,704
Other comprehensive (loss) income:
Unrealized (losses) gains from available-for-sale securities, net of tax of $0	$ (169)	$ 9
Total other comprehensive (loss) income	(169)	9
Comprehensive loss	$ (41,738)	$ (23,870)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Unrealized (losses) gains from available-for-sale securities, tax	$ 0	$ 0

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in-Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2015	$ 103,986	$ 22	$ 243,853	$ (97)	$ (139,792)
Balance (in shares) at Dec. 31, 2015		21,570,000
Issuance of common stock net of issuance costs	147,501	$ 10	147,491
Issuance of common stock net of issuance costs (in shares)		10,040,000
Issuance of common stock for equity awards	167		167
Issuance of common stock for equity awards (in shares)		30,000
Employee Stock Purchase Plan	476		476
Employee Stock Purchase Plan (in shares)		27,000
Stock-based compensation expense	6,770		6,770
Net loss	(71,894)				(71,894)
Other comprehensive income (loss)	26			26
Balance at Dec. 31, 2016	187,032	$ 32	398,757	(71)	(211,686)
Balance (in shares) at Dec. 31, 2016		31,667,000
Issuance of common stock net of issuance costs	132,177	$ 6	132,171
Issuance of common stock net of issuance costs (in shares)		5,520,000
Issuance of common stock for equity awards	3,858		3,858
Issuance of common stock for equity awards (in shares)		334,000
Employee Stock Purchase Plan	1,016		1,016
Employee Stock Purchase Plan (in shares)		90,000
Stock-based compensation expense	11,542		11,542
Portion of convertible debt proceeds allocated to equity component	62,466		62,466
Net loss	(137,481)			0	(137,481)
Other comprehensive income (loss)	(336)			(336)
Balance at Dec. 31, 2017	260,274	$ 38	609,810	(407)	(349,167)
Balance (in shares) at Dec. 31, 2017		37,611,000
Issuance of common stock for equity awards	$ 414		414
Issuance of common stock for equity awards (in shares)	22,000	22,000
Stock-based compensation expense	$ 3,657		3,657
Net loss	(41,569)				(41,569)
Other comprehensive income (loss)	(169)			(169)
Balance at Mar. 31, 2018	$ 222,607	$ 38	$ 613,881	$ (576)	$ (390,736)
Balance (in shares) at Mar. 31, 2018		37,633,000

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (41,569)	$ (23,879)	$ (137,481)	$ (71,894)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	563	453
Stock-based compensation expense	3,657	2,378
Other non-cash charges		11
Amortization of (discount) premium on marketable securities	(230)	155
Amortization of debt discount and debt issuance costs	1,884
Premium paid on securities purchased	(3)	(264)
Changes in operating assets and liabilities:
Accounts receivable	(1,987)
Inventory	(726)
Prepaid expenses, other current and long-term assets	(620)	(517)
Accounts payable	(2,806)	138
Accrued expenses and other current and long-term liabilities	(3,144)	131
Net cash used in operating activities	(44,981)	(21,394)
Cash flows from investing activities
Purchases of property and equipment	(334)	(1,018)
Purchases of marketable securities	(32,546)	(25,421)
Sale and redemption of marketable securities	99,462	60,328
Net cash provided by investing activities	66,582	33,889
Cash flows from financing activities
Payments on notes payable	(2,500)	(833)
Payments of public offering costs		(95)
Proceeds from the exercise of stock options	414	559
Net cash used in financing activities	(2,086)	(369)
Net increase in cash, cash equivalents, and restricted cash	19,515	12,126
Cash, cash equivalents, and restricted cash at beginning of period	128,389	31,395	31,395
Cash, cash equivalents, and restricted cash at end of period	147,904	43,521	$ 128,389	$ 31,395
Supplemental disclosures of cash flow information:
Cash paid for interest	326	469
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 13	$ 234

Overview and Nature of the Busi

Overview and Nature of the Business	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Overview and Nature of the Business	1. Flexion Therapeutics, Inc. (“Flexion” or the “Company”) was incorporated under the laws of the state of Delaware on November 5, 2007. Flexion is a biopharmaceutical company focused on the discovery, development and commercialization of novel, local therapies for the treatment of patients with musculoskeletal conditions, beginning with osteoarthritis, or OA, a type of degenerative arthritis. The Company has an approved product, ZILRETTA ® 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 the ability to secure additional capital to fund operations. Successfully commercializing ZILRETTA will require significant sales and marketing efforts and the Company’s pipeline programs may require significant additional research and development efforts, including extensive preclinical and clinical testing. These activities will in turn require significant amounts of capital, adequate personnel infrastructure and extensive compliance reporting capabilities. There can be no assurance when, if ever, the Company will realize significant revenue from the sales of ZILRETTA or if the development efforts supporting the Company’s pipeline, including future clinical trials, will be successful.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Basis of Presentation The accompanying condensed consolidated financial statements as of March 31, 2018, and for the three months ended March 31, 2018 and 2017, 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 8, 2018. The information presented in the condensed consolidated financial statements and related notes as of March 31, 2018, and for the three months ended March 31, 2018 and 2017, is unaudited. The December 31, 2017 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 months ended March 31, 2018 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2018, 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 March 31, 2018, the Company had cash, cash equivalents, marketable securities, and long-term investments of approximately $376.6 million. Management believes that current cash, cash equivalents and marketable securities on hand at March 31, 2018 should be sufficient to fund operations for at least the next twelve months from the issuance date of these financial statements. The future viability of the Company may be dependent on its ability to raise additional capital to finance its operations, including to support the commercialization of ZILRETTA and fund increased research and development costs in order to seek approval and commercialize its product candidates. The Company may not be able to obtain financing on acceptable terms, or at all. If the Company is unable to obtain funding on a timely basis the Company may need to curtail its operations, including the commercialization of ZILRETTA and research and development activities, which could adversely affect its prospects. Recent Accounting Pronouncements Accounting Standards Recently Adopted In January 2016, the Financial Accounting Standards Board, or FASB, issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In August 2016, the FASB issued ASU 2016-15, Statement of cash flows In November 2016, the FASB issued ASU 2016-18, Statement of cash flows In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation prospectively to an award modified on or after the adoption date. Accounting Standards Recently Issued In February 2016, the FASB issued ASU 2016-02, Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments 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 months ended March 31, 2018 and the year ended December 31, 2017. Revenue Recognition On October 6, 2017, the FDA approved ZILRETTA. The Company entered into a limited number of arrangements with specialty distributors and a specialty pharmacy in the U.S. to distribute ZILRETTA. These arrangements are the Company’s initial contracts with customers and, as a result the Company adopted Accounting Standards Codification (“ASC”) Topic 606 - Revenue from Contracts with Customers (“Topic 606”) as of January 1, 2017. There is no impact for the transition to Topic 606 because the Company had no historical revenue prior to the launch of ZILRETTA. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance and financial instruments. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to arrangements that meet the definition of a contract with a customer under Topic 606, including when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For a complete discussion of accounting for product revenue, see Product Revenue, Net (below). Product Revenue, Net — The Company sells ZILRETTA to its customers who then subsequently resell ZILRETTA to physicians, clinics and certain medical centers or hospitals. In addition to distribution agreements with customers, the Company enters into arrangements with government payers that provide for government mandated rebates and chargebacks with respect to the purchase of ZILRETTA. The Company recognizes revenue on product sales when the customer obtains control of the Company's product, which occurs at a point in time (upon delivery to the customer). The Company has determined that the delivery of ZILRETTA to its customers constitutes a single performance obligation. There are no other promises to deliver goods or services beyond what is specified in each accepted customer order. The Company has assessed the existence of a significant financing component in the agreements with its customers. The trade payment terms with customers do not exceed one year and therefore the Company has elected to apply the practical expedient and no amount of consideration has been allocated as a financing component. Product revenues are recorded net of applicable reserves for variable consideration, including discounts and allowances. Transaction Price, including Variable Consideration — Revenues from product sales are recorded at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established. Components of variable consideration include trade discounts and allowances, product returns, government chargebacks, discounts and rebates, and other incentives, such as voluntary patient assistance, and other fee for service amounts that are detailed within contracts between the Company and its customers relating to the Company’s sale of its products. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). These estimates take into consideration a range of possible outcomes which are probability-weighted in accordance with the expected value method in Topic 606 for relevant factors such as current contractual and statutory requirements, specific known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled based on the terms of the respective underlying contracts. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under the contract will not occur in a future period. The Company’s analyses also contemplated application of the constraint in accordance with the guidance, under which it determined a material reversal of revenue would not occur in a future period for the estimates detailed below as of March 31, 2018 and, therefore, the transaction price was not reduced further during the three months ended March 31, 2018. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s original estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such changes in estimates become known. Trade Discounts and Allowances —The Company compensates (through trade discounts and allowances) its customers for sales order management, data, and distribution services. However, the Company has determined such services received to date are not distinct from the Company’s sale of products to the customer and, therefore, these payments have been recorded as a reduction of revenue within the statement of operations and comprehensive loss through March 31, 2018, as well as a reduction to trade receivables, net on the condensed consolidated balance sheets. Product Returns — Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company based on the product’s expiration date. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized, as well as within accrued expenses and other current liabilities, net on the condensed consolidated balance sheets. The Company currently estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company has not received any returns to date and believes that returns of ZILRETTA will be minimal. The Company’s limited right of return allows for eligible returns of ZILRETTA in the following circumstances: • Shipment errors that were the result of an error by the Company; • Quantity delivered that is greater or less than the quantity ordered; • Product distributed by the Company that is damaged in transit prior to receipt by the customer; • Expired product, previously purchased directly from the Company, that is returned during the period beginning three months prior to the product’s expiration date and ending three months after the product’s expiration date; • Product subject to a recall; and • Product that the Company, at its sole discretion, has specified to be returned. Government Chargebacks, Discounts and Rebates — Chargebacks for fees and discounts to qualified government healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified VA hospitals and 340b entities at prices lower than the list prices charged to customers who directly purchase the product from the Company. The 340b Drug Discount Program is a US federal government program created in 1992 that requires drug manufacturers to provide outpatient drugs to eligible health care organizations and covered entities at significantly reduced prices. Customers charge the Company for the difference between what they pay for the product and the statutory selling price to the qualified government entity. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and trade receivables, net. Chargeback amounts are generally determined at the time of resale to the qualified government healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the Customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at each reporting period-end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. Government Rebates — The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses and other current liabilities on the condensed consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company anticipates its exposure to utilization from the Medicare Part D coverage gap discount program to be immaterial. For Medicaid programs, the Company estimates the portion of sales attributed to Medicaid patients and records a liability for the rebates to be paid to the respective state Medicaid programs. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives — Other incentives which the Company offers include voluntary patient assistance programs, such as the co-pay assistance program, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payers. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. To date, the Company’s only source of product revenue has been from the U.S. sales of ZILRETTA, which it began shipping to customers in October 2017. The following table summarizes activity in each of the product revenue allowance and reserve categories for the three months ended March 31, 2018: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Balance as of December 31, 2017 $ 60 $ 15 $ 2 $ 77 Provision related to sales in the current year 186 49 12 247 Credit and payments made (94 ) — — (94 ) Balance as of March 31, 2018 $ 152 $ 64 $ 14 $ 230 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, revenue and 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 estimates related to revenue, 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 Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and 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. Property and equipment includes construction-in-progress that is not yet in service. Foreign Currencies The Company maintains a bank account denominated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. The following tables present information about the Company’s assets that are measured at fair value on a recurring basis as of March 31, 2018 and December 31, 2017 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of March 31, 2018 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 127,468 $ — $ 127,468 Marketable securities — 229,276 — 229,276 $ — $ 356,744 $ — $ 356,744 Fair Value Measurements as of December 31, 2017 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 109,196 $ — $ 109,196 Marketable securities — 296,127 — 296,127 $ — $ 405,323 $ — $ 405,323 As of March 31, 2018 and December 31, 2017 the Company’s cash equivalents that are invested in money market funds and overnight repurchase contracts 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 three months ended March 31, 2018 and year ended December 31, 2017, there were no transfers between Level 1, Level 2, and Level 3. Amortization and accretion of discounts and premiums are recorded in other income. The Company has a term loan outstanding under its 2015 credit facility with MidCap Financial Funding XIII Trust and Silicon Valley Bank (the “2015 term loan”). The amount outstanding on its 2015 term loan is reported at its carrying value in the accompanying balance sheet. The Company determined the fair value of the 2015 term loan 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 2015 term loan was valued using Level 2 inputs as of March 31, 2018 and December 31, 2017. The result of the calculation yielded a fair value that approximates its carrying value. On May 2, 2017 the Company issued 3.375% convertible senior notes due 2024 (the “2024 Convertible Notes”) with embedded conversion features. The Company estimated the fair value of the 2024 Convertible Notes using a discounted cash flow approach to derive the value of a debt instrument using the expected cash flows and the estimated yield related to the convertible notes. The significant assumptions used in estimating the expected cash flows were: the estimated market yield based on an implied yield and credit quality analysis of a term loan with similar attributes, and the average implied volatility of the Company’s traded and quoted options available as of May 2, 2017. The Company recorded approximately $136.7 million as the fair value of the liability on May 2, 2017, with a corresponding amount recorded as a discount on the initial issuance of the 2024 Convertible Notes of approximately $64.5 million. The debt discount was recorded to equity and is being amortized to the debt liability over the life of the 2024 Convertible Notes using the effective interest method. The fair value of the 2024 Convertible Notes, which differs from their carrying value, is influenced by interest rates, stock price and stock price volatility and is determined by prices for the 2024 Convertible Notes observed in market trading. The market for trading of the 2024 Convertible Notes is not considered to be an active market and therefore the estimate of fair value is based on Level 2 inputs. The estimated fair value of the 2024 Convertible Notes, face value of $201.3 million, was $226.6 million at March 31, 2018.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. As of March 31, 2018 and December 31, 2017 the fair value of available-for-sale marketable securities by type of security was as follows: March 31, 2018 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper 22,383 — — 22,383 U.S. government obligations $ 84,254 $ — $ (156 ) $ 84,098 Corporate bonds 123,218 — (423 ) 122,795 $ 229,855 $ — $ (579 ) $ 229,276 December 31, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 22,436 $ — $ — $ 22,436 U.S. government obligations 121,470 $ — (136 ) 121,334 Corporate bonds 152,630 $ — (273 ) 152,357 $ 296,536 $ — $ (409 ) $ 296,127 As of March 31, 2018 and December 31, 2017, marketable securities consisted of approximately $224.3 and $264.6 million, respectively, of investments that mature within twelve months and approximately $4.9 million and $31.5 million, respectively of investments that mature after one year but within two years or less from the balance sheet date.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	3 Months Ended
	Mar. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	5 . Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets and other assets consisted of the following as of March 31, 2018 and December 31, 2017: (In thousands) March 31, 2018 December 31, 2017 Prepaid expenses $ 3,288 $ 2,359 Deposits 66 66 Interest receivable on marketable securities 669 978 Total prepaid expenses and other current assets $ 4,023 $ 3,403

Inventory

Inventory	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Inventory	6. Inventory Inventory consisted of the following as of March 31, 2018 and December 31, 2017: (In thousands) March 31, 2018 December 31, 2017 Raw materials $ 780 $ 928 Work in process 711 746 Finished goods 1,034 125 Total inventories $ 2,525 $ 1,799 Inventory acquired prior to receipt of the marketing approval for ZILRETTA, totaling approximately $3.7 million, was expensed as research and development expense as incurred. The Company began to capitalize the costs associated with the production of ZILRETTA upon receipt of FDA approval of ZILRETTA on October 6, 2017. Finished goods manufactured by the Company have a shelf life of approximately 24 months from the date of manufacture. The Company reduces its inventory to net realizable value for potentially excess, dated or obsolete inventory based on an analysis of forecasted demand compared to quantities on hand and any firm purchase orders, as well as product shelf life. During the three months ended March 31, 2018, the Company expensed $2.7 million to cost of sales primarily for fixed overhead costs related to the operation of the facility at Patheon. The Company determined that no write-downs to inventory for potentially excess, dated or obsolete inventory were required.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, Net	7. Property and Equipment, Net Property and equipment, net, as of March 31, 2018 and December 31, 2017 consisted of the following: (In thousands) March 31, 2018 December 31, 2017 Computer and office equipment $ 1,124 $ 1,124 Manufacturing equipment 11,827 11,780 Furniture and fixtures 456 456 Software 434 434 Leasehold improvements 478 474 Construction—in progress 592 305 14,911 14,573 Less: Accumulated depreciation (3,947 ) (3,384 ) Total property and equipment, net $ 10,964 $ 11,189 Depreciation expense for the three months ended March 31, 2018 and 2017 was approximately $0.6 million and $0.5 million, respectively. No property and equipment was disposed of during the three months ended March 31, 2018. Construction in progress consists of equipment purchased for the Company’s portfolio expansion efforts, as well as leasehold improvements related to the second floor space expansion of the Company’s Burlington, Massachusetts headquarters.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	8. Accrued Expenses and Othe Accrued expenses and other current liabilities consisted of the following: (In thousands) March 31, 2018 December 31, 2017 Research and development $ 788 $ 969 Payroll and other employee-related expenses 4,168 9,309 Professional services fees 2,163 2,591 Interest expense 2,933 1,249 Accrual for employee stock purchase plan 704 141 Other 304 124 Total accrued expenses and other current liabilities $ 11,060 $ 14,383

Debt

Debt	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Debt	9. Debt Term Loan On August 4, 2015, the Company entered into a credit and security agreement with MidCap Financial Trust, as agent, and MidCap Financial Funding XIII Trust and Silicon Valley Bank, as lenders, (the “Lenders”), to borrow up to $30.0 million in term loans. The Company concurrently borrowed an initial term loan of $15.0 million under the facility. 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 agreed not to encumber any of its intellectual property without the Lenders’ prior written consent. The Company also agreed to maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5% for so long as the Company maintains any cash or cash equivalents in non-secured bank accounts. On July 22, 2016, the Company borrowed the remaining $15.0 million under the credit and security agreement, in the form of a second term loan. The second term loan is subject to the same credit terms as the initial term loan under the facility. 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 March 31, 2018, the Company was compliant with all covenants. 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 straight line method, which approximates the effective interest method. As of March 31, 2018, the carrying value of the term loan was approximately $20.6 million, of which $10.0 million was due within 12 months and $10.6 million was due in greater than 12 months. In connection with the credit and security agreement, the Company incurred debt issuance costs totaling approximately $150,000. These costs are being amortized over the estimated term of the debt using the straight-line method which approximates the effective interest method. The Company deducted the debt issuance costs from the carrying amount of the debt as of March 31, 2018 and December 31, 2017. As of March 31, 2018, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments (in thousands) 2018 8,256 2019 10,449 2020 4,383 Total $ 23,088 Less interest (1,278 ) Less unamortized portion of final payment (1,231 ) Total $ 20,579 2024 Convertible Notes On May 2, 2017 the Company issued an aggregate of $201.3 million principal amount of the 2024 Convertible Notes. The 2024 Convertible Notes have a maturity date of May 1, 2024 are unsecured and accrue interest at a rate of 3.375% per annum, payable semi-annually on May 1 and November 1 of each year, beginning November 1, 2017. The Company received $194.8 million for the sale of the 2024 Convertible Notes, after deducting fees and expenses of $6.5 million. Upon conversion of the 2024 Convertible Notes, at the election of each holder of a 2024 Convertible Note (the Holder), the note will be convertible into cash, shares of the Company’s common stock, or a combination thereof, at the Company’s election (subject to certain limitations in the 2015 term loan), at a conversion rate of approximately 37.3413 shares of common stock per $1,000 principal amount of the 2024 Convertible Notes, which corresponds to an initial conversion price of approximately $26.78 per share of the Company’s common stock. The conversion rate is subject to adjustment from time to time upon the occurrence of certain events, including, but not limited to, fundamental change events and certain corporate events that occur prior to the maturity date of the notes. In addition, if the Company delivers a notice of redemption, the Company will increase, in certain circumstances, the conversion rate for a Holder who elects to convert its notes in connection with such a corporate event or notice of redemption, as the case may be. At any time prior to the close of business on the business day immediately preceding February 1, 2024, Holders may convert all, or any portion, of the 2024 Convertible Notes at their option only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2017 (and only during such calendar quarter), if the last reported sale price of the common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the “measurement period”) in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate on each such trading day; (3) if the Company calls any or all of the notes for redemption, at any time prior to the close of business on the business day immediately preceding the redemption date; and (4) upon the occurrence of specified corporate events. On or after February 1, 2024, until the close of business on the business day immediately preceding the maturity date, Holders may convert their notes at any time, regardless of the foregoing circumstances. The Company may redeem, for cash, all or any portion of the 2024 Convertible Notes, at its option, on or after May 6, 2020 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price for at least 20 trading days during any 30 consecutive day trading period, at a redemption price equal to 100% of the principal amount of the 2024 Convertible Notes to be redeemed, plus accrued and unpaid interest. The 2024 Convertible Notes are considered convertible debt with a cash conversion feature. Per ASC 470-20, Debt with Conversion and Other Options , The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The allocation was performed in a manner that reflected our non-convertible debt borrowing rate for similar debt. The equity component of the 2024 Convertible Notes was recognized as a debt discount and represents the difference between the proceeds from the issuance of the 2024 Convertible Notes and the fair value of the liability of the 2024 Convertible Notes on their respective dates of issuance. The excess of the principal amount of the liability component over its carrying amount (“debt discount”) is amortized to interest expense using the effective interest method over seven years. The equity component is not re-measured as long as it continues to meet the conditions for equity classification. In connection with the issuance of the 2024 Convertible Notes, the Company incurred approximately $6.5 million of debt issuance costs, which primarily consisted of underwriting, legal and other professional fees, and allocated these costs to the liability and equity components based on the allocation of the proceeds. Of the total d ebt issuance costs, $4.4 million were allocated to the liability component equity component and is recorded as a reduction to additional paid-in capital. Debt discount and issuance rate $1.7 The table below summarizes the carrying value of the 2024 Convertible Notes as of March 31, 2018: ( in thousands Gross proceeds $ 201,250 Portion of proceeds allocated to equity component (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion of issuance costs allocated to equity component (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 6,669 Carrying value 2024 Convertible Notes $ 138,983

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation Stock Option Valuation The fair value of each of the Company’s stock option grants is estimated on the date of grant using the Black-Scholes option-pricing model. The Company currently 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 publicly-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 three months ended March 31, 2018 and 2017 are as follows: Three months ended March 31, 2018 2017 Risk-free interest rates 2.72-2.74% 2.23-2.29% Expected dividend yield 0.00% 0.00% Expected term (in years) 6.0 6.0 Expected volatility 71.4-71.5% 71.0-71.6% The following table summarizes stock option activity for the three months ended March 31, 2018: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2017 3,800 $ 17.75 Granted 672 22.33 Exercised (22 ) 19.18 Cancelled (46 ) 20.07 Outstanding as of March 31, 2018 4,404 $ 18.42 Options vested and expected to vest at March 31, 2018 4,404 $ 18.42 Options exercisable at March 31, 2018 1,889 $ 15.42 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. Options to purchase a total of approximately 22,000 shares of the Company’s common stock, with an aggregate intrinsic value of approximately $0.2 million, were exercised during the three months ended March 31, 2018. At March 31, 2018 and 2017, there were options for the purchase of approximately 4,404,000 and 3,466,000 shares of the Company’s common stock outstanding, respectively, with a weighted average remaining contractual term of 8.1 years and 8.2, respectively, and with a weighted average exercise price of $18.42 and $16.20 per share, respectively. The weighted average grant date fair value of options granted during the three months ended March 31, 2018 and 2017 was $14.48 and $12.70, respectively. Restricted Stock Units On January 4, 2016, the Company granted 189,300 restricted stock units (“RSUs”) with performance and time-based vesting conditions to certain executives. These RSUs began vesting, and the underlying shares of common stock became deliverable, beginning when ZILRETTA was approved (the “Milestone”). The number of shares that vest varies based on the timing of achieving the Milestone. As a result of the Milestone being achieved on October 6, 2017, the number of shares of the Company’s common stock earned under these awards was 122,800, subject to ongoing employment with the Company for a period of 2 years. The 122,800 shares had an approximate value of $2.2 million as of the original grant date of which $1.6 million was recognized in the fourth quarter of 2017 upon achieving the Milestone and the remaining $0.6 million will be recognized over a period of two years. On February 1, 2018, the Company awarded 209,366 RSUs to employees at an average grant date fair value of $22.31 per share. The RSUs vest in four substantially equal installments on each of the first four anniversaries of the vesting commencement date, subject to the employee's continued employment with, or services to, the Company on each vesting date. Compensation expense is recognized on a straight-line basis. The following table summarizes the RSU activity for the three months ended March 31, 2018: (In thousands, except per share amounts) Number of Shares Weighted Average Grant Date Fair Value Nonvested balance as of December 31, 2017 82 $ 16.43 Granted 209 22.31 Cancelled (3 ) 22.31 Vested/Released — — Nonvested Balance as of March 31, 2018 288 $ 21.07 Stock-based Compensation The Company recorded stock-based compensation expense related to stock options and RSUs and shares purchased under the employee stock purchase plan for the three months ended March 31, 2018 and 2017 as follows: Three months ended March 31, (In thousands) 2018 2017 Research and development $ 1,150 $ 879 Selling, general and administrative 2,507 1,499 $ 3,657 $ 2,378 As of March 31, 2018, unrecognized stock-based compensation expense for stock options outstanding was approximately $31.6 million which is expected to be recognized over a weighted average period of 3.0 years. As of March 31, 2018, unrecognized stock-based compensation expense for RSUs outstanding was $4.9 million which is expected to be recognized over a period of 3.5 years.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	11. Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the three months ended March 31, 2018 and 2017: For the three months ended March 31, (In thousands) 2018 2017 Numerator: Net loss $ (41,569 ) $ (23,879 ) Net loss: $ (41,569 ) $ (23,879 ) Denominator: Weighted average common shares outstanding, basic and diluted 37,620 31,704 Net loss per share, basic and diluted $ (1.10 ) $ (0.75 ) The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated as including them would have an anti-dilutive effect: For the three months ended March 31, 2018 2017 Shares issuable upon conversion of the 2024 Convertible Notes 7,515 — Stock options 4,264 3,315 Restricted stock units 218 189 Total 11,997 3,504

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Operating Leases Burlington Lease In May 2013, the Company entered into a lease for office space in Burlington, Massachusetts (the “Lease”). The term of the Lease was for 42 months with minimum monthly lease payments beginning at $17,588 per month and escalating over the lease term. In July 2015, the Company amended the Lease to add approximately 4,700 square feet of additional office space, with the option to lease an additional 5,400 square feet in the same building in Burlington, Massachusetts. In addition, at the time, the Company leased approximately 6,700 square feet of temporary space for use prior to delivery of the additional space. This amendment also extended the term of the Lease through October 31, 2019. On September 30, 2015, the Company exercised its option for the additional 5,400 square feet of office space. On September 21, 2016, the Company entered into another amendment to extend the Lease for the 6,700 square feet of temporary space until October 31, 2017. On April 7, 2017, the Company further amended the Lease to extend the term to October 31, 2023 on the then-existing office space, including the temporary space, consisting of approximately 28,600 square feet of office space in Burlington, Massachusetts. From November 2016 through October 2017, the Company’s lease payment for this space was approximately $80,000 per month. Also, as part of this amendment to the Lease, the Company leased an additional 1,471 square feet of office space beginning in 2018. The lease payment for the 1,471 square feet of office space is approximately $4,100 per month. On October 6, 2017, the Company exercised its option for an additional 6,450 square feet of space, and the term for the space commenced in April 2018. The Company has approximately 36,500 square feet of office space in Burlington, Massachusetts under a lease term expiring on October 31, 2023. In addition to the base rent for the office space, which increases over the term of the amended Lease, the Company is responsible for its share of operating expenses and real estate taxes. Woburn Lease In February 2017, the Company entered into a five-year lease for laboratory space located in Woburn, Massachusetts with a monthly lease payment of approximately $15,000, which increases over the term of the lease, plus a share of operating expenses. The total cash obligations for the term of the lease are approximately $0.9 million. Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments (in thousands) 2018 1,033 2019 1,491 2020 1,533 2021 1,576 2022 1,447 2023 1,203 Total $ 8,283 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. Patheon 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 provided Patheon with certain equipment and materials necessary to manufacture ZILRETTA and pays Patheon a monthly fee for such activities and reimburses 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, or until October 6, 2027. The Company pays 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 also reimburses Patheon for purchases of materials and equipment made on its behalf, certain nominal expenses and additional services. The Manufacturing Agreement will remain in full effect unless and until it expires or is terminated. Upon termination of the Manufacturing Agreement (other than termination by Flexion in the event that Patheon does not meet the construction and manufacturing milestones or for a breach by Patheon), Flexion will be obligated to pay for the costs incurred by Patheon associated with the removal of our manufacturing equipment and for Patheon’s termination costs up to a capped amount. Future minimum payments under the Company’s agreed obligations are as follows: Year Aggregate Minimum Payments 2018 $ 5,362 2019 8,410 2020 8,410 2021 8,410 2022 8,410 2023 and thereafter 25,231 Total $ 64,233 Evonik Supply Agreement In November 2016, the Company entered into a Supply Agreement with Evonik Corporation (“Evonik”) for the purchase of PLGA which is used in the manufacturing of clinical and commercial supply of ZILRETTA. Pursuant to the Supply Agreement, Flexion is obligated to submit rolling monthly forecasts to Evonik for PLGA supply, a portion of which will constitute binding orders. In addition, Flexion agreed to certain minimum purchase requirements, which decrease over time, and which do not apply (i) during periods in which Evonik is in material breach of the Supply Agreement or is unable to perform its obligations due to a force majeure event, (ii) with respect to orders that Evonik is unable to supply in excess of binding orders, (iii) for orders Evonik is unable to timely deliver or does not deliver conforming product and provides a credit for such order, or (iv) during an uncured material quality failure by Evonik. Flexion agreed to purchase PLGA batches at a specified price per gram in U.S. dollars, subject to adjustment from time to time, including due to changes in price indices and in the event the initial term of the Supply Agreement is extended. The total term of the agreement is five years. Upon termination of the Supply Agreement (other than termination due to the bankruptcy of either Evonik or Flexion) Flexion is obligated to pay the costs associated with the binding supply forecast provided to Evonik. The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties. FX201 Related Agreements In December 2017, the Company entered into a definitive agreement with GeneQuine Biotherapeutics GmbH (“GeneQuine”) to acquire the global rights to FX201. As part of the asset purchase transaction with GeneQuine, the Company made an upfront payment to GeneQuine of $2 million. The Company may also be required to make additional milestone payments during the development of FX201, including up to $8.7 million through Phase 2 proof of concept (PoC) and, following successful PoC, up to an additional $54 million in development and global regulatory approval milestone payments. The transaction was accounted for as an asset acquisition, as it did not qualify as a business combination. The upfront fee was attributed to the intellectual property acquired, and recognized as research and development expense in December 2017 as the FX201 product candidate had not been commercially approved, and had no alternative future use. Future milestone payments earned prior to regulatory approval of FX201 would be recognized as research and development expense in the period when the milestone events become probable of being achieved. Future milestones earned upon regulatory approval would be recognized as an intangible asset and amortized to expense over the estimated life of FX201. As of March 31, 2018, none of the future milestone payments owed under the arrangement was probable of being achieved. As part of the transaction, the Company became the direct licensee of certain underlying Baylor College of Medicine (Baylor) patents and other proprietary rights related to FX201 for human applications. The Baylor license agreement grants the Company an exclusive, royalty-bearing, world-wide right and license (with a right to sublicense) for human applications under its patent and other proprietary rights directly related to FX201, with a similar non-exclusive license to certain Baylor intellectual property rights that are not specific to FX201. The license agreement with Baylor includes a low single-digit royalty on net sales of FX201 and requires the Company to use reasonable efforts to develop FX201 according to timelines set out in the license agreement. In December 2017, the Company also entered into a Master Production Services Agreement with SAFC Carlsbad, Inc., a part of MilliporeSigma, for the manufacturing of pre-clinical and initial clinical supplies of FX201.

Summary of Significant Accoun20

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements as of March 31, 2018, and for the three months ended March 31, 2018 and 2017, 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 8, 2018. The information presented in the condensed consolidated financial statements and related notes as of March 31, 2018, and for the three months ended March 31, 2018 and 2017, is unaudited. The December 31, 2017 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 months ended March 31, 2018 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2018, 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 March 31, 2018, the Company had cash, cash equivalents, marketable securities, and long-term investments of approximately $376.6 million. Management believes that current cash, cash equivalents and marketable securities on hand at March 31, 2018 should be sufficient to fund operations for at least the next twelve months from the issuance date of these financial statements. The future viability of the Company may be dependent on its ability to raise additional capital to finance its operations, including to support the commercialization of ZILRETTA and fund increased research and development costs in order to seek approval and commercialize its product candidates. The Company may not be able to obtain financing on acceptable terms, or at all. If the Company is unable to obtain funding on a timely basis the Company may need to curtail its operations, including the commercialization of ZILRETTA and research and development activities, which could adversely affect its prospects.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Accounting Standards Recently Adopted In January 2016, the Financial Accounting Standards Board, or FASB, issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In August 2016, the FASB issued ASU 2016-15, Statement of cash flows In November 2016, the FASB issued ASU 2016-18, Statement of cash flows In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation prospectively to an award modified on or after the adoption date. Accounting Standards Recently Issued In February 2016, the FASB issued ASU 2016-02, Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments
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 months ended March 31, 2018 and the year ended December 31, 2017.
Revenue Recognition	Revenue Recognition On October 6, 2017, the FDA approved ZILRETTA. The Company entered into a limited number of arrangements with specialty distributors and a specialty pharmacy in the U.S. to distribute ZILRETTA. These arrangements are the Company’s initial contracts with customers and, as a result the Company adopted Accounting Standards Codification (“ASC”) Topic 606 - Revenue from Contracts with Customers (“Topic 606”) as of January 1, 2017. There is no impact for the transition to Topic 606 because the Company had no historical revenue prior to the launch of ZILRETTA. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance and financial instruments. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to arrangements that meet the definition of a contract with a customer under Topic 606, including when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For a complete discussion of accounting for product revenue, see Product Revenue, Net (below). Product Revenue, Net — The Company sells ZILRETTA to its customers who then subsequently resell ZILRETTA to physicians, clinics and certain medical centers or hospitals. In addition to distribution agreements with customers, the Company enters into arrangements with government payers that provide for government mandated rebates and chargebacks with respect to the purchase of ZILRETTA. The Company recognizes revenue on product sales when the customer obtains control of the Company's product, which occurs at a point in time (upon delivery to the customer). The Company has determined that the delivery of ZILRETTA to its customers constitutes a single performance obligation. There are no other promises to deliver goods or services beyond what is specified in each accepted customer order. The Company has assessed the existence of a significant financing component in the agreements with its customers. The trade payment terms with customers do not exceed one year and therefore the Company has elected to apply the practical expedient and no amount of consideration has been allocated as a financing component. Product revenues are recorded net of applicable reserves for variable consideration, including discounts and allowances. Transaction Price, including Variable Consideration — Revenues from product sales are recorded at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established. Components of variable consideration include trade discounts and allowances, product returns, government chargebacks, discounts and rebates, and other incentives, such as voluntary patient assistance, and other fee for service amounts that are detailed within contracts between the Company and its customers relating to the Company’s sale of its products. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). These estimates take into consideration a range of possible outcomes which are probability-weighted in accordance with the expected value method in Topic 606 for relevant factors such as current contractual and statutory requirements, specific known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled based on the terms of the respective underlying contracts. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under the contract will not occur in a future period. The Company’s analyses also contemplated application of the constraint in accordance with the guidance, under which it determined a material reversal of revenue would not occur in a future period for the estimates detailed below as of March 31, 2018 and, therefore, the transaction price was not reduced further during the three months ended March 31, 2018. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s original estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such changes in estimates become known. Trade Discounts and Allowances —The Company compensates (through trade discounts and allowances) its customers for sales order management, data, and distribution services. However, the Company has determined such services received to date are not distinct from the Company’s sale of products to the customer and, therefore, these payments have been recorded as a reduction of revenue within the statement of operations and comprehensive loss through March 31, 2018, as well as a reduction to trade receivables, net on the condensed consolidated balance sheets. Product Returns — Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company based on the product’s expiration date. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized, as well as within accrued expenses and other current liabilities, net on the condensed consolidated balance sheets. The Company currently estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company has not received any returns to date and believes that returns of ZILRETTA will be minimal. The Company’s limited right of return allows for eligible returns of ZILRETTA in the following circumstances: • Shipment errors that were the result of an error by the Company; • Quantity delivered that is greater or less than the quantity ordered; • Product distributed by the Company that is damaged in transit prior to receipt by the customer; • Expired product, previously purchased directly from the Company, that is returned during the period beginning three months prior to the product’s expiration date and ending three months after the product’s expiration date; • Product subject to a recall; and • Product that the Company, at its sole discretion, has specified to be returned. Government Chargebacks, Discounts and Rebates — Chargebacks for fees and discounts to qualified government healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified VA hospitals and 340b entities at prices lower than the list prices charged to customers who directly purchase the product from the Company. The 340b Drug Discount Program is a US federal government program created in 1992 that requires drug manufacturers to provide outpatient drugs to eligible health care organizations and covered entities at significantly reduced prices. Customers charge the Company for the difference between what they pay for the product and the statutory selling price to the qualified government entity. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and trade receivables, net. Chargeback amounts are generally determined at the time of resale to the qualified government healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the Customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at each reporting period-end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. Government Rebates — The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses and other current liabilities on the condensed consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company anticipates its exposure to utilization from the Medicare Part D coverage gap discount program to be immaterial. For Medicaid programs, the Company estimates the portion of sales attributed to Medicaid patients and records a liability for the rebates to be paid to the respective state Medicaid programs. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives — Other incentives which the Company offers include voluntary patient assistance programs, such as the co-pay assistance program, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payers. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. To date, the Company’s only source of product revenue has been from the U.S. sales of ZILRETTA, which it began shipping to customers in October 2017. The following table summarizes activity in each of the product revenue allowance and reserve categories for the three months ended March 31, 2018: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Balance as of December 31, 2017 $ 60 $ 15 $ 2 $ 77 Provision related to sales in the current year 186 49 12 247 Credit and payments made (94 ) — — (94 ) Balance as of March 31, 2018 $ 152 $ 64 $ 14 $ 230
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, revenue and 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 estimates related to revenue, 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 Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and 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. Property and equipment includes construction-in-progress that is not yet in service.
Foreign Currencies	Foreign Currencies The Company maintains a bank account denominated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations.

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Summary of Product Revenue Allowance And Reserve Categories	The following table summarizes activity in each of the product revenue allowance and reserve categories for the three months ended March 31, 2018: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Balance as of December 31, 2017 $ 60 $ 15 $ 2 $ 77 Provision related to sales in the current year 186 49 12 247 Credit and payments made (94 ) — — (94 ) Balance as of March 31, 2018 $ 152 $ 64 $ 14 $ 230
Property Plant and Equipment Estimated Useful Lives	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5

Fair Value of Financial Asset22

Fair Value of Financial Assets and Liabilities (Tables)	3 Months Ended
	Mar. 31, 2018
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 March 31, 2018 and December 31, 2017 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of March 31, 2018 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 127,468 $ — $ 127,468 Marketable securities — 229,276 — 229,276 $ — $ 356,744 $ — $ 356,744 Fair Value Measurements as of December 31, 2017 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 109,196 $ — $ 109,196 Marketable securities — 296,127 — 296,127 $ — $ 405,323 $ — $ 405,323

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of March 31, 2018 and December 31, 2017 the fair value of available-for-sale marketable securities by type of security was as follows: March 31, 2018 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper 22,383 — — 22,383 U.S. government obligations $ 84,254 $ — $ (156 ) $ 84,098 Corporate bonds 123,218 — (423 ) 122,795 $ 229,855 $ — $ (579 ) $ 229,276 December 31, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 22,436 $ — $ — $ 22,436 U.S. government obligations 121,470 $ — (136 ) 121,334 Corporate bonds 152,630 $ — (273 ) 152,357 $ 296,536 $ — $ (409 ) $ 296,127

Prepaid Expenses and Other Cu24

Prepaid Expenses and Other Current Assets (Tables)	3 Months Ended
	Mar. 31, 2018
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 March 31, 2018 and December 31, 2017: (In thousands) March 31, 2018 December 31, 2017 Prepaid expenses $ 3,288 $ 2,359 Deposits 66 66 Interest receivable on marketable securities 669 978 Total prepaid expenses and other current assets $ 4,023 $ 3,403

Inventory (Tables)

Inventory (Tables)	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Schedule of Inventory	Inventory consisted of the following as of March 31, 2018 and December 31, 2017: (In thousands) March 31, 2018 December 31, 2017 Raw materials $ 780 $ 928 Work in process 711 746 Finished goods 1,034 125 Total inventories $ 2,525 $ 1,799

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2018
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment, net, as of March 31, 2018 and December 31, 2017 consisted of the following: (In thousands) March 31, 2018 December 31, 2017 Computer and office equipment $ 1,124 $ 1,124 Manufacturing equipment 11,827 11,780 Furniture and fixtures 456 456 Software 434 434 Leasehold improvements 478 474 Construction—in progress 592 305 14,911 14,573 Less: Accumulated depreciation (3,947 ) (3,384 ) Total property and equipment, net $ 10,964 $ 11,189

Accrued Expenses and Other Cu27

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: (In thousands) March 31, 2018 December 31, 2017 Research and development $ 788 $ 969 Payroll and other employee-related expenses 4,168 9,309 Professional services fees 2,163 2,591 Interest expense 2,933 1,249 Accrual for employee stock purchase plan 704 141 Other 304 124 Total accrued expenses and other current liabilities $ 11,060 $ 14,383

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Schedule of Annual Principal and Interest Payments Due Under Term Loan	As of March 31, 2018, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments (in thousands) 2018 8,256 2019 10,449 2020 4,383 Total $ 23,088 Less interest (1,278 ) Less unamortized portion of final payment (1,231 ) Total $ 20,579
Summary of Carrying Value of Convertible Notes	The table below summarizes the carrying value of the 2024 Convertible Notes as of March 31, 2018: ( in thousands Gross proceeds $ 201,250 Portion of proceeds allocated to equity component (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion of issuance costs allocated to equity component (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 6,669 Carrying value 2024 Convertible Notes $ 138,983

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants	The relevant data used to determine the value of the stock option grants for the three months ended March 31, 2018 and 2017 are as follows: Three months ended March 31, 2018 2017 Risk-free interest rates 2.72-2.74% 2.23-2.29% Expected dividend yield 0.00% 0.00% Expected term (in years) 6.0 6.0 Expected volatility 71.4-71.5% 71.0-71.6%
Summary of Stock Option Activity	The following table summarizes stock option activity for the three months ended March 31, 2018: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2017 3,800 $ 17.75 Granted 672 22.33 Exercised (22 ) 19.18 Cancelled (46 ) 20.07 Outstanding as of March 31, 2018 4,404 $ 18.42 Options vested and expected to vest at March 31, 2018 4,404 $ 18.42 Options exercisable at March 31, 2018 1,889 $ 15.42
Summary of of RSU Activity	The following table summarizes the RSU activity for the three months ended March 31, 2018: (In thousands, except per share amounts) Number of Shares Weighted Average Grant Date Fair Value Nonvested balance as of December 31, 2017 82 $ 16.43 Granted 209 22.31 Cancelled (3 ) 22.31 Vested/Released — — Nonvested Balance as of March 31, 2018 288 $ 21.07
Stock-Based Compensation Expense Related to Stock Options, and RSUs and Shares Purchased Under Employee Stock Purchase Plan	The Company recorded stock-based compensation expense related to stock options and RSUs and shares purchased under the employee stock purchase plan for the three months ended March 31, 2018 and 2017 as follows: Three months ended March 31, (In thousands) 2018 2017 Research and development $ 1,150 $ 879 Selling, general and administrative 2,507 1,499 $ 3,657 $ 2,378

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2018
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 three months ended March 31, 2018 and 2017: For the three months ended March 31, (In thousands) 2018 2017 Numerator: Net loss $ (41,569 ) $ (23,879 ) Net loss: $ (41,569 ) $ (23,879 ) Denominator: Weighted average common shares outstanding, basic and diluted 37,620 31,704 Net loss per share, basic and diluted $ (1.10 ) $ (0.75 )
Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated as including them would have an anti-dilutive effect: For the three months ended March 31, 2018 2017 Shares issuable upon conversion of the 2024 Convertible Notes 7,515 — Stock options 4,264 3,315 Restricted stock units 218 189 Total 11,997 3,504

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments under Operating Leases	Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments (in thousands) 2018 1,033 2019 1,491 2020 1,533 2021 1,576 2022 1,447 2023 1,203 Total $ 8,283
Schedule of Future Minimum Payments under Contractual Obligations	Future minimum payments under the Company’s agreed obligations are as follows: Year Aggregate Minimum Payments 2018 $ 5,362 2019 8,410 2020 8,410 2021 8,410 2022 8,410 2023 and thereafter 25,231 Total $ 64,233

Overview and Nature of the Bu32

Overview and Nature of the Business - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents and marketable securities	$ 376.6
ZILRETTA [Member]
Summary Of Significant Accounting Policies [Line Items]
Product return, period prior to expiration	3 months
Product return, period after expiration	3 months
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Trade Payment Term	1 year
ASU 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted Cash	$ 0.6	$ 0.6	$ 0.6	$ 0.5

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Summary of Product Revenue Allowance And Reserve Categories (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)
Trade Discounts, Allowances and Government Chargebacks [Member]
Summary Of Significant Accounting Policies [Line Items]
Beginning Balance	$ 60
Provision related to sales in the current year	186
Credit and payments made	(94)
Ending Balance	152
Government Rebates and Other Incentives [Member]
Summary Of Significant Accounting Policies [Line Items]
Beginning Balance	15
Provision related to sales in the current year	49
Ending Balance	64
Returns [Member]
Summary Of Significant Accounting Policies [Line Items]
Beginning Balance	2
Provision related to sales in the current year	12
Ending Balance	14
Product Revenue Allowance and Reserve [Member]
Summary Of Significant Accounting Policies [Line Items]
Beginning Balance	77
Provision related to sales in the current year	247
Credit and payments made	(94)
Ending Balance	$ 230

Summary of Significant Accoun35

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	3 Months Ended
	Mar. 31, 2018
Computers, Office Equipment, and Minor Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years

Fair Value of Financial Asset36

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 127,468	$ 109,196
Marketable securities	229,276	296,127
Assets, Total	356,744	405,323
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	127,468	109,196
Marketable securities	229,276	296,127
Assets, Total	$ 356,744	$ 405,323

Fair Value of Financial Asset37

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	May 02, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0
2024 Convertible Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of liability			$ 136,700,000
Unamortized debt discount			$ 64,500,000
Debt instrument interest rate	3.375%		3.375%
Convertible Notes, face value			$ 201,300,000
Estimated fair value of Convertible Notes	$ 226,600,000

Marketable Securities - Summary

Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 229,855	$ 296,536
Gross Unrealized Losses	(579)	(409)
Fair Value	229,276	296,127
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,383	22,436
Fair Value	22,383	22,436
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,254	121,470
Gross Unrealized Losses	(156)	(136)
Fair Value	84,098	121,334
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,218	152,630
Gross Unrealized Losses	(423)	(273)
Fair Value	$ 122,795	$ 152,357

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 224,338	$ 264,589
Marketable securities, noncurrent	$ 4,900	$ 31,500

Prepaid Expenses and Other Cu40

Prepaid Expenses and Other Current Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 3,288	$ 2,359
Deposits	66	66
Interest receivable on marketable securities	669	978
Total prepaid expenses and other current assets	$ 4,023	$ 3,403

Inventory - Schedule of Invento

Inventory - Schedule of Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 780	$ 928
Work in process	711	746
Finished goods	1,034	125
Total inventories	$ 2,525	$ 1,799

Inventory - Additional Informat

Inventory - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Inventory [Line Items]
Research and development expense	$ 11,551,000	$ 10,756,000
Inventory finished goods shelf life	24 months
Cost of sales expense for fixed overhead costs	$ 2,700,000
Write-downs to inventory	0
ZILRETTA [Member]
Inventory [Line Items]
Research and development expense	$ 3,700,000

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 14,911	$ 14,573
Less: Accumulated depreciation	(3,947)	(3,384)
Total property and equipment, net	10,964	11,189
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	1,124	1,124
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	11,827	11,780
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	456	456
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	478	474
Construction-in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	592	305
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 434	$ 434

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 563,000	$ 453,000
Property and equipment disposals	$ 0

Accrued Expenses and Other Cu45

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Research and development	$ 788	$ 969
Payroll and other employee-related expenses	4,168	9,309
Professional services fees	2,163	2,591
Interest expense	2,933	1,249
Accrual for employee stock purchase plan	704	141
Other	304	124
Total accrued expenses and other current liabilities	$ 11,060	$ 14,383

Debt - Additional Information (

Debt - Additional Information (Detail)	May 02, 2017USD ($)	Jul. 22, 2016USD ($)	Aug. 04, 2015USD ($)	Mar. 31, 2018USD ($)d$ / sharesshares	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Term loan due within 12 months				$ 9,967,000		$ 9,967,000
Term loan due in greater than 12 months				10,612,000		12,936,000
Equity component of Convertible Notes recorded as additional paid-in capital						$ 62,466,000
Amortization of debt discount				(230,000)	$ 155,000
2015 Term Loan [Member]
Debt Instrument [Line Items]
Carrying value of term loan				20,579,000
Liability component of Convertible Notes recorded as long-term debt				$ 23,088,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member]
Debt Instrument [Line Items]
Term loan, maximum borrowings			$ 30,000,000
Debt instrument description				The Company also agreed to maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5%.
Debt instrument percentage of cash or cash equivalents addition to principal balance			5.00%
Debt instrument interest rate			6.25%
Term loan, first periodic payment date			Mar. 1, 2017
Debt instrument maturity date			Feb. 1, 2020
Interest on final payment			9.00%
Term loan, payment description				Interest-only period of 19 months, principal will be due in 36 equal monthly installments.
Carrying value of term loan				$ 20,600,000
Term loan due within 12 months				10,000,000
Term loan due in greater than 12 months				$ 10,600,000
Debt issuance costs			$ 150,000
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 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 amount borrowed		$ 15,000,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.375%			3.375%
Debt instrument maturity date	May 1, 2024
Interest on final payment	3.375%			9.71%
Debt issuance costs	$ 6,500,000			$ 68,900,000
Debt instrument principal amount	201,300,000
Debt instrument frequency of periodic payment				semi-annually
Net proceeds from offering of convertible senior notes	194,800,000
Debt conversion, converted instrument, shares issued | shares				37.3413
Debt instrument principal amount denomination for conversion in to common stock | $ / shares				$ 1,000
Debt instrument convertible initial conversion price per share of common stock | $ / shares				$ 26.78
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock | d				20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock | d				30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible				130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period | d				5
Number of consecutive trading days before five consecutive business days during the note measurement period | d				10
Liability component of Convertible Notes recorded as long-term debt	136,700,000
Equity component of Convertible Notes recorded as additional paid-in capital	$ 64,500,000
Debt instrument effective interest rate period				7 years
Debt issuance costs				$ 6,500,000
Debt issuance costs allocated to liability component				4,400,000
Debt issuance costs allocated to equity component reduction to additional paid-in capital				2,100,000
Interest expense				3,400,000
Amortization of debt discount				$ 1,700,000
2024 Convertible Notes [Member] | On or After February 1, 2024 [Member]
Debt Instrument [Line Items]
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock | d				20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock | d				30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible				130.00%
Debt instrument redemption date				May 6, 2020
Redemption price percentage of principal amount to be redeemed				100.00%
2024 Convertible Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument conversion obligation trading price as percentage of product common stock closing sale price and conversion rate				98.00%

Debt - Schedule of Annual Princ

Debt - Schedule of Annual Principal and Interest Payments Due Under Term Loan (Detail) - 2015 Term Loan [Member] $ in Thousands	Mar. 31, 2018USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,018	$ 8,256
2,019	10,449
2,020	4,383
Total	23,088
Less interest	(1,278)
Less unamortized portion of final payment	(1,231)
Total	$ 20,579

Debt - Summary of Carrying Valu

Debt - Summary of Carrying Value of Convertible Notes (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amortization of debt discount and debt issuance costs	$ 1,884
Carrying value 2024 Convertible Notes	138,983	$ 137,107
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Gross proceeds	201,250
Gross proceeds, Portion of proceeds allocated to equity component (additional paid-in capital)	(64,541)
Debt issuance costs	(6,470)
Debt issuance costs, Portion of issuance costs allocated to equity component (additional paid-in capital)	2,075
Amortization of debt discount and debt issuance costs	6,669
Carrying value 2024 Convertible Notes	$ 138,983

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rates, Minimum	2.72%	2.23%
Risk-free interest rates, Maximum	2.74%	2.29%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years	6 years
Expected volatility, Minimum	71.40%	71.00%
Expected volatility, Maximum	71.50%	71.60%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Issuable Under Options, Beginning balance | shares	3,800,000
Shares Issuable Under Options, Granted | shares	672,000
Shares Issuable Under Options, Exercised | shares	(22,000)
Shares Issuable Under Options, Cancelled | shares	(46,000)
Shares Issuable Under Options, Ending balance | shares	4,404,000
Shares Issuable Under Options, Options vested and expected to vest at March 31, 2018 | shares	4,404,000
Shares Issuable Under Options, Options exercisable at March 31, 2018 | shares	1,889,000
Weighted Average Exercise Price, Beginning balance | $ / shares	$ 17.75
Weighted Average Exercise Price, Granted | $ / shares	22.33
Weighted Average Exercise Price, Exercised | $ / shares	19.18
Weighted Average Exercise Price, Cancelled | $ / shares	20.07
Weighted Average Exercise Price, Ending balance | $ / shares	18.42
Weighted Average Exercise Price, Options vested and expected to vest | $ / shares	18.42
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 15.42

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions	Feb. 01, 2018Installment$ / sharesshares	Oct. 06, 2017USD ($)shares	Jan. 04, 2016shares	Mar. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Mar. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options exercised | shares				22,000
Aggregate intrinsic value of stock options exercised				$ 0.2
Options to purchase common stock, outstanding | shares				4,404,000	3,800,000	3,466,000
Weighted average remaining contractual term				8 years 1 month 6 days		8 years 2 months 12 days
Weighted average exercise price | $ / shares				$ 18.42	$ 17.75	$ 16.20
Weighted average grant date fair value of options granted | $ / shares				$ 14.48		$ 12.70
Unrecognized stock-based compensation expense				$ 31.6
Unrecognized compensation costs, weighted-average recognition periods				3 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, number of shares granted | shares	209,366	122,800	189,300	209,000
Grant date fair value of restricted common stock units under performance-based vesting		$ 2.2			$ 1.6
Unrecognized stock-based compensation expense				$ 4.9
Unrecognized compensation costs, weighted-average recognition periods				3 years 6 months
Weighted average grant date fair value, shares granted, per share | $ / shares	$ 22.31			$ 22.31
Share based compensation, award vesting rights				The RSUs vest in four substantially equal installments on each of the first four anniversaries of the vesting commencement date, subject to the employee's continued employment with, or services to, the Company on each vesting date.
Restricted Stock Units (RSUs) [Member] | First Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, award vesting installments | Installment	4
Restricted Stock Units (RSUs) [Member] | Second Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, award vesting installments | Installment	4
Restricted Stock Units (RSUs) [Member] | Third Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, award vesting installments | Installment	4
Restricted Stock Units (RSUs) [Member] | Fourth Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, award vesting installments | Installment	4
Restricted Stock Units (RSUs) [Member] | January 4, 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense				$ 0.6
Unrecognized compensation costs, weighted-average recognition periods				2 years

Stock-Based Compensation - Su52

Stock-Based Compensation - Summary of of RSU Activity (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares	Feb. 01, 2018	Oct. 06, 2017	Jan. 04, 2016	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning balance				82,000
Number of Shares, Granted	209,366	122,800	189,300	209,000
Number of Shares, Cancelled				(3,000)
Number of Shares, Ending balance				288,000
Weighted Average Grant Date Fair Value, Beginning balance				$ 16.43
Weighted Average Grant Date Fair Value, Granted	$ 22.31			22.31
Weighted Average Grant Date Fair Value, Cancelled				22.31
Weighted Average Grant Date Fair Value, Ending balance				$ 21.07

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options, and RSUs and Shares Purchased Under Employee Stock Purchase Plan (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 3,657	$ 2,378
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,150	879
Selling, General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,507	$ 1,499

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 ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (41,569)	$ (23,879)	$ (137,481)	$ (71,894)
Net loss:	$ (41,569)	$ (23,879)	$ (137,481)	$ (71,894)
Denominator:
Weighted average common shares outstanding, basic and diluted	37,620	31,704
Net loss per share, basic and diluted	$ (1.10)	$ (0.75)

Net Loss per Share - Schedule55

Net Loss per Share - Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	11,997	3,504
Shares Issuable Upon Conversion of the 2024 Convertible Notes [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	7,515
Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	4,264	3,315
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	218	189

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)

May 01, 2018ft²

Oct. 06, 2017ft²

Apr. 07, 2017USD ($)ft²

Sep. 21, 2016ft²

Dec. 31, 2017USD ($)

Feb. 28, 2017USD ($)

Nov. 30, 2016Term

Jul. 31, 2015ft²

May 31, 2013USD ($)

Mar. 31, 2018USD ($)

Oct. 31, 2017USD ($)

Sep. 30, 2015ft²

Commitment And Contingencies [Line Items]

Operating lease term

42 months

Minimum monthly lease payments

$ 17,588

GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member]

Commitment And Contingencies [Line Items]

License agreement, upfront fee paid

$ 2,000,000

Future milestone payments owed

$ 0

GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member] | Maximum [Member]

Commitment And Contingencies [Line Items]

Additional milestone payments

54,000,000

Evonik Corporation [Member]

Commitment And Contingencies [Line Items]

Supply agreement period

5 years

Supply agreement renewal term

2 years

Number of renewal terms | Term

2

Supply agreement description

The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties.

Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]

Commitment And Contingencies [Line Items]

Manufacturing agreement period

10 years

Phase 2 Proof of Concept (PoC) [Member] | GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member] | Maximum [Member]

Commitment And Contingencies [Line Items]

Additional milestone payments

$ 8,700,000

Burlington Massachusetts [Member]

Commitment And Contingencies [Line Items]

Additional leased office space | ft²

6,450

1,471

4,700

Extended leased office space | ft²

5,400

5,400

Office space leased on temporary agreement | ft²

6,700

6,700

Lease termination date

Oct. 31, 2023

Oct. 31, 2019

Temporary lease termination date

Oct. 31, 2017

Office space leased | ft²

28,600

Lease payment per month for office space

$ 80,000

Lease payment per month for additional office space

$ 4,100

Operating lease commencement date

2018-04

Burlington Massachusetts [Member] | Scenario, Forecast [Member]

Commitment And Contingencies [Line Items]

Lease termination date

Oct. 31, 2023

Operating lease office space | ft²

36,500

Woburn, Massachusetts [Member]

Commitment And Contingencies [Line Items]

Operating lease term

5 years

Monthly lease payment

$ 15,000

Total cash obligations for lease

$ 900,000

Commitments and Contingencies57

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail) $ in Thousands	Mar. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 1,033
2,019	1,491
2,020	1,533
2,021	1,576
2,022	1,447
2,023	1,203
Total	$ 8,283

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]	Mar. 31, 2018USD ($)
Property Subject To Or Available For Operating Lease [Line Items]
2,018	$ 5,362
2,019	8,410
2,020	8,410
2,021	8,410
2,022	8,410
2023 and thereafter	25,231
Total	$ 64,233