Spark Therapeutics (ONCE) 10-Q 7 Aug 15 2015 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2015	Jul. 31, 2015
Document And Entity Information [Abstract]
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1,609,351
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Trading Symbol	ONCE
Document Period End Date	Jun. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		24,560,044

Balance sheets (unaudited)

Balance sheets (unaudited) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 223,047,436	$ 74,566,963
Other receivables	933,062	244,393
Prepaid expenses and deferred financing costs	1,111,171	2,551,912
Total current assets	225,091,669	77,363,268
Property and equipment, net	13,490,170	12,674,372
Other assets	408,211	408,211
Total assets	238,990,050	90,445,851
Current liabilities:
Accounts payable	4,015,432	2,676,697
Accrued expenses and other	3,234,730	3,163,154
Current portion of deferred rent	565,458	0
Current portion of deferred revenue	5,578,558	10,014,377
Total current liabilities	13,394,178	15,854,228
Long-term deferred rent	8,469,083	8,618,489
Long-term deferred revenue	11,640,137	10,767,414
Total liabilities	33,503,398	35,240,131
Stockholders’ equity:
Preferred stock, value	0	0
Common stock, $0.001 par value. Authorized, 150,000,000 shares; issued and outstanding, 6,290,317 and 24,543,923 shares at December 31, 2014 and June 30, 2015, respectively.	24,544	6,290
Additional paid-in capital	311,135,063	54,364,833
Accumulated deficit	(105,672,955)	(81,602,606)
Total stockholders’ equity	205,486,652	55,205,720
Total liabilities and stockholders’ equity	238,990,050	90,445,851
Series A Preferred Stock [Member]
Stockholders’ equity:
Preferred stock, value	0	10,000,000
Series B Preferred Stock [Member]
Stockholders’ equity:
Preferred stock, value	$ 0	$ 72,437,203

Balance sheets (unaudited) (Par

Balance sheets (unaudited) (Parenthetical) - $ / shares	Jun. 30, 2015	Dec. 31, 2014
Preferred stock , par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	24,543,923	6,290,317
Common stock, shares outstanding	24,543,923	6,290,317
Series A Convertible Preferred Stock [Member]
Preferred stock , par value		$ 0.001
Preferred stock, shares authorized		5,000,000
Preferred stock, shares issued		5,000,000
Preferred stock, shares outstanding		5,000,000
Series B Convertible Preferred Stock [Member]
Preferred stock , par value		$ 0.001
Preferred stock, shares authorized		45,186,334
Preferred stock, shares issued		45,186,334
Preferred stock, shares outstanding		45,186,334

Statements of operations (unaud

Statements of operations (unaudited) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Revenues	$ 1,288,629	$ 0	$ 3,563,096	$ 20,000
Operating expenses:
Research and development	9,343,972	2,129,573	17,678,080	5,517,306
General and administrative	6,333,123	2,038,909	10,018,003	3,055,032
Total operating expenses	15,677,095	4,168,482	27,696,083	8,572,338
Loss from operations	(14,388,466)	(4,168,482)	(24,132,987)	(8,552,338)
Interest income	51,624	98	62,638	98
Net loss	(14,336,842)	(4,168,384)	(24,070,349)	(8,552,240)
Preferred stock dividends	0	0	(634,794)	0
Net loss applicable to common stockholders	$ (14,336,842)	$ (4,168,384)	$ (24,705,143)	$ (8,552,240)
Basic and diluted net loss per common share (in dollars per share)	$ (0.60)	$ (0.79)	$ (1.17)	$ (1.65)
Weighted average basic and diluted common shares outstanding (in shares)	24,080,420	5,246,089	21,031,708	5,172,231

Statement of stockholders' equi

Statement of stockholders' equity (unaudited) - 6 months ended Jun. 30, 2015 - USD ($)	Total	Preferred Stock [Member]Series A Preferred Stock [Member]	Preferred Stock [Member]Series A Preferred Stock [Member]Initial Public Offering [Member]	Preferred Stock [Member]Series B Preferred Stock [Member]	Preferred Stock [Member]Series B Preferred Stock [Member]Initial Public Offering [Member]	Common Stock [Member]	Common Stock [Member]Series A Preferred Stock [Member]Initial Public Offering [Member]	Common Stock [Member]Series B Preferred Stock [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Series A Preferred Stock [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]Series B Preferred Stock [Member]Initial Public Offering [Member]	Accumulated Deficit [Member]
Beginning Balance (in shares) at Dec. 31, 2014		5,000,000		45,186,334		6,290,317
Beginning Balance at Dec. 31, 2014	$ 55,205,720	$ 10,000,000		$ 72,437,203		$ 6,290			$ 54,364,833			$ (81,602,606)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock and dividends to common stock upon initial public offering (in shares)			(5,000,000)		(45,186,334)		1,016,219	9,183,831
Conversion of preferred stock and dividends to common stock upon initial public offering			$ (10,000,000)		$ (72,437,203)		$ 1,016	$ 9,184		$ 9,998,984	$ 72,428,019
Issuance of common stock, net of issuance costs	168,892,493					$ 8,050			168,884,443
Issuance of common stock, net of issuance costs (in shares)						8,050,000
Issuance of common stock for services	193,909					$ 4			193,905
Issuance of common stock for services (in shares)						3,556
Stock-based compensation expense	5,264,879								5,264,879
Net loss	(24,070,349)											(24,070,349)
Ending Balance at Jun. 30, 2015	$ 205,486,652	$ 0		$ 0		$ 24,544			$ 311,135,063			$ (105,672,955)
Ending Balance (in shares) at Jun. 30, 2015		0		0		24,543,923

Statements of cash flows (unaud

Statements of cash flows (unaudited) - USD ($)	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities:
Net loss	$ (24,070,349)	$ (8,552,240)
Adjustments to reconcile net loss to net cash used in operating activities:
Noncash rent expense	416,052	138,648
Depreciation expense	797,177	4,933
Stock-based compensation expense	5,458,788	1,432,812
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,537,742)	(315,701)
Accounts payable and accrued expenses	3,178,565	1,912,549
Deferred revenue	(3,563,096)	1,395,723
Net cash used in operating activities	(19,320,605)	(3,983,276)
Cash flows from investing activities:
Purchases of property and equipment	(2,512,357)	(485,299)
Net cash used in investing activities	(2,512,357)	(485,299)
Cash flows from financing activities:
Proceeds from issuance of Series A convertible preferred units	0	4,861,285
Proceeds from issuance of Series B convertible preferred stock, net	0	72,728,904
Proceeds from issuance of common stock, net of financing costs	170,313,435	0
Net cash provided by financing activities	170,313,435	77,590,189
Net increase in cash and cash equivalents	148,480,473	73,121,614
Cash and cash equivalents, beginning of period	74,566,963	0
Cash and cash equivalents, end of period	223,047,436	73,121,614
Supplemental disclosure of cash flow information:
Deferred financing costs included in accounts payable	0	289,432
Property and equipment purchases included in accounts payable and accrued expenses	$ 247,818	$ 2,831,221

Background

Background	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Background	Background Spark Therapeutics, Inc. was formed on March 13, 2013 in the state of Delaware as AAVenue Therapeutics, LLC and amended its Certificate of Formation in October 2013 to change its name to Spark Therapeutics LLC. In May 2014, the Company converted from a limited liability company (LLC) to a C corporation, Spark Therapeutics, Inc. (the Company). The Company is a gene therapy company, seeking to transform the lives of patients suffering from debilitating genetic diseases by developing one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania. (a) Initial Public Offering (IPO) On February 4, 2015 , the Company completed its IPO, having sold 8,050,000 shares of common stock at an IPO price of $23.00 per share, for aggregate gross proceeds of $185.2 million . The Company received net proceeds from the IPO of $168.9 million , after deducting underwriting discounts and commissions and other offering expenses. As part of the IPO, all of the outstanding shares of preferred stock, including shares of preferred stock issued as accrued dividends, were converted into an aggregate of 10,200,050 shares of common stock.

Development-stage risks

Development-stage risks	6 Months Ended
	Jun. 30, 2015
Development Stage Enterprises [Abstract]
Development-stage risks	Development-stage risks The Company has incurred losses and negative cash flows from operations since inception and had an accumulated deficit of $105.7 million at June 30, 2015 . The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates in development. Additional financing will be needed by the Company to fund its operations and to commercially develop its product candidates. The Company’s future operations are highly dependent on a combination of factors, including: (i) the success of its research and development; (ii) regulatory approval and market acceptance of the Company’s proposed future products; (iii) the timely and successful completion of additional financing; and (iv) the development of competitive therapies by other biotechnology and pharmaceutical companies.

Summary of significant accounti

Summary of significant accounting policies	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Basis of presentation The accompanying unaudited interim financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the financial statements) considered necessary to present fairly the Company’s financial position as of June 30, 2015 , its results of operations for the three and six months ended June 30, 2014 and 2015 and cash flows for the six months ended June 30, 2014 and 2015 . Operating results for the three and six months ended June 30, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015. The interim financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual financial statements. The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2014 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014 . (b) Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (c) Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments. (d) Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of June 30, 2015 consisted primarily of money market funds. (e) Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception. (f) Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use. (g) Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: the milestone is commensurate with either: (1)the performance required to achieve the milestone or (2)the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; the milestone relates solely to past performance; and the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products. (h) Recapitalization On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split. (i) Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, the outstanding shares of convertible preferred stock, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of June 30, 2014 and 2015 as they would be anti-dilutive: June 30, 2014 2015 Convertible preferred shares 10,037,255 — Unvested restricted common shares 689,283 434,659 Options issued and outstanding 287,511 3,198,697 Amounts in the table above reflect the common stock equivalents of the noted instruments. (j) Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of June 30, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received.

Fair value of financial instrum

Fair value of financial instruments	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Fair value of financial instruments	Fair value of financial instruments The Company follows FASB accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements to maximize the use of “observable inputs.” The three-level hierarchy of inputs to measure fair value are as follows: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities • Level 2: Significant other observable inputs other than Level 1 prices such as quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity) The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2014: Assets: Money market funds (included in cash and cash equivalents) $ 74,025,841 — — At June 30, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 223,047,436 — —

Accrued expenses

Accrued expenses	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, June 30, Compensation and benefits $ 1,385,013 $ 1,873,734 Consulting and professional fees 1,327,942 255,199 Research and development 247,448 870,531 Other 202,751 235,266 $ 3,163,154 $ 3,234,730

Stockholders' equity

Stockholders' equity	6 Months Ended
	Jun. 30, 2015
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company’s certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. The Company has authorized 150,000,000 shares of common stock and 5,000,000 shares of preferred stock. (a) Convertible preferred October 2013 Series A financing In October 2013, the Company entered into an agreement with The Children’s Hospital of Philadelphia (CHOP) to sell 5,000,000 Series A Units at $2.00 per unit for proceeds of $10.0 million . Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder. The Series A Units were mandatorily convertible into common stock in the event of an IPO, as defined. May 2014 conversion to C corporation Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A Stock. May 2014 Series B financing In May 2014, the Company issued 45,186,334 shares of Series B convertible preferred stock (Series B Stock) for $72.4 million , net of transaction costs. In conjunction with the issuance of Series B Stock, certain Series A convertible preferred stock (Series A Stock) terms were amended. Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders, subject to certain customary antidilution adjustments contained in the Company’s certificate of incorporation. The Series A Stock and Series B Stock were entitled to receive cumulative dividends at 8% per annum, which accrued from day to day beginning November 23, 2014 and were payable upon conversion, an event of liquidation or a qualified IPO, in each case, in shares of Series A Stock and Series B Stock, as applicable. As of February 4, 2015, dividends of $1.3 million had accumulated, and in connection with the IPO, were declared and converted along with all outstanding shares of Series A Stock and Series B Stock into an aggregate of 10,200,050 shares of common stock. (b) Common Through May 1, 2014, the Board designated Series 1 Units, Series 2 Units and Series 3 common units (Series 3 Units). Capital distributions were to be made to and among the holders in the following order of priority: Series A Units, Series 1 Units, Series 2 Units and Series 3 Units. Upon conversion of the Company into a C corporation in May 2014, each outstanding Series 1 Unit converted into 0.2 shares (post-split) of common stock, each outstanding Series 2 Unit converted into 0.2 shares (post-split) of common stock and each outstanding Series 3 Unit converted into 0.03883773 shares (post-split) of common stock. In 2013 and 2014, the Company issued Series 2 Units and Series 3 Units to various employees, directors and consultants of the Company. The vesting terms of the common stock issued upon conversion of the Series 2 Units and Series 3 Units vary, but primarily, shares vest 25% on the first anniversary of the vesting commencement date and then quarterly over three years , with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company. Upon conversion, the vesting terms of the previously issued equity remained consistent. For the six months ended June 30, 2014 , the Company recorded compensation expense of $1.1 million and $0.3 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. For the six months ended June 30, 2015 , the Company recorded compensation expense of $31,616 and $1.4 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. At June 30, 2015 , there was $2.7 million of unrecognized compensation expense related to restricted common shares which is expected to be recognized over a weighted-average period of 1.9 years . The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2014 578,994 $ 4.46 Shares vested (144,335 ) $ 3.90 Nonvested shares at June 30, 2015 434,659 $ 4.65

Stock incentive plans

Stock incentive plans	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans In May 2014, the Company established the 2014 Stock Incentive Plan (the 2014 Plan), which allows for the granting of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards and other stock awards of the Company’s common stock to employees, officers, directors, consultants and advisors. In January 2015, upon the IPO, the plan was terminated and the 209,500 shares available for future grants under the 2014 Plan were rolled into the 2015 Stock Incentive Plan (the 2015 Plan). In January 2015, the Company established the 2015 Plan, which became effective immediately prior to the closing of the IPO. The 2015 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, officers, directors, consultants and advisors. Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; (2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors. As of June 30, 2015 , 1,105,300 shares were available for future grants under the 2015 Plan. In January 2015, the Company established the 2015 employee stock purchase plan (the 2015 ESPP), which became effective immediately prior to the closing of the IPO. The 2015 ESPP initially will provide participating employees with the opportunity to purchase an aggregate of 220,000 shares of common stock. The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. No shares were issued under the 2015 ESPP as of June 30, 2015 . The following table summarizes stock option activity: Number of shares Weighted- average exercise price Outstanding at December 31, 2014 2,264,497 $ 4.52 Granted 934,200 $ 61.16 Outstanding at June 30, 2015 3,198,697 $ 21.07 Vested at June 30, 2015 431,278 $ 4.11 Vested at June 30, 2015 and expected to vest 3,198,697 $ 21.07 The weighted average remaining contractual term of options outstanding as of June 30, 2015 is 9.3 years. The weighted average remaining contractual term of options exercisable as of June 30, 2015 is 9.1 years. During six months ended June 30, 2014 , the Company recorded compensation expense of $14,062 and $10,529 in research and development expense and general and administrative expense, respectively, related to stock options. During the six months ended June 30, 2015 , the Company recorded compensation expense of $1.7 million and $2.1 million in research and development expense and general and administrative expense, respectively, related to stock options. At June 30, 2015, there was $41.8 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 3.6 years. The weighted average grant date fair value of the options granted during the six months ended June 30, 2015 was estimated at $41.32 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions: Expected volatility 76.3 % Risk-free interest rate 1.67 % Expected term (in years) 6.10 Expected dividend yield 0.0 %

Related-party transactions

Related-party transactions	6 Months Ended
	Jun. 30, 2015
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2014 and June 30, 2015 , CHOP was considered a significant equity holder. In October 2013, the Company entered into technology and license agreements with CHOP for certain commercialization licenses to be provided to the Company in order to develop services, methods and marketable products for commercialization. The license agreement requires the Company to reimburse CHOP for the patent costs related to the underlying licensed rights incurred after the effective date. For the three months ended June 30, 2014 and 2015 , the Company recorded $34,095 and $0.2 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statement of operations. For the six months ended June 30, 2014 and 2015, the Company recorded $57,943 and $0.4 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statement of operations. In 2013, the Company entered into a number of services agreements with CHOP. The Master Research Services Agreement provides for certain research, development, and manufacturing services to be provided to the Company by CHOP. A separate Services Agreement provides for clinical, technical, and administrative services to be provided by CHOP to the Company. For the three months ended June 30, 2014 and 2015, the Company recorded $0.8 million and $1.3 million , respectively, as research and development expense and for the three months ended June 30, 2014, the Company recorded $18,544 , as general and administrative expense related to these agreements. For the six months ended June 30, 2014 and 2015, the Company recorded $3.2 million and $2.2 million , respectively, as research and development expense and for the six months ended June 30, 2014, the Company recorded $28,810 , as general and administrative expense related to these agreements. As of December 31, 2014 , $0.1 million and $0.9 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of June 30, 2015 , $0.2 million and $1.0 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Collaboration and license agree

Collaboration and license agreements	6 Months Ended
	Jun. 30, 2015
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements In March 2014, the Company entered into an agreement with Genable Technologies Limited (Genable) in which the Company will be the exclusive manufacturer and provide development advice and expertise in the ongoing development of Genable’s lead therapeutic product. Under a license agreement, the Company also granted certain rights to manufacturing patent applications. The Company is eligible to earn development milestone payments and mid-single-digit royalties on future product sales. During the six months ended June 30, 2014 , the Company received $20,000 for the license and recognized it as revenue. As the final product being manufactured is subject to final testing, the Company will recognize manufacturing revenues when product is released. As of June 30, 2015 , $0.4 million remains in current deferred revenue related to the Genable agreement. In April 2014, the Company began discussions with a biopharmaceutical company concerning a potential manufacturing technology agreement. The Company received a one-time, nonrefundable payment of $1.0 million to engage in due diligence. In March 2015, the Company concluded discussions on a potential arrangement with the biopharmaceutical company and, as a result, the Company recognized the nonrefundable payment of $1.0 million as revenue during the six months ended June 30, 2015 . In December 2014, the Company entered into a global collaboration agreement with Pfizer Inc. (Pfizer), for the development and commercialization of SPK-FIX product candidates for the treatment of hemophilia B. Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024. The Company will be primarily responsible for conducting all research and development activities through completion of Phase 1/2 clinical trials of hemophilia B product candidates. Pfizer and the Company will share development costs incurred under an agreed product development plan for each product candidate with the Company’s share of development costs under the agreement limited to $10.6 million . Following the completion of Phase 1/2 clinical trials, Pfizer will be primarily responsible for development, manufacture, regulatory approval and commercialization, including all costs associated therewith. In connection with this agreement, the Company received a $20.0 million upfront payment for the license in December 2014. As there is no stand-alone value for the license, the Company is recognizing revenue through the estimated completion date of Phase 1/2 clinical trials. During the six months ended June 30, 2015 , the Company recognized $2.6 million of revenue. As of June 30, 2015 , there is $5.2 million and $11.6 million of current and long term deferred revenue for this payment, respectively. During the six months ended June 30, 2015 , the Company recorded $0.8 million as a reduction to research and development expenses for the reimbursement of costs from Pfizer. The Company also is eligible to receive up to $260.0 million in aggregate milestone payments, $140.0 million of which relate to potential development, regulatory and commercial milestones for the first product candidate to achieve each milestone and $120.0 million of which relate to potential regulatory milestones for additional product candidates. In addition, the Company is entitled to receive royalties calculated as a low-teen percentage of net sales of licensed products. The royalties may be subject to certain reductions, including for a specified portion of royalty payments that Pfizer may become required to pay under any third-party license agreements, subject to a minimum royalty. Under the agreement, the Company remains solely responsible for the payment of license payments payable by the Company under specified license agreements. The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product; or (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements. In April 2015, the Company entered into a research, license and option agreement with Clearside Biomedical, Inc. (Clearside) under which the Company acquired exclusive rights to license Clearside’s microinjector technology and the option to further develop and commercialize gene therapy products delivered using the Clearside technology. Under the agreement, the companies will explore the feasibility of using Clearside’s microinjector technology to deliver viral vectors to the choroid and the retina through the suprachoroidal space. In connection with this agreement, the Company made an upfront payment of $0.5 million for services to be rendered in the development of licensed products. During the six months ended June 30, 2015, the Company recorded $0.2 million as research and development expense related to the upfront payment.

Summary of significant accoun16

Summary of significant accounting policies (Policies)	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying unaudited interim financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the financial statements) considered necessary to present fairly the Company’s financial position as of June 30, 2015 , its results of operations for the three and six months ended June 30, 2014 and 2015 and cash flows for the six months ended June 30, 2014 and 2015 . Operating results for the three and six months ended June 30, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015. The interim financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual financial statements. The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2014 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014 .
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses approximate fair value due to the short-term nature of those instruments.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of June 30, 2015 consisted primarily of money market funds.
Property and equipment	Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception.
Research and development and in-process research and development	Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use.
Revenue recognition	Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: the milestone is commensurate with either: (1)the performance required to achieve the milestone or (2)the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; the milestone relates solely to past performance; and the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products.
Recapitalization	Recapitalization On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split.
Net loss per common share	Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, the outstanding shares of convertible preferred stock, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same.
Deferred rent	Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of June 30, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received.

Summary of significant accoun17

Summary of significant accounting policies (Tables)	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding	The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of June 30, 2014 and 2015 as they would be anti-dilutive: June 30, 2014 2015 Convertible preferred shares 10,037,255 — Unvested restricted common shares 689,283 434,659 Options issued and outstanding 287,511 3,198,697

Fair value of financial instr18

Fair value of financial instruments (Tables)	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2014: Assets: Money market funds (included in cash and cash equivalents) $ 74,025,841 — — At June 30, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 223,047,436 — —

Accrued expenses (Tables)

Accrued expenses (Tables)	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, June 30, Compensation and benefits $ 1,385,013 $ 1,873,734 Consulting and professional fees 1,327,942 255,199 Research and development 247,448 870,531 Other 202,751 235,266 $ 3,163,154 $ 3,234,730

Stockholders' equity (Tables)

Stockholders' equity (Tables)	6 Months Ended
	Jun. 30, 2015
Equity [Abstract]
Summary of Restricted Stock Activity	The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2014 578,994 $ 4.46 Shares vested (144,335 ) $ 3.90 Nonvested shares at June 30, 2015 434,659 $ 4.65

Stock incentive plans (Tables)

Stock incentive plans (Tables)	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes stock option activity: Number of shares Weighted- average exercise price Outstanding at December 31, 2014 2,264,497 $ 4.52 Granted 934,200 $ 61.16 Outstanding at June 30, 2015 3,198,697 $ 21.07 Vested at June 30, 2015 431,278 $ 4.11 Vested at June 30, 2015 and expected to vest 3,198,697 $ 21.07
Summary of Weighted-Average Assumptions	The weighted average grant date fair value of the options granted during the six months ended June 30, 2015 was estimated at $41.32 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions: Expected volatility 76.3 % Risk-free interest rate 1.67 % Expected term (in years) 6.10 Expected dividend yield 0.0 %

Background - Additional Informa

Background - Additional Information (Detail) $ / shares in Units, $ in Millions	Feb. 04, 2015USD ($)$ / sharesshares	Jun. 30, 2015Segmentshares
Accounting Policies [Abstract]
Number of operating segment | Segment		1
Class of Stock [Line Items]
Common stock, shares issued during IPO	8,050,000
Aggregate gross proceeds from IPO | $	$ 185.2
Net proceeds from IPO | $	$ 168.9
Preferred stock, shares issued upon conversion	10,200,050
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO		8,050,000
Initial Public Offering [Member]
Class of Stock [Line Items]
IPO Completion date		Feb. 4, 2015
Initial Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock issuance price per share | $ / shares	$ 23

Development-Stage Risks - Addit

Development-Stage Risks - Additional Information (Detail) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Development Stage Enterprises [Abstract]
Retained earnings (accumulated deficit)	$ (105,672,955)	$ (81,602,606)

Summary of Significant Accoun24

Summary of Significant Accounting Policies - Additional Information (Detail)	Jan. 16, 2015	Jun. 30, 2015	Jun. 30, 2015USD ($)
Property, Plant and Equipment [Line Items]
Impairment charges of long-lived assets			$ 0
Reverse stock split, description		One share for every five shares previously held.
Conversion ratio of common stock	0.2
Computers [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life		3 years
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life		3 years
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life		5 years
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life		7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives		Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset

Summary of Significant Accoun25

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Detail) - shares	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Unvested Restricted Common Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	434,659	689,283
Options Issued and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	3,198,697	287,511
Convertible Preferred Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	0	10,037,255

Fair Value of Financial Instr26

Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - Money Market Funds [Member] - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 223,047,436	$ 74,025,841
Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 0	$ 0

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Compensation and benefits	$ 1,873,734	$ 1,385,013
Consulting and professional fees	255,199	1,327,942
Research and development	870,531	247,448
Other	235,266	202,751
Accrued expenses	$ 3,234,730	$ 3,163,154

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Feb. 04, 2015	May. 31, 2014	Oct. 31, 2013	Jun. 30, 2015	May. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Common stock, shares authorized				150,000,000		150,000,000		150,000,000
Preferred stock, shares authorized				5,000,000		5,000,000		5,000,000
Preferred stock, shares issued				0		0		0
Proceeds from issuance of Series B convertible preferred stock, net						$ 0	$ 72,728,904
Preferred stock, shares issued upon conversion	10,200,050
Preferred stock shares, Value				$ 0		0		$ 0
Accumulated dividends	$ 1,300,000
Unrecognized compensation expense related to restricted common shares				$ 2,700,000		$ 2,700,000
Unrecognized compensation expense related to restricted common shares expected to be recognized, weighted-average period						1 year 10 months 18 days
Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense						$ 1,400,000	300,000
General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense						$ 31,616	$ 1,100,000
Common Stock [Member]
Class of Stock [Line Items]
Preferred shares converted into number of common stock	10,200,050
Percentage of vesting of units on first anniversary				25.00%
Units, vesting description				The vesting terms of the common stock issued upon conversion of the Series 2 Units and Series 3 Units vary, but primarily, shares vest 25% on the first anniversary of the vesting commencement date and then quarterly over three years, with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company.
Common Stock [Member] | Share-based Compensation Award, Tranche Two [Member]
Class of Stock [Line Items]
Units, vesting period				3 years
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized								45,186,334
Preferred stock, shares issued								45,186,334
Convertible preferred stock terms of conversion				Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion				Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A Stock.
Preferred stock, shares issued upon conversion		0.2		1		1
Preferred stock shares, Value				$ 0		$ 0		$ 10,000,000
Preferred stock dividend rate		8.00%
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued		45,186,334
Preferred stock, shares issued upon conversion		0.2
Preferred stock shares, Value		$ 72,400,000		$ 0		$ 0		$ 72,437,203
Minimum percentage of approval by outstanding preferred share holders for preferred stock conversion to common stock		87.50%
Preferred stock dividend rate		8.00%
Series 1 Unit [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion					Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder.
Preferred stock, shares issued upon conversion			1
Series 1 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion		0.2
Series 2 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion		0.2
Series 3 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common units, shares issued upon conversion		0.03883773
CHOP [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued			5,000,000
Stock issuance price per share			$ 2
Proceeds from issuance of Series B convertible preferred stock, net			$ 10,000,000

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Restricted Stock Activity (Detail) - 6 months ended Jun. 30, 2015 - Restricted Stock [Member] - $ / shares	Total
Number of shares (in shares):
Nonvested beginning balance	578,994
Shares vested	(144,335)
Nonvested ending balance	434,659
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance	$ 4.46
Shares vested	3.90
Nonvested shares ending balance	$ 4.65

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2015	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual term of options exercisable			9 years 3 months
Weighted average remaining contractual term of options outstanding			9 years 1 month 12 days
Unrecognized compensation expense related to stock options		$ 41,800,000	$ 41,800,000
Weighted average period recognition for unrecognized compensation expense			1 year 10 months 18 days
Weighted average grant date fair value of options			$ 41.32
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 1,400,000	$ 300,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 31,616	1,100,000
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense			3 years 7 months 18 days
Employee Stock Option [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 1,700,000	14,062
Employee Stock Option [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 2,100,000	$ 10,529
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants	209,500	1,105,300	1,105,300
Common stock reserved for issuance	1,830,000
Maximum shares authorized	2,543,299
Shares of common stock	1,724,000
Percentage of common stock outstanding	4.00%
Stock incentive plans description		Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; 2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors.
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	220,000
Shares of common stock	440,000
Percentage of common stock outstanding	1.00%
Stock incentive plans description		The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. No shares were issued under the 2015 ESPP plan as of June 30, 2015
Number of shares issued			0

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Stock Option Activity (Detail) - Jun. 30, 2015 - $ / shares	Total
Number of shares (in share):
Outstanding at beginning of period	2,264,497
Granted	934,200
Outstanding at end of period	3,198,697
Vested at end of period	431,278
Vested and expected to vest at end of period	3,198,697
Weighted-average exercise price (in dollars per share):
Outstanding at beginning of period	$ 4.52
Granted	61.16
Outstanding at end of period	21.07
Vested at end of period	4.11
Vested and expected to vest at end of period	$ 21.07

Stock Incentive Plans - Summa32

Stock Incentive Plans - Summary of Weighted-Average Assumptions (Detail) - 3 months ended Jun. 30, 2015	Total
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected volatility	76.30%
Risk-free interest rate	1.67%
Expected term (in years)	6 years 1 month 6 days
Expected dividend yield	0.00%

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Research and development expense related to agreements	$ 9,343,972	$ 2,129,573	$ 17,678,080	$ 5,517,306
General and administrative expense related to agreements	(6,333,123)	(2,038,909)	(10,018,003)	(3,055,032)
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	200,000		200,000		$ 100,000
Accounts payable due to related party	1,000,000		1,000,000		$ 900,000
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements	1,300,000	800,000	2,200,000	3,200,000
General and administrative expense related to agreements		(18,544)		(28,810)
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs	$ 200,000	$ 34,095	$ 400,000	$ 57,943

Collaboration and License Agr34

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	Apr. 30, 2015	Dec. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Apr. 30, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Current portion of deferred revenue		$ 10,014,377	$ 5,578,558		$ 5,578,558		$ 10,014,377
Nonrefundable payment received								$ 1,000,000
Development costs			9,343,972	$ 2,129,573	17,678,080	$ 5,517,306
Long term deferred revenue		10,767,414	11,640,137		11,640,137		10,767,414
Revenue recognition from aggregate milestone payments					260,000,000
Genable License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue						$ 20,000
Current portion of deferred revenue			400,000		$ 400,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative Arrangement, Rights and Obligations					Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Collaboration And License Agreement Expiration Description					The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product or; (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.
Licenses revenue					$ 2,600,000
Current portion of deferred revenue			5,200,000		5,200,000
Development costs							$ 10,600,000
Upfront payment received for license		$ 20,000,000
Long term deferred revenue			$ 11,600,000		$ 11,600,000
Collaboration agreement expiration, period					15 years
First Product Candidate [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments					$ 140,000,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments					120,000,000
Clearside Biomedical [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development costs					200,000
Prepaid Expense, Current	$ 500,000
Research and Development Expense [Member] | Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement Revenue					$ 800,000