Merrimack Pharmaceuticals (MACK) 10-Q 10 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]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q2
Trading Symbol	MACK
Entity Registrant Name	MERRIMACK PHARMACEUTICALS INC
Entity Central Index Key	1,274,792
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		111,419,126

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 33,664	$ 35,688
Available-for-sale securities	33,991	88,340
Restricted cash	101	101
Accounts receivable	1,762	3,313
Prepaid expenses and other current assets	5,366	4,654
Total current assets	74,884	132,096
Restricted cash	584	584
Property and equipment, net	18,273	14,502
Other assets	132	144
Intangible assets, net	1,365	1,525
In-process research and development	6,200	6,200
Goodwill	3,605	3,605
Total assets	105,043	158,656
Current liabilities:
Accounts payable, accrued expenses and other	46,626	37,236
Deferred revenues	59,346	59,275
Deferred rent	1,582	1,285
Long-term debt, current portion	1,058	13,346
Total current liabilities	108,612	111,142
Deferred revenues, net of current portion	9,109	35,682
Deferred rent, net of current portion	5,652	5,401
Deferred tax incentives, net of current portion	417	496
Long-term debt, net of current portion	123,117	106,806
Accrued interest	1,200	1,200
Total liabilities	$ 248,107	$ 260,727
Commitments and contingencies (Note 10)
Non-controlling interest	$ 273	$ 69
Stockholders' deficit:
Preferred stock, $0.01 par value: 10,000 shares authorized at June 30, 2015 and December 31, 2014; no shares issued or outstanding at June 30, 2015 or December 31, 2014
Common stock, $0.01 par value: 200,000 shares authorized at June 30, 2015 and December 31, 2014; 110,798 and 106,697 shares issued and outstanding at June 30, 2015 and December 31, 2014, respectively	$ 1,108	$ 1,067
Additional paid-in capital	567,754	552,037
Accumulated other comprehensive loss	(6)	(74)
Accumulated deficit	(712,193)	(655,170)
Total stockholders' deficit	(143,337)	(102,140)
Total liabilities, non-controlling interest and stockholders' deficit	$ 105,043	$ 158,656

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
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	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	110,798,000	106,697,000
Common stock, shares outstanding	110,798,000	106,697,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$ 36,558	$ 27,815	$ 51,399	$ 40,849
Operating expenses:
Research and development	42,806	33,795	78,485	64,119
General and administrative	12,315	7,921	21,504	14,145
Total operating expenses	55,121	41,716	99,989	78,264
Loss from operations	(18,563)	(13,901)	(48,590)	(37,415)
Other income and expenses
Interest income	34	20	80	55
Interest expense	(4,482)	(4,570)	(9,048)	(9,081)
Other, net	110	161	224	397
Net loss	(22,901)	(18,290)	(57,334)	(46,044)
Less net income (loss) attributable to non-controlling interest	(123)	(181)	204	(350)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(22,778)	(18,109)	(57,538)	(45,694)
Other comprehensive income:
Unrealized gain on available-for-sale securities	21	5	68	21
Other comprehensive income	21	5	68	21
Comprehensive loss	$ (22,757)	$ (18,104)	$ (57,470)	$ (45,673)
Net loss per share available to common stockholders-basic and diluted	$ (0.21)	$ (0.17)	$ (0.53)	$ (0.44)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	109,975	103,809	108,662	103,351

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities
Net loss	$ (57,334)	$ (46,044)
Adjustments to reconcile net loss to net cash used in operating activities
Non-cash interest expense	4,038	4,267
Depreciation and amortization	2,092	2,241
Stock-based compensation	8,337	7,091
Changes in operating assets and liabilities
Purchased premiums and interest on available-for-sale securities		(4)
Accounts receivable	1,711	(748)
Accounts payable, accrued expenses and other	7,474	(6,027)
Deferred revenues	(26,502)	(26,169)
Other assets and liabilities, net	527	1,173
Net cash used in operating activities	(59,657)	(64,220)
Cash flows from investing activities
Purchases of available-for-sale securities		(20,100)
Proceeds from maturities of available-for-sale securities	53,963	94,733
Purchases of property and equipment	(4,105)	(2,983)
Net cash provided by investing activities	49,858	71,650
Cash flows from financing activities
Proceeds from exercise of common stock options and warrants	6,542	4,936
Proceeds from issuance of preferred stock of Silver Creek Pharmaceuticals, Inc.	1,233
Proceeds from convertible notes issued by Silver Creek Pharmaceuticals, Inc., net of issuance costs		300
Other financing activities, net		(1)
Net cash provided by financing activities	7,775	5,235
Net (decrease) increase in cash and cash equivalents	(2,024)	12,665
Cash and cash equivalents, beginning of period	35,688	65,086
Cash and cash equivalents, end of period	33,664	77,751
Non-cash investing and financing activities
Disposal of fully depreciated assets	106	670
Property and equipment in accounts payable and accrued expenses	2,060	564
Receivables related to stock option exercises	160
Supplemental disclosure of cash flows
Cash paid for interest	$ 4,946	$ 4,915

Nature of the Business

Nature of the Business	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Merrimack Pharmaceuticals, Inc. (the “Company”) is a biopharmaceutical company discovering, developing and preparing to commercialize innovative medicines consisting of novel therapeutics paired with companion diagnostics for the treatment of cancer. The Company has multiple targeted therapeutic oncology candidates in clinical development. The Company’s most advanced program is its investigational agent MM-398. The U.S. Food and Drug Administration (the “FDA”) has accepted for review a New Drug Application (“NDA”) for MM-398 in combination with 5-fluorouracil (“5-FU”) and leucovorin for the treatment of patients with metastatic adenocarcinoma of the pancreas who have been previously treated with gemcitabine-based therapy and granted priority review status to the NDA. The FDA has set a goal of October 24, 2015 as the action date for the NDA under the Prescription Drug User Fee Act. In addition, the European Medicines Agency (the “EMA”) accepted for review a Marketing Authorization Application (“MAA”) filed by Baxalta Incorporated, Baxalta US Inc. and Baxalta GmbH (collectively, “Baxalta”) for MM-398 in combination with 5-FU and leucovorin for the treatment of patients with metastatic adenocarcinoma of the pancreas who have been previously treated with gemcitabine-based therapy. The Company also has multiple product candidates in preclinical development and a discovery effort advancing additional candidate medicines. The Company also has an agreement to utilize its manufacturing expertise to develop, manufacture and exclusively supply bulk drug to a third party, who will in turn process the drug into finished product and commercialize it globally. The Company was incorporated in the Commonwealth of Massachusetts in 1993 and reincorporated in the State of Delaware in October 2010. The Company is subject to risks and uncertainties common to companies in the biopharmaceutical industry, including, but not limited to, its ability to secure additional capital to fund operations, success of clinical trials, development by competitors of new technological innovations, dependence on collaborative arrangements, protection of proprietary technology, compliance with government regulations and dependence on key personnel. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel, infrastructure and extensive compliance reporting capabilities. The Company has incurred significant losses and has not generated revenue from commercial sales. 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 may seek additional funding through public or private debt or equity financings, or through existing or new collaboration arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into additional collaborative arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or product candidates. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs or commercialization efforts, which could adversely affect its business prospects.

Basis of Presentation and Conso

Basis of Presentation and Consolidation	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Consolidation	2. Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements as of June 30, 2015, and for the three and six months ended June 30, 2015 and 2014, have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and generally accepted accounting principles in the United States of America (“GAAP”) for condensed consolidated financial information. 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 for the year ended December 31, 2014 filed with the SEC on February 27, 2015. The information presented in the condensed consolidated financial statements and related notes as of June 30, 2015, and for the three and six months ended June 30, 2015 and 2014, is unaudited. The December 31, 2014 condensed 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 six months ended June 30, 2015 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2015, or any future period. On September 23, 2014, the Company merged its wholly owned subsidiary, Merrimack Pharmaceuticals (Bermuda) Ltd. (“Merrimack Bermuda”), with and into the Company, with the Company being the surviving corporation (the “Merger”). These condensed consolidated financial statements include the consolidated accounts of Merrimack Bermuda prior to the Merger. These condensed consolidated financial statements also include the accounts of the Company and its majority owned subsidiary, Silver Creek Pharmaceuticals, Inc. (“Silver Creek”). All intercompany transactions and balances have been eliminated in consolidation. During the six months ended June 30, 2015, Silver Creek issued and sold a total of 1.0 million shares of Silver Creek Series B preferred stock at a price per share of $1.35 to investors and received net proceeds of $1.2 million, after deducting issuance costs. The Company’s ownership of Silver Creek was 57% and 60% as of June 30, 2015 and December 31, 2014, respectively. The consolidated financial statement activity related to Silver Creek was as follows: (in thousands) Non-Controlling Interest Balance at December 31, 2014 $ 69 Net income attributable to Silver Creek 204 Balance at June 30, 2015 $ 273 (in thousands) Non-Controlling Interest Balance at December 31, 2013 $ 337 Net loss attributable to Silver Creek (350 ) Balance at June 30, 2014 $ (13 )

Net Loss Per Common Share

Net Loss Per Common Share	6 Months Ended
	Jun. 30, 2015
Earnings Per Share [Abstract]
Net Loss Per Common Share	3. Net Loss Per Common Share Basic net loss per share is calculated by dividing the net loss available to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is computed by dividing the net loss available to common stockholders by the weighted-average number of common share equivalents outstanding for the period determined using the treasury-stock method. As discussed in Note 7, “Borrowings,” in July 2013, the Company issued $125.0 million aggregate principal amount of 4.50% convertible senior notes due 2020 (the “Notes”) in an underwritten public offering. Upon any conversion of the Notes while the Company has indebtedness outstanding under the Loan and Security Agreement (the “Loan Agreement”) with Hercules Technology Growth Capital, Inc. (“Hercules”), the Notes will be settled in shares of the Company’s common stock. Following the repayment and satisfaction in full of the Company’s obligations to Hercules under the Loan Agreement, upon any conversion of the Notes, the Notes may be settled, at the Company’s election, in cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock. For purposes of calculating the maximum dilutive impact, it is presumed that the conversion premium will be settled in common stock, inclusive of a contractual make-whole provision resulting from a fundamental change, and the resulting potential common shares included in diluted earnings per share if the effect is more dilutive. For purposes of this calculation, conversion of the Notes, stock options and warrants are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. The stock options, warrants and conversion premium on the Notes are excluded from the calculation of diluted loss per share because the net loss for the three and six months ended June 30, 2015 and 2014 causes such securities to be anti-dilutive. The potential dilutive effect of these securities is shown in the chart below: As of June 30, (in thousands) 2015 2014 Options to purchase common stock 19,426 21,245 Common stock warrants 377 2,407 Conversion premium on the Notes 25,000 25,000

License and Collaboration Agree

License and Collaboration Agreements	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License and Collaboration Agreements	4. License and Collaboration Agreements Baxalta On September 23, 2014, the Company and Baxter International Inc., Baxter Healthcare Corporation and Baxter Healthcare SA entered into a license and collaboration agreement (the “Baxalta Agreement”) for the development and commercialization of MM-398 outside of the United States and Taiwan (the “Licensed Territory”). In connection with Baxter International Inc.’s separation of the Baxalta business, the Baxalta Agreement was assigned to Baxalta during the second quarter of 2015. As part of the Baxalta Agreement, the Company granted Baxalta an exclusive, royalty-bearing right and license under the Company’s patent rights and know-how to develop and commercialize MM-398 in the Licensed Territory. Baxalta is responsible for using commercially reasonable efforts to develop, obtain regulatory approvals for and, following regulatory approval, commercializing MM-398 in the Licensed Territory. A joint steering committee comprised of an equal number of representatives from each of Baxalta and the Company is responsible for approving changes to the global development plan for MM-398, including all budgets, and overseeing the parties’ development and commercialization activities with respect to MM-398. Unless otherwise agreed, the Company will be responsible for conducting all clinical trials contemplated by the global development plan for MM-398 and manufacturing all clinical material needed for such trials. Under the terms of the Baxalta Agreement, the Company received a $100.0 million upfront, nonrefundable cash payment in September 2014. In addition, the Company is eligible to receive from Baxalta (i) up to an aggregate of $100.0 million upon the achievement of specified research and development milestones, of which the Company expects to receive $62.5 million from Baxalta in the second half of 2015, (ii) up to an aggregate of $520.0 million upon the achievement of specified regulatory milestones, of which the Company has received $20.0 million from Baxalta as of June 30, 2015, and (iii) up to an aggregate of $250.0 million upon the achievement of specified sales milestones. Under the terms of the Baxalta Agreement, the Company will bear up to the first $98.8 million of costs related to the development of MM-398 for pancreatic cancer patients who have not previously received gemcitabine-based therapy; however, the Company expects most of these costs to be offset by payments received upon the achievement of clinical trial-related milestones. The Company and Baxalta will share equally all other clinical trial costs contemplated by the global development plan. The Company is also entitled to tiered, escalating royalties ranging from sub-teen double-digits to low twenties percentages of net sales of MM-398 in the Licensed Territory. The Company and Baxalta expect to enter into a commercial supply agreement pursuant to which the Company will supply MM-398 bulk drug substance to Baxalta and, at Baxalta’s option, may manage fill and finish activities to be conducted by a third-party contract manufacturer for Baxalta. Baxalta also has the option to manufacture MM-398 itself, in which case the Company will perform a technology transfer of its manufacturing process to Baxalta. Under the Baxalta Agreement, the Company granted Baxalta a right of first negotiation to obtain a license to develop and commercialize MM-111, MM-141 and MM-302 outside of the United States. If not terminated earlier by either party, the Baxalta Agreement will expire upon expiration of all royalty and other payment obligations of Baxalta under the Baxalta Agreement. Either party may terminate the Baxalta Agreement in the event of an uncured material breach by the other party. Baxalta may also terminate the Baxalta Agreement on a product-by-product, country-by-country or sub-territory-by-sub-territory basis or in its entirety, for its convenience, upon 180 days’ prior written notice. In addition, the Company may terminate the Baxalta Agreement if Baxalta challenges or supports any challenge of the Company’s licensed patent rights. At the inception of the collaboration, the Company identified the following deliverables as part of the Baxalta Agreement: (i) license to develop and commercialize MM-398 in Baxalta’s territories, (ii) discovery, research, development and manufacturing services required to complete ongoing clinical trials related to MM-398, (iii) discovery, research, development and manufacturing services needed to complete future clinical trials in further indications related to MM-398, (iv) the option to perform a technology transfer of the Company’s manufacturing process related to the production of MM-398 to Baxalta and (v) participation on the joint steering committee. The Company concluded that none of the deliverables identified at the inception of the collaboration has standalone value from the other undelivered elements. As such, all deliverables represent a single unit of accounting. The Company has determined that the collaboration represents a services agreement and as such has estimated the level of effort expected to be completed as a result of providing the identified deliverables. The Company will recognize revenue from the nonrefundable upfront payment, forecasted non-substantive milestone payments and estimated payments related to discovery, research, development and technology transfer services based on proportional performance as effort is completed over the expected services period, which is estimated to be substantially complete by December 31, 2019. The Company will periodically review and, if necessary, revise the estimated service period related to its collaboration with Baxalta. Research, development and regulatory milestones that are considered substantive on the basis of the contingent nature of the milestone will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All sales milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. During the second quarter of 2015, the EMA accepted for review an MAA filed by Baxalta for MM-398. As a result of this acceptance, the Company recognized $20.0 million of revenue related to a substantive milestone payment owed from Baxalta. During the three and six months ended June 30, 2015, the Company recognized revenue based on the following components of the Baxalta Agreement: (in thousands) Three months ended 2015 Six months ended 2015 Proportional Performance Model $ 16,558 $ 31,399 Substantive Milestones 20,000 20,000 Total $ 36,558 $ 51,399 During the three and six months ended June 30, 2014, the Company recognized no revenue associated with the Baxalta Agreement. As of June 30, 2015 and December 31, 2014, the Company maintained the following assets and liabilities related to the Baxalta Agreement: (in thousands) As of As of December 31, 2014 Accounts receivable, billed $ 807 $ — Accounts receivable, unbilled 907 1,615 Deferred revenue $ 64,611 $ 91,156 Of the $64.6 million of deferred revenue related to the Baxalta Agreement, $59.2 million is classified as current in the condensed consolidated balance sheets based upon the Company’s estimate of revenue that will be recognized under the proportional performance model as a result of effort expected to be completed within the next twelve months. Sanofi On September 30, 2009, the Company and Sanofi entered into a license and collaboration agreement (the “Sanofi Agreement”) for the development and commercialization of MM-121. The Sanofi Agreement became effective on November 10, 2009, and Sanofi paid the Company a nonrefundable, noncreditable upfront license fee of $60.0 million. On June 17, 2014, the Company and Sanofi agreed to terminate the Sanofi Agreement effective December 17, 2014. In connection with the agreement to terminate the Sanofi Agreement, among other things, Sanofi transferred ownership of the investigational new drug application for MM-121 back to the Company in July 2014, and the Company waived Sanofi’s obligation to reimburse the Company for MM-121 development costs incurred after the effective termination date. Effective upon the termination of the Sanofi Agreement, the Company will not be entitled to receive any additional fees, milestone payments or reimbursements from the collaboration. The Company received total milestone payments of $25.0 million pursuant to the Sanofi Agreement. Under the Sanofi Agreement, Sanofi was responsible for all MM-121 development and manufacturing costs. Sanofi reimbursed the Company for direct costs incurred in both development and manufacturing and compensated the Company for its internal development efforts based on a full time equivalent rate. The Company recognized cost reimbursements for MM-121 development services within the period they were incurred and billable. Billable expenses were identified during each specified budget period. In the event that total development services expense incurred and expected to be incurred during the same period exceeded the total contractually allowed billable amount for development services during that period, the Company recognized only a percentage of the development services incurred as revenue during that period. At the inception of the collaboration, the Company determined that the license, the right to future technology, back-up compounds, participation on steering committees and manufacturing services performance obligations comprising the Sanofi Agreement represented a single unit of accounting. As the Company could not reasonably estimate its level of effort over the collaboration, the Company recognized revenue from the upfront payment, milestone payment and manufacturing services payments using the contingency-adjusted performance model over the expected development period, which was initially estimated to be 12 years from the effective date of the Sanofi Agreement. As a result of the Company and Sanofi agreeing to terminate the Sanofi Agreement, the development period was revised to end as of December 17, 2014. Accordingly, the balance of the deferred revenue remaining on April 1, 2014 was recognized prospectively on a straight-line basis over the remaining development period, ending on December 17, 2014, in accordance with current generally accepted principles on revenue recognition. During the three and six months ended June 30, 2015, the Company recognized no revenue under the Sanofi Agreement. During the three and six months ended June 30, 2014, the Company recognized revenue based on the following components of the Sanofi Agreement: (in thousands) Three months ended 2014 Six months ended 2014 Upfront payment $ 13,268 $ 14,518 Milestone payments 5,529 6,049 Development services 3,040 13,740 Manufacturing services and other 5,978 6,542 Total $ 27,815 $ 40,849 The Company performed development services for which revenue was recognized under the Sanofi Agreement in accordance with the specified budget period. Additionally, for the six months ended June 30, 2014, there was approximately $5.8 million of increased revenue recognized in the first quarter of 2014, related to the Company receiving budget approval for expenses incurred prior to December 31, 2013. As of June 30, 2015, the Company maintained no assets or liabilities related to the Sanofi Agreement. As of December 31, 2014, the Company maintained the following assets and liabilities related to the Sanofi Agreement: (in thousands) As of Accounts receivable, billed $ 369 Accounts receivable, unbilled 1,282 Deferred revenue — PharmaEngine, Inc. On May 5, 2011, the Company and PharmaEngine, Inc. (“PharmaEngine”) entered into an assignment, sublicense and collaboration agreement (the “PharmaEngine Agreement”) under which the Company reacquired rights in Europe and certain countries in Asia to MM-398. In exchange, the Company agreed to pay PharmaEngine a nonrefundable, noncreditable upfront payment of $10.0 million and up to an additional $80.0 million in aggregate development and regulatory milestones and $130.0 million in aggregate sales milestones. During the first quarter of 2012, the Company paid a development milestone of $5.0 million under the PharmaEngine Agreement in connection with dosing the first patient in a Phase 3 clinical trial of MM-398 in pancreatic cancer. PharmaEngine is also entitled to tiered royalties on net sales of MM-398 in Europe and certain countries in Asia. PharmaEngine is not responsible for any future development costs of MM-398 except those required specifically for regulatory approval in Taiwan. On September 22, 2014, the Company amended the PharmaEngine Agreement to redefine sublicense revenue and reduce the portion of sublicense revenue that the Company is required to pay to PharmaEngine. As a result of this amendment, the Company made a $7.0 million milestone payment to PharmaEngine in September 2014. Additionally, as a result of this amendment, a previously contingent $5.0 million milestone payment was paid to PharmaEngine on April 30, 2015. Prior to the amendment of the PharmaEngine Agreement, this milestone payment was contingent upon the award of certain specified regulatory designations. These milestone payments were recognized as research and development expense in September 2014. As a result of achieving a $20.0 million substantive milestone associated with the Baxalta Agreement during the second quarter of 2015, the Company was required to pay an $11.0 million milestone payment to PharmaEngine. The expense related to this obligation was recorded in the second quarter of 2015 and the payment to PharmaEngine was made in July 2015. During the three months ended June 30, 2015 and 2014, the Company recognized research and development expenses related to the PharmaEngine Agreement of $11.1 million and $0.1 million, respectively, and during the six months ended June 30, 2015 and 2014, the Company recognized research and development expenses related to the PharmaEngine Agreement of $11.3 million and $0.2 million, respectively. Actavis On November 25, 2013, the Company and Watson Laboratories, Inc. (“Actavis”) entered into a development, license and supply agreement (the “Actavis Agreement”) pursuant to which the Company will develop, manufacture and exclusively supply the bulk form of doxorubicin HCl liposome injection (the “Initial Product”) to Actavis. Under the Actavis Agreement, Actavis is responsible for all costs related to finished product processing and global commercialization. Pursuant to the Actavis Agreement, additional products may be developed for Actavis in the future, the identities of which will be mutually agreed upon. The Company is eligible to receive up to $15.1 million under the Actavis Agreement, of which $3.8 million has been received through June 30, 2015, and the remainder is expected to be received in development funding and development, regulatory and commercial milestone payments related to the Initial Product. The Company will also receive a double digit percentage of net profits on global sales of the Initial Product and any additional products. The Company will manufacture and supply the Initial Product to Actavis in bulk form at an agreed upon unit price. The Actavis Agreement will expire with respect to the Initial Product and any additional products developed in the future ten years after Actavis’ first sale of the applicable product, unless terminated earlier, and will automatically renew for additional two year periods thereafter unless either party provides notice of non-renewal. Either party may terminate the Actavis Agreement in the event of an uncured material breach or bankruptcy filing by the other party. Actavis may also terminate the Actavis Agreement for convenience in specified circumstances upon 90 days’ prior written notice. The Company applied revenue recognition guidance to determine whether the performance obligations under this collaboration, including the license, participation on steering committees, development services, and manufacturing and supply services could be accounted for separately or as a single unit of accounting. The Company determined that these obligations represent a single unit of accounting and will recognize revenue as product is supplied to Actavis. Therefore, the Company has recorded $3.8 million of billed and billable milestones and development expenses related to the Actavis Agreement as deferred revenue as of June 30, 2015 and December 31, 2014, of which $0.1 million is classified as short term as of June 30, 2015 related to revenue the Company expects to recognize in the first half of 2016.

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	5. Fair Value of Financial Instruments The carrying value of financial instruments, including cash and cash equivalents, restricted cash, available-for-sale securities, prepaid expenses, accounts receivable, accounts payable and accrued expenses, and other short-term assets and liabilities approximate their respective fair values due to the short-term maturities of these assets and liabilities. Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. Fair value is determined based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect certain market assumptions. As a basis for considering such assumptions, GAAP establishes a three-tier value hierarchy, which prioritizes the inputs used to develop the assumptions and for measuring fair value as follows: (Level 1) observable inputs such as quoted prices in active markets for identical assets; (Level 2) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level 3) unobservable inputs in which there is little or no market data, which requires the Company to develop its own assumptions. This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. Recurring Fair Value Measurements The following tables show assets and liabilities measured at fair value on a recurring basis as of June 30, 2015 and December 31, 2014 and the input categories associated with those assets and liabilities: (in thousands) As of June 30, 2015 Level 1 Level 2 Level 3 Assets: Cash equivalents – money market funds $ 15,187 $ — $ — Investments – commercial paper — 5,000 — Investments – corporate debt securities — 28,991 — As of December 31, 2014 Level 1 Level 2 Level 3 Assets: Cash equivalents – money market funds $ 33,199 $ — $ — Investments – commercial paper — 6,491 — Investments – corporate debt securities — 81,849 — The Company’s investment portfolio consists of investments classified as cash equivalents and available-for-sale securities. All highly liquid investments with an original maturity of three months or less when purchased are considered to be cash equivalents. The Company’s cash and cash equivalents are invested in U.S. treasury and various corporate debt securities that approximate their face value. All marketable securities with an original maturity when purchased of greater than three months are classified as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in other comprehensive income (loss). The amortized cost of securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. There have been no impairments of the Company’s assets measured and carried at fair value during the three and six months ended June 30, 2015. In addition, there were no changes in valuation techniques or transfers between the fair value measurement levels during the three and six months ended June 30, 2015. Other Fair Value Measurements The estimated fair value of the $125.0 million aggregate principal amount of the Notes was $261.7 million as of June 30, 2015. The Company estimated the fair value of the Notes by using a quoted market rate in an inactive market, which is classified as a Level 2 input. The carrying value of the Notes is $84.6 million due to the bifurcation of the conversion feature of the Notes as described more fully in Note 7, “Borrowings.” The estimated fair value and carrying value of the loans payable under the Loan Agreement with Hercules was $40.6 million as of June 30, 2015. The Company estimated the fair value of the loans payable by using publically available information related to Hercules’ portfolio of debt investments based on unobservable inputs, which is classified as a Level 3 input.

Accounts Payable, Accrued Expen

Accounts Payable, Accrued Expenses and Other	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Accounts Payable, Accrued Expenses and Other	6. Accounts Payable, Accrued Expenses and Other Accounts payable, accrued expenses and other as of June 30, 2015 and December 31, 2014 consisted of the following: (in thousands) As of As of Accounts payable $ 6,783 $ 2,510 Accrued goods and services 13,177 12,481 Accrued clinical trial costs 8,514 7,637 Accrued payroll and related benefits 3,866 6,166 Accrued interest 2,944 2,956 Accrued dividends payable 19 19 Accrued milestone payment owed to PharmaEngine 11,000 5,000 Deferred tax incentives 323 467 Total accounts payable, accrued expenses and other $ 46,626 $ 37,236

Borrowings

Borrowings	6 Months Ended
	Jun. 30, 2015
Debt Disclosure [Abstract]
Borrowings	7. Borrowings Future minimum payments under indebtedness agreements outstanding as of June 30, 2015 are as follows: (in thousands) As of June 30, 2015: 4.50% Convertible Senior Notes Loan Remainder of 2015 $ 2,813 $ 2,157 2016 5,625 13,575 2017 5,625 18,167 2018 5,625 16,639 2019 and thereafter 136,250 — 155,938 50,538 Less interest (30,938 ) (9,337 ) Less unamortized discount (40,396 ) (1,630 ) Less current portion — (1,058 ) Loans payable, net of current portion $ 84,604 $ 38,513 4.50% Convertible Senior Notes In July 2013, the Company issued $125.0 million aggregate principal amount of Notes in an underwritten public offering. As a result of the Notes offering, the Company received net proceeds of approximately $120.6 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. The Notes bear interest at a rate of 4.50% per year, payable semiannually in arrears on January 15 and July 15 of each year, beginning on January 15, 2014. The Notes are general unsecured senior obligations of the Company. The Notes will mature on July 15, 2020 (the “maturity date”), unless earlier repurchased by the Company or converted at the option of holders. Holders may convert their Notes at their option at any time prior to the close of business on the business day immediately preceding April 15, 2020 only under the following circumstances: • during any calendar quarter commencing after September 30, 2013 (and only during such calendar quarter), if the last reported sale price of the Company’s 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; • during the five business day period after any five consecutive trading day period (the “measurement period”) in which the trading price (as defined in the Notes) 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; or • upon the occurrence of specified corporate events set forth in the indenture governing the Notes. During the second quarter of 2015, the last reported sales price of the common stock for at least 20 trading days (whether or not consecutive) during the period of 30 consecutive trading days ending on the last trading day of the calendar quarter ended June 30, 2015 was greater than 130% of the conversion price for the Notes on each applicable trading day. As a result, holders may convert their Notes at their option at any time from July 1, 2015 through September 30, 2015. On or after April 15, 2020 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. Upon any conversion of Notes that occurs while the Company’s indebtedness to Hercules under the Loan Agreement remains outstanding, the Notes will be settled in shares of the Company’s common stock. Following the repayment and satisfaction in full of the Company’s obligations to Hercules under the Loan Agreement, upon any conversion of the Notes, the Notes may be settled, at the Company’s election, in cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock. The initial conversion rate of the Notes is 160 shares of the Company’s common stock per $1,000 principal amount of Notes, which is equivalent to an initial conversion price of $6.25 per share of common stock. The conversion rate will be subject to adjustment in some events. In addition, following certain corporate events that occur prior to the maturity date, the Company will increase the conversion rate for a holder who elects to convert its Notes in connection with such a corporate event in certain circumstances. The Company has separately accounted for the liability and equity components of the Notes by bifurcating gross proceeds between the indebtedness, or liability component, and the embedded conversion option, or equity component. This bifurcation was done by estimating an effective interest rate as of the date of issuance for similar notes which do not contain an embedded conversion option. The embedded conversion option was recorded in stockholders’ deficit and as debt discount, to be subsequently amortized as interest expense over the term of the Notes. Underwriting discounts and commissions and offering expenses totaled $4.4 million and were allocated to the indebtedness and the embedded conversion option based on their relative values. For the three and six months ended June 30, 2015, interest expense related to the outstanding principal balance of the Notes was $3.4 million and $6.8 million, respectively. For the three and six months ended June 30, 2014, interest expense related to the outstanding principal balance of the Notes was $3.4 million and $6.8 million, respectively. Loan Agreement In November 2012, the Company entered into the Loan Agreement with Hercules pursuant to which the Company received loans in the aggregate principal amount of $40.0 million. The Company, as permitted under the Loan Agreement, had previously extended the interest-only payment period with the aggregate principal balance of the loans to be repaid in monthly installments starting on June 1, 2014 and continuing through November 1, 2016. On June 25, 2014, the Company entered into an amendment to the Loan Agreement, whereby the Company and Hercules agreed to extend until October 1, 2014 the period during which the Company makes interest-only payments. On November 6, 2014, the Company entered into a further amendment to the Loan Agreement, whereby the Company and Hercules agreed to extend by four additional months the period during which the Company makes interest-only payments. On February 25, 2015, the Company entered into a fourth amendment to the Loan Agreement pursuant to which the Company and Hercules agreed to extend the maturity date and the period during which the Company makes interest-only payments on its current term loan in the aggregate principal amount of $40.0 million. As a result of this amendment, the Company will repay the outstanding aggregate principal balance of the term loan beginning on June 1, 2016 and continuing through November 1, 2018. If the FDA approves the Company’s NDA for MM-398 by May 1, 2016, the Company may elect to extend the interest-only period by an additional six months so that the Company would repay the outstanding aggregate principal balance of the term loan beginning on December 1, 2016 and continuing through November 1, 2018. In addition, if the FDA approves the Company’s NDA for MM-398 by May 1, 2016, the Company may elect to draw, at any time until August 1, 2016, an additional term loan advance of up to $15.0 million. Principal and interest payments on the additional term loan advance would be made in the same manner as the Company’s current term loan in the aggregate principal amount of $40.0 million. This amendment was treated as a debt modification for accounting purposes. Upon the earlier of full repayment of the loans or November 1, 2016, the Company is required to pay Hercules a fee of $1.2 million, which has been recorded as a discount to the loans and as a long-term liability on the Company’s condensed consolidated balance sheets. Additionally, the Company reimbursed Hercules for costs incurred related to the loans, which has been reflected as a discount to the carrying value of the loans. The Company is amortizing these loan discounts totaling $1.6 million to interest expense over the term of the loans using the effective interest method. For the three months and six months ended June 30, 2015, interest expense related to Hercules loans payable was $1.2 million and $2.3 million, respectively. For the three and six months ended June 30, 2014, interest expense related to Hercules loans payable was $1.2 million and $2.4 million, respectively. In connection with the Loan Agreement, the Company granted Hercules a security interest in all of the Company’s personal property now owned or hereafter acquired, excluding intellectual property but including the proceeds from the sale, if any, of intellectual property, and a negative pledge on intellectual property. The Loan Agreement also contains certain representations, warranties and non-financial covenants of the Company.

Common Stock

Common Stock	6 Months Ended
	Jun. 30, 2015
Equity [Abstract]
Common Stock	8. Common Stock As of June 30, 2015 and December 31, 2014, the Company had 200.0 million shares of $0.01 par value common stock authorized. There were approximately 110.8 million and 106.7 million shares of common stock issued and outstanding as of June 30, 2015 and December 31, 2014, respectively. The shares reserved for future issuance as of June 30, 2015 and December 31, 2014 consisted of the following: (in thousands) As of As of Options to purchase common stock 19,426 19,567 Common stock warrants 377 2,381 Conversion premium on the Notes 25,000 25,000 During the six months ended June 30, 2015, warrants to purchase 2.0 million shares of common stock were exercised in cash and cashless transactions at a weighted average exercise price of $3.00, and 1.5 million shares of common stock were issued. During the six months ended June 30, 2014, warrants to purchase 0.4 million shares of common stock were exercised in cash and cashless transactions at a weighted average exercise price of $3.41, and 0.3 million shares of common stock were issued.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation As of December 31, 2014, there were 2.0 million shares of common stock available to be granted under the Company’s 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan is administered by the Company’s board of directors and permits the Company to grant incentive and non-qualified stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. In February 2015, 3.7 million additional shares of common stock became available for grant to employees, officers, directors and consultants under the 2011 Plan. During the six months ended June 30, 2015 and 2014, the Company issued options to purchase 2.7 million and 3.1 million shares of common stock, respectively. At June 30, 2015, there were 3.3 million shares remaining available for grant under the 2011 Plan. The assumptions used to estimate the fair value of options granted to employees and directors at the date of grant for the three and six months ended June 30, 2015 were as follows: Three months ended Six months ended Risk-free interest rate 1.5-1.8 % 1.5-1.8 % Expected dividend yield 0 % 0 % Expected term 5.0-5.9 years 5.0-5.9 years Expected volatility 66-67 % 66-67 % Options granted to employees generally vest over a three year period. The Company recognized stock-based compensation expense as follows for the three and six months ended June 30, 2015 and 2014: Three months ended June 30, Six months ended June 30, (in thousands) 2015 2014 2015 2014 Employee awards: Research and development $ 2,369 $ 1,841 $ 4,335 $ 3,522 General and administrative 2,576 2,161 3,954 3,453 Stock-based compensation for employee awards 4,945 4,002 8,289 6,975 Stock-based compensation for non-employee awards 22 153 48 116 Total stock-based compensation $ 4,967 $ 4,155 $ 8,337 $ 7,091 The following table summarizes stock option activity during the six months ended June 30, 2015: (in thousands, except per share amounts and years) Number Weighted Weighted Aggregate Outstanding, December 31, 2014 19,556 $ 4.47 6.17 $ 133,599 Granted 2,723 $ 9.54 Exercised (2,595 ) $ 2.57 Forfeited (258 ) $ 6.82 Outstanding, June 30, 2015 19,426 $ 5.40 6.50 $ 135,208 Vested and expected to vest, June 30, 2015 19,086 $ 5.36 6.45 $ 133,696 Exercisable, June 30, 2015 14,210 $ 4.66 5.60 $ 109,441 The aggregate intrinsic value was calculated as the difference between the exercise price of the stock options and the fair value of the underlying common stock.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Operating Leases The Company leases its office, laboratory and manufacturing space under non-cancelable operating leases. Total rent expense under these operating leases was $1.9 million and $1.5 million for the three months ended June 30, 2015 and 2014, respectively. Total rent expense under these operating leases was $3.4 million and $3.0 million for the six months ended June 30, 2015 and 2014, respectively. Other Commitments In March 2015, the Company received an award of $1.4 million of tax incentives from the Massachusetts Life Sciences Center, which allows the Company to monetize approximately $1.2 million of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental 75 employees and to maintain the additional headcount through at least December 31, 2019. The Company has deferred and will amortize the benefit of the monetization on a straight-line basis over the five-year performance period, commencing with a cumulative catch-up when the pledge is achieved. Failure to achieve this commitment could result in the Company being required to repay some or all of these incentives.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	6 Months Ended
	Jun. 30, 2015
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	11. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2017, and early adoption is permitted for annual periods beginning after December 15, 2016. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on the consolidated financial statements. In August 2014, the FASB issued guidance outlining management’s responsibility to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and providing guidance on determining when and how to disclose going concern uncertainties in the financial statements. This guidance will be effective for annual and interim reporting periods ending after December 15, 2016, with early adoption permitted. The Company does not anticipate a material impact to the consolidated financial statements as a result of this change. In April 2015, the FASB issued ASU 2015-03, “Interest - Imputation of Interest,” which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. This update is effective for annual reporting periods beginning on or after December 15, 2015 and interim periods within fiscal years beginning after December 15, 2016. The Company is currently assessing the impact that adoption of ASU 2015-03 might have on its consolidated financial statements.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2015
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events On July 13, 2015, the Company entered into a Sales Agreement with Cowen and Company, LLC (“Cowen”) to sell, from time to time, shares of the Company’s common stock having an aggregate sales price of up to $40,000,000 through an “at the market offering” program under which Cowen will act as sales agent. Through August 7, 2015, the Company has generated approximately $7.7 million in net proceeds under this program. On July 22, 2015, the Company entered into an amendment to its facility lease. The lease amendment provides an additional 13,843 square feet of leased space at the Company’s current facility in Cambridge, Massachusetts, with a termination date of June 30, 2019, which is co-terminous with the Company’s existing lease. As a result of this amendment, the Company has additional lease payments totaling approximately $2.2 million through 2019. In addition, under the terms of the amendment, the landlord will provide the Company with an aggregate leasehold improvement allowance of up to approximately $0.5 million.

Recent Accounting Pronounceme18

Recent Accounting Pronouncements (Policies)	6 Months Ended
	Jun. 30, 2015
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	In May 2014, the Financial Accounting Standards Board (“FASB”) issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2017, and early adoption is permitted for annual periods beginning after December 15, 2016. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on the consolidated financial statements. In August 2014, the FASB issued guidance outlining management’s responsibility to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and providing guidance on determining when and how to disclose going concern uncertainties in the financial statements. This guidance will be effective for annual and interim reporting periods ending after December 15, 2016, with early adoption permitted. The Company does not anticipate a material impact to the consolidated financial statements as a result of this change. In April 2015, the FASB issued ASU 2015-03, “Interest - Imputation of Interest,” which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. This update is effective for annual reporting periods beginning on or after December 15, 2015 and interim periods within fiscal years beginning after December 15, 2016. The Company is currently assessing the impact that adoption of ASU 2015-03 might have on its consolidated financial statements.

Basis of Presentation and Con19

Basis of Presentation and Consolidation (Tables)	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Non-Controlling Interest in Subsidiary	The Company’s ownership of Silver Creek was 57% and 60% as of June 30, 2015 and December 31, 2014, respectively. The consolidated financial statement activity related to Silver Creek was as follows: (in thousands) Non-Controlling Interest Balance at December 31, 2014 $ 69 Net income attributable to Silver Creek 204 Balance at June 30, 2015 $ 273 (in thousands) Non-Controlling Interest Balance at December 31, 2013 $ 337 Net loss attributable to Silver Creek (350 ) Balance at June 30, 2014 $ (13 )

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	6 Months Ended
	Jun. 30, 2015
Earnings Per Share [Abstract]
Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares	The potential dilutive effect of these securities is shown in the chart below: As of June 30, (in thousands) 2015 2014 Options to purchase common stock 19,426 21,245 Common stock warrants 377 2,407 Conversion premium on the Notes 25,000 25,000

License and Collaboration Agr21

License and Collaboration Agreements (Tables)	6 Months Ended
	Jun. 30, 2015
Sanofi [Member]
Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements	During the three and six months ended June 30, 2015, the Company recognized no revenue under the Sanofi Agreement. During the three and six months ended June 30, 2014, the Company recognized revenue based on the following components of the Sanofi Agreement: (in thousands) Three months ended 2014 Six months ended 2014 Upfront payment $ 13,268 $ 14,518 Milestone payments 5,529 6,049 Development services 3,040 13,740 Manufacturing services and other 5,978 6,542 Total $ 27,815 $ 40,849 As of June 30, 2015, the Company maintained no assets or liabilities related to the Sanofi Agreement. As of December 31, 2014, the Company maintained the following assets and liabilities related to the Sanofi Agreement: (in thousands) As of Accounts receivable, billed $ 369 Accounts receivable, unbilled 1,282 Deferred revenue —
Baxalta [Member]
Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements	During the three and six months ended June 30, 2015, the Company recognized revenue based on the following components of the Baxalta Agreement: (in thousands) Three months ended 2015 Six months ended 2015 Proportional Performance Model $ 16,558 $ 31,399 Substantive Milestones 20,000 20,000 Total $ 36,558 $ 51,399 As of June 30, 2015 and December 31, 2014, the Company maintained the following assets and liabilities related to the Baxalta Agreement: (in thousands) As of As of December 31, 2014 Accounts receivable, billed $ 807 $ — Accounts receivable, unbilled 907 1,615 Deferred revenue $ 64,611 $ 91,156

Fair Value of Financial Instr22

Fair Value of Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following tables show assets and liabilities measured at fair value on a recurring basis as of June 30, 2015 and December 31, 2014 and the input categories associated with those assets and liabilities: (in thousands) As of June 30, 2015 Level 1 Level 2 Level 3 Assets: Cash equivalents – money market funds $ 15,187 $ — $ — Investments – commercial paper — 5,000 — Investments – corporate debt securities — 28,991 — As of December 31, 2014 Level 1 Level 2 Level 3 Assets: Cash equivalents – money market funds $ 33,199 $ — $ — Investments – commercial paper — 6,491 — Investments – corporate debt securities — 81,849 —

Accounts Payable, Accrued Exp23

Accounts Payable, Accrued Expenses and Other (Tables)	6 Months Ended
	Jun. 30, 2015
Payables and Accruals [Abstract]
Schedule of Accounts Payable, Accrued Expenses and Other	Accounts payable, accrued expenses and other as of June 30, 2015 and December 31, 2014 consisted of the following: (in thousands) As of As of Accounts payable $ 6,783 $ 2,510 Accrued goods and services 13,177 12,481 Accrued clinical trial costs 8,514 7,637 Accrued payroll and related benefits 3,866 6,166 Accrued interest 2,944 2,956 Accrued dividends payable 19 19 Accrued milestone payment owed to PharmaEngine 11,000 5,000 Deferred tax incentives 323 467 Total accounts payable, accrued expenses and other $ 46,626 $ 37,236

Borrowings (Tables)

Borrowings (Tables)	6 Months Ended
	Jun. 30, 2015
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments under the Loans Payable	Future minimum payments under indebtedness agreements outstanding as of June 30, 2015 are as follows: (in thousands) As of June 30, 2015: 4.50% Convertible Senior Notes Loan Remainder of 2015 $ 2,813 $ 2,157 2016 5,625 13,575 2017 5,625 18,167 2018 5,625 16,639 2019 and thereafter 136,250 — 155,938 50,538 Less interest (30,938 ) (9,337 ) Less unamortized discount (40,396 ) (1,630 ) Less current portion — (1,058 ) Loans payable, net of current portion $ 84,604 $ 38,513

Common Stock (Tables)

Common Stock (Tables)	6 Months Ended
	Jun. 30, 2015
Equity [Abstract]
Schedule of Shares Reserved for Future Issuance	The shares reserved for future issuance as of June 30, 2015 and December 31, 2014 consisted of the following: (in thousands) As of As of Options to purchase common stock 19,426 19,567 Common stock warrants 377 2,381 Conversion premium on the Notes 25,000 25,000

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees and Directors	The assumptions used to estimate the fair value of options granted to employees and directors at the date of grant for the three and six months ended June 30, 2015 were as follows: Three months ended Six months ended Risk-free interest rate 1.5-1.8 % 1.5-1.8 % Expected dividend yield 0 % 0 % Expected term 5.0-5.9 years 5.0-5.9 years Expected volatility 66-67 % 66-67 %
Schedule of Recognized Stock-Based Compensation Expense	Company recognized stock-based compensation expense as follows for the three and six months ended June 30, 2015 and 2014: Three months ended June 30, Six months ended June 30, (in thousands) 2015 2014 2015 2014 Employee awards: Research and development $ 2,369 $ 1,841 $ 4,335 $ 3,522 General and administrative 2,576 2,161 3,954 3,453 Stock-based compensation for employee awards 4,945 4,002 8,289 6,975 Stock-based compensation for non-employee awards 22 153 48 116 Total stock-based compensation $ 4,967 $ 4,155 $ 8,337 $ 7,091
Summary of Stock Option Activity	The following table summarizes stock option activity during the six months ended June 30, 2015: (in thousands, except per share amounts and years) Number Weighted Weighted Aggregate Outstanding, December 31, 2014 19,556 $ 4.47 6.17 $ 133,599 Granted 2,723 $ 9.54 Exercised (2,595 ) $ 2.57 Forfeited (258 ) $ 6.82 Outstanding, June 30, 2015 19,426 $ 5.40 6.50 $ 135,208 Vested and expected to vest, June 30, 2015 19,086 $ 5.36 6.45 $ 133,696 Exercisable, June 30, 2015 14,210 $ 4.66 5.60 $ 109,441

Basis of Presentation and Con27

Basis of Presentation and Consolidation - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Preferred stock issued	0	0
Net proceeds from stock issuance	$ 1,233
Silver Creek Series B Preferred Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock issued	1,000
Preferred stock price per share	$ 1.35
Net proceeds from stock issuance	$ 1,200
Silver Creek [Member]
Summary Of Significant Accounting Policies [Line Items]
Ownership interest percentage	57.00%	60.00%

Basis of Presentation and Con28

Basis of Presentation and Consolidation - Schedule of Non-Controlling Interest in Subsidiary (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Temporary Equity Disclosure [Abstract]
Balance at the beginning of the period			$ 69	$ 337
Net loss attributable to Silver Creek	$ (123)	$ (181)	204	(350)
Balance at the end of the period	$ 273	$ (13)	$ 273	$ (13)

Net Loss Per Common Share - Add

Net Loss Per Common Share - Additional Information (Detail) - 4.50% Convertible Senior Notes [Member] - USD ($) $ in Millions	1 Months Ended
	Jul. 31, 2013	Jun. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Aggregate principal amount	$ 125
Interest rate (as a percent)	4.50%	4.50%

Net Loss Per Common Share - Sch

Net Loss Per Common Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares (Detail) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Stock Compensation Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	19,426	21,245
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	377	2,407
Conversion Premium on the Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	25,000	25,000

License and Collaboration Agr31

License and Collaboration Agreements - Additional Information (Detail) - USD ($)

Nov. 25, 2013

May. 05, 2011

Nov. 10, 2009

Jul. 31, 2015

Sep. 30, 2014

Jun. 30, 2015

Jun. 30, 2014

Mar. 31, 2012

Dec. 31, 2015

Jun. 30, 2015

Jun. 30, 2014

Dec. 31, 2014

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration revenues

$ 36,558,000

$ 27,815,000

$ 51,399,000

$ 40,849,000

Deferred revenue, current

59,346,000

$ 59,346,000

$ 59,275,000

Agreement termination scheduled date

Dec. 17, 2014

Assets maintained related to Sanofi Agreement

105,043,000

$ 105,043,000

158,656,000

Liabilities maintained related to Sanofi Agreement

248,107,000

248,107,000

260,727,000

Research and development expenses

42,806,000

33,795,000

78,485,000

64,119,000

Baxalta [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration revenues

0

0

Substantive milestone achieved

20,000,000

$ 20,000,000

Baxalta [Member] | License and Collaboration Agreements [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront license fee received

$ 100,000,000

Royalty for improved products

The Company is also entitled to tiered, escalating royalties ranging from sub-teen double-digits to low twenties percentages of net sales of MM-398 in the Licensed Territory.

Notice period of termination

180 days

Revenue recognized related to a substantive milestone payment

20,000,000

Collaboration revenues

36,558,000

$ 51,399,000

Deferred revenue

64,611,000

64,611,000

91,156,000

Deferred revenue, current

59,200,000

59,200,000

Baxalta [Member] | License and Collaboration Agreements [Member] | Sales Milestone [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Maximum amount of milestone payments that can be received

250,000,000

Baxalta [Member] | License and Collaboration Agreements [Member] | Research and Development Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Maximum amount of milestone payments that can be received

100,000,000

Baxalta [Member] | License and Collaboration Agreements [Member] | Regulatory Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone license fee received

20,000,000

Maximum amount of milestone payments that can be received

520,000,000

Baxalta [Member] | License and Collaboration Agreements [Member] | Clinical Trials in Pancreatic Cancer [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement costs

98,800,000

Baxalta [Member] | License and Collaboration Agreements [Member] | Scenario Forecast [Member] | Research and Development Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments expects to receive

$ 62,500,000

Sanofi [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront license fee received

$ 60,000,000

Milestone license fee received

25,000,000

Collaboration revenues

0

Expected development period from the effective date of agreement

12 years

Assets maintained related to Sanofi Agreement

0

0

Liabilities maintained related to Sanofi Agreement

0

$ 0

Sanofi [Member] | License and Collaboration Agreements [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration revenues

27,815,000

40,849,000

Recognized revenue related to excess spending

5,800,000

PharmaEngine [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront license fees paid

$ 10,000,000

PharmaEngine [Member] | Subsequent Event [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payment

$ 11,000,000

PharmaEngine [Member] | Development and Regulatory Milestone [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Maximum milestone payment obligation

80,000,000

PharmaEngine [Member] | Sales Milestone [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Maximum milestone payment obligation

$ 130,000,000

PharmaEngine [Member] | Phase 3 Clinical Trial in Pancreatic Cancer [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payment

$ 5,000,000

PharmaEngine [Member] | License and Collaboration Agreements [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payment

$ 7,000,000

Milestone payment due date

Apr. 30, 2015

Research and development expenses

11,100,000

$ 100,000

$ 11,300,000

$ 200,000

PharmaEngine [Member] | License and Collaboration Agreements [Member] | New Drug Application [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration milestone obligation, accrued

5,000,000

Actavis [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone license fee received

$ 3,800,000

Maximum amount of milestone payments that can be received

$ 15,100,000

Notice period of termination

90 days

Agreement expiration term respect to each product

10 years

Additional renewal term

2 years

Milestones and development expenses

3,800,000

$ 3,800,000

$ 3,800,000

Milestones and development expenses - short term

$ 100,000

$ 100,000

License and Collaboration Agr32

License and Collaboration Agreements - Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	$ 36,558,000	$ 27,815,000	$ 51,399,000	$ 40,849,000
Baxalta [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total		0		0
Sanofi [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total			0
License and Collaboration Agreements [Member] | Baxalta [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proportional Performance Model	16,558,000		31,399,000
Substantive Milestones	20,000,000		20,000,000
Total	36,558,000		51,399,000
Accounts receivable, billed	807,000		807,000
Accounts receivable, unbilled	907,000		907,000		$ 1,615,000
Deferred revenue	$ 64,611,000		$ 64,611,000		91,156,000
License and Collaboration Agreements [Member] | Sanofi [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment		13,268,000		14,518,000
Milestone payments		5,529,000		6,049,000
Development services		3,040,000		13,740,000
Manufacturing services and other		5,978,000		6,542,000
Total		$ 27,815,000		$ 40,849,000
Accounts receivable, billed					369,000
Accounts receivable, unbilled					$ 1,282,000

Fair Value of Financial Instr33

Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	$ 15,187	$ 33,199
Level 2 [Member] | Commercial Paper [Member]
Assets:
Investments	5,000	6,491
Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Investments	$ 28,991	$ 81,849

Fair Value of Financial Instr34

Fair Value of Financial Instruments - Additional Information (Detail) - Jun. 30, 2015 - USD ($)	Total	Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities with an original maturity
Asset impairment charges	$ 0	$ 0
Transfers between fair value measurement levels	0	0
Hercules [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying value of debt	40,600,000	40,600,000
Level 3 [Member] | Hercules [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	40,600,000	40,600,000
Senior Convertible Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Principal amount	125,000,000	125,000,000
Carrying value of debt	84,600,000	84,600,000
Senior Convertible Notes [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	$ 261,700,000	$ 261,700,000
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquid investments with an original maturity		3 months

Accounts Payable, Accrued Exp35

Accounts Payable, Accrued Expenses and Other - Schedule of Accounts Payable, Accrued Expenses and Other (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Accounts payable	$ 6,783	$ 2,510
Accrued goods and services	13,177	12,481
Accrued clinical trial costs	8,514	7,637
Accrued payroll and related benefits	3,866	6,166
Accrued interest	2,944	2,956
Accrued dividends payable	19	19
Accrued milestone payment owed to PharmaEngine	11,000	5,000
Deferred tax incentives	323	467
Total accounts payable, accrued expenses and other	$ 46,626	$ 37,236

Borrowings - Schedule of Future

Borrowings - Schedule of Future Minimum Payments under the Loans Payable (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Less current portion	$ (1,058)	$ (13,346)
Loans payable, net of current portion	123,117	$ 106,806
4.50% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Remainder of 2015	2,813
2,016	5,625
2,017	5,625
2,018	5,625
2019 and thereafter	136,250
Loans payable, gross	155,938
Less interest	(30,938)
Less unamortized discount	(40,396)
Loans payable, net of current portion	84,604
Loans Payable [Member] | Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Remainder of 2015	2,157
2,016	13,575
2,017	18,167
2,018	16,639
Loans payable, gross	50,538
Less interest	(9,337)
Less unamortized discount	(1,630)
Less current portion	(1,058)
Loans payable, net of current portion	$ 38,513

Borrowings - 4.50% Convertible

Borrowings - 4.50% Convertible Senior Notes - Additional Information (Detail) - USD ($)	Jul. 31, 2013	Jul. 31, 2013	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule Of Debt Instruments [Line Items]
Debt maturity date					Jul. 15, 2020
Interest expense			$ 4,482,000	$ 4,570,000	$ 9,048,000	$ 9,081,000
4.50% Convertible Senior Notes [Member]
Schedule Of Debt Instruments [Line Items]
Aggregate principal amount		$ 125,000,000
Net proceeds from the debt issuance		$ 120,600,000
Interest rate (as a percent)	4.50%	4.50%	4.50%		4.50%
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	30 days		30 days
Convertibility of debt, closing price of stock test, percentage of stock price to conversion price for the notes that must be exceeded	130.00%		130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period	5 days
Number of consecutive trading days before five consecutive business days during the note measurement period	5 days
Conversion ratio, principal amount	$ 1,000	$ 1,000
Conversion rate of common stock shares per $1,000 principal amount	160
Initial conversion price of shares (in dollars per share)	$ 6.25	$ 6.25
Underwriting discounts and commissions and offering expenses	$ 4,400,000
Interest expense			$ 3,400,000	$ 3,400,000	$ 6,800,000	$ 6,800,000
4.50% Convertible Senior Notes [Member] | Minimum [Member]
Schedule Of Debt Instruments [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	20 days		20 days
4.50% Convertible Senior Notes [Member] | Maximum [Member]
Schedule Of Debt Instruments [Line Items]
Convertibility of debt, trading price of debt test, percentage of closing price of stock used in calculation	98.00%

Borrowings - Loan Agreement - A

Borrowings - Loan Agreement - Additional Information (Detail) - USD ($) $ in Thousands	Feb. 25, 2015	Nov. 06, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Nov. 30, 2012
Schedule Of Debt Instruments [Line Items]
Extension period for interest-only payment		4 months
Interest expense			$ 4,482	$ 4,570	$ 9,048	$ 9,081
Loan and Security Agreement [Member]
Schedule Of Debt Instruments [Line Items]
Discount on loan recorded as a non-current liability to be paid upon full repayment or maturity of the loans			1,200		1,200
Interest expense			1,200	$ 1,200	2,300	$ 2,400
Hercules [Member]
Schedule Of Debt Instruments [Line Items]
Less unamortized discount			$ 1,600		$ 1,600
Hercules [Member] | Loan and Security Agreement [Member]
Schedule Of Debt Instruments [Line Items]
Aggregate principal amount of loans received	$ 40,000						$ 40,000
Outstanding principal balance repayment period	Jun. 1, 2016
Outstanding principal balance repayment period	Nov. 1, 2018
Extended maturity period on principal payment	6 months
Additional term loan advance by company	$ 15,000

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - $ / shares	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Class of Stock [Line Items]
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares issued	110,798,000	106,697,000
Common stock, shares outstanding	110,798,000	106,697,000
Common Stock Warrants [Member]
Class of Stock [Line Items]
Number of warrants cashless exercised	2,000,000		400,000
Weighted average exercise price of warrants	$ 3		$ 3.41
Number of common stock issued as a result of cash and cashless exercise of warrants	1,500,000		300,000

Common Stock - Schedule of Shar

Common Stock - Schedule of Shares Reserved for Future Issuance (Detail) - shares shares in Thousands	Jun. 30, 2015	Dec. 31, 2014
Options To Purchase Common Stock [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	19,426	19,567
Common Stock Warrants [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	377	2,381
Conversion Premium On Notes [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	25,000	25,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Feb. 28, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted	2,723
Stock Incentive Plan 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock available for grant (in shares)	3,300		3,700	2,000
Number of Shares, Granted	2,700	3,100

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees and Directors (Detail) - Jun. 30, 2015 - Options to Purchase Common Stock [Member]	Total	Total
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.50%	1.50%
Expected term	5 years	5 years
Expected volatility	66.00%	66.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.80%
Expected term	5 years 10 months 24 days	5 years 10 months 24 days
Expected volatility	67.00%	67.00%

Stock-Based Compensation - Sc43

Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$ 4,945	$ 4,002	$ 8,289	$ 6,975
Stock-based compensation expense for non-employee awards	22	153	48	116
Total stock-based compensation	4,967	4,155	8,337	7,091
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	2,369	1,841	4,335	3,522
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$ 2,576	$ 2,161	$ 3,954	$ 3,453

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Shares, Outstanding, Beginning balance	19,556
Number of Shares, Granted	2,723
Number of Shares, Exercised	(2,595)
Number of Shares, Forfeited	(258)
Number of Shares, Outstanding, Ending balance	19,426	19,556
Number of shares vested and expected to vest	19,086
Number of stock option exercisable	14,210
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 4.47
Weighted Average Exercise Price, Granted	9.54
Weighted Average Exercise Price, Exercised	2.57
Weighted Average Exercise Price, Forfeited	6.82
Weighted Average Exercise Price, Outstanding, Ending balance	5.40	$ 4.47
Weighted average exercise price of shares vested and expected to vest (in dollars per share)	5.36
Weighted average exercise price for exercisable option (in dollars per share)	$ 4.66
Weighted Average Remaining Contractual Term	6 years 6 months	6 years 2 months 1 day
Weighted Average Remaining Contractual Term, Vested and expected to vest	6 years 5 months 12 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding, Beginning balance	$ 133,599
Aggregate Intrinsic Value, Outstanding, Ending balance	135,208	$ 133,599
Aggregate Intrinsic Value, Vested and expected to vest	133,696
Aggregate Intrinsic Value, Exercisable	$ 109,441

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)Employees	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)
Operating Leased Assets [Line Items]
Total rent expense	$ 1.9		$ 1.5	$ 3.4	$ 3
Life Sciences Tax Incentive Program 2015 [Member]
Operating Leased Assets [Line Items]
Tax incentive awarded		$ 1.4
State research and development tax credits monetized		$ 1.2
Number of incremental employees pledged to be hired | Employees		75
Performance period		5 years

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member]	Aug. 07, 2015USD ($)	Jul. 22, 2015USD ($)ft²	Jul. 13, 2015USD ($)
Cambridge, Massachusetts [Member] | July 22, 2015 Amendment [Member]
Subsequent Event [Line Items]
Leased space | ft²		13,843
Additional future minimum lease payments due through expiration		$ 2,200,000
Aggregate leasehold improvement allowance		$ 500,000
Cowen and Company Llc [Member]
Subsequent Event [Line Items]
Net proceeds from issuance of common stock	$ 7,700,000
Maximum [Member] | Cowen and Company Llc [Member] | At the Market Offering [Member]
Subsequent Event [Line Items]
Aggregate sales price of offering			$ 40,000,000