Nature of the Business	12 Months Ended
Dec. 31, 2012
Nature of the Business
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. The Company has six targeted therapeutic oncology candidates in clinical development (MM-398, MM-121, MM-111, MM-302, MM-151 and MM-141), multiple product candidates in preclinical development and a discovery effort advancing additional candidate medicines. The Company's discovery and development effort is driven by Network Biology, which is its proprietary systems biology-based approach to biomedical research. 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, ability to secure additional capital to fund operations, 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 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. In April 2012, the Company closed the initial public offering of its common stock pursuant to a registration statement on Form S-1, as amended. The Company sold an aggregate of 15,042,459 shares of common stock under the registration statement at a public offering price of $7.00 per share, including 742,459 shares pursuant to the exercise by the underwriters of an over-allotment option. Net proceeds were approximately $98.1 million, after deducting underwriting discounts and commissions and other offering expenses but prior to the payment of dividends on the Company's Series B convertible preferred stock. At the time of the initial public offering, the Company's convertible preferred stock and warrants to purchase convertible preferred stock automatically converted to common stock and warrants to purchase common stock, as discussed more fully in Note 3. On November 8, 2012, the Company entered into a Loan and Security Agreement (the "Loan Agreement"), with Hercules Technology Growth Capital, Inc. ("Hercules"). As more fully discussed in Note 12, the Loan Agreement provides for an initial term loan advance of $25.0 million, which closed on November 8, 2012, and an additional term loan of $15.0 million, which closed on December 14, 2012, which resulted in aggregate net proceeds of $39.6 million during the fourth quarter of 2012. 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.

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. The Company has six targeted therapeutic oncology candidates in clinical development (MM-398, MM-121, MM-111, MM-302, MM-151 and MM-141), multiple product candidates in preclinical development and a discovery effort advancing additional candidate medicines. The Company's discovery and development effort is driven by Network Biology, which is its proprietary systems biology-based approach to biomedical research. 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, ability to secure additional capital to fund operations, 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 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.

In April 2012, the Company closed the initial public offering of its common stock pursuant to a registration statement on Form S-1, as amended. The Company sold an aggregate of 15,042,459 shares of common stock under the registration statement at a public offering price of $7.00 per share, including 742,459 shares pursuant to the exercise by the underwriters of an over-allotment option. Net proceeds were approximately $98.1 million, after deducting underwriting discounts and commissions and other offering expenses but prior to the payment of dividends on the Company's Series B convertible preferred stock. At the time of the initial public offering, the Company's convertible preferred stock and warrants to purchase convertible preferred stock automatically converted to common stock and warrants to purchase common stock, as discussed more fully in Note 3.

On November 8, 2012, the Company entered into a Loan and Security Agreement (the "Loan Agreement"), with Hercules Technology Growth Capital, Inc. ("Hercules"). As more fully discussed in Note 12, the Loan Agreement provides for an initial term loan advance of $25.0 million, which closed on November 8, 2012, and an additional term loan of $15.0 million, which closed on December 14, 2012, which resulted in aggregate net proceeds of $39.6 million during the fourth quarter of 2012.

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.