CAMBRIDGE, Mass., July 17, 2013 (GLOBE NEWSWIRE) -- Merrimack Pharmaceuticals, Inc. (Nasdaq:MACK) today announced the closing of its sale of 5,750,000 shares of its common stock at a price to the public of $5.00 per share and $125,000,000 aggregate principal amount of its 4.50% convertible senior notes due 2020 (the "Notes") in concurrent underwritten public offerings pursuant to Merrimack's effective Registration Statement on Form S-3 (File No. 333-186369) (the "Registration Statement") and a related prospectus filed with the Securities and Exchange Commission (the "SEC"). The total number of shares of common stock sold reflects the full exercise by the underwriters of their option to purchase an additional 750,000 shares of common stock. Merrimack has granted the underwriters in the Notes offering an option, exercisable for 30 days, to purchase up to an additional $18.75 million in aggregate principal amount of Notes. The Notes bear interest at a rate of 4.50% per year, payable semiannually on January 15 and July 15 of each year, beginning January 15, 2014. Interest accrues from July 17, 2013. The Notes mature on July 15, 2020. The Notes are convertible, under certain circumstances and during certain periods, at the option of the holder, based on an initial conversion rate of 160.0000 shares of 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, subject to adjustment in certain circumstances. The initial conversion price represents a conversion premium of 25% over the public offering price in the common stock offering. Following certain corporate events that occur prior to the maturity date, Merrimack will increase the conversion rate for a holder who elects to convert its Notes in connection with such a corporate event in certain circumstances. Upon any conversion of the Notes that occurs while Merrimack's indebtedness to Hercules Technology Growth Capital, Inc. ("Hercules"), under the Loan and Security Agreement dated November 8, 2012 between Merrimack and Hercules, remains outstanding, the Notes will be settled in shares of Merrimack's common stock. Following the repayment and satisfaction in full of Merrimack's obligations to Hercules under the Loan and Security Agreement, upon any conversion of the Notes, the Notes may be settled, at Merrimack's election, in cash, shares of Merrimack's common stock or a combination of cash and shares of Merrimack's common stock.