Merrimack Pharmaceuticals Inc., of Cambridge, Mass., said it closed a private placement of $175 million in aggregate principal amount of its senior secured notes due 2022. The notes bear an interest rate of 11.5 percent per year, payable semi-annually. The company intends to use a portion of the net proceeds to repay all outstanding obligations under its loan and security agreement, as amended, with Hercules Technology Growth Capital Inc., with the remainder for working capital and other general corporate purposes.