 CAMBRIDGE, Mass., July 25, 2019 PRNewswire -- Merrimack Pharmaceuticals, Inc. MACK today announced that its Board of Directors has authorized and declared a special cash dividend of 20 million on the Companys common stock. The special dividend is payable on September 5, 2019 to stockholders of record as of the close of business on August 28, 2019. The ex-dividend date for the special dividend will be September 6, 2019, the first trading day following the payment date. 