Subjects will initiate study medication at an oral dose of 50 mg/day (25 mg/day for East Asians), which will be increased or decreased as clinically indicated to the lowest dose that maintains a stable platelet count 20,000/(micro)L above baseline while maximizing tolerability. Treatment response is defined as platelet count increases to 20,000/(micro)L above baseline at three months. Subjects with response at 3 months may continue study medication (extended access) until they meet an off study criteria.