Patients will be enrolled into 1 of 3 dose groups and receive 4 weekly IV doses of PRTX-100. A Safety Monitoring Committee will review safety data through Day 28 for the first 5 patients in a dose group before escalation to the next higher dose level. Patients will be followed for 8 weeks after dosing for safety, PK, immunogenicity and effect on platelet count(pharmacodynamics).