Terminated due to recruitment not meeting expectations.

The primary purpose of this study is to evaluate the safety of multiple doses of Staphylococcal protein A (PRTX-100) in adult patients with Idiopathic Thrombocytopenia Purpura (ITP). The pharmacokinetics, immunogenicity and pharmacodynamics will also be studied.

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).

Inclusion Criteria:

• Diagnosis of chronic ITP > 4 months
• Mean platelet count <50 x 10^9/L for patients not receiving corticosteroids; or mean platelet count >=50 x 10^9/L for patients receiving stable dose of corticosteroids

Exclusion Criteria:

• Splenectomy within 45 days of screening
• Rituximab within 6 months prior to screening
• Cyclophosphamide, vincristine, or any other non-monoclonal antibody treatment for ITP within 3 months prior to screening
• IVIG, WinRho or other anti-RhD within 30 days prior to screening