This trial will study different dose levels of hA20 (IMMU-106) to see if they are safe and effective for treating ITP.

The goal of current treatment guidelines for most patients with chronic adult ITP is to maintain platelet levels above 30 x 109/L. The conventional first-line therapy is corticosteroids with or without intravenous immunoglobulins, but many patients relapse when steroids are tapered. Standard therapy then is splenectomy, but patients with refractory ITP who do not respond require further therapy. Unfortunately, immunosuppressive agents or other available treatments typically produce only short-term responses. Because of the lack of medical options after first-line therapy, the target population for this first study of anti-CD20 immunotherapy with hA20 are adult patients with chronic ITP who failed at least one standard ITP therapy (i.e., received at least one standard ITP therapy and now present with platelet levels below 30 x 109/L).

In autoimmune disease, rituximab as well as other anti-CD20 antibodies currently being considered for commercialization have focused on a different dosing schedule in rheumatoid arthritis, and use fixed dosages rather than variable doses based on body surface area. In addition, recent studies of these newer anti-CD20 antibodies in rheumatoid arthritis have reported that lower doses indeed appear effective when administered twice, 2 weeks apart. Based upon these considerations, patients in this study will receive hA20 twice, 2 weeks apart, and administered at one of 3 dose levels.

Inclusion Criteria:

• Male or female, >18 years old, with or without prior splenectomy
• Signed written informed consent obtained prior to study entry
• ITP according to ASH guidelines, with other potential causes of thrombocytopenia excluded
• Platelet levels < 150 x 109/L for more than 6 months
• Received an adequate course of at least one standard ITP treatment (an inadequate course of standard ITP therapy does not qualify as meeting this requirement)
• Platelet count < 30 x 109/L at study entry and on at least one other occasion at least 1 week apart within the past month. (Phase I only: platelet count also > 10 x 109/L at study entry).
• Bleeding assessment score of Grade 0 or 1. See full protocol for all inclusion criteria

Exclusion Criteria:

See full protocol for exclusion criteria or contact study staff for details