PRTX-100-203 Open-Label, Dose Escalation Study in Adult Patients With ITP

Inclusion Criteria: 1. Willing and able to provide written informed consent prior to initiation of any study-related procedures 2. Male or female ≥ 18 years of age 3. ITP that has persisted for ≥ 3 months. ITP must be diagnosed in accordance The American Society of Hematology 2011 Evidence-based Practice Guideline for Immune Thrombocytopenia (Neunert et al. 2011) or the International Consensus Report on The Investigation and Management of Primary Immune Thrombocytopenia (Provan et al. 2010), as locally applicable. 4. Received ≥ 1 typical regimen for the treatment of ITP. Splenectomy is considered one regimen. 5. A mean platelet count of \< 30,000/μL with no individual platelet count \> 55,000/μL. The mean platelet count must be determined based on 2 platelet counts including one obtained within ≤ 7 days of first PRTX-100 dose and the other within ≤ 30 days of the first dose of PRTX-100. 6. If on corticosteroids, a dose of \< 1 mg/kg prednisone per day or equivalent that has been stable for ≥ 21 days prior to the first dose of PRTX-100. High-dose pulse steroid therapy is NOT allowed within 14 days prior to the first dose of PRTX-100. 7. If on steroid-sparing adjunctive immunosuppression with cyclosporine, azathioprine, mycophenolate, or 6-mercaptopurine, the dose must have been stable for ≥ 30 days prior to the first dose of PRTX-100 and must be expected to remain stable through study Day 29, unless dose reduction is required due to toxicities. Treatment with other cytotoxic agents (e.g. cyclophosphamide, vincristine) are not allowed within three months prior to the first dose of PRTX- 100. 8. Any prior treatment with rituximab or any other anti-CD20 agent must have been \> 6 months prior to the first dose of PRTX-100 9. If female, must not be pregnant (as indicated by screening negative pregnancy test), must not be nursing and must be one of the following: * Surgically sterile (bilateral tubal ligation, hysterectomy) * Postmenopausal with last natural menses \> 24 months prior * Premenopausal and using an acceptable form of birth control. Acceptable forms of birth control include: hormonal contraceptives (implantable, oral, patch) used for ≥ 2 months prior to screening or double barrier methods (any combination of two of the following: intrauterine device \[IUD\], male or female condom with spermicidal gel, diaphragm, sponge, cervical cap). All premenopausal females must have a negative urine or serum pregnancy test at screening and on Day 1 prior to first PRTX-100 treatment. Exclusion Criteria: 1. Splenectomy ≤ 90 days prior to the first dose of PRTX-100 2. Exposure to TPO-RA within 2 weeks before inclusion 3. Previous treatment with rituximab within \<6 months prior to the first dose of PRTX-100 4. Bleeding score ≥ 8 (Khellaf M et al. Haematologica 2005) 5. Unstable coronary artery disease or other medical condition (such as type 1 diabetes) that, in the investigator's opinion, might increase the risk to the patient 6. Evidence of active infection requiring antibiotic therapy ≤ 14 days prior to the first dose of PRTX-100 7. Myelodysplastic syndrome. If clinically significant anemia or pancytopenia exists, documentation of a bone marrow aspirate within 24 months prior to the first dose of PRTX- 100 showing no evidence of myelodysplasia is required. 8. Medical history systemic lupus erythematosus or any cause of secondary ITP 9. History of any treatment for cancer within the past two years other than basal cell or squamous cell carcinoma of the skin that has been treated with curative intent 10. Seropositive for human immunodeficiency virus (HIV) 11. History of acute/chronic hepatitis B or C and/or carriers of hepatitis B or C (positive for hepatitis B surface antigen or positive anti-hepatitis C antibody test) 12. History suggestive of substance abuse 13. Clinically significant abnormalities in screening laboratory tests, including: * Absolute neutrophil count \< 1.0 x109/L * Hemoglobin \< 10 g/dL * Absolute lymphocyte count \< 0.8 x109/L * Alanine transaminase (ALT) or aspartate transaminase (AST) \> 2 x upper limit of normal (ULN) * Lactate dehydrogenase \> 3 x ULN * Total bilirubin level \>1.5 x ULN * Serum creatinine level \> 0.14 mmol/L (1.6 mg/dL) in males or 0.12 mmol/L (1.4 mg/dL) in females 14. Treatment with IVIG ≤ 14 days prior to the first dose of PRTX-100 15. Treatment with an anti-Rh D antigen agent (e.g. WinRho®) ≤ 14 days prior to the first dose of PRTX-100 16. Use of any investigational drug ≤ 30 days or 5 half-lives of the investigational drug (whichever is longer) prior to the first dose of PRTX-100