Study to Evaluate Safety, Tolerability and Efficacy of UCB7665 in Subjects With Primary Immune Thrombocytopenia

Inclusion Criteria: * Subject has a diagnosis of primary immune thrombocytopenia (ITP) for a minimum of 3 months prior to Screening Visit * Subject has a platelet count \<30x10\^9/L at Screening and \<35x10\^9/L at Baseline (Visit 2) * Subject has a current or history of a peripheral blood smear consistent with ITP * Subject has responded to previous ITP therapy (according to the judgment of the investigator) Exclusion Criteria: * Subject has an immunoglobulin G (IgG) level \<=6g/L at Screening Visit * Subject has a partial thromboplastin time (PTT) \>=1.5x upper limit of normal (ULN) or International Normalized Ratio (INR) \>=1.5 at Screening Visit * Subject has renal and/or liver impairment defined as: * Serum creatinine level of \>=1.4 mg/dL for females and \>=1.5 mg/dL for males at Screening Visit * Subject has planned an elective surgical procedure in the coming 6 months * Subject has evidence of a secondary cause of primary immune thrombocytopenia purpura * Subject has a history of clinically relevant ongoing chronic infections * Subject has a family history of primary immunodeficiency * Subject has a clinically relevant active infection or has had a serious infection within 6 weeks prior to the first dose of IMP * Subject has a history of known inflammatory bowel disease, diverticular disease, and gastric or esophageal ulceration * Subject has experienced gastrointestinal bleed in the last 6 months prior to Screening Visit and/or has current gastritis or esophagitis * Subject has a medical history of thrombosis * Subject has a history of coagulopathy disorders other than ITP * Subject has received a live vaccination within 8 weeks prior to the Baseline Visit; or intends to have a live vaccination during the course of the study or within 7 weeks following the final dose of IMP * Subject has had prior treatment with rituximab in the 6 months prior to the Baseline Visit * Subject has not completed the washout period for the immunosuppressants, biologics and other therapies