A Study of Pirtobrutinib in Participants With Immune Thrombocytopenia

Inclusion Criteria: * Have a diagnosis of primary ITP, defined as isolated thrombocytopenia not associated with another known disease process * Have documented history of response, defined as 2 or more platelet counts greater than or equal to 50,000/microliter (μL), to at least 1 prior line of therapy. Splenectomy is considered a line of therapy * Have relapsed or treatment-resistant primary ITP, with no available therapies known to provide clinical benefit * Have a platelet count less than 30,000/μL on 2 occasions at least 5 days apart in the 15 days before randomization * Have adequate liver, renal, and hematologic functions as defined by a table * Are willing to follow contraception requirements Exclusion Criteria: * Have a history of any thrombotic or embolic event within 12 months before screening * Had a transfusion with blood or blood products or plasmapheresis within 14 days (Phase 1) or within 28 days (Phase 2) of randomization * Have significant cardiovascular disease * Have a diagnosis or history of hematologic malignancy * Have hepatitis B virus (HBV) defined as positive for antigen of hepatitis B (HBsAg) or polymerase chain reaction (PCR) positive for HBV deoxyribonucleic acid (DNA) * Have hepatitis C virus (HCV) defined as positive for anti-HCV antibodies and PCR positive for HCV ribonucleic acid (RNA)