The purpose of the phase 1 part of this study was to evaluate how well pirtobrutinib is tolerated and what side effects may occur. The phase 2 part of the study will further investigate efficacy and safety of multiple pirtobrutinib dosages versus placebo.

The study drug will be administered orally in participants with Primary Immune Thrombocytopenia (ITP). Blood tests will be performed to check how much pirtobrutinib gets into the bloodstream and how long it takes the body to eliminate it.

The study will last up to approximately 16 weeks for phase 1 dose-escalation and 28 weeks for phase 2 dose-optimization, excluding screening.

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)

Phase 1-Open label, Phase 2-Double-blind