A Study Evaluating the Efficacy and Safety of Pirtobrutinib in Participants With Relapsed or Refractory Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma

The main purpose of Part 1 of this study is to assess the efficacy and safety of 3 dose levels of Pirtobrutinib in participants with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL), who have received 1-3 lines of treatment including a covalent Bruton tyrosine kinase (BTK) inhibitor. The purpose of Part 2 of this study is to evaluate pirtobrutinib monotherapy in participants with treatment-naïve CLL/SLL with 17p deletions. Participation in Part 1 is expected to last approximately 3 years. Participation in Part 2 is expected to last up to 2 years.

Inclusion Criteria: * Have confirmed diagnosis of CLL/SLL as defined by iwCLL 2018 criteria. * Part 1: Have received prior CLL/SLL treatment * Have received at least 1, but not more than 3 lines of prior treatment for CLL/SLL * Have received a covalent BTK inhibitor * Part 2: Have received no prior treatment for CLL/SLL * Part 1 - Known 17p deletion status (positive or negative) * Part 2 - Must have 17p deletion (positive) * Have a requirement for therapy consistent with iwCLL 2018 criteria for initiation of therapy * Capable of swallowing oral study medication. * Have an Eastern Cooperative Oncology Group Performance Status (ECOG) score of 0 to 2. Exclusion Criteria: * Have received prior treatment with a BTK degrader or a noncovalent BTK inhibitor * Have a history of greater than or equal to (\>=) Grade 3 bleeding due to treatment with a BTK inhibitor * Have known or suspected Richter's transformation * Have known or suspected history of central nervous system involvement by CLL/SLL * Previous or concurrent cancer distinct from CLL/SLL within 3 years before randomization. Exceptions may occur with documented sponsor approval. Examples include: * nonmelanoma skin cancer or lentigo malignant melanoma * cervical carcinoma in situ * localized prostate cancer undergoing active surveillance, and * localized (for example, lymph node negative) breast cancer with no evidence of active disease present for more than 3 years. Individual may be receiving adjuvant hormonal therapy