Phase II Study of Combined Pirtobrutinib, Venetoclax and Obinutuzumab (PVO) Time-limited Treatment for Patients With Recurrent Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (CLL/SLL).

Eligibility Criteria: 1. Age 18 years or older. 2. Diagnosis of CLL/SLL per 2018 iwCLL criteria (See Appendix 1). 3. Participants with previously treated CLL requiring therapy based on 2018 iwCLL criteria. 4. The participant is able to take oral medications. 5. Willing and capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol. 6. Prior or ongoing therapy with covalent BTKi is allowed, but not required. 7. Prior or ongoing therapy (at least for six months) with BCL2i is allowed, but not required. Prior therapy with combined BTKi and BCL2i or triplet BTKi, BCL2 and anti-CD20 mAb is allowed, but Participants need to be at least six months after completion of combination therapy. Participants with history of prior venetoclax therapy should have achieved at least a partial response or better while receiving venetoclax therapy. 8. Participants are required to have the following washout periods prior to planned Cycle 1 Day1 (C1D1). * Targeted agents, investigational agents, therapeutic monoclonal antibodies or cytotoxic chemotherapy: 5 half-lives or 2 weeks, whichever is shorter * immunoconjugated antibody treatment within 10 weeks * broad field radiation (≥ 30% of the bone marrow or whole brain radiotherapy) must be completed 14 days prior to enrollment * palliative limited field radiation must be completed 7 days prior to enrollment 9. Prior treatment-related AEs must have recovered to Grade ≤ 1 with the exception of alopecia and Grade 2 peripheral neuropathy. 10. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤2. 11. Participants must have adequate renal and hepatic function: * Serum bilirubin ≤1.5 x upper limit of normal (ULN) or ≤3 x ULN for Participants with Gilbert's disease or disease involvement by CLL/SLL. * Serum creatinine clearance of ≥30ml/min (calculated or measured). * ALT and AST ≤3.0 x ULN, unless clearly due to documented disease involvement, in which case ALT and AST ≤5.0 x ULN 12. Adequate bone marrow function: * Platelet count of ≥50,000/μl, with no platelet transfusion in prior 2 weeks. * ANC ≥750/μl in the absence of growth factor support within 7 days of screening assessment. * Hemoglobin ≥8g/dL, independent of transfusions within 7 days of screening assessment. Please refer to Appendix 4 for details of adjustments of toxicities in participants with abnormal baseline values) 13. Adequate coagulation, defined as activated partial thromboplastin time (aPTT) or partial thromboplastin time and prothrombin time (PT) or international normalized ratio (INR) not greater than 1.5 x ULN. 14. Women of childbearing potential must have a negative serum beta human chorionic gonadotropin (β-hCG) pregnancy test result at the time of screening and serum or urine β-hCG pregnancy test within 7 days prior to the first dose of study drugs and must agree to use both a highly effective method of birth control (eg, implants, injectables, combined oral contraceptives, some intrauterine devices \[IUDs\], complete abstinence, or sterilized partner) and a barrier method (eg., condoms, vaginal ring, sponge, etc) during the period of therapy and for 6 months after the last dose of study drug (pirtobrutinib and Obinutuzumab) and 12 months after the last dose of obinutuzumab. Women of nonchildbearing potential are those who are postmenopausal (defined as absence of menses for ≥1 year) or who have had a bilateral tubal ligation or hysterectomy. Men who have partners of childbearing potential must agree to use effective contraception, defined above, during the study and for 30 days following the last dose of study drug Exclusion Criteria: 1. Participants who experienced progression of disease according to 2018 iwCLL criteria while on venetoclax will be excluded. 2. Patient with prior history of Richter's syndrome or current Richter's Syndrome. 3. Participants with known hypersensitivity to any of the excipients of pirtobrutinib, venetoclax,obinutuzumab or to any intended study medications. 4. Known or suspected history of central nervous system (CNS) involvement by CLL/SLL. 5. History of bleeding diathesis. 6. Participants who experienced a major bleeding event on a prior BTK inhibitor.• NOTE: Major bleeding is defined as bleeding having one or more of the following features: life-threatening bleeding with signs or symptoms of hemodynamic compromise; bleeding associated with a decrease in the hemoglobin level of at least 2 g/dL; or bleeding in a critical area or organ (e.g., retroperitoneal, intraarticular, pericardial, epidural, or intracranial bleeding or intramuscular bleeding with compartment syndrome). 7. History of stroke or intracranial hemorrhage within 6 months of enrollment. 8. Participants requiring therapeutic anticoagulation with warfarin or another vitamin K antagonists. 9. Major surgery within 4 weeks of planned start of study therapy. 10. A significant history of renal, neurologic, psychiatric, endocrine, metabolic or immunologic disorder, that, in the opinion of the Investigator, would adversely affect the participant's participation in this study or interpretation of study outcomes. 11. History of allogeneic or autologous stem cell transplant (SCT) or chimeric antigen receptor-modified Tcell (CAR-T) therapy within 60 days of enrollment or presence of any of the following, regardless of prior SCT and/or CAR-T therapy timing: * active graft versus host disease (GVHD); * cytopenia from incomplete blood cell count recovery post-transplant; * need for anti-cytokine therapy for toxicity from CAR-T therapy; residual symptoms of neurotoxicity \> Grade 1 from CAR-T therapy; * ongoing immunosuppressive therapy (\> 20 mg prednisone or equivalent daily). 12. Active uncontrolled auto-immune cytopenia (e.g., autoimmune hemolytic anemia \[AIHA\], idiopathic thrombocytopenic purpura \[ITP\]) for which new therapy was