A Study of BGB-16673 Compared to Investigator's Choice in Participants With Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma Previously Exposed to Both Bruton Tyrosine Kinase (BTK) and B-cell Leukemia/Lymphoma 2 Protein (BCL2) Inhibitors

The purpose of this study is to investigate the efficacy and safety of BGB-16673 compared with investigator's choice (idelalisib plus rituximab \[for CLL only\] or bendamustine plus rituximab or venetoclax plus rituximab retreatment) in participants with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) previously exposed to both BTK inhibitors (BTKi) and BCL2 inhibitors (BCL2i).

Chronic lymphocytic leukemia is a type of blood cancer that affects people around the world. People with CLL suffer from enlarged lymph nodes, spleen, or liver, or have symptoms like night sweats, weight loss and fever. They have shorter life expectancy compared to healthy people. There is an urgent need for new treatment to prolong life and control disease-related symptoms. In this study, participants with relapsed/refractory (R/R) CLL who were previously exposed to a BTKi and a BCL2i will receive BGB-16673 or the investigator's choice of idelalisib plus rituximab (for CLL only) or bendamustine plus rituximab or venetoclax plus rituximab retreatment. The main purpose of this study is to compare the length of time that participants live without their CLL or SLL worsening between those participants who receive BGB-16673 versus the investigator's choice of treatment (idelalisib plus rituximab or bendamustine plus rituximab, or venetoclax plus rituximab). Approximately 250 participants will be included in this study around the world. Participants will be randomly allocated to receive either BGB-16673 or the investigator's choice of treatment. "Our company, previously known as BeiGene, is now officially BeOne Medicines. Because some of our older studies were sponsored under the name BeiGene, you may see both names used for this study on this website."

Inclusion Criteria: 1. Confirmed diagnosis of CLL or SLL, requiring treatment, based on 2018 international workshop on chronic lymphocytic leukemia (iwCLL) criteria. 2. Previously received treatment for CLL/SLL with both a BTKi and a BCL2i. 3. Participants with SLL must have measurable disease by computer tomography (CT)/magnetic resonance imaging (MRI) 4. Eastern Cooperative Oncology Group (ECOG) score 0, 1, or 2 5. Adequate liver function 6. Adequate blood clotting function Exclusion Criteria: 1. Known prolymphocytic leukemia or history of, or currently suspected, Richter's transformation 2. Prior autologous stem cell transplant or chimeric antigen receptor-T cell therapy in the last 3 months 3. Known central nervous system involvement 4. Prior exposure to any BTK protein degraders 5. Active fungal, bacterial and/or viral infection requiring parenteral systemic therapy 6. Clinically significant cardiovascular disease NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.