A Study Evaluating the Safety, Efficacy, and Pharmacokinetics of Mosunetuzumab and a Combined Regimen of Mosunetuzumab and Venetoclax in Participants With Relapsed or Refractory Chronic Lymphocytic Leukemia

Must have received: BTK inhibitor — Arm B only

Participants must have been taking a BTKi for at least 12 months, have demonstrated evidence of progressive disease while receiving the BTKi and require additional salvage therapy as assessed by their treating physician. Participants should be able to continue their previously prescribed BTKi at a stable dose throughout the study screening period and for the first two cycles of mosunetuzumab administration

Cannot have received: CD20/CD3-directed bispecific antibody (mosunetuzumab)

treatment with mosunetuzumab or other CD20/CD3-directed bispecific antibodies

Cannot have received: allogeneic stem cell transplant

allogenic stem cell transplant

Cannot have received: radiotherapy

radiotherapy within 2 weeks prior to the first dose of study treatment

Cannot have received: autologous stem cell transplant

autologous stem cell transplant within 100 days prior to first study treatment

Cannot have received: CAR-T cell therapy

CAR T-cell therapy within 30 days before first study treatment

Cannot have received: monoclonal antibody

Exception: for anti-CLL treatment within 4 weeks before first dose of study treatment

prior use of any monoclonal antibodies, radioimmunoconjugates, or antibody-drug conjugates for anti-CLL treatment within 4 weeks before first dose of study treatment

Cannot have received: systemic immunosuppressive medication (cyclophosphamide, azathioprine, methotrexate, thalidomide, anti-tumor necrosis factor agents)

systemic immunosuppressive medications (including but not limited to cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor agents) within 2 weeks prior to the first dose of study treatment

Cannot have received: chemotherapy

Exception: except for participants enrolled in Arm B, where overlapping therapy is permitted

any other anti-cancer therapy, whether investigational or approved, including but not limited to chemotherapy within 4 weeks prior to initiation of study treatment (except for participants enrolled in Arm B, where overlapping therapy is permitted; other prior cancer immunotherapy not explicitly defined by the protocol is to be discussed with the medical monitor to determine eligibility

Cannot have received: venetoclax (venetoclax)

Exception: Arm C only, within 12 months prior to first study treatment administration

Have received venetoclax therapy within 12 months prior to first study treatment administration