MOSUN: This phase I study is assessing how safe, tolerable and effective targeted therapy (mosunetuzumab) is in people with relapsed or refractory chronic lymphocytic leukaemiaA Phase IB Open-Label, Multicenter Study Evaluating the Safety, Efficacy, and Pharmacokinetics of Mosunetuzumab in Patients With Relapsed or Refractory Chronic Lymphocytic Leukemia

Exclusion

• Pregnant or breastfeeding, or intending to become pregnant during the study or within 3 months after the final dose of mosunetuzumab and tocilizumab or within 30 days after the final dose of venetoclax (if applicable)
• Participants who have received any of the following treatments prior to study entry: treatment with mosunetuzumab or other CD20/CD3-directed bispecific antibodies; allogenic stem cell transplant
• Participants who have received any of the following treatments, whether investigational or approved, within the respective time periods prior to initiation of study treatment: radiotherapy within 2 weeks prior to the first dose of study treatment; autologous stem cell transplant within 100 days prior to first study treatment; CAR T-cell therapy within 30 days before first 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; 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; 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
• Received a live, attenuated vaccine within 4 weeks before the first dose of study treatment, or in whom it is anticipated that such a vaccine will be required during the study period or within 5 months after the final dose of study treatment
• Transformation of CLL to aggressive non-Hodgkin's lymphoma (NHL)
• History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibody therapy (or recombinant antibody-related fusion proteins)
• Contraindication to tocilizumab
• History of prior malignancy except for conditions defined by the protocol
• Participants with infections requiring intravenous (IV) treatment with antibiotics or hospitalization within the last 4 weeks prior to enrollment or known active bacterial, viral (including SARS-CoV-2), fungal, mycobacterial, parasitic, or other infection (excluding fungal infections of nail beds) at study enrollment
• Evidence of any significant concomitant disease that could affect compliance with the protocol or interpretation of results
• Recent major surgery within 4 weeks prior to first study treatment administration, with the exception of protocol-mandated procedures (e.g., tumor biopsies and bone marrow biopsies)
• Positive SARS-CoV-2 test within 7 days prior to enrollment

Exclusion Criteria Specific to Arm C:

• Have received venetoclax therapy within 12 months prior to first study treatment administration
• Participants with known infection with HIV or human T-cell leukemia virus 1 (HTLV1)
• HIV testing will be performed in countries where mandatory testing by health authorities is required
• HTLV testing is required in participants from endemic countries
• Participants with uncontrolled autoimmune hemolytic anemia or immune thrombocytopenia
• Participants who have received the following: strong and moderate CYP3A inhibitors within 7 days prior to the initiation of study treatment; strong and moderate CYP3A inducers within 7 days prior to the initiation of study treatment; steroid therapy for anti-neoplastic intent with the exception of inhaled steroids for asthma, topical steroids, or replacement/stress corticosteroids within 7 days prior to the first dose of study drug administration
• Have consumed grapefruit, grapefruit products, Seville oranges (including marmalade containing Seville oranges), or star fruit within 3 days prior to the first dose of study drug and throughout venetoclax administration
• Inability to swallow a large number of tablets
• Malabsorption syndrome or other condition that precludes enteral route of administration
• Known allergy to both xanthine oxidase inhibitors and rasburicase