A Study Evaluating the Safety, Efficacy, and Pharmacokinetics of Mosunetuzumab Monotherapy in Participants With Select B-Cell Malignancies

Inclusion Criteria:

• At least one bi-dimensionally measurable nodal lesion, defined as >1.5 cm in itslongest dimension, or one bi-dimensionally measurable lesion, defined as >1.0 cm inits longest diameter by computed tomography (CT) scan, positivie emission tomography

• computed tomography (PET- CT), or magnetic resonance imaging (MRI)

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

• Adequate hematologic function

• No active infection

• Negative HIV test at screening, with the following exception: Individuals with apositive HIV test at screening are eligible provided they are stable onantiretroviral therapy for at least 4 weeks, have a CD4 count ≥ 200/µL, have anundetectable viral load, and have not had a history of opportunistic infectionattributable to AIDS within the last 12 months

• For women of childbearing potential (except those in Cohort B): agreement to remainabstinent (refrain from heterosexual intercourse) or use contraceptive measures, andagreement to refrain from donating eggs, as defined by the protocol

• For men: agreement to remain abstinent (refrain from heterosexual intercourse) oruse a condom, and agreement to refrain from donating sperm, as defined by theprotocol

Inclusion Criteria Specific to Cohorts A1 and A2

• Previously untreated FL with indication to start systemic therapy

• Adequate renal function

Inclusion Criteria Specific to Cohort B

• Aged ≥ 80 years at the time of signing informed consent form (ICF), or aged 65-79years and considered ineligible for chemoimmunotherapy (R-CHOP) with at least one ofthe following: Impairment in ≥ 2 Activities of Daily Living (ADL); impairment in ≥ 2Instrumental Activities of Daily Living (IADL); or Cumulative Illness RatingScale-Geriatric (CIRS-G) score of ≥ 1 comorbidity with a severity of 3-4 or a scoreof 2 in ≥ 8 comorbidities

• Histologically confirmed DLBCL according to WHO 2016 classification expected toexpress the CD20 antigen (Swerdlow et al. 2016)

• Previously untreated DLBCL with indication to start systemic therapy and are noteligible for curative therapy

• High-grade B-cell lymphomas, not otherwise specified (HGBL NOS) and HGBL with MYCand B-cell lymphoma (BCL)-2 and/or BCL-6 rearrangements

• Adequate end-organ function

Inclusion Criteria Specific to Cohort C

• Histologically conformed MZL (splenic, nodal, and extra-nodal)

• Previously untreated MZL with indication to start systemic therapy

• Helicobacter pylori-positive disease that has remained stable, progressed, orrelapsed following antibiotic therapy and requires therapy, as assessed by theinvestigator (for cases of gastric/MALT MZL)

• Adequate renal function

Inclusion Criteria Specific to Cohort D

• Histologically confirmed MCL

• Relapsed after or failed to respond to at least one prior treatment regimencontaining a Bruton's tyrosine kinase (BTK) inhibitor

• Adequate renal function

• Adverse events from prior anti-cancer therapy resolved to Grade

Inclusion Criteria Specific to Cohort E

• Histologically confirmed RT or tFL

• Relapsed after or failed to respond to at least one prior systemic treatment regimenfor RT or tFL

• Adequate renal function

• Absolute lymphocyte count

• Adverse events from prior anti-cancer therapy resolved to Grade

Exclusion Criteria:

• Current or past history of central nervous system (CNS) lymphoma or leptomeningealinfiltration

• Prior treatment with mosunetuzumab

• History of severe allergic or anaphylactic reactions to humanized or murinemonoclonal antibodies or known sensitivity or allergy to murine products

• History of confirmed progressive multifocal leukoencephalopathy (PML)

• Known active SARS-CoV-2 infection

• Known or suspected chronic active Epstein-Barr virus (CAEBV) infection

• Patients with history of macrophage activation syndrome (MAS)/hemophagocyticlymphohistiocytosis (HLH)

• Positive test results for chronic hepatitis B infection (HBV), acute or chronichepatitis C virus (HCV) infection, or known or suspected HIV infection

• Administration of a live, attenuated vaccine within 4 weeks before firstmosunetuzumab administration or anticipation that such a live, attenuated vaccinewill be required during the study

• Prior solid organ transplantation

• Prior allogenic stem cell transplant

• Treatment with CAR-T therapy within 30 days prior to C1D1

• History of autoimmune disease, including, but not limited to myasthenia gravis,myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis,inflammatory bowel disease, vascular thrombosis associated with anti-phospholipidsyndrome, Wegener granulomatosis, Sjögren syndrome, Guillain-Barré syndrome,multiple sclerosis, vasculitis, or glomerulonephritis

• Received systemic immunosuppressive medications (including, but not limited to,cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosisfactor agents) with the exception of corticosteroid treatment

• Current or past history of CNS disease, such as stroke, epilepsy, CNS vasculitis, orneurodegenerative disease

• History of other malignancy that could affect compliance with the protocol orinterpretation of results

• Evidence of significant, uncontrolled concomitant diseases that could affectcompliance with the protocol or interpretation of results or that could increaserisk to the patient

• Known active bacterial, viral, fungal, mycobacterial, parasitic, or other infection (excluding fungal infections of nail beds) at study enrollment or any major episodeof infection requiring treatment with intravenous antibiotics or hospitalization (relating to the completion of the course of antibiotics) within 4 weeks before C1D1

• Clinically significant history of liver disease, including viral or other hepatitis,or cirrhosis

• Recent major surgery within 4 weeks before the start of C1D1, other than superficiallymph node biopsies for diagnosis

• Prior treatment with radiotherapy within 2 weeks prior to C1D1

• Adverse events from prior anti-cancer therapy not resolved to Grade

• Significant cardiovascular disease (such as New York Heart Association Class III orIV cardiac disease, congestive heart failure, myocardial infarction within theprevious 6 months, unstable arrhythmias, or unstable angina) or significantpulmonary disease (including obstructive pulmonary disease and history ofbronchospasm)

• History of severe allergic or anaphylactic reaction to humanized, chimeric or murinemonoclonal antibodies (MAbs)

• Contraindication to tocilizumab

• Prior anti-lymphoma treatment with monoclonal antibodies, radioimmunoconjugates, orantibody-drug conjugates within 4 weeks before first mosunetuzumab administration

Exclusion Criteria Specific to Cohorts D and E

• Prior anti-lymphoma treatment with any monoclonal antibody (e.g., anti-CD20),radioimmunoconjugate, or antibody-drug conjugate therapy within 4 weeks before firstmosunetuzumab administration