The purpose of this study is to assess the therapeutic efficacy of mosunetuzumab, a bispecific antibody targeting CD20 and CD3 in patients who have detectable chronic lymphocytic leukemia (CLL) after receiving Bruton's tyrosine kinase inhibitors (BTKis) for at least 6 months and have no clinical or laboratory evidence of disease progression.

Primary Objective:

The primary objective is to assess the activity of mosunetuzumab in eliminating MRD when added to continuous BTKi therapy. The primary endpoint will be the rate of undetectable MRD4 (uMRD4) in the bone marrow, using ClonoSEQ at the end of Cycle 17 of mosunetuzumab treatment. The target bone marrow uMRD4 rate at the end of Cycle 17 is 50%.

Secondary Objectives:

• Assessment of safety and tolerability of mosunetuzumab when added to continuous BTKi therapy
• Proportion of patients who stop all treatment after C17 assessment
• Elimination of residual lymph nodes (≥1.5 cm) with mosunetuzumab
• Clinical response rates (ORR, CR, PR) at the end of Cycle 17
• MRD clearance by ClonoSEQ (uMRD6) at the end of Cycle 8 of mosunetuzumab treatment
• Assessment of PFS and OS
• uMRD4 and uMRD6 rates in blood and bone marrow at end of Cycle 17
• uMRD4 and uMRD6 rates in blood and bone marrow and clinical response rates at end of cycle 17, stratified by TP53/del(17p)/IGHV status, prior venetoclax treatment, line of therapy
• uMRD4 and uMRD6 status every 6 months after end of combined treatment for 3 years
• Assessment of MRD kinetics in peripheral blood and bone marrow at serial time points

Inclusion Criteria:

Participants are eligible to be included in the study only if ALL the following criteria apply:

1. Age ≥18 years at the time of signing the Informed Consent Form
2. Ability to comply with the study protocol and procedures and required
3. Patients must have received ≥2 prior lines of systemic therapy, including the current BTKi
4. Patients with high-risk CLL/SLL defined as the presence of any of the following factors:

progression of disease on prior covalent BTKi, or progression of disease on or within 6 months of venetoclax-based treatment, or 3 or more prior treatments, or presence of del(17p) and/or TP53 mutation, or unmutated IGHV e. Patients with CLL/SLL on continuous BTKi therapy (covalent or non-covalent) for ≥12 months and detectable bone marrow MRD4 by ClonoSEQ f. Eastern Cooperative Oncology Group (ECOG) performance score (PS) of 0 or 1 g. Adequate BM function independent of growth factor or transfusion support, within 2 weeks of screening, at screening as follows unless cytopenia is clearly due to marrow involvement of CLL:

• Platelet count ≥50,000/µL; in cases of thrombocytopenia clearly due to marrow involvement of CLL (per the discretion of the investigator), platelet count should be

  ≥30,000/mm3

• ANC ≥1.5x109 cells/L unless neutropenia is clearly due to marrow involvement of CLL (per the discretion of the investigator)

• Total hemoglobin ≥10 g/dL unless anemia is due to marrow involvement of CLL (per the discretion of the investigator) h. Adequate liver function as indicated by a total bilirubin ≤1.5 x ULN, AST, and ALT ≤3 times the institutional ULN value

• In patients with CLL involvement of the liver; AST and ALT <5 times institutional ULN and total bilirubin <3 times institutional ULN i. In patients with Gilbert syndrome: total bilirubin <3 times institutional ULN j. Adequate renal function: serum creatinine ≤1.5 ULN or eGFR ≥50 mL/min k. Life expectancy >6 months l. Resolution to Grade ≤1 for clinically significant toxicities attributable to prior therapies before commencement of the first study drug administration with the following exceptions:

• Any grade alopecia or vitiligo

• Grade 2 peripheral sensory or motor neuropathy

• Endocrinopathy managed and controlled using replacement therapy m. Patients who have a negative HIV test at screening, with the following exception.

• Patients with a positive HIV test at screening are eligible provided that, prior to enrollment, they are stable on anti-retroviral therapy for at least 4 weeks, have a CD4 count ≥200/μL, have an undetectable viral load, and have not had a history of an AIDSdefining opportunistic infection within the past 12 months.

  n. For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of <1% per year, and agreement to refrain from donating eggs, during the treatment period and for at least 3 months after the last dose of mosunetuzumab and 3 months after the last dose of tocilizumab (if applicable). A woman is considered to be of childbearing potential if she is postmenarchal, has not reached a postmenopausal state (≥12 continuous months of amenorrhea with no identified cause other than menopause), and is not permanently infertile due to surgery (i.e., removal of ovaries, fallopian tubes, and/or uterus) or another cause as determined by the investigator (e.g., Müllerian agenesis). The definition of childbearing potential may be adapted for alignment with local guidelines or regulations. Examples of contraceptive methods with a failure rate of <1% per year include bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the individual. Periodic abstinence (e.g., calendar, ovulation, symptom-thermal, or post ovulation methods) and withdrawal are not adequate methods of contraception. If required per local guidelines or regulations, locally recognized adequate methods of contraception and information about the reliability of abstinence will be described in the local Informed Consent Form.

  o. For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agreement to refrain from donating sperm, as defined below:

• With a female partner of childbearing potential or pregnant female partner, men must remain abstinent or use a condom during the treatment period and for 2 months after the final dose of tocilizumab (if applicable), to avoid exposing the embryo. Men must refrain from donating sperm during this same period. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the individual.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

1. Patients with high disease burden, defined as having either absolute lymphocyte count >5 x 109 cells/L, or largest lymph node >2 cm, or bone marrow with >50% CLL involvement
2. Pregnant or breastfeeding or intending to become pregnant during the study or within 3 months after the final dose of mosunetuzumab and tocilizumab (if applicable). Women of childbearing potential must have a negative serum pregnancy test result within 14 days prior to initiation of study treatment. If a serum pregnancy test has not been performed within 14 days prior to receiving first study treatment, a negative urine pregnancy test result (performed within 7 days prior to study treatment) must be available.

b. Participants who previously received any of the following treatments prior to study entry:

• Treatment with mosunetuzumab or other CD20/CD3-directed bispecific antibodies
• Allogeneic stem cell transplant within 90 days of enrollment or with active GVHD c. 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
• CAR T-cell therapy within 90 days before first study treatment
• Use of monoclonal antibodies or antibody-drug conjugates within 4 weeks prior to first study treatment d. Systemic immunosuppressive medications (including, but not limited to, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor agents) within 2 weeks prior to first dose of study treatment
• Systemic corticosteroid treatment ≤25 mg/day prednisone or equivalent and inhaled corticosteroids are permitted.
• Administration of acute, low-dose, systemic immunosuppressant medications (e.g., single dose of dexamethasone for nausea or B-symptoms) is permitted.
• The use of mineralocorticoids for management of orthostatic hypotension and corticosteroids for management of adrenal insufficiency is permitted. e. Any other anti-cancer therapy, whether investigational or approved, including but not limited to chemotherapy, within 4 weeks or 5 half-lives of the drug, whichever is shorter, prior to initiation of study treatment. f. Prior cancer immunotherapy not explicitly described in this protocol g. Received a live, attenuated vaccine within 4 weeks before first dose of study treatment, or in whom it is anticipated that such a live attenuated vaccine will be required during the study period or within 5 months after the final dose of study treatment h. Transformation of CLL to aggressive NHL (e.g., Richter's transformation, prolymphocytic leukemia, or diffuse large B-cell lymphoma [DLBCL]) or CNS involvement by CLL i. History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibody therapy (or recombinant antibody-related fusion proteins) j. History of prior malignancy, except for conditions as listed below if patients have recovered from the acute side effects incurred as a result of previous therapy:
• Malignancies treated with curative intent and with no known active disease present for ≥2 years before enrollment
• Adequately treated non-melanoma skin cancer or lentigo maligna without evidence of disease
• Adequately treated cervical carcinoma in situ without evidence of disease
• Surgically/adequately treated low grade, early stage, localized prostate cancer without evidence of disease k. Participants with infections requiring IV treatment with antibiotics or hospitalization (Grade 3 or 4) 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 l. Evidence of any significant, concomitant disease that could affect compliance with the protocol or interpretation of results, including, but not limited to:
• Significant cardiovascular disease (e.g., New York Heart Association Class III or IV cardiac disease, myocardial infarction within the previous 6 months, unstable arrhythmia, or unstable angina)
• Significant pulmonary disease (such as obstructive pulmonary disease or history of bronchospasm)
• Clinically significant history of liver disease, including viral or other hepatitis, or cirrhosis
• Current or past history of CNS disease, such as stroke, epilepsy, CNS vasculitis, or neurodegenerative disease
• Participants with a history of stroke who have not experienced a stroke or transient ischemic attack in the past 1 year and have no residual neurologic deficits as judged by the investigator are allowed.
• Participants with a history of epilepsy who have had no seizures in the past 2 years with or without anti-epileptic medications can be eligible only for the expansion cohort m. History of confirmed progressive multifocal leukoencephalopathy (PML) n. Positive test results for chronic hepatitis B infection (defined as positive hepatitis B surface antigen [HBsAg] serology)
• Participants with occult or prior hepatitis B infection (defined as positive total hepatitis B core antibody and negative HBsAg) may be included if hepatitis B virus (HBV) DNA is undetectable at the time of screening.
• These participants must be willing to undergo monthly DNA testing and appropriate prophylactic antiviral therapy as indicated. o. Acute or chronic hepatitis C virus (HCV) infection
• Participants who are positive for HCV antibody must be negative for HCV by polymerase chain reaction (PCR) to be eligible for study participation. p. Known or suspected chronic active Epstein-Barr virus infection q. Known or suspected history of HLH r. 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 antiphospholipid syndrome, Wegener granulomatosis, Sjögren syndrome, Guillain-Barré syndrome, multiple sclerosis, vasculitis, or glomerulonephritis
• Participants with a history of autoimmune-related hypothyroidism on a stable dose of thyroid replacement hormone may be eligible.
• Participants with controlled Type 1 diabetes mellitus who are on an insulin regimen are eligible for the study.
• Participants with a history of disease-related immune thrombocytopenic purpura or autoimmune hemolytic anemia may be eligible.
• Participants with a remote history of, or well-controlled autoimmune disease, with a treatment-free interval from immunosuppressive therapy for 12 months may be eligible after review by the investigator. s. Evidence of other clinically significant uncontrolled condition(s) including but not limited to active or uncontrolled systemic infections (e.g., viral, bacterial, or fungal) t. Recent major surgery within 4 weeks prior to first study treatment administration, except for protocol-mandated procedures (e.g., tumor biopsies and bone marrow biopsies) u. Participants with a left ventricular ejection fraction (LVEF) <40% v. Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes an individual's safe participation in and completion of the study