CHOP Plus Mosunetuzumab as First Line in Patients With Richter´s Syndrome: a Phase II Study of the Spanish Group of CLL

Inclusion Criteria: 1. Capable of giving signed informed consent as described in Section 13.2, which includes compliance with the requirements and restrictions listed in the Informed Consent Form and this protocol. 2. Aged between 18 and 79 years at the time of signing the Informed Consent Form 3. Ability to comply with the study protocol and procedures and required hospitalizations, in the investigator's judgement. 4. Eastern Cooperative Oncology Group (ECOG) performance status (PS) of ≤2. 5. Adult patients with previously untreated, histologically proven Richter's syndrome, diffuse large B cell variants, following WHO 2008 criteria (Swerdlow SH, 2008). 6. Screening flow cytometry or immunohistochemistry (IHC) evidence of CD20 positive disease as per central review (dim expression of CD20 is acceptable) 7. Adequate BM function independent of growth factor or transfusion at screening as follows unless cytopenia is clearly due to marrow involvement of CLL: 1. Platelet count ≥75 x 109/L; in cases of thrombocytopenia clearly due to marrow involvement of CLL (per the discretion of the investigator), platelet count should be ≥ 30 x 109/L. 2. ANC ≥1 x 109/L unless neutropenia is clearly due to marrow involvement of CLL (per the discretion of the investigator) 3. Total hemoglobin ≥ 9 g/dL unless anemia is due to marrow involvement of CLL (per the discretion of the investigator) 8. Measured or estimated creatinine clearance ≥ 45 mL/min by institutional standard method. 9. Life expectancy \> 3 months 10. For women of childbearing potential (WOCBP): 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). 1. It is recommended to remain abstinent or use contraception for 12 months after the final dose of cyclophosphamide, doxorubicin, or vincristine. 2. A woman is considered to be of childbearing potential (WOCBP) 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. 3. 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. 4. 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, symptothermal, or postovulation methods) and withdrawal are not adequate methods of contraception. 5. 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 Informed Consent Form. 11. For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agreement to refrain from donating sperm, as defined below: 1. 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 60 days after the final dose of tocilizumab (if applicable) and must remain abstinent or use a condom for 12 months after the final dose of cyclophosphamide, doxorubicin, or vincristine to avoid exposing the embryo. Men must refrain from donating sperm during this same period. 2. 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, symptothermal, or postovulation methods) and withdrawal are not adequate methods of preventing drug exposure. If required per local guidelines or regulations, information about the reliability of abstinence will be described in the Informed Consent Form. Exclusion Criteria: 1. Pregnant or breastfeeding or intending to become pregnant during the study or within 3 months after the final dose of mosunetuzumab. a) WOCBP 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. 2. Participants who have received any of the following treatments prior to study entry: a) Treatment with mosunetuzumab or other CD20/CD3-directed bispecific antibodies. 3. Participants who have received any of the following treatments, whether investigational or approved, given to treat RS, within the respective time periods prior to initiation of study treatment: 1. Autologous SCT within 100 days prior to first mosunetuzumab administration. 2. Allogeneic stem cell transplant for CLL 3. CAR T-cell therapy for CLL within 100 days before first study treatment 4. Systemic corticosteroid treatment ≤ 20 mg/day prednisone or equivalent to control symptoms related to disease progression for a maximum of 5 days before starting C1D1 and inhaled corticosteroids are permitted. 4. Central nervous system (CNS) involvement as documented by spinal fluid cytology or imaging. 5. Transformation of CLL to prolymphocytic leukemia. 6. History of prior malignancy, except for conditions as listed below if patients have recovered from the acute side