Early administration of CD20 x CD3 bispecific antibodies 4-6 weeks after CAR-T infusion for patients with residual or progressive large B cell lymphomas

Abstract Background: CD19-directed CAR T-cell therapy (CAR-T) has improved outcomes and altered the treatment landscape for patients with relapsed/refractory large B-cell lymphomas (r/r LBCL). Despite these improvements, 60-70% of patients do not have long term remissions after CAR-T. CD20 x CD3 bispecific antibodies (BsAbs), such as mosunetuzumab and glofitamab, have demonstrated efficacy in LBCL relapsing after CAR-T (Chong Blood Advances 2025). We hypothesized that BsAbs could enhance the efficacy of CAR-T by reducing antigen-negative escape and enhancing CAR-T cell activation and persistence. To evaluate this hypothesis, we designed a phase IIa trial of early administration of mosunetuzumab or glofitamab within 31-45 days of CAR T-cell infusion. Methods: This is a multi-center clinical trial of early administration of BsAb for patients with r/r LBCL who receive standard of care CAR-T and have a partial response (PR), stable disease (SD), or progressive disease (PD) at day 30 post CAR-T infusion. BsAb is administered day 31-45 post CAR-T. Patients receive 2 cycles of BsAb (Cohort 1, mosunetuzumab; Cohort 2, glofitamab) and are assessed for response. Patients with complete response (CR) or PD after 2 cycles of BsAb discontinue BsAb; patients with PR or SD are continue BsAb every 3 weeks for up to 1 year and every 24 months during the second year. Efficacy is measured by the CR rate at 24 weeks after initiation of BsAb. CAR-T expansion in blood is assessed by qPCR. Enrollment to Cohort 1 (mosunetuzumab) is complete and Cohort 2 (glofitamab) enrollment is ongoing (NCT04889716). Results: Eight patients, 5 male and 3 female, with a median age of 63 years (range 47-78) were enrolled between January 2022 and May 2025, and included 7 patients with diffuse large B-cell lymphoma NOS (GCB-like [n=4], ABC-like [n=2]) and 1 patient with high grade B-cell lymphoma (double-hit). Patients had a median of 3 prior lines of therapy (range 2-7); 5 patients were primary refractory, 6 patients had extranodal disease, and 5 patients had elevated LDH at CAR-T infusion. Prior CAR-T products included tisagenlecleucel (n-2) and lisocabtagene maraleucel (n=6). The median time from CAR-T cell infusion to BsAb treatment was 42 days (range 33-45). Pre-BsAb responses to CAR-T at Day 30 included 4 PR, 1 SD, 3 PD. Within 30 days after CAR-T infusion and prior to treatment with BsAb, three patients had cytokine release syndrome (CRS) (n=2, grade 1; n=1, grade 2); no ICANS was observed. Mosunetuzumab (n=6) was generally well tolerated; CRS occurred in 3 of the 6 (50%) patients and was low grade (n=2, grade 1; n=1 grade 2). One patient received corticosteroids. One patient had a grade > 3 adverse event related to mosunetuzumab (2 episodes of grade 4 neutropenia, which responded to G-CSF and delay of mosunetuzumab). No CRS occurred in the 2 patients who received glofitamab. No patients developed ICANS. No unexpected adverse events have occurred. The best overall response rate (ORR) in the mosunetuzumab cohort (n=6) was 67% (1 CR, 3 PR, 2 SD, 1 PD). Four patients improved their CAR-T response status after the addition of mosunetuzumab (1 PD to SD, 2 SD/PD to PR, and 1 PR to CR). At 24 weeks, the best ORR was 50% (1 CR, 2 PR, 3 PD). With median follow-up of over 3 years, 1 year progression-free survival is 33% (95%CI 5-68); 1 year duration of response is 50% (95%CI 6-84). Response assessment in the glofitamab cohort is forthcoming. We also assessed changes in T cells and CAR-T cells in both cohorts. After starting BsAb, CAR-T cells in peripheral blood increased between cycle 1 day 1 and cycle 1 day 8 in 5/7 patients with available data; median fold change in CAR-T expansion was 0.25 (25% increase) copies/ug gDNA (range -0.42-12.62). Two patients with responses to mosunetuzumab had undetectable CAR-T at baseline and developed detectable CAR-T by cycle 1 day 8. All patients who continued to receive bispecific antibodies (5/7), had detectable CAR transgene at 12 weeks to 3 months. Two patients underwent biopsy at PD; both tumors expressed CD19 and CD20 and had minimal to no infiltration by T cells. Additional samples are undergoing evaluation and will be presented at the meeting. Conclusions: Early administration of CD20 x CD3 bispecific antibodies after CAR-T appears safe and may enhance CAR-T expansion. The sequential combination of CD19 and CD20 targeted therapies may improve clinical responses in certain patients with r/r LBCL.