Impact of prior CAR-T cell therapy on mosunetuzumab efficacy in patients with relapsed or refractory B-cell lymphomas

• 1. A longer interval between prior CAR-T therapy and mosunetuzumab is associated with a higher response rate to mosunetuzumab

• 2. After prior CAR-T, mosunetuzumab responders have greater increases in CD4 and CD8 T-cells and longer CAR-T persistence than nonresponders

Mosunetuzumab and other CD20/CD3 bispecific antibodies (BsAbs) have efficacy in B-cell lymphomas relapsing after or refractory to CD19-directed chimeric antigen receptor-modified T cells (CAR-T). The optimal timing of BsAbs and biomarkers of BsAb response after CAR-T are unknown. We addressed these questions using clinical data and blood samples from patients previously treated with CAR-T treated on a phase I/II study of mosunetuzumab (clinicaltrials.gov; NCT02500407). Thirty patients had paired samples at baseline and after 1 cycle of mosunetuzumab. Median time from CAR-T to mosunetuzumab was significantly longer for responding than for nonresponding patients (p=0.006, unadjusted for multiple comparisons). Most patients (20/30) did not receive intervening therapy between CAR-T administration and mosunetuzumab. The remainder of patients received one intervening therapy after a protocol-mandated drug washout. After mosunetuzumab, responding patients had higher lymphocytes (995 vs 400 cells/µL, p = 0.02) and greater increases in CD4 and CD8 cells (median change, 73 vs -90 cells/µL, p=0.005; 243 vs -103 cells/µL, p=0.004, respectively). Additionally, responding patients had an increase in activated CD8 cells (median 1.7-fold-change, p=0.02). Non-responders had a relative decrease in CAR transgene levels (N=16; p=0.04). This is the first study to assess changes in lymphocytes, T cells, and CAR transgene levels in patients treated with BsAb after CAR-T. These findings suggest an interaction between prior CAR-T and BsAb outcomes, and have implications for optimal timing of BsAb after CAR-T.