Abstract 1869: Preclinical evaluation of mosunetuzumab for the treatment of relapsed/refractory chronic lymphocytic leukemia

Abstract Background: CD20 x CD3 T-cell engaging bispecific antibodies, such as Mosunetuzumab (Mosun), have shown promising efficacy in Non-Hodgkin’s lymphoma. However, their role in treating chronic lymphocytic leukemia (CLL) is still uncertain. CLL, a clonal disease of mature B cells, is associated with high B/T cell ratios and diminished T cell activation and function, including the inability to form strong immunosynapses (Ramsay et al, J Clinical Investigation, 2008). Herein, we explore whether Mosun could overcome these barriers and elicit strong T-cell mediated B-cell killing in CLL samples. Methods: PBMCs from CLL patients and healthy donors were exposed to a titration of Mosun for 48 and 72 hours and analyzed for B cell death with flow cytometry. Markers of T cell activation and exhaustion were quantified in CD4 and CD8T cells and within their CD45RA/CCR7 subsets. The concentrations of secreted cytokines were measured by ELISA. To study the impact of high B/T ratio on B cell killing, B cells were isolated from healthy donor PBMC and added back to autologous PBMC to attain B/T ratios seen in CLL patient samples. Results: After 48 hours with 0.04ug/ml Mosun, the percentage of dead B cells in healthy-donor samples (n=9) exceeded that in CLL samples (n=7) by four-fold (54.2 + 17.9 vs: 12.9 + 17.5%, p=0.0004). Under the same conditions, significantly more T cells expressed activation markers in healthy donors than in CLL patients, with 58.8 +17.1% of healthy donor CD8T cells expressing both CD69 and CD25 compared to and 31.4 +21.4% of CLL CD8T cells (p=0.013). The mean concentration of Granzyme B secreted by healthy donor samples upon Mosun treatment was over ten times higher than by CLL patient samples. Reconstitution of a healthy donor sample to a B/T ratio of 10 resulted in decreased B cell killing (29.7% dead B cells vs: 70.5% in un-reconstituted) without affecting T cell activation (62.9% CD69+CD25+ CD8T cells versus 56.2% in un-reconstituted). Conclusions: Although single-agent Mosun treatment can activate T-cells in CLL patient samples and result in B-cell death, the response is suboptimal due to a number of factors, including deficient T cell activation and high B/T ratio. Ongoing work is focusing on evaluating drug partners to debulk CLL B cells or prime CLL T cells for better B-cell killing by Mosun. Citation Format: Kathy Barrett, Kai Lu, Hyun Yong Jin, Rosie Millen, Marcus Lefebure, Yanwen Jiang. Preclinical evaluation of mosunetuzumab for the treatment of relapsed/refractory chronic lymphocytic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1869.