Abstract 6251: Evaluation of a dual-targeting BCMA-CS1 HLE BiTE® molecule for multiple myeloma

Abstract Multiple myeloma (MM) is a cancer of the antibody-producing plasma cells (PC). MM is invariably fatal due to frequent disease relapse and/or treatment refractoriness, and therapies that provide deeper and more durable responses are needed. BiTE® (Bispecific T Cell Engager) molecules are immunotherapy agents that redirect a patient’s T cells to lyse tumor cells by simultaneously engaging a tumor associated antigen (TAA) on cancer cells and CD3ε on T cells. Clinical activity has been observed in MM using BiTE® molecules and other T cell engagers that target B-cell maturation antigen (BCMA). BCMA is an attractive MM TAA due to its broad prevalence and restricted normal tissue profile, mainly PCs and plasmablasts. However, heterogenous expression in MM cancer cells, potential antigen loss, and the presence of high levels of soluble BCMA in MM patient sera present challenges that may prevent BCMA-only-targeted therapies from achieving their full potential. To address these issues, we generated a BiTE® molecule capable of engaging BCMA and a second MM TAA, CS1 (also known as SLAMF7). This BiTE® molecule also contains an Fc-based domain to provide half-life extension (HLE). Here, we evaluated the in vitro and in vivo properties of this BCMA-CS1 HLE BiTE® molecule. The BCMA-CS1 HLE BiTE® molecule had nanomolar binding affinity for human BCMA, CS1, and CD3ε and the non-human primate (NHP) orthologues. In vitro, we observed picomolar activity against human MM cell lines with a range of BCMA and CS1 expression in a T cell dependent cellular cytotoxicity (TDCC) assay using human T cells or NHP peripheral blood mononuclear cells. The BCMA-CS1 HLE BiTE® molecule retained TDCC activity in the presence of soluble BCMA up to 2500 ng/mL as well as against human MM cells engineered to express only BCMA or CS1. In vivo, this BiTE® molecule inhibited tumor growth in a MM xenograft model. We also evaluated the BCMA-CS1 HLE BiTE® molecule in NHP over 15 days (IV dosing; intra-animal dose escalation from 60→240 μg/kg or 180→540 μg/kg on days 1 and 8). We observed hallmarks of BiTE® molecule activity in all groups, including transient decreases in circulating lymphocytes and moderate increases in cytokines like MCP-1. We measured the PC-specific transcripts BCMA and J-chain in NHP bone marrow and blood as surrogates for PC levels using ddPCR. These transcripts were reduced in both treatment groups (≥90%) with the strongest effects occurring in the 180→540 μg/kg group. Lastly, we showed that CS1 is highly and broadly expressed in MM patient samples and is restricted to a few normal hematopoietic cell types including PCs, NK cells, T cells, and some monocytes. These data suggest that the BCMA-CS1 HLE BiTE® molecule has potent in vitro and in vivo activity and may provide therapeutic benefit for MM patients by expanding the population of MM cancer cells that can be eliminated by a BiTE® molecule while overcoming common mechanisms that can impair BCMA-only-targeted MM therapies. Citation Format: Elizabeth T. Andrews, Stephanie C. Casey, Mohammad Farhad Amani, Grit Lorenczewski, Mozhgan Farshbaf, Lisa Winkel, Matthias Klinger, John M. Harrold, Famke Aeffner, Ana Goyos, Matthias Friedrich, Tara Arvedson, Matthew G. Chun. Evaluation of a dual-targeting BCMA-CS1 HLE BiTE® molecule for multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6251.