Abstract 3183: GPRC5D and BCMA bi-specific CAR-T: Optimized CAR design to mitigate antigen escape and elicit deep and durable response in heterogeneous multiple myeloma

Abstract Introduction: BCMA CAR-T cell therapies have demonstrated superior efficacy, including in patients with extramedullary disease and high-risk cytogenetics, compared to BCMA-directed TCE or ADC. The longitudinal single cell analysis revealed that pre-existing T cell exhaustion and presence of highly proliferative plasma cells with Ag downregulation predisposed to early relapse post BCMA CAR-T. Moreover, shedding BCMA and suppressive TME also pose significant challenges for sustained clinical response. To tackle these obstacles, BCMA and GPRC5D bi-specific CAR-Ts were designed. GPRC5D is a promising therapeutic target for multiple myeloma (MM), as it - expresses exclusively on MM cells but not in normal tissues (except for hair follicles) and is independent of BCMA expression. Clinical studies confirmed the co-existence of single or double Ag positive MM subsets within the same patient, along with a rare occurrence of double-negative multiple myeloma plasma cells. Methods: Aiming to broadly and deeply eradicate MM subsets and achieve long-term remission, we’ve developed bi-specific CAR-Ts exclusively targeting BCMA and GPRC5D. Our design incorporates novel VHH binders with optimized CAR design to enhance sensitivity and efficacy toward heterogenous MM cells in the presence of soluble BCMA, mitigate Ag escape, restore T cell functionality and improve expansion. We’ve conducted the pre-clinical investigations to validate the activity, functionality and druggability of the bi-specific CAR-Ts. Results: Binding to GPRC5D and BCMA resulted in bi-specific CAR-T exhibiting improved immune cell activation, superior anti-tumor activity, and functionality over mono-CAR-T or benchmark, and enhanced target-dependent expansion under re-stress. Moreover, the bi-specific CAR-T retained its functionality against BCMA.KO or GPRC5D.KO cells, whereas the mono-CAR-T could not. Importantly, the optimized bi-specific CAR designs incorporating the novel BCMA VHH binder(s) showed robust cytotoxicity against MM cells with low Ag expression in the presence of high concentrations of soluble BCMA, suggesting their potential to effectively counteract the inhibitory effects of soluble BCMA in the serum of MM patients. Compared to mono-specific CAR-Ts, the bi-specific CAR-T showed robust efficacy in tumor eradication at low dose(s) in NSG mice xenografted with heterogeneous MM cell lines. Conclusions: The pre-clinical data of GPRC5D and BCMA bi-specific CAR-T exhibited superior anti-tumor efficacy and persistence, mitigated Ag escape and preserved potency with shedding BCMA. Our findings provide support for the clinical investigation of the bi-specific CAR-T as a therapeutic approach to broadly and deeply deplete heterogenetic MM cell clones and subsets, potentially leading to durable responses in both newly diagnosed and R/R MM patients. Citation Format: Jincai Zhou, Xinran Luo, Kai Wu, Feifei Shi, Bixia Lei, Yanhong Xu, Joy Zhou, Xiaowen He. GPRC5D and BCMA bi-specific CAR-T: Optimized CAR design to mitigate antigen escape and elicit deep and durable response in heterogeneous multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3183.