Abstract 3406: GPRC5D multispecific antibodies with potent anti-tumor activity against multiple myeloma

Abstract Background: GPRC5D is a G protein-coupled receptor that is expressed on multiple myeloma cells but absent from most healthy tissues. It is an attractive target for anti-tumor modalities including T cell-engaging bispecific antibodies. Recent clinical data demonstrate that combination therapy using T cell-engaging molecules, which individually target GPRC5D and BCMA on tumor cells, offers unprecedented therapeutic benefits for patients with relapsed/refractory multiple myeloma. A trispecific antibody that simultaneously engages these molecules while activating T cells is expected to provide comparable and additional benefits as a monotherapy. Methods: Multipass membrane proteins are valuable therapeutic targets but are largely inaccessible for antibody discovery due to structural complexity and high conservation. We developed an antibody discovery platform (MPS) tailored for membrane proteins that utilizes advanced immunization techniques including DNA, mRNA, and Lipoparticles (virus-like particles) and also utilizes evolutionarily divergent host species. From the parental antibodies isolated, we engineered panels of GPRC5DxCD3 bispecific and GPRC5DxBCMAxCD3 trispecific antibodies using multiple formats and CD3 arms that encompass different geometries and binding stoichiometries. GPRC5D multispecific antibodies were tested for target specificity using Integral Molecular’s Membrane Proteome Array (MPA). Both bispecific and trispecific antibodies were tested for in vitro potency and cytokine release using MM.1R and RPMI 8226 multiple myeloma cell lines. In vivo efficacy for a subset of antibodies was determined using xenograft models in humanized mice. Results: We immunized chickens with GPRC5D to obtain high-titer immune responses and configured resulting antibodies as T cell-engaging bispecific molecules with a CD3 binding arm. A subset of GPRC5DxCD3 bispecific antibodies displayed potent T cell-mediated cytotoxicity with picomolar potency on multiple myeloma cell lines. Antibody specificity profiling showed high specificity binding to its intended targets, and not to other proteins across the membrane proteome. Bispecific molecules were then re-engineered as trispecific antibodies with additional binding to BCMA, a well-validated multiple myeloma target. GPRC5DxBCMAxCD3 trispecific antibodies showed improved cytotoxic activity against multiple myeloma cells. Both bispecific and trispecific antibodies demonstrated a large window between cell killing and cytokine release. GPRC5D multispecific antibodies also showed robust tumor control against multiple myeloma xenografts in a humanized mouse model. Conclusions: GPRC5D multispecific antibodies hold promise as potent and safe therapeutics for multiple myeloma. Citation Format: Hayley Roth, Daniel Rogers, Ileine Sanchez, Breanna Tyrell, Amelia Snyder, Kyle Doolan, Benjamin Doranz, Ross Chambers, Joseph Rucker. GPRC5D multispecific antibodies with potent anti-tumor activity against 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 3406.