Background GPRC5D is a G protein-coupled receptor that is expressed on multiple myeloma cells but absent from most healthy tissues except for hair follicles. Tumor-associated antigens like GPRC5D with little or no expression in healthy tissue are attractive targets for potent anti-tumor modalities including T cell-engaging bispecific antibodies. There is immense interest in GPRC5D as a multiple myeloma target, but currently no clinically approved therapies against it.

Methods Multipass membrane proteins are valuable therapeutic targets in oncology and other disease areas but are largely inaccessible as antibody targets due to their poor expression, membrane-dependent structure, small extracellular regions, and poor immunogenicity due to sequence conservation. We developed an antibody discovery platform (MPS) that specifically addresses each of these challenges. This platform utilizes advanced immunization techniques including DNA, mRNA, and Lipoparticles (virus-like particles). It also employs chickens as an evolutionarily divergent host species for robust immune responses against conserved targets. Antibodies raised in chickens are directly humanized prior to isolation reducing the need for downstream engineering. From the parental antibodies isolated, we engineered panels of GPRC5DxCD3 bispecific antibodies using multiple formats and CD3 arms, that encompass different geometries and binding stoichiometries, as these factors are expected to play a critical role for in vitro and in vivo potency.

Results We immunized chickens with GPRC5D and obtained high-titer immune responses. A subset of antibodies from this discovery program was configured as bispecific molecules using a CD3-targeting arm to bring tumor cells into close proximity with cytotoxic T cells that mediate cell killing. The molecules comprised multiple bispecific formats bearing different stoichiometries, geometries, and sizes, to enable identification of lead molecules with favorable activities and safety profiles. GPRC5D bispecifics displayed potent T cell-mediated cytotoxicity with picomolar potency. They also exhibited high specificity and bound to only their target, and not the other 6,000 proteins tested using a Membrane Proteome Array (MPA). Furthermore, these molecules showed good developability profiles and minimal cytokine release. GPRC5DxCD3 bispecifics are currently undergoing additional testing for developability and in vivo potency.

Conclusions GPRC5DxCD3 bispecific antibodies hold promise as potent and safe therapeutics for multiple myeloma