Preclinical Proof-of-Concept of an mRNA-Based T Cell Engager Targeting BCMA, FcRH5, and GPRC5D for the Treatment of Multiple Myeloma

Introduction Multiple myeloma (MM) remains incurable due to the inevitable development of resistance to available therapies. Targeting multiple tumor associated antigens (TAAs) simultaneously may circumvent target-mediated resistance and achieve superior outcomes to single TAA targeting. The messenger RNA (mRNA) platform enables multiplexing of mRNA sequences to encode multiple T cell engagers (TCEs) in a single drug product. mRNA-3xTCE is a multiplexed antibody therapeutic that encodes three distinct bispecific TCEs directed against clinically validated MM TAAs BCMA, FcRH5, and GPRC5D. Preclinical data demonstrate efficient translation of all TCEs, potent activity of mRNA-3xTCE against primary MM cells ex vivo and murine tumor xenografts in vivo, and target cell depletion in non-human primates (NHP). Methods mRNA-3xTCE is a lipid nanoparticle (LNP) encapsulating four mRNA constructs that encode three TCEs targeting BCMA, FcRH5, and GPRC5D upon in vivo translation. Recombinant BCMAxCD3, FcRH5xCD3, and GPRC5DxCD3 proteins were used for in vitro and ex vivo studies. mRNA-3xTCE was administered to naive or tumor-bearing mice for the evaluation of in vivo protein production and efficacy, respectively. Cynomolgus monkeys received single or repeat (QWx4) injections of mRNA-3xTCE, and peripheral blood and bone marrow were collected for PK/PD analyses. Results mRNA-3xTCE encoded proteins bind to CD3 and TAAs with low nM affinity, induce T cell-mediated cytotoxicity with pM EC50s in MM cell lines in vitro, and are active in primary MM patient samples ex vivo. In mice, mRNA-3xTCE exhibited robust protein expression of all three TCEs, and the multiplexed therapeutic resulted in superior efficacy compared to a single TAA-targeted TCE in murine xenograft tumor models in vivo. In NHP, weekly IV dosing resulted in therapeutically relevant levels of protein expression, potent on target pharmacology, and an acceptable safety profile. Robust protein expression of all three TCEs was detected after the first dose, and expression was maintained above cytotoxicity EC50s for more than one week post dose. mRNA-3xTCE administration led to TAA+ target (B cell) depletion 6-24 hours post dose, dose-dependent T cell activation 24 hours post dose in the periphery, and T cell proliferation 7 days post dose in the periphery and 24 hours post fourth dose in bone marrow. Dose-dependent increases in IL-6 were observed following IV administration but returned to baseline by 24 hours post injection. No changes in IFNγ or TNFα levels were observed. Conclusions These preclinical data demonstrate potent activity of mRNA-3xTCE encoded BCMAxCD3, FcRH5xCD3, and GPRC5DxCD3 proteins in vitro and in vivo. mRNA-3xTCE demonstrated efficient TCE protein production and on target tumor cell killing in mice and TAA+cell depletion in NHP. mRNA-encoded, multiplexed TCEs are a promising application of the mRNA platform to potentially overcome mechanisms of resistance and improve anti-tumor responses compared to conventional recombinant, single TAA-targeted TCEs. Study Support ModernaTX, Inc.