Forimtamig, a novel GPRC5D-targeting T-cell bispecific antibody with a 2+1 format, for the treatment of multiple myeloma

• Bivalent binding of forimtamig to GPRC5D conveys superior potency compared to other GPRC5D- and BCMA-targeting T cell bispecific antibodies.

• Forimtamig combination with BCMA-directed T cell bispecific antibodies and Cereblon E3 Ligase Modulatory Drugs prevents tumor relapse.

Despite several approved therapies, multiple myeloma (MM) remains an incurable disease with high unmet medical need. "Off-the-shelf" T-cell bispecific antibodies (TCBs) targeting BCMA and GPRC5D have demonstrated high objective response rates (ORR) in heavily pre-treated MM patients, however, primary resistance, short duration of response and relapse driven by antigen shift frequently occurs. Although GPRC5D represents the most selective target in MM, recent findings indicate antigen loss occurs more frequently than with BCMA. Thus, anti-GPRC5D immunotherapies must hit hard during a short period of time to kill as many myeloma cells as possible. Here, we characterize forimtamig, a novel GPRC5D-targeting TCB with 2+1 format, using preclinical models of MM. Bivalent binding of forimtamig to the N-terminus of GPRC5D confers higher affinity as compared to classical 1+1 TCB formats correlating with formation of more stable immunological synapses and higher potency in tumor cell killing and T cell activation. Using an orthotopic mouse model of MM, forimtamig recruited T effector cells to the bone marrow and induced rapid tumor killing even after the introduction of step-up dosing to mitigate cytokine release. Combination of forimtamig with standard-of-care (SoC) agents including anti-CD38 antibodies, immunomodulatory drugs and proteasome inhibitors improved depth and duration of response. The combination of forimtamig with novel therapeutic agents including BCMA-TCB and Cereblon E3 Ligase Modulatory Drugs (CELMoDs) was potent and prevented occurrence of GPRC5D-negative tumor relapse. Forimtamig is currently being evaluated in Phase 1 clinical trials in relapsed and refractory myeloma (RRMM) patients for monotherapy and in combination treatments. NCT04557150