Characterization of Linvoseltamab's BCMA Binding Epitope and Efficacy Against BCMA Mutations in Relapsed/Refractory Multiple Myeloma

Patient outcomes in relapsed/refractory multiple myeloma (RRMM) have improved significantly with the adoption of BCMA targeted CAR-T and CD3 bispecific antibodies (bsAb). However, despite improvements in MM therapies, relapse remains an issue. Recent data from patients who relapsed following treatment with approved BCMAxCD3 bsAbs led to the identification of rare mutations in the extracellular domain of BCMA that can negatively impact the binding and cytotoxic activity of these molecules in vitro (Lee, H., et al, Nat Med. 2024). Linvoseltamab is a fully human BCMAxCD3 bsAb that demonstrated high and durable responses in RRMM. Here we characterize the binding epitope of linvoseltamab and evaluate its binding and activity on cells with identified BCMA mutations. A 3.4 Å cryo-EM structure of the linvoseltamab Fab fragment bound to the BCMA extracellular domain shows that linvoseltamab features a long heavy chain CDR3, stabilized by a disulfide bond, which plays a major role in binding by inserting into a groove on the surface of BCMA. Linvoseltamab contacts a total of 16 BCMA residues which largely overlap with the reported epitopes of teclistamab and elranatamab (Lee, H., et al, Nat Med. 2024; Josic, M., et al, ASCO 2024). However, linvoseltamab also binds to the very N-terminal end of BCMA, suggesting that it is tilted toward the BCMA N-terminus with a distinct binding orientation. Binding assays on cells expressing wild-type or mutant BCMA variants demonstrated that, unlike teclistamab and elranatamab, the BCMA R27P mutation had minimal impact on linvoseltamab binding. Additionally, utilizing a Jurkat NFAT-luc reporter assay system which measures BCMAxCD3 specific activation of T-cells, linvoseltamab mediated potent NFAT-luc activation in the presence of cells expressing BCMA-R27P. While the structural analysis showed linvoseltamab contacts BCMA R27, it is located at the edge of the binding interface and thus appears to not be crucial for binding. In addition, BCMA P33 does not contact linvoseltamab, consistent with the observation that the P33S mutation has no effect on linvoseltamab binding or activity. Cell binding and functional reporter bioassay and primary T cell cytotoxicity studies are underway for additional reported BCMA mutations. This studyidentified a unique binding orientation for linvoseltamab and demonstrated that linvoseltamab retains its binding and functional activity even in the presence of an identified BCMA mutation (R27P) that negatively impacts other BCMAxCD3 bispecific antibodies. This suggests that linvoseltamab may be less susceptible to resistance mechanisms that involve these mutations, resulting in improved patient outcomes.