Characterization of the BCMA epitope bound by BCMA-CD3 T cell engager elranatamab.

Abstract

7546 Background: Elranatamab is a BCMA-CD3 bispecific antibody with an accelerated approval in the US and EU for relapsed/refractory multiple myeloma (RRMM). Elranatamab binds to CD3 on T cells and BCMA on MM tumor cells; this dual binding results in T cell activation, cytokine release, and tumor cell killing. Engagement of the BCMA surface receptor by its ligands, APRIL and BAFF, mediates plasma cell proliferation and survival. Elevated BCMA, as well as BAFF and APRIL are present in MM patients, yet in vitro functional assays have shown physiologically relevant soluble APRIL or BAFF concentrations did not significantly impact elranatamab activity. Moreover, emerging clinical data from relapsed patients has identified novel, yet relatively rare BCMA protein mutations that may impact BCMA ligand or targeted modality binding. Here we characterized the BCMA binding affinity, evaluated the impact on BCMA binding to BAFF and APRIL interactions, and identified the BCMA binding epitope. We also generated a model from crystal structure data to map reported BCMA mutations identified in patients. Methods: Elranatamab binding affinity to recombinant human BCMA and characterization of BAFF and APRIL interactions with elranatamab-bound BCMA were calculated via SPR at 37˚C with a classical sandwich format. The elranatamab BCMA binding epitope was identified by a co-crystal structure with a parental anti-BCMA Fab that has identical CDR sequences to elranatamab. Alpha-fold modeling was used to map identified BCMA mutations on the antibody/BCMA interface. Results: Elranatamab binds to BCMA with higher affinity (~38 pM) compared to reported binding affinities of BAFF (μM range) and APRIL (nM range) ligands. The identified elranatamab epitope largely overlaps with known ligand epitopes, supporting that elranatamab higher affinity BCMA binding blocks ligand binding. Alpha fold modeling of four recently reported rare BCMA mutations shows that they lie along the binding interface. Conclusions: Due to its high affinity for BCMA, elranatamab potency is unlikely to be impacted by elevated levels of soluble BAFF or APRIL in RRMM patients. Future studies are needed to characterize the impact of emerging MM patient BCMA mutations on the binding and function of anti-BCMA targeting modalities, particularly given the small extracellular domain of BCMA available for therapeutic targeting and the highly overlapping binding epitopes reported for approved BCMA agents.