Abstract
B-cell maturation antigen (BCMA) is a clinically validated target for multiple myeloma. T-cell engineered with chimeric antigen receptors (CARs) directed against BCMA have demonstrated robust therapeutic activity in clinical trials, but toxicities remain a significant concern for a subset of patients, supporting continued investigation of other engineered T-cell platforms that may offer equal efficacy with an improved toxicity profile. The authors recently described a BCMA-specific, T-cell-centric synthetic antigen receptor, the T-cell antigen coupler (TAC) receptor, that can be used to engineer T-cell with robust anti-myeloma activity. Here the authors describe the creation of a fully humanized BCMA-specific TAC receptor. Single-chain variable fragments (scFvs) were developed from BCMA-specific F(ab)s that were identified in a fully human phage display library. Twenty-four configurations of the F(ab)s were evaluated in a medium-throughput screening using primary T-cell, and a single F(ab), TRAC 3625, emerged as the most robust following in vitro and in vivo evaluation. An optimized BCMA-specific TAC receptor was developed through iterations of the BCMA-TAC design that evaluated a next-generation TAC scaffold sequence, different domains connecting the TAC to the 3625 scFv and different orientations of the TRAC 3625 heavy and light variable regions.