Rational engineering of sdAb-based CAR T cells targeting BCMA enhances antitumor efficacy and persistence in Multiple Myeloma

BCMA-directed CAR T therapies have transformed the treatment of relapsed/refractory multiple myeloma (MM), yet durable responses remain limited by insufficient persistence and functional exhaustion. The contribution of antigen-binding domain properties to CAR T cell performance is not fully defined. Here, we report the rational discovery and engineering of BCMA-targeting single-domain antibodies (sdAbs) and their incorporation into second-generation CAR T cells. We generated sdAbs recognizing distinct BCMA epitopes with diverse binding kinetics. Comprehensive biophysical and functional characterization identified sdAb5 as a lead candidate with balanced binding kinetics and a non-overlapping epitope relative to clinically approved constructs. sdAb-based CAR T cells exhibited potent antigen-specific cytotoxicity, minimal tonic signaling, and preserved IL-2 production without excessive inflammatory activation. In xenograft models, sdAb5-based CAR T cells induced durable tumor control, enhanced persistence, and improved responses to tumor rechallenge compared with ide-cel and cilta-cel constructs. Notably, this superior functionality correlated with balanced binding kinetics rather than maximal affinity. Single-cell transcriptomic analyses corroborated these findings, revealing a restrained inflammatory activation in sdAb5-based CAR T cells with preservation of transcriptional plasticity. Collectively, these findings support antigen-binding optimization as a key determinant of CAR T-cell durability and identify sdAb5 as a candidate for next-generation BCMA-directed therapies in MM.