J6M0-mcMMAF selectively inhibits MM cell viability and colony formation via caspase 3/7-dependent apoptosis. (A) J6M0 ADCs (-mcMMAF or -vcMMAE) were added in the culture for 6 days followed by luminescence-based viability assays. Gray bars represent the normalized mean fluorescence intensity of BCMA expression in each cell line. Black triangles and red circles represent the growth inhibitory IC₅₀ for J6M0-mcMMAF and J6M0-vcMMAE, respectively, calculated from triplicate measurements. (B) ANBL6 cells were treated for 3 days with J6M0-mcMMAF/J6M0-vcMMAE, or iso-mcMMAF/iso-vcMMAE (isotype control ADCs), in the presence of absence of BMSCs. (C) CD138⁺ patient MM cells were incubated for 3 days with indicated ADCs followed by annexin V/PI staining and flow cytometry analysis. CD138⁺ cells from additional MM patients were treated with J6M0-mcMMAF for 3 days, followed by viability (D) and caspase 3/7 activity (E) assays. (F) J6M0-mcMMAF was added to MM cell culture in methylcellulose for 21 days to determine effects on colony formation. Representative colony formation of INA6 and OPM2 cells are shown on the right.