Novel Fc-Engineered Anti-B Cell Maturation Antigen-Monomethyl Auristatin F Antibody-Drug Conjugate (GSK2857916) Induces Potent and Selective Anti-Multiple Myeloma Activity Via Enhanced Effector Function and Direct Tumor Cell Killing

B cell maturation antigen (BCMA), which is highly expressed on malignant plasma cells in human multiple myeloma (MM), has not been effectively targeted with therapeutic monoclonal antibodies (mAbs). We here investigated the anti-MM activity of J6M0-mcMMAF (GSK2857916), a humanized and afucosylated anti-BCMA antibody-drug conjugate (ADC) via uncleavable linker. This novel antagonist anti-BCMA antibody shows binding against all CD138-expressing MM cell lines (n=13) and patient MM cells (n=18), confirming universal BCMA expression on the surface of myeloma cells. Real-time qRT-PCR also showed significantly upregulated BCMA mRNA in CD138+ cells purified from MM patients vs. normal donors (p < 0.03). In contrast, BCMA is undetectable in CD138-negative cells from MM patients (n=3). J6M0-mcMMAF strongly blocks cell growth and induces caspase 3-dependent apoptosis in both drug-sensitive and -resistant MM cell lines and patient CD138+ MM cells, alone and in co-culture with BMSCs. In contrast, an isotype control antibody-drug conjugate (iso-mcMMAF) had no effect on viability of ANBL6 MM cells, alone or cocultured with BMSC. J6M0-mcMMAF specifically induces cell death in CD138-positive patient MM cells but not CD138-negative cells, demonstrating the minimal bystander killing against surrounding BCMA-negative cells. J6M0-mcMMAF completely blocks colony formation of MM cell lines (n=6) via induction of G2/M arrest, followed by apoptosis. This ADC does not affect viability of BCMA-negative NK, PBMC, and BMSCs, cultured alone or together, confirming its specific targeting of BCMA-positive MM cells. J6M0-mcMMAF, which has enhanced Fc-receptor binding due to afucosylation, significantly improved autologous antibody-dependent cellular cytotoxicity (ADCC) potency and maximum MM cell lysis against MM patient cells (n=5), when compared to J6M0 with normal Fc. Such augmented ADCC and maximum patient MM cell lysis by J6M0-mcMMAFis more pronounced in the autologous setting vs. the allogenic setting where MM cells and healthy donor effectors were used. Pretreatment of PBMC effector cells with lenalidomide further increased J6M0-mcMMAF-induced ADCC against MM cells in the presence or absence of BMSC. The in vivo efficacy of J6M0-mcMMAF was evaluated in murine subcutaneous xenograft models using NCI-H929 and OPM2 cells, as well as in NK-deficient SCID-beige mice with diffuse human MM bone lesions using MM1Sluc cells. Administration of J6M0-mcMMAF at 4 mg/kg (q3d x 4, ip) completely eliminated NCI-H929 and OPM2 xenograft tumors in all mice which remained tumor-free until the termination of studies at 60 and 100 days, respectively. In the MM1Sluc bone marrow dissemination model, J6M0-mcMMAF eradicates detectable tumors after 2 doses at 0.4 mg/kg (q3d x 9, ip), which resulted in extended survival (p<0.0001) and no weight loss of mice following 120 days. J6M0 treatment, although less effective than J6M0-mcMMAF, also had significantly prolonged survival (p<0.03) and diminished tumor burden when compared with control vehicle and isotype-treated groups, indicating a potential role of macrophage-mediated phagocytosis. Indeed, J6M0-mcMMAF recruits macrophage and mediates phagocytosis of target MM cells. Taken together, our studies show that J6M0-mcMMAF potently and selectively induce direct and indirect killing of MM tumor cells both in vitro and in vivo, providing a very promising next-generation immunotherapeutic in this cancer.