Immunotherapeutically targeting CD138 has not been successfully translated into substantial clinical benefit in multiple myeloma (MM) patients. We here developed and determined in preclinical models of human MM the efficacy of VIS832, a novel humanized monoclonal antibody (MoAb) with differentiated CD138 target binding to the anti-CD138 MoAb BB4 within indatuximab ravtansine (BT082). Compared with BB4, VIS832 has significantly improved binding (P<.005, up to 1-log increases) to all MM cell lines (n>8). In the reporter-based antibody-dependent cellular cytotoxicity (ADCC) bioassay, EC50 of VIS832 was approximately 99.31 ng/ml (0.66 nM), whereas the BB4 possessing an isotype matched human IgG1 minimally induced ADCC against U266 MM cells. Evaluation of VIS832-induced ADCC in NK-MM cell co-cultures determined its EC50 values ranged from 2.22 + 0.37 to 15.3 + 2.71 ng/ml, with % maximal lysis of 37.06 + 1.45 % to 97.3 + 3.34 %, across tested MM cell lines (n=12), both sensitive or resistant to current therapies including dexamethasone, IMiDs, and bortezomib. While VIS832 effectively induced ADCC against parental cells with high and low CD138 levels, no lysis was detected in paired CD138-knockout MM cells. In the presence of BM stromal cells or osteoclasts, two key MM-supporting accessory cells in the BM milieu, VIS832-induced MM cytolysis was minimally affected. Importantly, VIS832 eliminated autologous MM cells of bone marrow (BM) samples from relapsed/refractory (RRMM) patients, with > 60% maximal lysis and > 90% killing in 4 out of 7 patient BM samples. EC50 values of VIS832 were 8.58-86.04 ng/ml in this patient cohort, comparable to those (17.28-179.3 ng/ml) of its non-humanized predecessor mAb 2810 against another RRMM patient cohort (n=6). In contrast to its potent autologous CD138+ patient cell lysis, VIS832 did not affect CD138-negative patient BM cells. VIS832 induced higher maximal lysis (~3-fold) of all target MM cell lines than CD38-targeting daratumumab (dara), regardless of resistance to lenalidomide and pomalidomide. These results also confirmed significantly higher CD138 vs CD38 levels in all MM cell lines and patient MM cells (P<.0001). Specifically, VIS832 remained active against MM cells resistant to dara, either via loss of CD38 or acquired drug resistance following extended dara treatment in ex vivo co-cultures. Unlike dara, VIS832 selectively induced MM cell lysis via NK activation, without inducing NK cell apoptosis due to CD38 expression. When combined with lenalidomide, either pretreatment or added concomitantly, VIS832-induced MM cell lysis was significantly enhanced, with combination index (CI) of < 1 indicating synergism. Furthermore, antibody-dependent cellular phagocytosis (ADCP) represented another effector-mediated killing of VIS832 against MM cells, with phagocytosis dependent on both VIS832 and the E:T ratio. The ADCP activity of VIS832 was significantly higher than that observed for dara (P<.05) against MM cell lines sensitive or resistant to IMiDs and dara, and VIS832 induced significant ADCP against dara-resistant MM cells. Finally, in a disseminated MM1S xenograft model of human MM in CB-17 SCID mice, VIS832 alone reduced median tumor burden and improved overall survival from 30 to >60 days (P<.0001). This efficacy was sustained after discontinuation of treatment 3 weeks prior to study termination. Combined treatment of tumor-bearing mice with both VIS832 and bortezomib at sub-optimal doses rapidly eradicated tumors, resulting in 100% survival of all tumor-free animals to the end of study at day 73. The superior efficacy of combined VIS832 and bortezomib when compared to either agent alone suggests synergistic in vivo activity, consistent with synergistically enhanced in vitro cytotoxicity of this combination (CI < 1). Taken together, the significant in vivo efficacy of VIS832, coupled with its mechanisms of action and potent in vitro MM cytotoxicity, strongly support clinical development of VIS832, as monotherapy and in combination, to overcome multidrug resistance and improve patient outcome in MM. Once its efficacy is established in RRMM, its favorable therapeutic index should allow for moving rapidly to earlier stages of disease, newly diagnosed MM and smoldering MM.