Anti-Myeloma Activity of the Maytansinoid Immunoconjugate of Internalizing Human Monoclonal Antibody Specific for HM1.24/BST2 (CD317).

HM1.24/BST-2 (CD317) is a type II transmembrane protein that was originally identified as a cell surface antigen overexpressed on multiple myeloma (MM) cells. Our previous studies have demonstrated that HM1.24 has a potential as an attractive target molecule for antibody-based or cellular-mediated immunotherapy of MM. Subsequent studies have revealed that rat homologue of HM1.24 at the cell surface can be internalized and localized to the intracellular trans-Golgi network, suggesting that HM1.24 may play an important role in trafficking and signaling. To evaluate the potential effect of HM1.24-targeting therapy in MM, we generated internalizing fully human monoclonal antibodies specific for human HM1.24 by using KM mice. Among these monoclonal antibodies, b-76-8 had the highest affinity to HM1.24 and induced ADCC activity and complement-dependent cytotoxicity in the presence of immune effectors. Importantly, b-76-8 internalized rapidly after cell surface binding and approximately 80% of b-76-8 was delivered to the intracellular location within 30 min as determined by immunofluorescence staining of b-76-8-treated RPMI 8226 cells. We next developed the immunoconjugate of b-76-8 with the analog of the cytotoxic drug maytansine, DM1 [N2′-deacetyl-N2′-(3-mercapto-1-oxopropyl)-maytansine] and further evaluated its potential impact on MM cells. The ratio of DM1 molecules linked per antibody molecule was 1.8–3.0. Several MM cell lines as well as primary MM cells were cultured with either b-76-8, DM1, b-76-8-DM1, or control IgG-DM1, and cell viability was determined by a WST-8 assay. Treatment with b-76-8-DM1 (2–10 nM, based on the concentration of DM1) induced 50–70% of cell death in RPMI 8226, KMS12-BM, and IM-9 cells. In contrast, the relevant amount of b-76-8, DM1 (10 nM), or control IgG-DM1 (10 nM) did not inhibit growth of these MM cells. Of interest, T-cell lines such as MOLT-4 and CEM also expressed HM1.24; however, these cells showed minimal cytotoxicity (