213Bi Radioimmunotherapy with an Anti-mCD138 Monoclonal Antibody in a Murine Model of Multiple Myeloma

New multiple myeloma (MM) treatments-such as high-dose melphalan therapy plus autologous stem cell transplantation or regimens incorporating bortezomide, thalidomide, and lenalidomide-substantially increase the rate of complete response that is associated with longer patient survival. Thus, maintaining the complete response status by improving the minimal residual disease after induction therapy is a key goal for MM management. Here, we address the question of radioimmunotherapy efficacy in MM minimal residual disease treatment in mice with a low tumor burden. alpha-emitters are particularly well adapted to this approach because the short range of alpha-particles enables localized irradiation of tumor cells within the bone marrow and a cytotoxic effect on isolated cells due to the high LET (linear energy transfer) of alpha-particles. The CD138 antigen was used as a target because of its strong expression on myeloma cells in 100% of patients. Method: Intravenous injection of 10(6) 5T33 mouse myeloma cells into the Syngeneic mouse strain C57BL/KaLwRij resulted in a rapid invasion of the marrow and limb paralysis, as illustrated by bioluminescence imaging with luciferase-transfected 5T33 cells. Radioimmunotherapy was performed 10 d after 5T33 cell engraftment with an intravenous injection of an antimouse CD138 antibody radiolabeled with Bi-213 at activities of 1.85, 3.7, 7.4, and 11.1 MBq. A blood cell count was performed weekly to monitor hematologic toxicity. The levels of blood Flt3 ligand were also measured to evaluate the radioimmunotherapy-related myelotoxicity. Disease progression was monitored by titrating the monoclonal IgG2b antibody produced by 5T33 cells. Results: The groups treated with 3.7 and 7.4 MBq exhibited a median survival greater than 300 and 227 d, respectively, compared with 45.5 d in the control untreated group. The highest activity (11.1 MBq) showed short-term toxicity whereas the lowest activity (1.85 MBq) gave results similar to those of the controls. With activities of 3.7 and 7.4 MBq, mice exhibited a transient hematologic toxicity whereas only temporary and moderate myelotoxicity was observed with 7.4 MBq. Conclusion: This study demonstrates promising therapeutic efficacy of Bi-213-labeled anti-mCD138 for the treatment of residual disease in the case of MM, with only moderate and transient toxicity.