Pb-212 alpha-Radioimmunotherapy Targeting CD38 in Multiple Myeloma: A Preclinical Study

Multiple myeloma (MM) is a plasma cell cancer and represents the second most frequent hematologic malignancy. Despite new treatments and protocols, including high-dose chemotherapy associated with autologous stem cell transplantation, the prognosis of MM patients is still poor. alpha-radioimmunotherapy (alpha-RIT) represents an attractive treatment strategy because of the high-linear-energy transfer and short pathlength of alpha-radiation in tissues, resulting in high tumor cell killing and low toxicity to surrounding tissues. In this study, we investigated the potential of alpha-RIT with Pb-212-daratumumab (anti-hCD38), in both in vitro and in vivo models, as well as an antimouse CD38 antibody using in vivo models. Methods: Inhibition of cell proliferation after incubation of the RPM18226 cell line with an increasing activity (0.185-3.7 kBq/mL) of Pb-212-isotypic control or Pb-212-daratumumab was evaluated. Biodistribution was performed in vivo by SPECT/CT imaging and after death. Dose-range-finding and acute toxicity studies were conducted. Because daratumumab does not bind the murine CD38, biodistribution and dose-range finding were also determined using an antimurine CD38 antibody. To evaluate the in vivo efficacy of Pb-212-daratumumab, mice were engrafted subcutaneously with 5 x 10(6) RPM18226 cells. Mice were treated 13 d after engraftment with an intravenous injection of Pb-212-daratumumab or control solution. Therapeutic efficacy was monitored by tumor volume measurements and overall survival. Results: Significant inhibition of proliferation of the human myeloma RPM18226 cell line was observed after 3 d of incubation with Pb-212-daratumumab, compared with Pb-212-isotypic control or cold anti-bodies. Biodistribution studies showed a specific tumoral accumulation of daratumumab. No toxicity was observed with Pb-212-daratumumab up to 370 kBq because of lack of cross-reactivity. Nevertheless, acute toxicity experiments with Pb-212-anti-mCD38 established a toxic activity of 277.5 kBq. To remain within realistically safe treatment activities for efficacy studies, mice were treated with 185 kBq or 277.5 kBq of 212Pb-daratumumab. Marked tumor growth inhibition compared with controls was observed, with a median survival of 55 d for 277.5 kBq of Pb-212-daratumumab instead of 11 d for phosphate-buffered saline. Conclusion: These results showed Pb-212-daratumumab to have efficacy in xenografted mice, with significant tumor regression and increased survival. This study highlights the potency of alpha-RIT in MM treatment.