Synergistic activity of the proteasome inhibitor PS-341 with non-myeloablative 153-Sm-EDTMP skeletally targeted radiotherapy in an orthotopic model of multiple myeloma.

Multiple myeloma is a highly radiosensitive skeletal malignancy, but bone-seeking radionuclides have not yet found their place in disease management. We previously reported that the proteasome inhibitor PS-341 selectively sensitizes myeloma cells to the lethal effects of ionizing radiation. To extend these observations to an in vivo model, we combined PS-341 with the bone-seeking radionuclide 153-Sm-EDTMP. In vitro clonogenic assays demonstrated synergistic killing of myeloma cells exposed to both PS-341 and 153-Sm-EDTMP. Using the orthotopic, syngeneic 5TGM1 myeloma model, the median survivals of mice treated with saline, 2 doses of PS-341 (0.5 mg/kg), or a single nonmyeloablative dose of 153-Sm-EDTMP (22.5 MBq) were 21, 22, and 28 days, respectively. In contrast, mice treated with combination therapy comprising 2 doses of PS-341 (0.5 mg/kg), 1 day prior to and 1 day following 153-Sm-EDTMP (22.5 MBq) showed a significantly prolonged median survival of 49 days (P < .001). In addition to prolonged survival, this treatment combination yielded reduced clonogenicity of bone marrow-resident 5TGM1 cells, reduced serum myeloma-associated paraprotein levels, and better preservation of bone mineral density. Myelosuppression, determined by peripheral blood cell counts and clonogenicity assays of hematopoietic progenitors, did not differ between animals treated with 153-Sm-EDTMP alone versus those treated with the combination of PS-341 plus 153-Sm-EDTMP. PS-341 is a potent, selective in vivo radiosensitizer that may substantially affect the efficacy of skeletal-targeted radiotherapy in multiple myeloma.