Multiple myeloma is a highly radiosensitive malignancy but, at the present time, radionuclide-based interventions have no proven place in disease management. Bone-seeking radionuclides such as 153-Sm-EDTMP and 166-Ho-DOTMP are promising agents for systemic delivery of ionizing radiation to sites of myeloma disease activity, but they are associated with significant myelosuppression at therapeutically effective doses and have therefore been used only in the context of myeloma stem cell transplantation protocols. We previously reported that the proteasome inhibitor PS-341 potently and selectively sensitizes myeloma cell lines and primary myeloma cells to the lethal effects of ionizing X-irradiation (

Goel et al,
Exp Hematol
.
33
,
784
,
2005
). To determine whether PS-341 is equally effective in sensitizing myeloma cells to ionizing beta-radiation and to extend our initial observations to an in vivo model, we combined PS-341 with the bone-seeking radionuclide 153-Sm-EDTMP. In vitro clonogenic assays were performed using a panel of myeloma cell lines and demonstrated synergistic killing following co-treatment with PS-341 and 153-Sm-EDTMP. Using the orthotopic, syngeneic 5TGM1 myeloma model, the median survivals of mice treated with saline, two doses of PS-341 (0.5 mg/kg), or a single non-myeloablative dose of 153-Sm-EDTMP (22.5 MBq) were 21, 22 and 28 days, respectively. In contrast, mice treated with combination therapy comprising two doses of PS-341 (0.5 mg/kg), one day prior to and one day following 153-Sm-EDTMP (22.5 MBq) showed a greatly prolonged median survival of 49 days. Correlative studies indicated that, compared to single-agent therapy, combination treatment with PS-341 and 153-Sm-EDTMP rapidly reduced the clonogenicity of bone-marrow resident 5TGM1 cells, slowed the elevation of serum myeloma-associated paraprotein levels, and was associated with longer term preservation of bone mineral density. The myelotoxicity of single agent and combination therapies was evaluated by comparing peripheral blood cell counts in each of the treatment groups, and by clonogenicity assays of hematopoietic progenitors isolated form normal mice receiving identical treatment regimens. Treatment with 153-Sm-EDTMP led to significant, but transient, myelosuppression which did not differ between animals treated with 153-Sm-EDTMP alone versus those treated with the combination of PS-341 plus 153-Sm-EDTMP. In summary, PS-341 is a potent in vivo radiosensitizer that greatly enhances the therapeutic potency, without increasing myelotoxicity, of skeletal targeted radiotherapy in the syngeneic, orthotopic 5TGM1 myeloma model. Based on these findings, we propose to conduct a phase I clinical trial at Mayo Clinic to evaluate the combination of PS-341 plus non-myeloablative skeletal targeted radiotherapy (using 153-Sm-EDTMP) in patients with advanced or treatment-refractory multiple myeloma.

Topics:
multiple myeloma, proteasome inhibitors, radioisotopes, myelosuppression, radiation therapy, cancer, combined modality therapy, hematopoietic stem cell transplantation, paraproteins, radiation-sensitizing agents

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2005, The American Society of Hematology
2005
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