Sublethal doses of ionizing radiation induce a complex cellular survival response, one component of which is activation of the nuclear factor kappa B (NF-kB) pathway. PS-341 is a selective inhibitor of the 26S proteasome which prevents the proteasomal degradation of I-kB and thereby inhibits the activation of NF-kB. We therefore hypothesized that PS-341 should synergistically enhance the anti-myeloma effect of ionizing radiation (IR). The individual and combined effects of PS-341 and IR were analyzed using human MM cell lines (MM1, RPMI 8226, JJN-3 and ARH-77) and bone marrow samples from myeloma patients.

Treatment of myeloma cell lines with 10nM PS-341 produced a modest anti-proliferative effect with G2/M-phase arrest and p21cip/waf-1 induction but minimal procaspase-3 cleavage and minimal apoptotic cell death: Similar sublethal damage was observed after single exposure to IR at a dose of 6 Gy. In contrast, sequential exposure to PS-341 (10 nM) and IR (6 Gy) resulted in potent (synergistic) induction of apoptosis, assayed by both fluorescein isothiocyanate conjugated annexin V-propidium iodide assay and cell cycle analysis. Overall cell viability was greatly reduced by the combination of PS3451 and IR, but not by either modality alone, as determined by 3-(4,5 dimethylthiazol-2yl) 2–5 diphenyltetrazolium bromide (MTT) assay and analysis of polyadenosine-5′-diphosphate-ribose polymerase (PARP) cleavage -product after 72 hours. As predicted, exposure to 6 Gy IR led to potent induction of nuclear NF-kB activity and this response was almost completely inhibited by simultaneous treatment with PS-341. The combination of PS341 and IR was next evaluated on CD138+ve and CD138-ve cell fractions from bone marrow aspirates of myeloma patients. Synergistic killing of primary myeloma (CD138+) cells was observed with relative sparing of normal marrow elements (CD138−). We conclude that PS-341 is a potent radiosensitizer for myeloma cell lines and primary myeloma cells. This provides a strong rationale to combine PS-341 with conventional external beam or with targeted radionuclide therapy for effective treatment of MM patients.

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