Because of the pivotal role the proteasome plays in apoptosis, inhibitors of this enzyme, such as PS-341, provide a great opportunity for exploring synergy between proteasome inhibition and other apoptosis-inducing agents. Tumor necrosis factor—related apoptosis-inducing ligand (TRAIL) can selectively induce apoptosis in tumor cells. In overnight assays, combinations of PS-341 and TRAIL were much more effective than either agent alone in promoting apoptosis of a murine myeloid leukemia, C1498, and a murine renal cancer, Renca. For C1498 cells, apoptosis sensitization by PS-341 affected neither the activity of nuclear factor κB (NF-κB) nor the levels of most antiapoptotic proteins. However, reductions in the antiapoptotic protein c-FLIP in response to PS-341 were observed in both C1498 and Renca cells. Treatment of normal bone marrow mixed with C1498 tumor cells for 18 hours with a combination of PS-341 and TRAIL resulted in a specific depletion of the tumor cells. Upon transfer to irradiated syngeneic recipient mice, mixtures treated with the PS-341 plus TRAIL combination resulted in enhanced long-term tumor-free survival of mice. These data therefore support the targeting of apoptotic pathways in tumor cells, using combinations of agents such as PS-341 and TRAIL that interact synergistically to preferentially promote tumor cell apoptosis. (Blood. 2003;102:303-310)