Synergistic antitumor effects of a selective proteasome inhibitor and TNF in mice

The ubiquitin-proteasome pathway is becoming an attractive target in cancer therapy. The inhibitors of proteasomes have recently been shown to induce apoptosis of tumor cells in vitro and to exert significant antitumor effects in murine tumor models in vivo. Proteasome inhibitors, also prevent NF-κB activation. Since this transcription factor is responsible for counteracting apoptosis induced by numerous agents, and proteasome inhibitors have already proved efficacious in increasing the proapoptotic activity of TNF in vitro, we decided to evaluate the antitumor effects of the combined PSI and TNF treatment against a murine C-26 carcinoma. Both agents separately exerted moderate antitumor efficacy. However, their combination proved to exert dramatic antitumor activity with retardation of tumor growth and prolongation of mice survival time. Moreover, 50% of the mice were completely cured by this drug combination. Unexpectedly, there was no potentiation of the cytostatic/cytotoxic effects of these drugs in in vitro assays which argues against the direct influence on C-26 cells. Similarly, the influence of these drugs on tumor induced angiogenesis does not seem to explain the observed antitumor effects. Further studies are necessary to explain the striking antitumor effects of the PSI and TNF combination.