The proteasome inhibitor PS-341 markedly enhances sensitivity of multiple myeloma tumor cells to chemotherapeutic agents.

Increased nuclear factor kappaB (NF-kappaB) activity is associated with increased tumor cell survival in multiple myeloma. The function of NF-kappaB is inhibited through binding to its inhibitor, IkappaB. Release of activated NF-kappaB follows proteasome-mediated degradation of IkappaB resulting from phosphorylation of the inhibitor and, finally, conjugation with ubiquitin. We report that myeloma cells have enhanced IkappaBalpha phosphorylation and increased NF-kappaB activity compared with normal hematopoietic cells. The proteasome inhibitor PS-341 blocked nuclear translocation of NF-kappaB, blocked NF-kappaB DNA binding, and demonstrated consistent antitumor activity against chemoresistant and chemosensitive myeloma cells. The sensitivity of chemoresistant myeloma cells to chemotherapeutic agents was markedly increased (100,000-1,000,000-fold) when combined with a noncytotoxic dose of PS-341 without affecting normal hematopoietic cells. Similar effects were observed using a dominant negative super-repressor for IkappaBalpha. Thus, these results suggest that inhibition of NF-kappaB with PS-341 may overcome chemoresistance and allow doses of chemotherapeutic agents to be markedly reduced with antitumor effects without significant toxicity.