Abstract
The recent success of bortezomib/proteasome inhibitor PS-341 (VelcadeTm) provides evidence that proteasome inhibitors can be safely utilized clinically. Specifically, bortezomib is currently being used to treat MM patients (pts) who have relapsed after initial therapy and refractory to current therapy and is under evaluation for treatment of newly diagnosed pts. Although bortezomib kills MM cells, prolonged exposure is associated with attendant toxicity and development of bortezomib-resistance. Here we show that a novel non-peptide proteasome inhibitor NPI-0052 triggers apoptosis even in bortezomib-resistant patient MM cells, without affecting the viability of normal cells. NPI-0052 overcomes MM cell-resistance to conventional therapies including melphalan (LR5), doxorubicin (Dox-40), dexamethasone (MM.1R, U266, RPMI-8226) and retains its anti-tumor activity in cells harboring two major mechanisms of drug-resistance: overexpression of P-glycoprotein and altered topoisomerase-II activity. Importantly, NPI-0052, like bortezomib, blocks MM cell growth induced by both adhesion to bone marrow stromal cells (BMSCs) and related cytokine secretion, thereby depriving MM cells of the growth and survival advantage conferred by the BM microenvironment. Moreover, NPI-0052-triggers MM cells apoptosis despite the presence of interleukin-6 or insulin-like growth factor-1 (IGF-1), two major growth and anti-apoptotic factors for MM cells. NPI-0052 also overcomes the cytoprotective effects of anti-apoptotic proteins Bcl2 and Hsp27, as well as NF-kB-mediated growth/survival, and drug-resistance in MM cells. Initial in vivo studies in animal models using either oral or intravenous administration have established the maximum tolerated dose (MTD) based on the whole blood proteasome activity. NPI-0052 decreased proteasome activity in packed whole blood lysates in a dose dependent fashion, and these data also suggest that NPI-0052 is orally active. The mechanisms whereby NPI-0052-trigger MM cell death includes: loss of mitochondrial membrane potential; Superoxide generation; release of mitochondrial proteins cytochrome-c/Smac; and activation of caspase-8/9/3. Conversely, the pan caspase inhibitor Z-VAD-fmk abrogates NPI-0052-induced apoptosis. Finally, combining NPI-0052 and bortezomib triggers synergistic apoptosis. Together, these findings provide the framework for clinical evaluation of NPI-0052, either alone or in combination with bortezomib, to inhibit tumor cell growth, reduce bortezomib-related toxicity, overcome bortezomib-resistance, and improve patient outcome in MM.
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