A novel non-peptide and orally active proteasome inhibitor overcomes bortezomib-resistance in multiple myeloma (MM) cells

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Bortezomib/PS-341 is a novel proteasome inhibitor for the treatment of relapsed/refractory multiple myeloma (MM) patients; however, some patients have intrinsic and others acquire resistance. Here we demonstrate that a novel non-peptide proteasome inhibitor NPI-0052 triggers apoptosis even in bortezomib-resistant patient MM cells, without altering the viability of normal cells. NPI-0052 overcomes MM cell-resistance to conventional therapies including melphalan (LR5), doxorubicin (Dox-40), and dexamethasone (MM.1R, U266, RPMI-8226) Moreover, NPI-0052 retains its anti-tumor activity in cancer cells overexpressing P-glycoprotein and altered topoisomerase-II activity. Importantly, NPI-0052 blocks MM cell growth triggered by adhesion to bone marrow stromal cells (BMSCs) and related cytokine secretion. Moreover, NPI-0052 induces MM cells apoptosis even in the presence of interleukin-6 or insulin-like growth factor-1, two major survival/anti-apoptotic factors for MM cells. NPI-0052 also overcomes the cytoprotective effects of anti-apoptotic proteins Bcl2 and Hsp27 in MM cells. NPI-0052-triggered MM cell death is associated with these events: 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. 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. Finally, combining NPI-0052 and bortezomib triggers synergistic apoptosis (Combination Index < 1.0). Maximal anti-MM activity of NPI-0052 was observed when given concomitantly, rather than other treatment schedules. Low doses of combined NPI-0052 and bortezomib does not significantly affect viability of normal lymphocytes. Studies using a human plasmacytoma xenograft mouse model show that NPI-0052 is orally active and has significant anti-MM activity at doses that are well tolerated, inhibit MM cell growth, and prolong survival. Together, these findings provide the rationale for clinical protocols of NPI-0052, either alone or in combination with bortezomib, to inhibit tumor cell growth, reduce bortezomib-related toxicity, overcome bortezomib-resistance in MM.