Inosine monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme required for the de novo synthesis of guanine nucleotides from IMP. VX-944 (Vertex Pharmaceuticals, Cambridge, MA) is a small molecule, selective, uncompetitive novel inhibitor directed against human IMPDH enzyme. IMPDH inhibitors have been demonstrated to induce growth arrest, and extensively investigated as immunosuppressants. Here we show that VX-944 inhibits growth of human multiple myeloma (MM) cell lines, including those resistant to conventional agents, via induction of apoptosis and S phase arrest in vitro. Interleukin-6, insulin-like growth factor-1, or co-culture with bone marrow stromal cells (BMSCs), do not protect against VX-944-induced MM cell growth inhibition. We next delineated the molecular mechanism of VX-944-induced MM cell death in the MM.1S human MM cell line. VX-944 induced apoptosis in MM.1S cells, confirmed by PARP cleavage as well as flow cytometric detection of the mitochondrial membrane protein 7A6 and TdT-mediated dUTP nick-end labelling (TUNEL) positive cells, without significant cleavage of caspases 3, 8 and 9. While the pan-caspase inhibitor z-VAD-fmk did not inhibit the VX-944-induced apoptosis and cell death suggesting that VX-944 triggers apoptosis in MM1.S cells primarily via caspase-independent pathway. Importantly, VX-944 augments the cytotoxicity of doxorubicin, melphalan and bortezomib, all of which activate caspases in MM cells and induce apoptosis, even in the presence of BMSCs. Taken together, our data demonstrate non-caspase-dependent apoptotic pathway triggered by VX-944 thereby providing a rationale to enhance MM cell cytotoxicity by combining this agent with conventional and/or novel agents which trigger caspase activation. Our ongoing studies are delineating the mechanisms whereby VX-944 induces MM cell apoptosis.

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