Methylation of DNA is an epigenetic modification that plays an important role in the regulation of gene expression in mammalian cells. Although DNA methylation is required for normal cell development and function, aberrant methylation and the resulting aberrant expression of genes, such as tumor suppressor genes and oncogenes, contribute to the development of malignancies. Interestingly, aberrant DNA methylation has recently emerged as one of the most frequent molecular alterations in hematologic malignancies, providing a powerful rationale to use inhibitors of DNA methylation as a novel means of targeting hematologic malignancies. Vidaza™ [Pharmion Corporation] (5-azacytidine), an FDA approved drug for the treatment of myelodysplastic syndromes, is an inhibitor of DNA methylation. Multiple myeloma (MM) is currently an incurable hematological malignancy despite all the conventional and novel therapies, and we here examined the biological effects using Vidaza™ on human MM cells. We demonstrate here that Vidaza™ has significant cytotoxicity in both conventional therapy sensitive (MM1S, RPMI-8226) and resistant (MM1R, RPMI-Dox40, RPMI-LR5) MM cell lines, as well as freshly isolated patient MM tumor cells, with an IC50 of 1.25–4 mM at 72 hours in vitro. Importantly, no cytotoxic effects of Vidaza™ were detected in peripheral blood mononuclear cells (PBMNC) obtained from healthy volunteers at ≤20 mM, suggesting a therapeutic index. Moreover, Vidaza™ overcame the survival and growth advantages conferred by interleukin-6 (IL-6) and insulin-like growth factor-1 (IGF-1), or by adherence of MM cells to bone marrow stromal cells (BMSC). Vidaza™ induced apoptosis in MM cells, as determined by flow cytometric analysis using PI and Annexin V staining. Vidaza™-induced apoptosis was associated with PARP, caspase 8 and caspase 9 cleavage. Importantly, pan-caspase inhibitor zVAD-fmk, significantly, but only partially, inhibited apoptosis induced by Vidaza™, suggesting the involvement of both caspase dependent and independent pathways. Taken together, our studies therefore demonstrate that Vidaza™ induces apoptosis and overcomes in vitro drug resistance in MM cells. Ongoing studies are delineating the mechanism of action of Vidaza™ against MM cells to further provide the preclinical rationale for clinical evaluation of Vidaza™, alone or in combination with other agents, to improve patient outcome in MM.

Disclosures: Pharmion Corp.; Pharmion Corp.

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