Combined azacitidine and romidepsin enhances cytotoxicity in azacitidine-sensitive but not in azacitidine-resistant multiple myeloma cell lines

In hematopoietic tumor cells, aberrant epigenetic alterations of hypermethylation or histone deacetylation are usually observed. In multiple myeloma (MM), histone deacetylase inhibitors (HDIs) have proven anti-tumor activity, whereas the effects of DNA demethylating agents are obscure. In this study, we examined the effect of DNA demethylating agent azacitidine and HDI romidepsin in human MM cell lines RPMI8226 and U266. In RPMI8226 cells, azacitidine restored p16 expression accompanied by disruption of its main target molecules DNA methyltransferases (DNMTs), thereby showing anti- tumor effect. However, in U266 cells, azacitidine-mediated demethylation was abrogated, thereby losing its anti-myeloma effect. The combination of azacitidine and romidepsin enhanced induction of apoptosis by activation of the caspase pathway in RPMI8226 cells but not in U266 cells. Furthermore, isobologram analyses showed that this combination had an additive inhibitory effect on the growth of RPMI8226 cells, whereas in U266 cells it had a nearly subtractive effect. These results thus suggest that the combination is effective in azacitidine-sensitive but not in azacitidine-resistant MM cells. Taken together, the results support the utility of this combination as a potential therapy for MM; however, this therapy should be considered based on the sensitivity of the particular MM cells to azacitidine. (Keywords:multiple myeloma, azacitidine, romidepsin, DNA demethylating agent, histone deacetylase Abstract