Hydroxycarbamide in combination with azacitidine or decitabine is antagonistic on DNA methylation inhibition

Azacitidine and decitabine are cytidine analogues that inhibit DNA methylation, and are used to treat myeloid haematological malignancies. Hydroxycarbamide (HC) (also known as hydroxyurea), a ribonucleotide reductase (RR) inhibitor, blocks the conversion of ribonucleotides to deoxyribonucleotides, and is also used to treat leukaemia and sickle-cell disease. Azacitidine is a ribonucleoside and decitabine is a deoxyribonucleoside; therefore, we hypothesized that inhibition of RR by HC would be antagonistic to azacitidine and synergistic to decitabine. HL-60 and T24 cancer cell lines were treated with azacitidine or decitabine in combination with HC and DNA methylation of LRE1, MAGEA1 and CDKN2A was quantitatively measured by bisulphite-polymerase chain reaction pyrosequencing. Surprisingly, we found that HC blocked the ability of both azacitidine and decitabine to inhibit DNA methylation and this antagonistic effect was attributable to the arrest of the cell cycle induced by HC. However, this antagonism could be avoided with sequential treatment of HC followed by azacitidine or decitabine. This data suggest that concurrent combination of HC blocks the ability of azacitidine and decitabine to inhibit DNA methylation and therefore these drugs should be used sequentially.