DNA methyltransferase inhibitors represent promising new drugs for
cancer therapies. The first of these compounds (5-azacytidine, Vidaza) has recently been approved as an
antitumor agent, and others are presently in various stages of their preclinical or clinical development. Most of the archetypal inhibitors have been established and characterized in different experimental systems, which has thus far precluded their direct comparison. We have now established defined experimental conditions that allowed a comparative analysis of the six most widely known
DNA methyltransferase inhibitors:
5-azacytidine (5-aza-CR),
5-aza-2'-deoxycytidine (5-aza-CdR),
zebularine,
procaine, (-)-epigallocatechin-3-gallate (EGCG), and
RG108. Of these, 5-aza-CR, 5-aza-CdR,
zebularine, and EGCG were found to exhibit significant cytotoxicity in human
cancer cell lines. 5-aza-CdR and EGCG were also found to be genotoxic, as evidenced by the induction of micronuclei. In addition, 5-aza-CR, 5-aza-CdR,
zebularine, and
RG108 caused concentration-dependent demethylation of genomic
DNA, whereas
procaine and EGCG failed to induce significant effects. Finally, the experiments in
cancer cell lines were complemented by a cell-free in vitro assay with purified
recombinant DNA methyltransferase, which indicated that
RG108 is the only
drug capable of direct
enzyme inhibition. These results show a substantial diversity in the molecular activities of
DNA methyltransferase inhibitors and provide valuable insights into the developmental potential of individual drugs.