Covalent DNMT Inhibitors

The potency of zebularine is about 10-fold lower than for the azacytosines [73]. Zebularine also inhibits cytidine deaminase [75] which is involved in nucleoside catabolism and deactivates also for example azacitidine and its desoxy analog [76]. Thus, it increases the concentrations of nucleoside triphosphates for incorporation into DNA, the efficacy of DNA methylation and ultimately the anticancer activity of for example azacitidine [77, 78[. Zebularine is metabolized by aldehyde oxidase and ithasbeen shovm that its activity can be increases if an inhibitor of that enzyme, for example raloxifene is given in combination [79]. One big question about all epigenetic drugs is the origin of the observed selectivity towards cancer cells. For zebularine, it has been shown that much less activation towards triphosphate metabolites that can be incorporated into DNA occurs in normal muscle tissue as compared to cancer tissue [80]. 

The covalent trapping of the enzyme leads to a depletion of the cellular pool of DNMTs and subsequent DNA hypomethylation. This in turn results in activation with respect to the reactivation of silenced genes. Additionally, the covalently trapped DNMT may inhibit RNA and DNA polymerases, which leads to an inhibition of protein biosynthesis and DNA strand breaks. This may lead to apoptosis and hence cytotoxicity. Thus, it is not easy to dissect the reasons for the clinical efficacy of these inhibitors in terms of real epigenetic and plain cytotoxic effects [81]. 

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Generally, DNMT inhibitors can be divided in two big dasses. One group consists of base analogs which are incorporated into DNA and act as suidde substrates for DNMT via a covalent adduct formation. The other group acts on the free enzyme in the same way as classic enzyme inhibitors do. 

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