Radiosensitizing potential of epigenetic anticancer drugs

Over the last few decades, epigenetic tumor changes characterized by promoter hypermethylation and histone modifications have become a topic of intense research. Of particular interest is the potential reversibility of these processes that has led to the development of epigenetic anticancer drugs such as demethylating agents and histone deacetylase inhibitors (HDAC-I). Besides single agent clinical activity in both hematological and solid malignancies, combinations of both types of epigenetic drugs with classic chemotherapeutics have shown promising results. In addition, as demethylating agents and HDAC-I act synergistically to reverse gene silencing, treatment schedules combining both epigenetic strategies could theoretically enhance tumor response. This assumption has been validated in vitro and in vivo for several hematological and solid cancer types, and awaits further clinical investigation. Nowadays, the majority of patients with cancer are treated with radiotherapy. To optimize the results obtained with this treatment modality, efforts are being put in strategies enhancing tumor response selectively in favor of normal tissue response. The combination of epigenetic drugs with radiotherapy is particularly valuable since a drug- and dose-dependent radiosensitizing potential of several classes of HDAC-I has been proven in vitro and in vivo. The molecular mechanisms underlying this radiosensitization have not been fully clarified yet. In general, higher concentrations of HDAC-I are believed to exert cell cycle redistribution, induction of apoptosis, and downregulation of surviving signals. The radiosensitizing effect of lower, non-toxic doses of HDAC-I has been attributed to, at least in part, acetylation-induced changes leading to altered double strand break (DSB) formation and repair. Although promising so far, further research is needed before HDAC-I administered alone or in combination with demethylating agents will be implemented in the clinic to act as radiosensitizers.