A SYSTEMATIC ASSESSMENT OF RADIATION DOSE ENHANCEMENT BY 5-AZA-2′-DEOXYCYTIDINE AND HISTONE DEACETYLASE INHIBITORS IN HEAD-AND-NECK SQUAMOUS CELL CARCINOMA

Purpose: Investigations of epigenetic drugs have shown that radiotherapy can be successfully combined with histone deacetylase inhibitors (HDAC-Is) for the treatment of head-and-neck squamous cell carcinoma (HNSCC). Whether the reversal of epigenetic silencing by demethylating agents with or without HDAC-Is can also act as radiosensitizing remains unclear. This study therefore aimed to investigate whether 5-aza-2'-deoxycytidine (DAC) alone or in combination with the HDAC-Is trichostatin A, LBH589, or MGCD0103 could radiosensitize HNSCC tumor cell lines. Methods and Materials: Histone acetylation status and expression of epigenetically silenced genes at the DNA, RNA, and protein levels were assessed as measures of drug effectiveness in six HNSCC cell lines. Based on their colony-forming capacity, colony assays were performed in four of six cell lines to evaluate the radiosensitizing potential of DAC with or without HDAC-Is. Additional assays of cell survival, apoptosis, cell proliferation, and DNA damage were performed. Results: Radiosensitization was observed in two HNSCC cell lines treated with noncytotoxic doses of DAC with or without HDAC-Is before irradiation. The radiosensitizing doses induced histone hyperacetylation and reversal of gene silencing to variable extents and increased radiation-induced cell-cycle arrest. Conclusions: A role for low-dose DAC with or without HDAC-Is as radiosensitizers in HNSCC seems promising and is supportive of future clinical use, especially for combinations of DAC with LBH589 or MGCD0103, although the mechanisms by which they work will require further study. (C) 2009 Elsevier Inc.