Evaluation of Cytotoxic Activity of Epigenetic Drugs in Oesophageal Squamous Cell Carcinoma

Oesophageal squamous cell carcinoma (OESCC) is a poor prognosis cancer with current treatments demonstrating limited efficacy. Recently, the field of epigenetics has gained equal importance to that of genetics in cancer, including in oesophageal cancer. The increasing number of epigenetic drugs is driving the need to identify the most efficacious and potent of these drugs in various cancers. This study investigated both the metabolic activity and importantly, the long-term cytotoxic effects of different families of epigenetic drugs including DNA methyltransferase inhibitors (DNMTi: azacitidine, 5-aza-2ʹ-deoxycytidine, 5-fluoro-2ʹ-deoxycytidine, RG108, procaine hydrochloride, zebularine), histone deacetylase inhibitors (HDACi: valproic acid, panobinostat, vorinostat, belinostat, romidepsin, RGFP966 and santacruzamate A) and bromodomain and extra-terminal motif inhibitors (BETi: JQ1, OTX015, I-BET151A and I-BET762) in OESCC cells (KE-8 and KYSE-450) as well as non-neoplastic oesophageal epithelial cells (HET-1A). Effects on metabolic activity were measured using the MTT bio-reduction assay, while cytotoxic effects were assessed using the long-term clonogenic assay. Dose-dependent effects were observed with specific drugs from all three classes of epigenetic drugs. OESCC cells were most vulnerable to the FDA approved drugs (valproic acid, panobinostat, vorinostat, belinostat and romidepsin) in both the MTT bio-reduction and clonogenic assays. Interestingly, the non-neoplastic HET-1A epithelial cells demonstrated resistance to some DNMTi and HDACi drugs at concentrations that were cytotoxic to OESCC. Thus, our findings have identified a subset of epigenetic drugs that can impair the metabolic activity of OESCC and importantly, are cytotoxic. This may aid in the design of future therapeutic regimes for treatment of this drug resistant cancer.