HDACis inhibit the enzyme HDAC responsible for gene silencing through hypoacetylation of histones. Histone deacetylation increases the electrostatic attraction between the positive charges of the histones and negative charges of the DNA, which ensures tight binding and renders promoter regions inaccessible to polymerases for gene transcription. Cancer is linked to histone SNS-032 cell line hypoacetylation, due to overexpression of HDACs, and the anticancer effects of HDACis have been attributed to the restoration of the histone acetylation balance [20]. However, the developing story is more complex, involving at least six human HDAC enzymes, a broad spectrum of protein classes, multiple

mechanisms that include induction of reactive oxygen species (ROS), and pleiotropic biologic effects for which the putative target is unknown or uncertain [21]. Acetylation has broad regulatory functions on histones and nonhistone proteins. Substrates of nonhistone acetylation are multiple and include important cellular factors involved in cellular homeostasis such as p53, nuclear factor κB, and hypoxia-inducible factor 1α [22] that overlap with the DNA methylation inhibitors described

above and RRx-001 described below. In particular, the effect on p53 is highlighted for this review: The p53 tumor suppressor protein and glycolytic regulator are activated directly through deacetylation [23] and indirectly through ROS-induced DNA damage [24]. The HDACi, vorinostat, PF 01367338 has been approved by Acyl CoA dehydrogenase the FDA for the treatment of cutaneous T-cell lymphoma, whereas entinostat has received breakthrough therapy status

in estrogen receptor (ER)-positive breast cancer. However, their evaluation as combination chemotherapy in clinical trials in different tumor types hints obliquely at a resensitization potential [1]. Two of the trials in non–small lung cancer were not promising and the lack of activity may be related to dosing considerations; however in a phase II breast cancer trial of the aromatase inhibitor exemestane with the HDACi entinostat versus exemestane alone, the combination significantly improved overall survival by 8.3 months (P = .04), warranting additional testing to determine whether the improvement was due both to increased susceptibility of the tumor to the aromatase inhibitor and resensitization to subsequent therapies. Romidepsin, a unique HDAC prodrug, which is converted intracellularly to a reduced form that binds to and inhibits class 1 HDACs, was approved for the treatment of cutaneous T-cell lymphoma on the basis of studies that demonstrated an objective RR rather than overall survival, which unfortunately does not allow for an assessment of its resensitization potential. These epigenetic therapies are representative of the current paradigm for rational drug discovery, which emphasizes structures that specifically target and antagonize the chromatin-modifying enzymes.