Targeting the Zinc-Dependent Histone Deacetylases (HDACs) for Drug Discovery

In humans, the zinc-dependent histone deacetylases (HDACs) are a family of 11 nonredundant isoforms that catalyze the dynamic reversal of posttranslationally modified acyl-lysine residues back to lysine. At the epigenetic level, HDACs have a critical gene silencing effect, promoting the compaction of histone tails with DNA to prevent transcription. In addition, HDACs deacylate many nonhistone substrates in diverse cellular compartments to profoundly influence protein structure and function. While the action of HDACs is indispensable to normal physiology, their abnormal overexpression is linked to the majority of human diseases. Consequently, the inhibition of HDACs has become a valuable target for therapeutic applications. Numerous potent small molecules are known, of both natural product and synthetic origin, that inhibit HDACs, primarily by reversibly interacting with the zinc cation within the enzyme active site. At the present time, five such HDAC inhibitors have received regulatory approval for the treatment of hematological cancers. This review focuses on the typical zinc-binding groups employed in HDAC inhibitors and the major advances within each class in terms of potency, isoform selectivity, and clinical applications

HDAC Inhibition and Graft Versus Host Disease

Histone deacetylase (HDAC) inhibitors are currently used clinically as anticancer drugs. Recent data have demonstrated that some of these drugs have potent antiinflammatory or immunomodulatory effects at noncytotoxic doses. The immunomodulatory effects have shown potential for therapeutic benefit after allogeneic bone marrow transplantation in several experimental models of graft versus host disease (GVHD). These effects, at least in part, result from the ability of HDAC inhibitors (HDACi) to suppress the function of host antigen presenting cells such as dendritic cells (DC). HDACi reduce the dendritic cell (DC) responses, in part, by enhancing the expression of indoleamine 2,3-dioxygenase (IDO) in a signal transducer and activator of transcription-3 (STAT-3) dependent manner. They also alter the function of other immune cells such as T regulatory cells and natural killer (NK) cells, which also play important roles in the biology of GVHD. Based on these observations, a clinical trial has been launched to evaluate the impact of HDAC inhibitors on clinical GVHD. The experimental, mechanistic studies along with the brief preliminary observations from the ongoing clinical trial are discussed in this review