Histone deacetylase inhibitors: design, structure-activity relationships and therapeutic implications for cancer

Histone deacetylases (HDACs) remove acetyl groups from the tails of lysine residues of histone protein in nuclear chromatin and also from acetylated sites in non-histone proteins. HDACs and histone acetyltransferases (HATs) are major influences on the level of cellular protein acetylation, and an imbalance in acetylation levels, particularly under-acetylated (hypoacetylated) histone protein has been associated with precancerous or malignant states. Consequently, small molecule inhibitors of HDACs have been synthesised and some now form a newly emerging class of anti-cancer agents that can regulate transcription and inhibit proliferation of cancer cells by inducing cell cycle arrest, differentiation and/or apoptosis, among other major biological phenomena. The different mechanism(s) of action of HDAC inhibitors compared to conventional anti-neoplastic agents provides a possibility that HDAC inhibitors may be effective for refractory cancers. Accordingly, a number of programs for the development of HDAC inhibitors as anti-cancer drugs have been initiated. This review highlights recent developments in the design, synthesis and biological properties of HDAC inhibitors in the context of potential cancer therapy.