HDAC inhibitors: clinical update and mechanism-based potential

Recently, the role of transcriptional repression through epigenetic modulation in carcinogenesis has been clinically validated with several inhibitors of histone deacetylases and DNA methyltransferases. It has long been recognized that epigenetic alterations of tumor suppressor genes was one of the contributing factors in carcinogenesis. Inhibitors of histone deacetylase (HDAC) de-repress genes that subsequently result in growth inhibition, differentiation and apoptosis of cancer cells. Vorinostat (SAHA), romidepsin (depsipeptide, FK-228), belinostat (PXD101) and LAQ824/LBH589 have demonstrated therapeutic benefit as monotherapy in cutaneous T-cell lymphoma (CTCL) and have also demonstrated some therapeutic benefit in other malignancies. The approval of the HDAC inhibitor vorinostat (Zolinzatrade mark) was based on the inherent sensitivity of this type of lymphoma to alterations in acetylation patterns that resulted in the induction of repressed apoptotic pathways. However, the full potential of these inhibitors (epigenetic modulators) is still on the horizon, as the true breadth of their utility as anti-cancer agents will be determined by the careful analysis of gene expression changes generated by these inhibitors and then combined with conventional chemotherapy to synergistically improve response and toxicity for an overall enhanced therapeutic benefit to the patient. The question that must be considered is whether the current HDACIs are being utilized to their fullest potential in clinical trials based on their mechanism-based alterations in disease processes.