Histone deacetylase inhibitors in cancer therapy

Histone deacetylase inhibitors (HDAC inhibitors) represent a novel class of antineoplastic agents that act by promoting acetylation of histones, leading in turn to uncoiling of chromatin and activation of a variety of genes implicated in the regulation of cell surivival, proliferation, differentiation, and apoptosis. The major classes of HDIs include shortchain fatty acids, hydroxamic acid derivatives, synthetic benzamide derivatives, and cyclic tetrapeptides. Members of each of these classes have now entered clinical trials in humans. Despite their shared capacity to trigger histone deacetylation, individual HDIs exert diverse actions on cell cycle regulatory, signal transduction, and survival-related proteins which in all probability accounts for their disparate actions. Major areas of investigation surrounding HDIs include elucidating the mechanisms by which they induce apoptosis in neoplastic cells, and characterizing the factors responsible for the decision of such cells to undergo maturation versus cell death in the response to these agents. In this context, attention has recently focused on the ability of HDIs to induce perturbations in cell cycle regulatory proteins (e.g., p21(CIP1)), downregulation of survival signaling pathways (e.g., Raf/MEK/ERK), and disruption of cellular redox state (e.g., induction of reactive oxygen species; ROS). Aside from efforts to combine HDIs with established cytotoxic drugs, attempts are underway to establish a rational basis for combining HDIs with differentiation- inducing agents (e.g., ATRA, hypomethylating agents such as 5'-deoxyazacytine) with the goal of triggering re-expression of turn or suppressor and/or differentiation-associated genes. Finally, the results of recent preclinical studies provide a strong rationale for combining HDIs with other novel, molecularly targeted agents, including inhibitors of survival signaling pathways or cell cycle progression. Collectively, these findings should provide a fertile environment for the development of novel HDI-containing regimens in the treatment of cancer for many years to come.