Clinical Studies of Histone Deacetylase Inhibitors

Over the last 5 years, a plethora of histone deacetylase inhibitors (HDACi) have been evaluated in clinical trials. These drugs have in common the ability to hyperacetylate both histone and nonhistone targets, resulting in a variety of effects on cancer cells, their microenvironment, and immune responses. To date, responses with single agent HDACi have been predominantly observed in advanced hematologic malignancies including T-cell lymphoma, Hodgkin lymphoma, and myeloid malignancies. Therefore, in this review we focus upon hematologic malignancies. Generally HDACi are well tolerated with the most common acute toxicities being fatigue, gastrointestinal, and transient cytopenias. Of note, few patients have been treated for prolonged periods of time and little is known about long-term toxicities. The use of the biomarker of histone hyperacetylation has been useful as a guide to target specificity, but generally does not predict for response and the search for more clinically relevant biomarkers must continue.