HDAC inhibitors have shown activity in Phase I monotherapy trials for AML and advanced MDS. A Phase I trial of panobinostat as monotherapy in primarily AML yielded transient hematologic responses with reduction in peripheral blood blast counts in 8 of 11 patients consistent with the documented in vitro activity of the drug. Major toxicities included nausea, diarrhea, hypokalemia, anorexia, thrombocytopenia and reversible QTcF prolongation. Similar responses with rare complete responses have also been seen with the HDAC inhibitors romidepsin and MGCD0103 in AML and MDS.