MGCD0103 is an isotype-selective inhibitor of histone deacetylases (HDACs) targeted to isoforms 1, 2, 3, and 11. In a phase 1 study in patients with leukemia or myelodysplastic syndromes (MDS), MGCD0103 was administered orally 3 times weekly without interruption. Twenty-nine patients with a median age of 62 years (range, 32-84 years) were enrolled at planned dose levels (20, 40, and 80 mg/m²). The majority of patients (76%) had acute myelogenous leukemia (AML). In all, 24 (83%) of 29 patients had received 1 or more prior chemotherapies (range, 0-5), and 18 (62%) of 29 patients had abnormal cytogenetics. The maximum tolerated dose was determined to be 60 mg/m², with dose-limiting toxicities (DLTs) of fatigue, nausea, vomiting, and diarrhea observed at higher doses. Three patients achieved a complete bone marrow response (blasts ≤ 5%). Pharmacokinetic analyses indicated absorption of MGCD0103 within 1 hour and an elimination half-life in plasma of 9 (± 2) hours. Exposure to MGCD0103 was proportional to dose up to 60 mg/m². Analysis of peripheral white cells demonstrated induction of histone acetylation and dose-dependent inhibition of HDAC enzyme activity. In summary, MGCD0103 was safe and had antileukemia activity that was mechanism based in patients with advanced leukemia.