A phase I study of AVN944, an inosine monophosphate dehydrogenase (IMPDH) inhibitor, was carried out to assess its safety, tolerability, pharmacokinetics, and pharmacodynamics. Healthy male volunteers (N = 25) participated in this double-blind, randomized, placebo-controlled trial. Sixteen received oral doses ranging from 25 to 250 mg on 3 separate occasions at intervals of 3 or 6 days after overnight fasting. Six participants received two 100-mg doses, and 2 participants received 2 placebo doses, one with food and the other after fasting overnight. Clinical and laboratory parameters, including excretion of AVN944 and thiocyanate and IMPDH inhibition, were made at intervals for 48 hours postdosing. There were 13 mild and 2 moderate but no serious adverse events. One mild and 1 moderate event could be treatment related. AVN944 disappeared rapidly from plasma, but clearance decreased at doses > 50 mg. Food reduced absorption, with a geometric mean Cmax ratio of 33% and a geometric mean AUC0-infinity ratio of 44%. Urinary excretion was negligible. AVN944 doses > 100 mg showed definite IMPDH inhibition lasting at least 4 to 6 hours. AVN944, when administered orally to healthy volunteers, is well tolerated, absorbs better with fasting, and exhibits a pharmacodynamic profile that suggests potential for significant anticancer activity.