[4S-[4alpha,7alpha(R*),12bbeta]]-7[[2-(acetylthio)-1-oxo-3-phenylpropyl]amino]-1,2,3,4,6,7,8,12b-octahydro-6-oxo-pyrido[2,1-a][2]benzazepine-4-carboxylic acid (M100240) is an acetate thioester of MDL 100,173-a dual angiotensin-converting enzyme/neutral endopeptidase inhibitor currently in phase II development. The mass balance of [(14)C]M100240 was assessed following oral administration of [(14)C]M100240. Healthy male subjects were given a single 25-mg dose of [(14)C]M100240 (50 microCi) as an oral solution under fasting conditions. Blood samples and excreta were collected postdose. (14)C-radioactivity was measured by liquid scintillation counting. Plasma concentrations of M100240 and MDL 100,173 were determined by LC/MS/MS methods. Pharmacokinetic parameters were calculated. About 98% of the total radioactive dose was recovered within 7 days of oral administration, with most of the radioactivity recovered within 72 hours. Of the recovered radioactive dose, 49.4% and 48.5% were recovered in the urine and feces, respectively. Unchanged M100240 and MDL 100,173 were not detected in the excreta. On average, 76% of the total radioactivity in the blood was associated with the plasma fraction. M100240 accounted for less than 0.06% of the (14)C-radioactivity in plasma and MDL 100,173 accounted for 15.8% (AUC( infinity )) of (14)C-radioactivity in plasma after oral dosing. These data suggest that the drug was absorbed but rapidly converted to its metabolites either presystemically or postsystemically. Up to 78% of the total radioactivity was identified as MDL 100,173. The apparent terminal elimination half-life of MDL 100,173 was longer than that of (14)C-radioactivity, attributable to assay sensitivity and the saturable binding phenomenon commonly associated with angiotensin-converting enzyme inhibitors. M100240 undergoes extensive metabolism in humans, and its metabolites are excreted relatively equally in feces and urine.