Metabolism of carprofen, a nonsteroidal anti-inflammatory agent, in rats, dogs, and humans

The metabolic disposition of 14C-labeled carprofen [(±)-6-chloro-α-methylcarbazole-2-acetic acid] was investigated in rats, dogs, and humans. Carprofen is eliminated predominantly by biotransformation in these three species. In dogs and rats, the direct conjugation of carprofen to form an ester glucuronide and oxidation to the C-7 and the C-8 phenols followed by their conjugation represent the major metabolic pathways. Small amounts of the α-hydroxy derivative also are formed by these species and are excreted free in the urine. In dogs, biliary secretion predominates, and 70% of an intravenous dose of carprofen is excreted in the feces while 8–15% of the dose is excreted in the urine. In rats, fecal excretion due to biliary secretion varies from 60 to 75%, and urinary excretion accounts for 20–30% of an intravenous dose. In humans, direct conjugation of carprofen represents the only significant pathway of metabolism. Between 65 and 70% of the orally administered carprofen was found to be excreted as the ester glucuronide in the urine, and most of the remaining dose was estimated to be secreted as this metabolite in the bile. Due to enterohepatic circulation, only a fraction of the biliary metabolite was recovered in the feces in humans. Less than 5% of the dose was excreted in human urine as free, intact carprofen. In dogs and humans, plasma levels of carprofen and of total radioactivity exhibit a multiphasic decline. In the three human subjects studied, the terminal component declined with a 13–26-hr half-life; the terminal half-life was ∼40hr in dogs.