Excretion and biotransformation of ketanserin after oral and intravenous administration in rats and dogs

Abstract

The excretion and biotransformation of ketanserin, a novel serotonin S2-receptor blocking agent used in hypertension and related diseases, were studied after single po (1 or 10 mg/kg) and iv (2.5 mg/kg) administration in rats and dogs, using two different radiolabels. Orally administered ketanserin was well absorbed and almost completely metabolized in both species. The excretion of the metabolites was rapid and amounted to about 90% within 4 days. In the various groups of rats and dogs, excretion of the radioactivity with the feces (48 to 67%) exceeded that with urine (27 to 43%). In po dosed bile-cannulated rats, 57% was excreted with the bile within 24 hr, whereas about 30 to 40% of the biliary radioactivity was subjected to enterohepatic circulation. The major urinary, biliary, and fecal metabolites were characterized by HPLC and mass spectrometric analysis. The main metabolic pathways of ketanserin were the aromatic hydroxylation at the quinazolinedione moiety, the oxidative N-dealkylation at the piperidine nitrogen, reduction of the ketone function and piperidine oxidation, followed by ring scission. The mass balance of the metabolites, as estimated by reversed-phase HPLC with on-line radioactivity detection, was very similar between male and female rats, as well as between rats po dosed at 10 mg/kg and iv dosed at 2.5 mg/kg. In rats, major urinary metabolites resulted from the N-dealkylation pathway, whereas hydroxyketanserin was the main biliary and fecal metabolite. In dogs, the N-dealkylation pathway was less abundant, whereas higher doses resulted in smaller relative amounts of hydroxylated metabolites and larger relative amounts of ketanserin-ol, resulting from the ketone reduction. Dog plasma levels of ketanserin-ol exceeded those of the parent drug from about 5 hr after po dosing.