Pharmacokinetics and first-pass metabolism of levomepromazine in the rat

The time course of the blood concentrations of levomepromazine and its two major non-polar metabolites in man were studied in the rat after single oral and intraarterial doses of levomepromazine hydrochloride. The blood levels of N-monodesmethyl levomepromazine were on average 179% of the levomepromazine levels after oral doses, but only 15% of the levomepromazine levels after intraarterial doses. The blood levels of levomepromazine sulfoxide, relative to the levomepromazine levels, were also generally higher after oral doses than after parenteral doses, on average 65% and 25%, respectively. Large interindividual variations were observed in the blood levels of levomepromazine and in the systemic availability of the drug after oral doses. The distribution phase lasted for about 8 hrs, the mean apparent volume of distribution was 16.6 l/kg, and the total body clearance was on average 12.3 ml/min. It is concluded that the rat provides a suitable model for the kinetics of levomepromazine in man, and that the sulfoxide and N-monodesmethyl metabolites of the drug are mainly formed by first-pass metabolism after oral doses in the rat, either in the liver or in the gut.