Pharmacokinetics of methotrimeprazine after single and multiple doses.

Concentrations of methotrimeprazine and a metabolite, methotrimeprazine sulfoxide, were measured in plasma after a single intramuscular dose and after single and multiple oral doses of methotrimeprazine. The highest plasma concentrations of methotrimeprazine were found 30 to 90 min after intramuscular injection, and 1 to 3 hr after oral administration. On average 50% of orally administered drug reached the general circulation as unchanged methotrimeprazine. The apparent volume of distribution (Vbeta) was 23 to 42 L/kg body weight, and the biologic half-life, 15 to 30 hr. The sulfoxide could not be traced in plasma after a 25-mg intramuscular dose, but was found in higher plasma concentrations than the unmetabolized drug after single and multiple oral doses. This could be due to oxidation of the drug either in the gastrointestinal lumen or in the intestinal wall, or during its first passage through the liver. The apparent half-life of the sulfoxide was on average 30% shorter than the half-life of methotrimeprazine.