Absorption and disposition of epithiosteroids in rats (1): Route of administration and plasma levels of epitiostanol.

1. Absorption and disposition of epitiostanol (EP) were determined following intramuscular (i.m.), intravenous (i.v.) and oral administration of 14C-epitiostanol (14C-EP) to rats. 2. When 14C-EP was administered orally, radioactivity was absorbed from the gastrointestinal tract and appeared in the systemic circulation, but no unchanged EP was detected in the plasma. EP was extensively metabolized by first-pass metabolism in the intestinal mucosa and liver, and because of the low biological activity when given orally, EP should be given by injection. 3. The EP plasma concentration/time curve following i.m. administration is analysed by a flip-flop model (ka less than kel), because the ka for EP remaining at the i.m. injection site was less than the kel for i.v. injection.