Phenacetin concentrations in portal and hepatic venous blood in man

After intra-gastric or intra-duodenal administration of phenacetin (200 mg in solution), concentrations of phenacetin and its 0-dealkylated metabolite, acetaminophen, in portal and hepatic veins and in peripheral blood were monitored. A sharp increase in the portal concentration of phenacetin indicated rapid absorption. The concentration ratio of acetaminophen to phenacetin in early portal blood was 0.01--0.11 and lower than the ratio in hepatic vein. The hepatic extraction ratio of phenacetin was calculated to be 0.59--0.78. It is concluded that 0-dealkylation occurs mainly in the liver and only to a limited extent in the gastrointestinal wall.