[Enteral resorption and bioavailability of norfenefrine]

Four volunteers received orally an aqueous solution of 4.5 mg of 14C norfenefrine. Urine was collected for the following 24 h and assayed for unchanged drug and metabolites. Blood was drawn for the determination of free norfenefrine at frequent intervals over an 8-h period. The highest serum levels of free norfenefrine were seen 30-60 min after ingestion with an average of 2 ng/ml. The half-lives ranged from 163-402 min. The cumulative urinary excretion of 14C radioactivity exceeded 90% of the dose, more than 50% being conjugated norfenefrine. As result of the area under the plasma level curve bioavailability has to be set at 20% when compared to other sympathomimetic amines. The result in our patient with porto-caval anastomosis did not differ from normal. Thus, the significance of the gut wall as the site of conjugation during the first-pass is demonstrated.