Pharmacokinetics of 3H-phenylephrine in man

7-3H-phenylephrine was given to 15 volunteers by a short-infusion n = 4) or p.o. (10 volunteers, 1 patient with porto-caval anastomosis). Analysis of serum for free 3H-phenylephrine and fractionation of urinary radioactivity was performed by ion-exchange and thin-layer chromatography. As almost the same 3H-activity was excreted in urine after i.v. and p.o. administration, 86% and 80% of the dose respectively, complete enteral absorption can be assumed. A considerable difference was seen in the fraction of free phenylephrine, i.v. 16% of the dose versus p.o. 2.6%, which suggested reduced bioavailability. This was confirmed by comparison of the areas under the serum curve, which showed a bioavailability factor of 0.38. The result for the patient with porto-caval anastomosis was comparable to that in the normal volunteers. The biological half-life of 2 to 3h was comparable to that of structurally related amines, as were the total clearance of 2 l/h, and the volume of distribution of 340 l. Metabolism to phenolic conjugates mainly after oral ingestion, and to m-hydroxymandelic acid after i.v. injection, again demonstrated that m-hydroxylated amines are predominantly conjugated during the "first-pass" metabolism.