Propranolol concentrations in the blood have been measured after oral and i.v. administration of several doses to 6 subjects. After oral administration of doses less than 30 mg, only trace amounts of the drugs were detected in the systemic circulation. With oral doses exceeding 40 mg, the area under the blood concentration/time curve was linearly related to dose. These data suggest that at the low portal venous concentrations occurring after i.v. administration or following small oral doses, the hepatic extraction of propranolol is essentially complete. With the increasing portal venous concentrations that follow oral doses greater than about 30 mg, extraction falls to about 72% and, thereafter, was independent of the dose over the range examined. These findings illustrate a novel form of dose dependent kinetics of elimination which, in subjects receiving doses giving fully effective blood levels, results in a difference in propranolol disposition according to the route of administration. In addition, the presence of the apparent threshold for orally administered drug and the fact that propranolol probably reduces liver blood flow result in a lower estimate of the bioavailability of the drug than that obtained previously.

Keywords:
Propranolol disposition, Bioavailability in man, Elimination during absorption
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© 1972 S. Karger AG, Basel
1972
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