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Pharmacology: Pharmacodynamics: The substance has no β-1 receptor selectivity (cardioselectivity) and has no intrinsic sympathomimetic activity (ISA). Propranolol is a strongly lipophilic substance and has a membrane stabilising effect.
Pharmacokinetics: Absorption: Propranolol is highly lipophilic and almost completely absorbed after oral administration. However it undergoes high first-pass metabolism by the liver and on average, only about 25% of propranolol reaches the systemic circulation. Peak plasma concentrations occur about 1 to 4 hours after an oral dose.
Administration of protein-rich foods increase the bioavailability of propranolol by about 50% with no change in time to peak concentration, plasma binding, half-life, or the amount of unchanged drug in the urine.
Distribution: Approximately 90% of circulating propranolol is bound to plasma proteins (albumin and alpha 1 acid glycoprotein). The binding is enantiomer-selective. The S(-)-enantiomer is preferentially bound to alpha 1 glycoprotein and the R(+)-enantiomer preferentially bound to albumin. The volume of distribution of propranolol is approximately 4 liters/kg. Propranolol crosses the blood-brain barrier and the placenta, and is distributed into breast milk.
Metabolism and Elimination: Propranolol is extensively metabolized with most metabolites appearing in the urine. Propranolol is metabolized through three primary routes: aromatic hydroxylation (mainly 4-hydroxylation), N-dealkylation followed by further side-chain oxidation, and direct glucuronidation. It has been estimated that the percentage contributions of these routes to total metabolism are 42%, 41% and 17%, respectively, but with considerable variability between individuals. The four major metabolites are propranolol glucuronide, naphthyloxylactic acid and glucuronic acid, and sulfate conjugates of 4-hydroxy propranolol. In vitro studies have indicated that the aromatic hydroxylation of propranolol is catalyzed mainly by polymorphic CYP2D6. Side-chain oxidation is mediated mainly by CYP1A2 and to some extent by CYP2D6.
4-hydroxy propranolol is a weak inhibitor of CYP2D6. Propranolol is also a substrate of CYP2C19 and a substrate for the intestinal efflux transporter, p-glycoprotein (p-gp). Studies suggest however that p-gpis not dose-limiting for intestinal absorption of propranolol in the usual therapeutic dose range. In healthy subjects, no difference was observed between CYP2D6 extensive metabolizers (EMs) and poor metabolizers (PMs) with respect to oral clearance or elimination half-life. Partial clearance of 4-hydroxy propranolol was significantly higher and of naphthyloxylactic acid significantly lower in Ems than PMs. The plasma half-life of propranolol is from 3 to 6 hours.
