General

Propranolol is a nonselective, beta-adrenergic receptor-blocking agent possessing no other autonomic nervous system activity. It specifically competes with beta-adrenergic receptor- stimulating agents for available receptor sites. When access to beta-receptor sites is blocked by propranolol, chronotropic, inotropic, and vasodilator responses to beta- adrenergic stimulation are decreased proportionately. At dosages greater than required for beta blockade, propranolol also exerts a quinidine-like or anesthetic-like membrane action,

which affects the cardiac action potential. The significance of the membrane action in the treatment of arrhythmias is uncertain.

Mechanism of Action

The mechanism of the antihypertensive effect of propranolol has not been established. Among factors that contribute to the antihypertensive action are: (1) decreased cardiac output, (2) inhibition of renin release by the kidneys, and (3) diminution of tonic sympathetic nerve outflow from vasomotor centers in the brain. Although total peripheral resistance may increase initially, it readjusts to or below the pretreatment level with chronic use of propranolol. Effects of propranolol on plasma volume appear to be minor and somewhat variable.

Pharmacokinetics And Drug Metabolism

Absorption

Propranolol is highly lipophilic and is almost completely absorbed after oral administration. However, it undergoes high first-pass metabolism by the liver and on average, only about 25% of propranolol reach the systemic circulation.

A single-dose, food-effect study in 36 healthy subjects showed that a high fat meal administered with InnoPran XL at 10 PM, increased the lag time from 3 to 5 hours and the time to reach the maximum concentration from 11.5 to 15.4 hours, under fed conditions, with no effect on the AUC. (See DOSAGE AND ADMINISTRATION).

Following multiple-dose administration of InnoPran XL at 10PM under fasting conditions, the steady state lag time was between 4-5 hours and propranolol peak plasma concentrations were reached approximately 12-14 hours after dosing. Propranolol trough levels were achieved 24-27 hours after dosing, and persisted for 3-5 hours after the next dose. The elimination half-life of propranolol was approximately 8 hours.

The plasma levels of propranolol showed dose proportional increases after single and multiple administration of 80, 120, and 160 mg of InnoPran XL.

At steady state, the bioavailability of 160-mg dose of InnoPran XL and propranolol hydrochloride long acting capsules did not differ significantly.

Distribution

Approximately 90% of circulating propranolol is bound to plasma proteins (albumin and alpha1 acid glycoprotein). The binding is enantiomer-selective. The S-isomer is preferentially bound to alpha1 glycoprotein and the R-isomer preferentially bound to albumin. The volume of distribution of propranolol is approximately 4 liters.

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 for CYP2C19 and a substrate for the intestinal efflux transporter, p-glycoprotein (p-gp). Studies suggest however that p-gp is not dose-limiting for intestinal absorption of propranolol in the usual therapeutic dose range.

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