Recent Developments in Beta Adrenergic Blocking Drugs

Mutagenic activity of a fluorinated analog of the beta-adrenoceptor ligand carazolol in the Ames test

S-1'[(18)F]-Fluorocarazolol (FCAR) is a fluorinated analog of the nonmutagenic beta-blocker carazolol (CAR). In former studies FCAR proved to be suitable for quantification of beta-adrenoceptors in vivo with positron emission tomography (PET). We report here that FCAR displays no acute toxicity in either rats or mice. However, FCAR induces a strong dose-related increase in the number of revertants in the Ames test. We conclude that FCAR yields mutagenic activity as measured by the Ames test.

Polymorphic metabolism of the beta-adrenoreceptor blocking drugs and its clinical relevance

Although beta-blockers are structurally closely related, there are marked differences in the extent of metabolism, related mainly to relative lipophilicity. Lipophilic beta-blockers are metabolized by C-oxidative pathways and glucuronidation. Metabolism of lipophilic beta-blockers is important in determining pharmacokinetics, formation of active metabolites, stereoselectivity and isomer preference, and interphenotypic variation. The oxidative clearance of metoprolol, timolol and bufuralol is regulated/influenced by the debrisoquine hydroxylation gene locus. The metabolism of these lipophilic beta-blockers thus exhibits polymorphic characteristics, there being significant interphenotype differences in pharmacokinetics (bioavailability, peak plasma level, plasma terminal t1/2) between the poor and extensive metabolizers of debrisoquine. There are similar interphenotype differences in beta-blocker pharmacodynamics in terms of beta-blockade. A number of adverse effects of lipophilic beta-blockers have been hypothesized to predominate in the poor metabolizer phenotype including unacceptable bradycardia, loss of cardioselectivity, greater CNS side-effects, and interactions with drugs metabolized by the same polymorphic systems. However, objective evidence for this is lacking.

Absorption of the beta-adrenergic-blocking agent, nadolol, by mice, rats, hamsters, rabbits, dogs, monkeys, and man: an unusual species difference

1. Dogs absorbed [14C]nadolol almost completely (88--104%) after oral administration of 10--75 mg/kg. In contrast, mice, rats, hamsters, and rabbits, given oral doses of 20 mg/kg, absorbed, at most, 25%. After administration of oral doses of 2--75 mg/kg, monkeys absorbed no more than an average of 30%. Six monkeys given oral doses of 10 mg/kg absorbed 12--44%. 2. In rats, [14C]nadolol was orally absorbed to an average of 13.7--17.5%, whether given in suspension or in solution, indicating that absorption was not limited by rate of dissolution nor by solubility. 3. Mildly hypertensive humans given single doses of [14C]nadolol (2 or 80 mg, orally) and normal subjects given single doses (10 mg, orally) absorbed an average of 20.4--33.3%.

The metabolism of tolamolol in the mouse, rat, guinea-pig, rabbit and dog

1. [3H, 14C]Tolamolol was well absorbed after oral administration to mice, rats, guinea-pigs, rabbits and dogs. 2. The major route for excretion of radioactivity by mice, rats and guinea-pigs was the faeces; in rabbits the major route was the urine. Dogs excreted similar amounts of radioactivity by both routes. Biliary excretion of radioactivity by the rat and guinea-pig was demonstrated. 3. Tolamolol was extensively metabolized by all five species. The major metabolite in mice, rats, guinea-pigs and rabbits was the product of hydroxylation of the tolyl ring, which was excreted as such as the glucuronide and sulphate conjugates. 4. In the dog the major metabolite was the acid resulting from hydrolysis of the carbamoyl group. This acid was also excreted by the rabbit, but was only a minor metabolite in the other species studied.