Comparison of disposition and effect of timolol and propranolol on exercise tachycardia

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Abstract
Summary The kinetic disposition and beta-adrenergic blocking action in relation to plasma level of a single oral dose of either timolol or propranolol has been compared in healthy male volunteers. The disposition profiles clearly disclosed different properties of the two drugs, although their half-lives were similar. The available fraction of timolol in the systemic circulation was estimated to be approximately 60% of the dose, and 17.4% was exereted unchanged in urine. The logarithm of plasma concentration showed a significant correlation with the beta-blocking activity assessed by an exercise test. The mean potency ratios of timolol to propranolol as an antagonist of chronotropic effects on exercise tachycardia were 11 to 17 and 3.6 to 5.5 in dose- and concentration-effect relationships, respectively. The absolute reduction of exercise heart rate gave the best coefficient of all measures of beta-blockade. When drug action was measured as beta-blockade assessed by a given response to exercise tachycardia, the effect declined linearly with time, even though plasma levels fell exponentially. This results suggest that the pharmacokinetic t1/2 is much shorter than the pharmacological t1/2.