Pharmacokinetics and pharmacodynamics of the enantiomers of gallopamil

The pharmacokinetics and pharmacodynamics of the enantiomers of the calcium antagonist gallopamil have been investigated in six healthy volunteers. Each subject was studied on five occasions after receiving, in randomized order: placebo, 25 mg of (R)-gallopamil, 25 mg of (S)-gallopamil, 50 mg of pseudoracemic [25 mg of deuterated (S)-gallopamil and 25 mg of (R)-gallopamil] and 100 mg of (R)-gallopamil HCl orally. After separate administration, the apparent oral clearances of both enantiomers were similar [(R), 15.1 +/- 9.9 liters/min; (S), 11.0 +/- 6.0 liters/min], indicating that gallopamil first-pass metabolism is not stereoselective. After coadministration, the apparent oral clearance of each enantiomers decreased [(R), 5.9 +/- 2.8 liters/min; (S), 5.8 +/- 2.66 liters/min], suggesting that a partial saturation of first-pass metabolism occurs because the dose was twice as high than for the single enantiomers. Serum protein binding and renal elimination of gallopamil are stereoselective, favoring (S)-gallopamil. Analysis of urine samples revealed a marked degree of stereoselectivity in the formation of O- and N-dealkyl metabolites. Because these showed opposite stereoselectivity, canceling out each other, the net result was no or only marginal stereoselectivity. Twenty-five milligrams of (S)-gallopamil prolonged the PR interval in all subjects; however, a greater effect was elicited by 50 mg of (RS)-gallopamil. (R)-Gallopamil (100 mg) did not significantly alter the PR interval, although higher concentrations were attained than after the pseudoracemate. Based on a consideration of (S)-gallopamil serum concentrations, a comparable relationship between (S)-gallopamil level and effect occurred after (S)- and (RS)-gallopamil, indicating that the pharmacological effect produced by the racemate could be totally accounted for by the higher concentrations of (S)-gallopamil attained.