The Pharmacokinetics and Pharmacodynamics of d- and dl-Verapamil in Rabbits

Abstract

The pharmacokinetics and pharmacodynamics of d- and dl-verapamil were studied in conscious rabbits in randomized cross-over fashion. Following a single intravenous dose, there was a biexponential decline in plasma concentration with time. No differences were observed in the pharmacokinetic properties of the compounds. The mean (+/- SD) clearances of d- and dl-verapamil were 0.13 +/- 0.03 and 0.12 +/- 0.05 L/min/kg, respectively. The mean (+/- SD) steady-state volume of distribution was 9.7 +/- 5.2 L/kg for d-verapamil and 8.1 +/- 4.1 L/kg for dl-verapamil. No difference was observed between the compounds in their binding to plasma proteins. The mean (+/- SD) half-life in plasma was 98.7 +/- 63.8 min for d-verapamil and 96.3 +/- 38.0 min for dl-verapamil. In contrast to the lack of stereoselective differences in the pharmacokinetic properties of verapamil, there were marked differences in the pharmacodynamics of d- and dl-verapamil. dl-Verapamil appeared to prolong the PR interval to a greater degree than did d-verapamil, consistent with the more potent calcium channel effects of the l-enantiomer. Similarly, dl-verapamil had more potent hypotensive effects compared with the d-enantiomer, which produced no effects on systemic arterial pressure. Chronotropic effects, judged to be caused by autonomic reflexes in response to the hypotensive effects of the compound, were also statistically greater for dl-verapamil than for d-verapamil. These results demonstrate stereo-selective pharmacodynamic effects in vivo of verapamil.