1 Verapamil is a racemic mixture of two optical isomers, the (-)-form being the more active component. Recent studies indicate a rapid hepatic transformation of (-)-verapamil, which results in different concentration-effect relationships after oral and intravenous administration. In practice the important pharmacokinetic properties of verapamil are low bioavailability (20%), predominant elimination by metabolism (> 95%) and a relatively short half-life (tl/2,, is 3-5 h). After repeated dosing, the rate of hepatic drug clearance seems to decrease. 2 Slow release (SR) formulations of verapamil may offer certain therapeutic advantages during long-term treatment. A comparison of conventional (C) and SR tablets in a 1-week treatment of eight cardiac patients showed a relative bioavailability (AUCsR/AUCc) of 90 + 30%. More stable serum drug levels were maintained by 12-hourly administration of SR verapamil. 3 A further study using a new 240 mg SR preparation in patients with arterial hypertension showed that even a single daily dose can be sufficient for adequate blood pressure control over 24 h.
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