Clinical pharmacokinetics of felodipine. A summary.

Felodipine is completely absorbed from the gastrointestinal tract. However, the amount reaching the systemic circulation is reduced to about 15% because of first-pass degradation. The bioavailability is constant within the dose interval of 5 to 40mg orally. The frequency histogram of the area under the plasma concentration-time curve (AUC) seems to be normally distributed. The disposition of felodipine is independent of the administered dose over the intravenous dose interval (1-3 mg). The plasma concentration-time curve declines in 3 distinct phases. The mean elimination half-life of felodipine is approximately 25h. Felodipine is extensively distributed to extravascular tissues. The volume of distribution of felodipine is about 10 L/kg, implying that less than 1% of the amount of drug in the body is localised in the blood. Felodipine is more than 99% bound to plasma proteins. Mean total clearance from the blood is 1 to 1.5 L/min and, therefore, felodipine is considered a high clearance drug. Felodipine is metabolised completely and no unchanged drug is eliminated in the urine. The first step in the metabolism involves oxidation to the corresponding pyridine derivative by the cytochrome P-450 system. Identified metabolites in plasma and urine are devoid of vasodilating activity. Long term treatment, and the presence of hypertension and impaired renal function do not affect the disposition of felodipine. Elderly people may have higher plasma levels than the young and middle-aged. Impaired liver function significantly decreases systemic clearance. Cimetidine and food affect felodipine kinetics, but with negligible clinical implications. Therapeutic concentrations of felodipine do not interact with highly protein-bound drugs and these drugs have no effect on the binding of felodipine to human plasma proteins in vitro. Plasma levels of digoxin and metoprolol tended to increase during felodipine treatment. There is a significant correlation between plasma concentrations of felodipine and haemodynamic effects in both healthy subjects and hypertensive patients during short term as well as during long term treatment.