PT  - JOURNAL ARTICLE
AU  - U G Eriksson
AU  - J Lundahl
AU  - C Bäärnhielm
AU  - C G Regårdh
TI  - Stereoselective metabolism of felodipine in liver microsomes from rat, dog, and human.
DP  - 1991 Sep 01
TA  - Drug Metabolism and Disposition
PG  - 889--894
VI  - 19
IP  - 5
4099  - http://dmd.aspetjournals.org/content/19/5/889.short
4100  - http://dmd.aspetjournals.org/content/19/5/889.full
SO  - Drug Metab Dispos1991 Sep 01; 19
AB  - Felodipine, a 1,4-dihydropyridine derivative, is a potent, vasoselective calcium antagonist that is used clinically as a racemic mixture of two stereoisomers. In the rat, dog, and human, the bioavailability of an oral dose is about 15% because of high first-pass metabolism. Oxidation of the dihydropyridine ring to the corresponding achiral, pharmacologically inactive pyridine metabolite is the predominant metabolic step. In the liver, this metabolism is catalyzed by cytochrome P-450. In the present study, the metabolism of (R)- and (S)- felodipine was compared in vitro in liver microsomes from rats, dogs, and humans. Slightly higher rates of metabolism were found for the (S)-enantiomer in rat and dog liver microsomes. However, no significant differences were observed in the Michaelis-Menten parameters, Vmax or KM. In human liver microsomes, the (R)-enantiomer was metabolized more readily than (S)-felodipine. The mean value of KM was lower for (R)-felodipine, while the Vmax values of the enantiomers were similar. The intrinsic clearance, defined as the ratio of Vmax and KM, was about two times higher for (R)-felodipine. Assuming complete absorption and that the bioavailability is determined by the first-pass metabolism in the liver, these in vitro results suggest that the bioavailability of (S)-felodipine in vivo is about two times higher than that of (R)-felodipine.