Cytochrome P450 isoforms involved in metabolism of the enantiomers of verapamil and norverapamil

• Published in: British journal of clinical pharmacology 1999-05, Vol.47 (5), p.545-552
• Main Authors: Tracy, Timothy S., Korzekwa, Kenneth R., Gonzalez, Frank J., Wainer, Irving W.
• Format: Article
• Language: English
• Subjects: Antiarythmic agents; Biological and medical sciences; Cardiovascular system; Cell Line; Cytochrome P-450 CYP2D6 - metabolism; Cytochrome P-450 CYP2E1 - metabolism; Cytochrome P-450 CYP3A; Cytochrome P-450 Enzyme System - metabolism; cytochrome P450; enantiomer; Humans; Isoenzymes - metabolism; Kinetics; Medical sciences; metabolism; Mixed Function Oxygenases - metabolism; Nitriles; norverapamil; Original; Pharmacology. Drug treatments; Stereoisomerism; Substrate Specificity; Tumor Cells, Cultured; verapamil; Verapamil - analogs & derivatives; Verapamil - chemistry; Verapamil - metabolism
• ISSN: 0306-5251; 1365-2125
• DOI: 10.1046/j.1365-2125.1999.00923.x

Aims The present study was conducted to evaluate metabolism of the enantiomers of verapamil and norverapamil using a broad range of cytochrome P450 isoforms and measure the kinetic parameters of these processes. Methods Cytochrome P450 cDNA‐expressed cells and microsomes from a P450‐expressed lymphoblastoid cell line were incubated with 40 μm concentrations of R‐ or S‐verapamil and R‐ or S‐norverapamil and metabolite formation measured by h.p.l.c. as an initial screening. Those isoforms exhibiting substantial activity were then studied over a range of substrate concentrations (2.5–450 μm ) to estimate the kinetic parameters for metabolite formation. Results P450s 3A4, 3A5, 2C8 and to a minor extent 2E1 were involved in the metabolism of the enantiomers of verapamil. Estimated Km values for the production of D‐617 and norverapamil by P450 s 3A4 and 3A5 were similar (range=60–127 μm ) regardless of the enantiomer of verapamil studied while the Vmax estimates were also similar (range=4–8 pmol min−1 pmol−1 P450). Only nominal production of D‐620 by these isoforms was noted. Interestingly, P450 2C8 readily metabolized both S‐ and R‐verapamil to D‐617, norverapamil and PR‐22 with only slightly higher Km values than noted for P450s 3A4 and 3A5. However, the Vmax estimates for P450 2C8 metabolism of S‐ and R‐verapamil were in general greater (range=8–15 pmol min−1 pmol−1 P450) than those noted for P450 s 3A4 and 3A5 with preference noted for metabolism of the S‐enantiomer. Similarly, P450 s 3A4, 3A5 and 2C8 also mediated the metabolism of the enantiomers of norverapamil with minor contributions by P450 s 2D6 and 2E1. P450s 3A4 and 3A5 readily formed the D‐620 metabolite with generally a lower Km and higher Vmax for S‐norverapamil than for the R‐enantiomer. In contrast, P450 2C8 produced both the D‐620 and PR‐22 metabolites from the enantiomers of norverapamil, again with stereoselective preference seen for the S‐enantiomer. Conclusions These results confirm that P450s 3A4, 3A5 and 2C8 play a major role in verapamil metabolism and demonstrate that norverapamil can also be further metabolized by the P450s.