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Cytochrome P450 isoforms involved in metabolism of the enantiomers of verapamil and norverapamil
Author(s) -
Tracy Timothy S.,
Korzekwa Kenneth R.,
Gonzalez Frank J.,
Wainer Irving W.
Publication year - 1999
Publication title -
british journal of clinical pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.216
H-Index - 146
eISSN - 1365-2125
pISSN - 0306-5251
DOI - 10.1046/j.1365-2125.1999.00923.x
Subject(s) - metabolite , verapamil , cytochrome p450 , chemistry , microsome , metabolism , unspecific monooxygenase , cyp3a4 , drug metabolism , pharmacology , biochemistry , biology , cyp1a2 , enzyme , calcium , organic chemistry
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 K m 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 V max 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 K m values than noted for P450s 3A4 and 3A5. However, the V max 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 K m and higher V max 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.