title: Cytochrome P450 isoforms involved in metabolism of the enantiomers of verapamil and norverapamil.
abstract: OBJECTIVE
  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 microm 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 microm ) 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 microm ) 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.
authors.affiliation: Department of Basic Pharmaceutical Sciences, School of Pharmacy, West Virginia University, Morgantown, West Virginia, USA.
authors.forename: T S
authors.initials: TS
authors.lastname: Tracy
authors.forename: K R
authors.initials: KR
authors.lastname: Korzekwa
authors.forename: F J
authors.initials: FJ
authors.lastname: Gonzalez
authors.forename: I W
authors.initials: IW
authors.lastname: Wainer
mesh_terms: D002460:Cell Line; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000378:metabolism; D009570:Nitriles; D013237:Stereoisomerism; D013379:Substrate Specificity; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; D009570:Nitriles; C016906:alpha-(3-aminopropyl)-3,4-dimethoxy-alpha-(1-methylethyl)benzeneacetonitrile; C482241:2-(3,4-dimethoxyphenyl)-5-amino-2-isopropylvaleronitrile; D003577:Cytochrome P-450 Enzyme System; C016904:norverapamil; D014700:Verapamil; D006899:Mixed Function Oxygenases; D019392:Cytochrome P-450 CYP2E1; C104464:CYP3A protein, human; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A