Use of Human Plasma Samples to Identify Circulating Drug Metabolites that Inhibit Cytochrome P450 Enzymes.
Drug interactions elicited through inhibition of cytochrome P450 (P450) enzymes are important in pharmacotherapy. Recently, greater attention has been focused on not only parent drugs inhibiting P450 enzymes but also on possible inhibition of these enzymes by circulating metabolites. In this report, an ex vivo method whereby the potential for circulating metabolites to be inhibitors of P450 enzymes is described. To test this method, seven drugs and their known plasma metabolites were added to control human plasma at concentrations previously reported to occur in humans after administration of the parent drug. A volume of plasma for each drug based on the known inhibitory potency and time-averaged concentration of the parent drug was extracted and fractionated by high-pressure liquid chromatography-mass spectrometry, and the fractions were tested for inhibition of six human P450 enzyme activities (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4). Observation of inhibition in fractions that correspond to the retention times of metabolites indicates that the metabolite has the potential to contribute to P450 inhibition in vivo. Using this approach, norfluoxetine, hydroxyitraconazole, desmethyldiltiazem, desacetyldiltiazem, desethylamiodarone, hydroxybupropion, erythro-dihydrobupropion, and threo-dihydrobupropion were identified as circulating metabolites that inhibit P450 activities at a similar or greater extent as the parent drug. A decision tree is presented outlining how this method can be used to determine when a deeper investigation of the P450 inhibition properties of a drug metabolite is warranted.
Pfizer Inc., Groton, Connecticut.
Heather
H
Eng
Pfizer Inc., Groton, Connecticut r.scott.obach@pfizer.com.
R Scott
RS
Obach
D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065609:Cytochrome P-450 CYP1A2 Inhibitors / Q000097:blood / Q000494:pharmacology; D065731:Cytochrome P-450 CYP2C19 / Q000378:metabolism; D065689:Cytochrome P-450 CYP2C19 Inhibitors / Q000097:blood / Q000494:pharmacology; D065727:Cytochrome P-450 CYP2C8 / Q000378:metabolism; D065687:Cytochrome P-450 CYP2C8 Inhibitors / Q000097:blood / Q000494:pharmacology; D065729:Cytochrome P-450 CYP2C9 / Q000378:metabolism; D065688:Cytochrome P-450 CYP2C9 Inhibitors / Q000097:blood / Q000494:pharmacology; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism; D065690:Cytochrome P-450 CYP2D6 Inhibitors / Q000097:blood / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism; D065692:Cytochrome P-450 CYP3A Inhibitors / Q000097:blood / Q000494:pharmacology; D065607:Cytochrome P-450 Enzyme Inhibitors / Q000097:blood* / Q000494:pharmacology*; D003663:Decision Trees; D004347:Drug Interactions; D006801:Humans; D013058:Mass Spectrometry; D013997:Time Factors
D065609:Cytochrome P-450 CYP1A2 Inhibitors; D065689:Cytochrome P-450 CYP2C19 Inhibitors; D065687:Cytochrome P-450 CYP2C8 Inhibitors; D065688:Cytochrome P-450 CYP2C9 Inhibitors; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065692:Cytochrome P-450 CYP3A Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; C504573:CYP1A2 protein, human; C045793:CYP2C19 protein, human; C450259:CYP2C8 protein, human; D019388:Cytochrome P-450 CYP1A2; D065731:Cytochrome P-450 CYP2C19; D065727:Cytochrome P-450 CYP2C8; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human