OBJECTIVE: The purpose of the present study was to elucidate the cytochrome P450 (P450) isoform(s) involved in the metabolism of loperamide (LOP) to N-demethylated LOP (DLOP) in human liver microsomes. METHODS: Three established approaches were used to identify the P450 isoforms responsible for LOP N-demethylation using human liver microsomes and cDNA-expressed P450 isoforms: (1) correlation of LOP N-demethylation activity with marker P450 activities in a panel of human liver microsomes, (2) inhibition of enzyme activity by P450-selective inhibitors, and (3) measurement of DLOP formation by cDNA-expressed P450 isoforms. The relative contribution of P450 isoforms involved in LOP N-demethylation in human liver microsomes were estimated by applying relative activity factor (RAF) values. RESULTS: The formation rate of DLOP showed biphasic kinetics, suggesting the involvement of multiple P450 isoforms. Apparent Km and Vmax values were 21.1 microM and 122.3 pmol/min per milligram of protein for the high-affinity component and 83.9 microM and 412.0 pmol/min per milligram of protein for the low-affinity component, respectively. Of the cDNA-expressed P450 s tested, CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyzed LOP N-demethylation. LOP N-demethylation was significantly inhibited when coincubated with quercetin (a CYP2C8 inhibitor) and ketoconazole (a CYP3A4 inhibitor) by 40 and 90%, respectively, but other chemical inhibitors tested showed weak or no significant inhibition. DLOP formation was highly correlated with CYP3A4-catalyzed midazolam 1-hydroxylation (rs=0.829; P<0.01), CYP2B6-catalzyed 7-ethoxy-4-trifluoromethylcoumarin O-deethylation (rs=0.691; P<0.05), and CYP2C8-catalyzed paclitaxel 6alpha-hydroxylation (rs=0.797; P<0.05). CONCLUSION: CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyze LOP N-demethylation in human liver microsomes, and among them, CYP2C8 and CYP3A4 may play a crucial role in LOP metabolism at the therapeutic concentrations of LOP. Coadministration of these P450 inhibitors may cause drug interactions with LOP. However, the clinical significance of potential interaction of LOP metabolism by CYP2C8 and CYP3A4 inhibitors should be studied further.
OBJECTIVE: The purpose of the present study was to elucidate the cytochrome P450 (P450) isoform(s) involved in the metabolism of loperamide (LOP) to N-demethylated LOP (DLOP) in human liver microsomes. METHODS: Three established approaches were used to identify the P450 isoforms responsible for LOP N-demethylation using human liver microsomes and cDNA-expressed P450 isoforms: (1) correlation of LOP N-demethylation activity with marker P450 activities in a panel of human liver microsomes, (2) inhibition of enzyme activity by P450-selective inhibitors, and (3) measurement of DLOP formation by cDNA-expressed P450 isoforms. The relative contribution of P450 isoforms involved in LOP N-demethylation in human liver microsomes were estimated by applying relative activity factor (RAF) values. RESULTS: The formation rate of DLOP showed biphasic kinetics, suggesting the involvement of multiple P450 isoforms. Apparent Km and Vmax values were 21.1 microM and 122.3 pmol/min per milligram of protein for the high-affinity component and 83.9 microM and 412.0 pmol/min per milligram of protein for the low-affinity component, respectively. Of the cDNA-expressed P450 s tested, CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyzed LOP N-demethylation. LOP N-demethylation was significantly inhibited when coincubated with quercetin (a CYP2C8 inhibitor) and ketoconazole (a CYP3A4 inhibitor) by 40 and 90%, respectively, but other chemical inhibitors tested showed weak or no significant inhibition. DLOP formation was highly correlated with CYP3A4-catalyzed midazolam 1-hydroxylation (rs=0.829; P<0.01), CYP2B6-catalzyed 7-ethoxy-4-trifluoromethylcoumarin O-deethylation (rs=0.691; P<0.05), and CYP2C8-catalyzed paclitaxel 6alpha-hydroxylation (rs=0.797; P<0.05). CONCLUSION:CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyze LOP N-demethylation in human liver microsomes, and among them, CYP2C8 and CYP3A4 may play a crucial role in LOP metabolism at the therapeutic concentrations of LOP. Coadministration of these P450 inhibitors may cause drug interactions with LOP. However, the clinical significance of potential interaction of LOP metabolism by CYP2C8 and CYP3A4 inhibitors should be studied further.
Authors: K Venkatakrishnan; L L von Moltke; M H Court; J S Harmatz; C L Crespi; D J Greenblatt Journal: Drug Metab Dispos Date: 2000-12 Impact factor: 3.922
Authors: Diane L DeHaven-Hudkins; Alan Cowan; Luz Cortes Burgos; Jeffrey D Daubert; Joel A Cassel; Robert N DeHaven; George B Kehner; Virendra Kumar Journal: Life Sci Date: 2002-10-25 Impact factor: 5.037
Authors: Mikko Niemi; Aleksi Tornio; Marja K Pasanen; Hanna Fredrikson; Pertti J Neuvonen; Janne T Backman Journal: Eur J Clin Pharmacol Date: 2006-04-27 Impact factor: 2.953
Authors: Nicholas Seneca; Sami S Zoghbi; H Umesha Shetty; Edward Tuan; Pavitra Kannan; Andrew Taku; Robert B Innis; Victor W Pike Journal: Nucl Med Biol Date: 2010-04 Impact factor: 2.408
Authors: Garrett R Ainslie; Kristina K Wolf; Yingxin Li; Elizabeth A Connolly; Yolanda V Scarlett; J Heyward Hull; Mary F Paine Journal: J Pharmacol Exp Ther Date: 2014-09-24 Impact factor: 4.030