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Research ArticleArticle

Evaluation of 3-O-Methylfluorescein as a Selective Fluorometric Substrate for CYP2C19 in Human Liver Microsomes

Sirimas Sudsakorn, Jeffrey Skell, David A. Williams, Thomas J. O'Shea and Hanlan Liu
Drug Metabolism and Disposition June 2007, 35 (6) 841-847; DOI: https://doi.org/10.1124/dmd.106.014472
Sirimas Sudsakorn
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Jeffrey Skell
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David A. Williams
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Thomas J. O'Shea
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Hanlan Liu
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Abstract

Cytochrome P450 (P450) fluorometric high-throughput inhibition assays have been widely used for drug-drug interaction screening particularly at the preclinical drug discovery stages. Many fluorometric substrates have been investigated for their selectivity, but most are found to be catalyzed by multiple P450 isozymes, limiting their utility. In this study, 3-O-methylfluorescein (OMF) was examined as a selective fluorescence substrate for CYP2C19 in human liver microsomes (HLMs). The kinetic studies of OMF O-demethylation in HLMs using a liquid chromatography/mass spectrometry method exhibited two-enzyme kinetics with apparent Km and Vmax values of 1.14 ± 0.90 μM and 11.3 ± 4.6 pmol/mg/min, respectively, for the high affinity component(s) and 57.0 ± 6.4 μM and 258 ± 6 pmol/mg/min, respectively, for the low affinity component(s). Studies utilizing cDNA-expressed individual P450 isoforms and P450-selective chemical inhibitors showed that OMF O-demethylation to fluorescein was selective for CYP2C19 at substrate concentrations ≤1 μM. At substrate concentrations ≥10 μM, other P450 isozymes were found to catalyze OMF O-demethylation. In HLMs, analysis of the two-enzyme kinetics in the presence of P450 isozyme-selective chemical inhibitors (ticlopidine for CYP2C19, sulfaphenazole for CYP2C9, and furafylline for CYP1A2) indicated that CYP2C19 was the high affinity component and CYP2C9 was the low affinity component. Based on these findings, a fluorometric assay was developed using 1 μM OMF and 2 μM sulfaphenazole for probing CYP2C19-mediated inhibition in HLMs. The IC50 data of 13 substrates obtained from the fluorometric assay developed in this study correlated well with that reported in the literature using nonfluorescence assays.

Footnotes

  • doi:10.1124/dmd.106.014472.

  • ABBREVIATIONS: DDI, drug-drug interaction; P450, cytochrome P450; HLM, human liver microsome; OMF, 3-O-methylfluorescein; FL, fluorescein; LC/MS, liquid chromatography/mass spectrometry; MAMC, 7-methoxy-4-(aminomethyl)-coumarin.

    • Received December 21, 2006.
    • Accepted February 27, 2007.
  • The American Society for Pharmacology and Experimental Therapeutics
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Drug Metabolism and Disposition: 35 (6)
Drug Metabolism and Disposition
Vol. 35, Issue 6
1 Jun 2007
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Research ArticleArticle

Evaluation of 3-O-Methylfluorescein as a Selective Fluorometric Substrate for CYP2C19 in Human Liver Microsomes

Sirimas Sudsakorn, Jeffrey Skell, David A. Williams, Thomas J. O'Shea and Hanlan Liu
Drug Metabolism and Disposition June 1, 2007, 35 (6) 841-847; DOI: https://doi.org/10.1124/dmd.106.014472

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Research ArticleArticle

Evaluation of 3-O-Methylfluorescein as a Selective Fluorometric Substrate for CYP2C19 in Human Liver Microsomes

Sirimas Sudsakorn, Jeffrey Skell, David A. Williams, Thomas J. O'Shea and Hanlan Liu
Drug Metabolism and Disposition June 1, 2007, 35 (6) 841-847; DOI: https://doi.org/10.1124/dmd.106.014472
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