TY - JOUR T1 - Substrate-Dependent Modulation of CYP3A4 Catalytic Activity: Analysis of 27 Test Compounds with Four Fluorometric Substrates JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1440 LP - 1448 VL - 28 IS - 12 AU - David M. Stresser AU - Andrew P. Blanchard AU - Stephanie D. Turner AU - John C. L. Erve AU - Andre A. Dandeneau AU - Vaughn P. Miller AU - Charles L. Crespi Y1 - 2000/12/01 UR - http://dmd.aspetjournals.org/content/28/12/1440.abstract N2 - Inhibition of cytochrome P450 catalytic activity is a principal mechanism for pharmacokinetic drug-drug interactions. Rapid, in vitro testing for cytochrome P450 inhibition potential is part of the current paradigm for identifying drug candidates likely to give such interactions. We have explored the extent that qualitative and quantitative inhibition parameters are dependent on the cytochrome P450 (CYP) 3A4 probe substrate. Inhibition potential (e.g., IC50values from 8-point inhibition curves) or activation potential for most compounds varied dramatically depending on the fluorometric probe substrates for CYP3A4 [benzyloxyresorufin (BzRes), 7-benzyloxy-4-trifluoromethylcoumarin (BFC), 7-benzyloxyquinoline (BQ), and dibenzylfluorescein (DBF)]. For 21 compounds that were primarily inhibitors, the range of IC50 values for the four substrates varied from 2.1- to 195-fold with an average of 29-fold. While the rank order of sensitivity among the fluorometric substrates varied among the individual inhibitors, on average, BFC dealkylation was the most sensitive to inhibition, while BQ dealkylation was least sensitive. Partial inhibition was observed with BzRes and BQ but not for BFC and DBF. BzRes was more prone to activation, whereas dramatic changes in IC50 values were observed when the BQ concentration was below the S50. Three different correlation analyses indicated that IC50 values with BFC, BQ, and DBF correlated well with each other, whereas the response with BzRes correlated more weakly with the other substrates. One of these correlation analyses was extended to the percent inhibition of 10 μM inhibitor with the standard CYP3A4 probe substrates testosterone, midazolam, and nifedipine. In this analysis the responses with BQ, BFC and DBF correlated well with testosterone and midazolam but more poorly with nifedipine. In the aggregate, BFC and DBF appear more suitable as an initial screen for CYP3A4 inhibition. However, the substrate-dependent effects reported here and by others indicate that all CYP3A4 inhibition data should be interpreted with caution. The American Society for Pharmacology and Experimental Therapeutics ER -