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The proton gradient from the intracellular space to plasma creates an unbound drug gradient for weak acids and bases that could modulate apparent drug clearance and drug-drug interactions. Cytochrome P450 intrinsic clearance and inhibitor potency are routinely determined in vitro at the plasma pH of 7.4 rather than the intrahepatocyte pH of 7.0. We determined the impact of pH on in vitro enzyme kinetic parameters and inhibition potency for substrates (bufuralol, dextromethorphan), reversible inhibitors (quinidine, amiodarone, desethylamiodarone, clozapine), and mechanism-based inhibitors (paroxetine, desethylamiodarone) of the major drug metabolizing-enzyme CYP2D6. The lower intracellular pH 7.0 compared with pH 7.4 resulted in a 60 and 50% decrease in intrinsic clearance for the substrates bufuralol and dextromethorphan, respectively. Reversible inhibition constants for three of the four inhibitors tested were unaffected by pH, whereas for the inhibitor quinidine, a 2-fold increase in the inhibition constant was observed at pH 7.0. For time-dependent inhibitors desethylamiodarone and paroxetine, changes in time-dependent inhibition parameters were different for each inhibitor. These results were incorporated into physiologically based pharmacokinetic models indicating that the changes in in vitro parameters determined at pH 7.0 offset the effect of increased unbound intracellular concentrations on apparent clearance and extent of drug-drug interactions. However, this offset between concentration and enzyme activity cannot be generalized for all substrates, inhibitors, and enzymes, as the effect of a lower pH in vitro varied significantly; therefore, it would be prudent to determine in vitro enzyme parameters at the hepatocyte-appropriate pH 7.0.

The Impact of the Hepatocyte-to-Plasma pH Gradient on the Prediction of Hepatic Clearance and Drug-Drug Interactions for CYP2D6 Substrates