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Surrogate assays for drug metabolism and inhibition are traditionally performed in buffer systems at pH 7.4, despite evidence that hepatocyte intracellular pH is 7.0. This pH gradient can result in a pK a -dependent change in intracellular/extracellular concentrations for ionizable drugs that could affect predictions of clearance and P450 inhibition. The effect of microsomal incubation pH on in vitro enzyme kinetic parameters for CYP2C9 (diclofenac, (S)-warfarin) and CYP3A4 (midazolam, dextromethorphan, testosterone) substrates, enzyme specific reversible inhibitors (amiodarone, desethylamiodarone, clozapine, nicardipine, fluconazole, fluvoxamine, itraconazole) and a mechanism-based inhibitor (amiodarone) was investigated. Intrinsic clearance through CYP2C9 significantly increased (25% and 50% for diclofenac and (S)-warfarin respectively) at intracellular pH 7.0 compared with traditional pH 7.4. The CYP3A4 substrate dextromethorphan intrinsic clearance was decreased by 320% at pH 7.0, while midazolam and testosterone remained unchanged. Reversible inhibition of CYP2C9 was less potent at pH 7.0 compared with 7.4, while CYP3A4 inhibition potency was variably affected. Maximum enzyme inactivation rate of amiodarone toward CYP2C9 and CYP3A4 decreased at pH 7.0, while the irreversible inhibition constant remained unchanged for CYP2C9, but decreased for CYP3A4 at pH 7.0. Predictions of clearance and drug-drug interactions made through physiologically based pharmacokinetic models were improved with the inclusion of predicted intracellular concentrations based at pH 7.0 and in vitro parameters determined at pH 7.0. No general conclusion on the impact of pH could be made and therefore a recommendation to change buffer pH to 7.0 cannot be made at this time. It is recommended that the appropriate hepatocyte intracellular pH 7.0 be used for in vitro determinations when in vivo predictions are made.

The Impact of the Hepatocyte-to-Plasma pH Gradient on the Prediction of Hepatic Clearance and Drug-Drug Interactions for CYP2C9 and CYP3A4 Substrates