Drug–drug interactions and cooperative effects detected in electrochemically driven human cytochrome P450 3A4

Inhibition of cytochrome P450-mediated drug metabolism by a concomitantly administered second drug is one of the major causes of drug–drug interactions in humans. The present study reports on the first electrochemically-driven drug–drug interactions of human cytochrome P450 3A4 probed with erythromycin, ketoconazole, cimetidine, diclofenac and quinidine. Cytochrome P450 3A4 was immobilized on glassy carbon electrodes in the presence of a cationic polyelectrolyte, PDDA (poly(diallyldimethylammonium chloride)). Inhibition of the turnover of its substrate, erythromycin, was subsequently measured using chronoamperometry at increasing concentrations of different known inhibitors of this enzyme namely ketoconazole, cimetidine and diclofenac for which IC 50 values of 135 nM, 80 μM and 311 μM were measured, respectively. Furthermore, heterotrophic cooperativity where the turnover of a first substrate is enhanced in the presence of a second one, was tested for the immobilized P450 3A4 enzyme. In this case, diclofenac 5-hydroxylation was stimulated by the presence of quinidine resulting in doubling of the potency of this inhibitor i.e. lowering the measured IC 50 of diclofenac from 311 μM down to 157 μM. The results obtained in this work confirm that bioelectrochemistry can be employed for in vitro studies of not only drug–drug interactions but also prediction of adverse drug reactions in this important P450 isozyme.