Identification of CYP Mechanism-Based Inhibitors

Metabolic drug-drug interactions occur when a drug A alters the pharmacokinetics of a coadministered drug B by either inhibiting, activating, or inducing the activity of the enzyme(s) that metabolize drug B. Inhibitory drug-drug interactions could result in serious adverse effects, including fatalities in patients receiving multiple medications. Cytochrome P450 superfamily (CYPs) are the major oxidative enzymes that participate in the metabolism of commercially available drugs. In addition to direct inhibition (reversible), these enzymes could be subjected to metabolism- or mechanism-based inhibition. Metabolism-based inhibition results from a metabolic product of the drug that is a more potent reversible inhibitor, whereas mechanism-based inhibition results from a metabolic product that binds irreversibly to the enzyme, rendering it inactive. Potent CYP inhibitors, including metabolism- and mechanism-based inhibition, are usually excluded from further consideration for development. The potential of new chemical entities (NCEs) to inhibit human CYPs, including metabolism- and mechanism-based inhibition, is assessed during the discovery stage in major pharmaceutical companies using in vitro screens. Metabolism- and mechanism-based inhibition is differentiated from direct inhibition primarily by being time dependent and involves catalytic steps. Metabolism- and mechanism-based inhibitors are differentiated by extensive dialysis, which would result in the recovery of enzyme activity for metabolism-based inhibition but not for mechanism-based inhibition. This chapter discusses the importance of identifying metabolism- and mechanism-based inhibitors and provides detailed experimental procedures for their identification early in drug discovery.