Abstract

The use of selective chemical inhibitors of human cytochrome P450 (P450) enzymes represents a powerful method by which the relative contributions of various human P450 enzymes to the metabolism of drugs can be determined. However, the identification of CYP2B6 in the metabolism of drugs has been more challenging because of the lack of a well established inhibitor of this enzyme. In this report, we describe the selectivity of 2-phenyl-2-(1-piperidinyl)propane (PPP) as an inactivator of CYP2B6 and compare this selectivity versus other CYP2B6 inactivators: 1,1′,1″-phosphinothioylidynetrisaziridine (thioTEPA), clopidogrel, and ticlopidine. Values of K_I and k_inact for PPP were 5.6 μM and 0.13/min for bupropion hydroxylase catalyzed by pooled human liver microsomes, and values for thioTEPA were similar (4.8 μM and 0.20/min, respectively). Intrinsic inactivation capability was considerably greater for clopidogrel because of a greater k_inact value (1.9/min). Ticlopidine was potent with K_I and k_inact values of 0.32 μM and 0.43/min, respectively. The selectivity of these four agents for CYP2B6 was determined by testing their effects on other human P450 enzyme activities using conditions that yield ∼90% inactivation of CYP2B6 activity. The results showed that preincubation of human liver microsomes with PPP at 30 μM for 30 min provided more selective inhibition for CYP2B6 than thioTEPA, clopidogrel, and ticlopidine. Furthermore, the use of clopidogrel is complicated by the observation that this agent is not stable in the presence of human liver microsomes, even without addition of NADPH. Therefore, PPP can serve as a selective chemical inactivator of CYP2B6 and be used to define the role of CYP2B6 in the metabolism of drugs.