Received for publication July 25, 2008.
Revised September 11, 2008.
Accepted for publication September 12, 2008.

SKF525A Revisited: Comparative Cytochrome P450 Inhibition in Human Liver Microsomes by SKF525A, its Metabolites, and SKF-Acid and SKF-Alcohol

Abstract

When incubated with human liver microsomes, SKF525A (2-diethylaminoethyl-2,2-diphenylvalerate.HCl) undergoes cytochrome P450-dependent oxidative N-deethylation to the secondary amine metabolite (SKF8742). CYP-selective inhibitors indicated CYP3As catalyzed this reaction, and the deethylation rate correlated best with the CYP3A activity across a range of human liver microsomes. SKF525A and its metabolite, and primary amine analog all inhibited CYP2B6-, CYP2C9-, CYP2C19-, CYP2D6- and CYP3A-selective reactions to varying degrees, but had little effect on CYP1A2-, CYP2A6-, and CYP2E1-reactions. Only the inhibition of CYP3A showed major enhancement when the inhibitors were pre-incubated with NADPH-fortified microsomes, and the extent of metabolic intermediate (MI) complex formation approximated typical CYP3A content. Two "lost with time" SKF525A derivatives devoid of the ethylamine moiety, SKF-Alcohol (2,2-diphenylpropylethanol) and SKF-Acid (2,2-diphenylpropylacetic acid) did not form a MI complex and were identified as selective inhibitors of CYP2C9. Although without detectable metabolism, their CYP2C9 inhibition fitted best with a competitive mechanism. Thus not all human P450s are inhibited by SKF525A and related compounds, and the mechanisms contributing to those that are inhibited varies with the isoform. P450 MI-complex formation only appears to play a role with CYP3As.

Key words: CYP inhibition, CYP2C, CYP3A, cytochrome P450 catalyzed oxidations, human CYP enzymes, mechanism-based inhibition, MI complex, metabolite-inhibitor complex