Influence of amlodipine enantiomers on human microsomal cytochromes p450: Stereoselective time-dependent inhibition of CYP3A enzyme activity

Abstract

Amlodipine (AML) is available as a racemate, i.e., a mixture of R- A nd S-enantiomers. Its inhibitory potency towards nine cytochromes P450 (CYP) was studied to evaluate the drug-drug interactions between the enantiomers. Enzyme inhibition was evaluated using specific CYP substrates in human liver microsomes. With CYP3A, both enantiomers exhibited reversible and time-dependent inhibition. S-AML was a stronger reversible inhibitor of midazolam hydroxylation: The Ki values of S- A nd R-AML were 8.95 M, 14.85 M, respectively. Computational docking confirmed that the enantiomers interact differently with CYP3A: The binding free energy of S-AML in the active site was greater than that for R-AML (-7.6-vs.-6.7 kcal/mol). Conversely, R-AML exhibited more potent time-dependent inhibition of CYP3A activity (KI 8.22μM, Kinact 0.065 min-1) than S-AML (KI 14.06μM, Kinact 0.041 min-1). R-AML was also a significantly more potent inhibitor of CYP2C9 (Ki 12.11μM/S-AML 21.45μM) and CYP2C19 (Ki 5.97μM/S-AML 7.22μM. In conclusion, results indicate that clinical use of S-AML has an advantage not only because of greater pharmacological effect, but also because of fewer side effects and drug-drug interactions with cytochrome P450 substrates due to absence of R-AML.