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 <i>R</i>- and <i>S</i>-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. <i>S</i>-AML was a stronger reversible inhibitor of midazolam hydroxylation: the K_i values of <i>S</i>- and <i>R</i>-AML were 8.95 µM, 14.85 µM, respectively. Computational docking confirmed that the enantiomers interact differently with CYP3A: the binding free energy of <i>S</i>-AML in the active site was greater than that for <i>R</i>-AML (-7.6- vs. -6.7 kcal/mol). Conversely, <i>R</i>-AML exhibited more potent time-dependent inhibition of CYP3A activity (K_I 8.22 µM, K_inact 0.065 min^-1) than <i>S</i>-AML (K_I 14.06 µM, K_inact 0.041 min^-1). <i>R</i>-AML was also a significantly more potent inhibitor of CYP2C9 (K_i 12.11 µM/<i>S</i>-AML 21.45 µM) and CYP2C19 (K_i 5.97 µM/<i>S</i>-AML 7.22 μM. In conclusion, results indicate that clinical use of <i>S</i>-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 <i>R</i>-AML.