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- and 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 K-i values of S- and R-AML were 8.95 mu M, 14.85 mu 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 (K-I 8.22 mu M, K-inact 0.065 min(-1)) than S-AML (K-I 14.06 mu M, K-inact 0.041 min(-1)). R-AML was also a significantly more potent inhibitor of CYP2C9 (K-i 12.11 mu M/S-AML 21.45 mu M) and CYP2C19 (K-i 5.97 mu M/S-AML 7.22 M.
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.