Studies on the enantioselective disposition of amlodipine : A challenge in bioanalysis

Sammanfattning: To study the disposition of the enantiomers of the calcium channel blocker amlodipine in human plasma, an enantioselective high performance liquid chromatographic (HPLC) assay was developed. Several HPLC techniques such as chiral ion-pair forming, chiral derivatisation and direct separation on chiral columns were evaluated. A robust assay withhigh sensitivity for amlodipine's enantiomers, suitable for large scale analysis of plasma samples, was established by combining enantiomeric separation on Chiral-AGP i.e., an alfa1 acid glycoprotein material, with achiral HPLC using electrochemical detection, in a coupledcolumn system. The enantiomeric separation on the AGP material was evaluated andoptimised by performing chemometric i.e., multivariate analysis.Enantioselective pharmacokinetics, in healthy human subjects, after a single oral dose of amlodipine (Norvasc®) was shown. The pharmacologically active (S)-enantiomer was found in higher plasma concentrations than the (R)-enantiomer in all subjects, but the individual SIR ratios ranged from about 50:50 to 70:30. Comparable inter-individual differences in amlodipine concentration SIR ratio was found in patients during long-term treatment with Norvasc®. However, the changes over time, in the individual subjects, were low and were not significantly influenced if the dose was increased. A difference in systemic blood clearance of the two enantiomers is the most likely cause of the observed difference in pharmacokinetic behavior. At present, however, it cannot be discerned whether this is caused by differences in protein-binding or in intrinsic clearance of the unbound drug.Administration together with grapefruit juice was found to increase the amlodipine plasma concentrations in healthy volunteers without markedly change the plasma concentration SIR ratio. A nonstereoselective inhibition of the first-pass metabolism is proposed. Even though the effects seen were small the clinical significance of this food/drug interaction cannot be totally neglected since there are considerable differences seen between individuals.