Stereospecific Interaction of Bupivacaine Enantiomers with Lipid Membranes

Background and Objectives: S (−)-Bupivacaine has the pharmacotoxicological advantage over its antipode and racemate. The interaction with lipid membranes was compared between S (−)-, R(+)- and racemic bupivacaine. Methods: The bupivacaine-induced changes in membrane property were determined by turbidity and fluorescence polarization measurements of membrane preparations to which bupivacaine stereoisomers of 1.0-5.0 mmol/L were applied. Liposomal membranes were made of 1,2-dipalmitoyl- sn -glycero-3-phosphocholine without or with cholesterol (5 to 15 mol%), and nerve cell model membranes of 55 mol% different phospholipids and 45 mol% cholesterol. The purity and hydrophobic interaction of bupivacaine were analyzed by reversed-phase high-performance liquid chromatography. Results: Both S (−)- and R (+)-bupivacaine were not different in lowering the phase transition temperature of membrane 1,2-dipalmitoyl- sn -glycero-3-phosphocholine. S (−)-, R (+)- and racemic bupivacaine disordered 100 mol% 1,2-dipalmitoyl- sn -glycero-3-phosphocholine liposomal membranes, although the potency was indistinguishable between stereoisomers. By adding cholesterol to membranes, however, the membrane-disordering effects showed stereostructure-specificity that was enhanced with increasing the cholesterol content (0 to 15 mol%). The enantio-differentiating effects resulted from neither impurities in enantiomers nor hydrophobic interaction with phosphatidylcholine acyl chains. Bupivacaine disordered nerve cell model membranes with the potency being S (−)-enantiomer < racemate < R (+)-enantiomer, which resembled their relative stereopotency in nerve and cardiac channel inhibition. Membrane-disordering stereospecificity disappeared in the membranes without containing cholesterol. Conclusions: Bupivacaine stereostructure-specifically interacts with membranes containing cholesterol, which is consistent with the clinical features of S (−)-bupivacaine. Membrane cholesterol appears to increase the chirality of lipid bilayers and enable them to interact with S (−)-, racemic and R (+)-bupivacaine differently.