Interaction of Charged and Uncharged Calcium Channel Antagonists with Phospholipid Membranes. Binding Equilibrium, Binding Enthalpy, and Membrane Location

Abstract

The membrane location and the binding mechanism of two calcium channel antagonists, amlodipine and nimodipine, in pure lipid membranes were investigated with different physical-chemical methods. Deuterium nuclear magnetic resonance shows that the non-charged, hydrophobic nimodipine is distributed homogenously across the whole hydrocarbon layer. In contrast, the positively charged, amphiphilic amlodipine adopts a well defined position in the lipid bilayer. The charged ethanolamine side group of amlodipine is located at the water-lipid interface, interacting with the dipoles of the headgroup region while the non-polar ring system is located at the level of the hydrocarbon segment interacting specifically with the cis double bond of the lipid, forming a weak association complex. The binding equilibrium of amlodipine to phosphatidylcholine membranes was studied by measuring the electrophoretic mobility of lipid vesicles and with a centrifugation assay. Both experimental methods lead to identical results. The binding of amlodipine to a lipid membrane can be described by a surface partition equilibrium with an intrinsic partition constant Kp = 15500 M , yielding a free enthalpy of △G = -8.1 kcal/mol. The enthalpy of amlodipine binding to POPC membranes was independently measured with a high sensitivity titration calorimeter yielding △H = -9.2 kcal/mol, at 27°C.The partitioning of the amphiphilic drug into the lipid bilayer is an enthalpy-driven process.