Comparison of location and binding for the positively charged 1,4-dihydropyridine calcium channel antagonist amlodipine with uncharged drugs of this class in cardiac membranes

The distinctive pharmacokinetic and pharmacodynamic activity of amlodipine, including long onset and duration of activity as a calcium channel antagonist, may be related to its interactions with membranes. We have used X-ray crystallography and small-angle X-ray scattering to examine and compare the crystal structure of amlodipine and its location in cardiac sarcolemmal lipid bilayers with that of uncharged dihydropyridines (DHPs) such as nimodipine. Crystallographic analysis demonstrated that the DHP ring of amlodipine is considerably more planar than that of nimodipine, that amlodipine has a greater torsion angle between the DHP and aryl rings, and that the protonated amino group extends away from the DHP ring structure. Despite the positive charge of amlodipine at physiological pH, membrane electron density profile structures showed amlodipine to have a time-averaged location near the hydrocarbon core/water interface similar to that observed for several uncharged DHPs. However, unlike uncharged DHPs, this location is consistent with an ionic interaction between the protonated amino function of amlodipine and the negatively charged phospholipid headgroup region, in addition to a hydrophobic interaction with the fatty acyl chain region near the glycerol backbone similar to other DHPs. This location may also provide an appropriate conformation and orientation for amlodipine binding to its receptor site at this depth in the membrane. Finally, we have measured the nonspecific partitioning of amlodipine into native sarcoplasmic reticulum membranes from rabbit skeletal muscle and compared these data with those for the uncharged DHPs. The partition coefficient into light sarcoplasmic reticulum for amlodipine was higher than that observed for most uncharged DHPs and rates of incorporation of amlodipine into membranes were very high, as with other DHPs, whereas the "washout time" of amlodipine from these membranes was longer by over 1 order of magnitude. These data suggest differences in membrane interactions for amlodipine, compared with uncharged DHPs, that may be correlated with its novel pharmacodynamic and pharmacokinetic profile.