INTERACTION OF CALCIUM-CHANNEL ANTAGONISTS WITH CALCIUM - STRUCTURAL STUDIES ON NICARDIPINE AND ITS CA2+ COMPLEX

Conformational features of nicardipine in acetonitrile, in the absence and presence of Ca2+ were investigated by one-dimensional NMR and difference absorption spectroscopy techniques. The data show that in acetonitrile solution the antiperiplanar form of nicardipine is dominant. The addition of Ca2+ to the drug solution caused marked changes in the difference absorbance spectra in the 200-400 nm region and in many of its H-1 and C-13 NMR resonances. The changes were most significant up to a ratio of 0.5 Ca2+:drug. Analysis of the binding data showed the predominant species to be a 2:1 drug:Ca2+ ''sandwich'' complex with an estimated dissociation constant-of 100 mu M at 25 degrees C. One-dimensional nuclear Overhauser effect (NOE) experiments revealed through-space connectivities in the drug before and after Ca2+ binding. These changes in conjunction with the changes in H-1 and C-13 chemical shifts suggest a structure in which the 4-aryl ring substituent of the pyridine moiety moves closer to the C3-side chain in the presence of Ca2+. This attraction is achieved via the chelation of the Ca2+ ion by the oxygen atoms in the m-NO2 of the aryl group and the COOCH2 group in the side chain of the dihydropyridine ring, and gives rise to a stable synperiplanar conformation. A preference for this conformation was also observed in the Ca2+ complex of nifedipine in acetonitrile as inferred from the rather limited NOE data obtained. Our study provides a detailed solution structure for nicardipine and also leads to a suggestion of a role for Ca2+ in the action of this and possibly other dihydropyridines.