Polymorphs and solvates of nicardipine hydrochloride. Selective stabilization of different diastereomeric racemates

Molecular understanding of the drug nicardipine hydrochloride (NHc) is provided within this study. For this reason, the polymorphism and crystal structures, including stereochemistry, of the known and the new discovered polymorphs of NHc are discussed. Three new crystalline forms of the nicardipine hydrochloride drug have been isolated: (i) a bishydrated phase, (ii) a chloroform solvate and (iii) a toluene hemisolvate. The crystal structures of these new solvated phases and those of the previously known α and β polymorphs have been determined from conventional single-crystal X-ray diffraction analysis (α phase and chloroform solvate) or from high quality powder X-ray diffraction data using direct-space methods (β phase, bishydrate and toluene hemisolvate). The analysis of the crystal structures revealed that nicardipine hydrochloride crystallizes, in all studied phases, as a racemate with the organic moiety adopting different diastereoisomeric configurations (addressed by the presence of two stereocenters, the C1 and N3 atoms) depending on the actual solvent or polymorph. The chirality of the protonated nicardipine molecules is driven, in the solids, by the strong electrostatic interactions between chloride ions and the protonated nitrogen atoms, which result, in the α phase and in the chloroform solvate, in centrosymmetric dimers built by R,R and S,S molecules. At variance, the β polymorph contains R,S and S,R molecules, still arranged in dimers, but possessing a markedly different molecular shape. Interestingly, in the bishydrated and toluene hemisolvate phases, a slightly disordered crystal structure about the positively charged ammonium group is formed, and both diasteroisomeric couples are present (although with different site occupation factors).