Study of inclusion complexes of acridine with β- and (2,6-di-O-methyl)-β-cyclodextrin by use of solubility diagrams and NMR spectroscopy

Previous molecular modeling studies, in our laboratory, have shown that some esters of type RCOO(CH2)(n)C5H5N+Cl- are potentially active against Alzheimer's disease. We have also demonstrated that acridine, which has strong anticholinesterase activity appears to be a suitable R substituent. The main obstacle to the possible pharmaceutical application of these compounds is their limited solubility in water, which is due to the poor aqueous solubility of acridine itself (0.26 mM). Inclusion complexation with cyclodextrins may overcome this problem. Solubility diagrams and NMR spectroscopy were used to study the inclusion of acridine (Acr) within beta-cyclodextrin (betaCD) and heptakis(2,6-di-O-methyl)cyclomaltoheptose (DMbetaCD). A 1:1 complex was formed for the Acr-betaCD system and both 1:1 and 2:1 complexes for the Acr-DMbetaCD system (apparent K-a 215+/-20 and 1150+/-100 M-1, respectively). Data from H-1 NMR studies corrected the assignment of the acridine H1, H8 and H4, H5 protons in D2O, which have been erroneously assigned in previous publications. H-1 spin-lattice relaxation times T-1 measured with selective, non-selective and null inversion characterize the dynamics of the inclusion complexes. Geometric features of the host-guest inclusion complexes were inferred from intermolecular dipolar interactions obtained by 2D adiabatic off-resonance ROESY experiments. A more detailed picture of the structure of the inclusion complexes was obtained by combining NMR structural data and molecular modeling (docking, dynamics under NMR constraints and energy calculations). Copyright (C) 2002 John Wiley Sons, Ltd.