Theoretical and experimental study of a praziquantel and beta-cyclodextrin inclusion complex using molecular mechanic calculations and H1-nuclear magnetic resonance.

Praziquantel (PZQ) is a broadly effective anthelminthic drug available for human and veterinary use, being the drug of choice for the treatment of all forms of schistosomiasis. Nevertheless, large doses are required in order to achieve adequate concentrations at the target site due to the poor solubility of PZQ and its significant first pass metabolism. To improve it, avoiding efficiency loss, we have designed a controlled-release system, in which PZQ was encapsulated in beta-cyclodextrin (beta-CD). The inclusion complexes between PZQ/beta-CD were studied at two different stoichiometries 1:1 and 1:2, through experimental and theoretical analysis. Molecular modeling calculations were used to foresee the better stoichiometry of the complex formed as well as the possible orientations of PZQ inside the beta-CD cavity. The complexes prepared were analyzed through H1 two-dimensional nuclear magnetic resonance (H1 2D-NMR) experiments, which provide (evidences) for the 1:1 complexation of PZQ/beta-CD. H1 2D-NMR also revealed details of PZQ/beta-CD molecular interaction, in which the isoquinoline ring of praziquantel is located inside the beta-CD cavity. Finally, phase-solubility diagrams revealed a five-fold increase in praziquantel water solubility upon addition of increasing beta-CD concentrations up to 16 mM, corresponding to the solubility of beta-CD itself. The solubilization profile is consistent with 1:1 stoichiometry of the PZQ/beta-CD complex while the solubilization effect will certainly increase the pharmacological activity of praziquantel.