NMR, CD and UV spectroscopic studies reveal uncommon binding modes of dapoxetine to native cyclodextrins

Abstract

Complex formation between the selective serotonin reuptake inhibitor drug (S)-dapoxetine (Dpx) and [small beta]-, [gamma]-, and methylated [gamma]-cyclodextrins (CyDs) was studied by complementary experimental techniques. Phase solubility studies indicated 1 : 1 stoichiometry for all CyDs, with [small beta]-CyD being the most effective to enhance the solubility of and form the most stable complex with Dpx. 1H NMR titrations and Job's method of continuous variation were also employed to corroborate the stoichiometry and association constants with [gamma]- and [small beta]-CyDs. The simultaneous evaluation of titration datasets of several Dpx and CyD protons revealed for both CyDs that the best fit was achieved when the coexistence of Dpx[middle dot]CyD and 2Dpx[middle dot]CyD complexes were assumed. This finding was rather unanticipated considering the smaller cavity size of [small beta]-CyD, which inspired further investigations. The UV hypochromism and circular dichroism spectroscopic data suggested the inclusion of two Dpx molecules into the CyD cavity (albeit to a less extent with [small beta]-CyD). This conclusion was fully supported by 2D ROESY and 1D NOESY NMR spectra, observing NOEs between the inner methine protons of CyDs and both the phenyl and naphthyl ring protons of Dpx. Additional molecular dynamics calculations identified the energetically most favoured species for both the Dpx[middle dot]CyD and 2Dpx[middle dot]CyD complexes.