Host–guest inclusion complex of quercetin and hydroxypropyl-β-cyclodextrin: A calorimetric study

Quercetin (QCT), a flavonoid derived from many fruits and vegetables, is endowed with manifold biological properties, such as the ability to elicit a strong inhibitory effect on the growth of several tumor cell lines. Unfortunately, the pharmacological application of QCT is severely restricted by its inherent hydrophobicity and consequent low in vivo bioavailability. The therapeutic potential of QCT can be unraveled by enhancing its solubility through the formation of a host–guest complex with hydroxypropyl-β-cyclodextrin (HPβCD). In this study, HPβCD·QCT complex has been obtained in liquid phase, at 37 °C and under a prolonged mixing (72 h), and using two buffers at pH = 3.6 and pH = 8.0. Phase solubility and differential scanning calorimetry (DSC) studies revealed that, at pH = 8.0, the complex was obtained with a 1:1 stoichiometric ratio and a strong enhancement of QCT solubility, while in acidic buffer complex formation was significantly thwarted. The affinity constant was calculated by isothermal calorimetry at pH = 8 and was found to be 489 ± 38 M⁻¹, in good agreement with the value indirectly obtained from phase solubility tests 394 ± 101 M⁻¹. The results confirmed the formation of the inclusion complex between QCT and HPβCD and highlight the importance of the choice of the appropriate solvent, pH, temperature and mixing time on the formation of host guest inclusion complex with active ingredient(s) and HPβCD.