Abstract: AimTo investigate the complexation of prostaglandin E₁(PGE₁) with hydroxylpropyl-β-cyclodextrin (HP-β-CD) in aqueous solutions, inclusion molar ratio of the host and guest molecules and change of thermodynamic parameters during the complexation process. MethodsThe measurements of the complexation mechanism, inclusion molar ratio of the host and guest molecules and change of thermodynamic parameters were carried out by the following methods separately: phase solubility method, UV absorption spectroscopy, circular dichroism spectroscopy, equimolar series method and thermodynamic method, respectively. ResultsThat all the phase solubility diagrams showed a typical A_L-type in various pH buffered solutions, suggested the formation of a soluble complex of 1∶1 molar ratio. Both UV absorption spectroscopy and circular dichroism spectroscopy confirmed that the significant interaction between the host and guest molecules was probably due to the inclusion of chromophore moiety of PGE₁molecule into the hydrophobic cavity of HP-β-CD molecule. The change in the thermodynamic parameters suggested that the complexation could proceed spontaneously along with the release of heat and the decrease of entropy. ConclusionAn 1∶1 molar ratio inclusion complex of PGE₁with HP-β-CD could be formed spontaneously and, hence, the solubility of PGE₁in aqueous solution increased. Appropriate temperature and suitable media pH probably favor the progress of complexation procedure.