Characterization of an inclusion complex of cholesterol and hydroxypropyl-β-cyclodextrin

Interactions between endogenous cholesterol and cyclodextrins have been investigated by several researchers, and they found altered skin penetration of some drugs, membrane disruption, and extraction of cholesterol from the large lipoprotein particles or animal fat. In the present study, an inclusion complex composed of cholesterol and hydroxypropyl-β-cyclodextrin (HPβCD) prepared by lyophilization was investigated and characterized in order to confirm these interactions. Five grams of cholesterol were dispersed in 50 ml of 73.2 mM HPβCD aqueous solution, mixed for 2 days, and the filtrate lyophilized. A phase solubility study was performed by mixing an excess amount of cholesterol with an aqueous solution containing increasing amounts of HPβCD. The amount of cholesterol in solution after mixing for 2 days at 25°C was determined by HPLC. The inclusion complex was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry, and differential scanning calorimetry (DSC). An Ap-type Higuchi phase solubility diagram, DSC, FTIR, and X-ray diffraction demonstrated the formation of an inclusion complex. DSC thermograms indicated that the endothermic peaks of cholesterol and physical mixture of cholesterol with HPβCD due to the fusion of drug crystals, were absent in DSC thermograms obtained on the freeze dried inclusion complex. FTIR spectra indicated that some of the absorption peaks in the lyophilized inclusion complex were different from that of the physical mixture of cholesterol and HPβCD. X-ray diffraction patterns showed that the pure cholesterol and a physical mixture of cholesterol and HPβCD exhibited crystalline characteristics whereas the lyophilized inclusion complex and HPβCD displayed amorphous characteristics. The results indicated that the formation of a cholesterol/HPβCD inclusion complex is more water soluble than cholesterol alone.