Evaluation of amorphous ursodeoxycholic acid by thermal methods

PURPOSE: The purpose of this study was to characterize the amorphous state of ursodeoxycholic acid (UDCA) samples by using isothermal microcalorimetry, X-ray diffraction, infrared (IR) spectroscopy and solid state carbon 13 nuclear magnetic resonance (13C-NMR) spectroscopy, and to demonstrate the application of the thermal methods (microcalorimetry and differential scanning calorimetry (DSC) for studying the amorphous state and clarifying the dissolution mechanism of UDCA. METHODS: Amorphous UDCA was prepared by grinding and rapid cooling of the melts. The heat of solution of UDCA was measured by an isothermal heat-conduction twin microcalorimeter at 25.0 degrees C. Some physicochemical properties of amorphous UDCA were also studied. RESULTS: The intensities of X-ray diffraction peaks of crystalline UDCA decreased with an increase in grinding time. The heat levels of solution of crystalline UDCA and UDCA ground for 1 min were endothermic, and became exothermic with an increase in grinding time. A good correlation was obtained between the heat of solution and the heat of crystallization determined from the peak area in DSC. Although no significant difference was observed in X-ray diffraction patterns of amorphous UDCA prepared by the two methods, significant differences were recognized in DSC, IR and 13C-NMR, and the heat of solution indicated different values among the two samples. The stability of amorphous UDCA samples stored under 74.5% relative humidity at 40 degrees C was found to depend upon the preparation methods. CONCLUSIONS: Different states of amorphous UDCA were obtained depending on the preparation method. The application of thermal methods to evaluate the amorphous state was demonstrated. The mechanism of dissolution of UDCA was discussed from the results of the heat of solution examination.