Nucleation in Amorphous Terfenadine at a Temperature Much Lower than the Glass Transition Temperature and Its Impact on Physical Stability

In this study, the crystallization behavior of amorphous terfenadine (TFD) was investigated with a focus on nucleation temperature. The cold crystallization behavior of amorphous TFD annealed at various temperatures and the resultant crystal form were evaluated by using differential scanning calorimetry and powder X-ray diffraction. Samples annealed at -20 °C provided the lowest cold crystallization temperature and the highest proportion of form II, indicating that nucleation for form II was enhanced at a temperature much lower than the glass transition temperature (58 °C). Initiation time for isothermal crystallization of TFD at 100 °C was shortened by 30% by applying annealing at -20 °C beforehand. In contrast, dissolution and supersaturation behaviors were not affected by nucleation, as the crystal form of the precipitate was different from that induced by annealing. The isoconversional Kissinger-Akahira-Sunose model was applied to analyze the cold crystallization kinetics to find an increase in the apparent frequency factor for samples annealed at -20 °C, suggesting the presence of preformed crystal nuclei. Lastly, the anomalous nucleation behavior of amorphous TFD at low temperatures was discussed from the framework of classical nucleation theory and molecular mobility. This study provides important insights into the nucleation and crystal growth behaviors of amorphous pharmaceutical compounds.