Evaluation of different calorimetric methods to determine the glass transition temperature and molecular mobility below Tg for amorphous drugs

The purpose of the present study was to compare different calorimetric methods used to determine the glass transition temperature (Tg) and to evaluate the relaxation behaviour and hence the stability of amorphous drugs below their Tg. Data showed that the values of the activation energy for the transition of a glass to its super-cooled liquid state qualitatively correlate with the values of the mean molecular relaxation time constant of ketoconazole, itraconazole and miconazole, three structurally related drugs. Estimation of the molecular mobility by activation energy calculation indicated that loperamide was more stable than its two building blocks T263 and R731. It was further shown that the most commonly used approach to determine Tg (Tg1/2cp) leads to erroneous values when enthalpy recovery is significant. In this case, an alternative method based on enthalpic considerations leads to results in accordance to basic thermodynamics. Estimation of molecular mobility based on activation energy calculations is therefore considered to be a valuable alternative for the method based on measurement of the extent of relaxation. When enthalpy relaxation is important, the use of Tg1/2cp leads to an overestimation of the Tg.