Vitrification of two active pharmaceutical ingredients by fast scanning calorimetry: From structural relaxation to nucleation phenomena

Cinchonidine and Theophylline vitrification abilities have been investigated by differential and fast scanning calorimetry. These active pharmaceutical compounds are known in the literature to have a very high tendency to crystallize which has been confirmed by classical differential scanning calorimetry. Due to the growing interest in amorphous pharmaceutical compounds, their possible vitrifications have been investigated by fast scanning calorimetry. This work shows the high potential of this advanced thermal analysis technique to investigate the vitrification of active pharmaceutical compounds by melt-quenching protocol. For the first time, glass transitions of Cinchonidine and Theophylline were measured. From Cinchonidine, it has been shown that complete glassy state can be obtained by cooling from the melt at 2000 K/s. Crystallization has also been suppressed by cooling down from the melt at 2 K/s. However, such rate does not avoid the formation of nuclei. Theophylline crystallization process has been suppressed by a melt-quenching protocol carried out with a cooling rate of 4000 K/s. However, the phenomenon of nuclei formation upon cooling seems unavoidable at this cooling rate. For both active pharmaceutical compounds, physical aging has been observed to play a role on the nuclei formation below the glass transition leading to modify the subsequent crystallization.