Real-time in situ X-ray micro-computed tomography study of the effect of impurities on the crystallization of amorphous nifedipine

Controlling the physical stability of noncrystalline active pharmaceutical ingredients remains a major challenge in the development of amorphous formulations such as amorphous solid-dispersion (ASD) formulations. To establish new evaluation and formulation strategies, the spatial distribution of the crystal phase in bulk amorphous nifedipine (NFD) was investigated as a model. The crystallization of amorphous NFD and the effect of a deliberately added impurity were investigated using powder X-ray diffraction (PXRD), differential scanning calorimetry and real-time in situ X-ray micro-computed tomography (X-ray CT). The stability data of amorphous samples, i.e., NFD and a mixture of NFD with an oxidative degradation product of NFD, impurity A (Imp A), at a weight ratio of 90:10, presented as percent amorphous remaining, suggests that Imp A accelerates the bulk crystal growth of NFD. Real-time in situ X-ray CT results showed surface-enhanced crystal growth and cavity formation in solid NFD samples. Moreover, the crystals were heterogeneous in density. These results suggest that Imp A affects the physical stability of the amorphous NFD. X-ray CT equipped with a heating unit can aid in-situ evaluation and assessment of physicochemical properties and physical stability of amorphous samples and formulations.