Implications of crystal disorder on the solid-state stability of olanzapine

Mechanical perturbations of drug during solid pharmaceutical processing like milling can often generate crystal disorder posing serious implications to drug's stability. While physical changes like amorphization, recrystallization, polymorphism of the disordered drugs are extensively studied and reported in the literature, the propensities and inter-dependencies of recrystallization and degradation of disordered drugs have seldom received deep attention. Previous investigations from our lab have explored some of these interplays, aiming to develop predictive stability models. As a follow-up, the implication of crystal disorder on the oxidative instability of Olanzapine (OLA) during accelerated storage is investigated in this work. Cryo-milling OLA at varied time intervals generated different extents of crystal disorder. The milled samples were characterized using calorimetry and infrared (IR) spectroscopy to examine the physical state, while their degradation was evaluated using ultra-performance liquid chromatographic methods. An X-ray amorphous OLA sample was generated by melt-cooling, and used as an amorphous reference. The crystallinity of the cryo-milled samples was quantified using a partial least square regression model based on ATR-FTIR spectroscopic data. The cryo-milled samples were exposed to different accelerated stability conditions along with crystalline (unmilled) and quench cooled (amorphous) samples, serving as controls. At periodic intervals, samples were removed from the stability storage, and analyzed using ATR-FTIR and UPLC methods to quantify the crystallinity- and degradation extents. A positive relation was witnessed between the initial degree of crystallinity and degradation kinetics of the disordered OLA samples during stability storage indicating a strong dependency of degradation on the disorder contents for such disordered solids. The results obtained in this study can potentially explain consequences of inter-batch variations of drugs during stability storage, in addition to enabling de-risking strategies towards eliminating solid drug instabilities in product development.