Application and limitations of thermogravimetric analysis to delineate the hot melt extrusion chemical stability processing window.

Thermogravimetric analysis (TGA) is frequently used to define the threshold of acceptable processing temperatures for hot melt extrusion. Herein, evaluation of chemical stability of amorphous drug and polymer systems was assessed by a critical evaluation of TGA nonisothermal and isothermal methods. Nonisothermal analysis of three crystalline APIs of high glass-forming ability (posaconazole, indomethacin, and bicalutamide), as well as six common polymers, identified a degradation onset temperature that ranged from 52 to 170 °C, depending on heating rate and degradation detection method employed. In particular, the tangent method significantly overestimated the onset of acceptable levels of degradation, while weight loss threshold criteria were more suitable. Isothermal analysis provided a more direct indication of chemical stability, however neat amorphous materials are likely to recrystallize. By forming an amorphous solid dispersion, the polymer can stabilize the amorphous drug against recrystallization, enabling isothermal analysis of chemical degradation. However, TGA mass loss of volatiles should be considered only as an approximate indicator of degradation, as actual potency loss is likely to be significantly higher; this was confirmed by high performance liquid chromatographic analysis of samples. TGA methods should be selected to generate highly sensitive outcomes, and caution should be applied when extrapolating suitability of processing conditions.