Simultaneous XRD-DSC identifies correct drug-polymer solubility and miscibility for enantiotropic solid forms.

Thermodynamic properties, including solubility and miscibility, which are highly correlated with amorphous solid dispersion physical stability were identified for the complex solid forms of bromopropamide using simultaneous X-ray diffraction (XRD)-differential scanning calorimetry (DSC). The most stable solid form of bromopropamide was crystallized and its crystal structure was solved. The crystallized material was characterized using simultaneous XRD-DSC measurements, which allowed dual analyses of a single sample. Transitions of bromopropamide during heating resulted in observation of the unique diffraction patterns of its different solid forms. The dissolution endpoint (T_end) was measured for various mixtures of bromopropamide and polyvinylpyrrolidone-vinyl acetate random copolymer (PVPVA). The use of XRD-DSC allowed confident and accurate measurements of the T_end for a large range of compositions, assisting in the estimation of drug-polymer solubility and miscibility. Thermodynamic properties identified using combined XRD-DSC were further compared to those obtained using only DSC data. It was found that DSC data in isolation can lead to ambiguity, misinterpretations, and incorrect conclusions, especially for a solid demonstrating multiple, closely related forms.