Solid dispersions of diflunisal-PVP: polymorphic and amorphous states of the drug

Coprecipitates of diflunisal and polyvinylpyrrolidone (PVP K15, K30, and K90) and physical mixtures were studied using x-ray diffraction analysis, infrared (IR) spectroscopy, differential scanning calorimetry (DSC), and hot-stage microscopy. X-ray diffraction results revealed an almost amorphous state, even in coprecipitates with a high content of drug, next to 70%, which was independent of the polymer molecular weight. The IR spectra of 70:30 drug-PVP solid dispersions suggest the formation of diflunisal-PVP hydrogen bonds. For 70:30 drug-polymer ratio, the physical mixture showed linear dissolution kinetics of free crystals, but the corresponding coprecipitates exhibit two different dissolution processes. When the 25:75 drug-polymer dispersion is analyzed by hot-stage microscopy, only solid plates of PVP are observed; the absence of drug particles may be due to a molecular dispersion of the drug into the polymer. Moreover, polymorphic changes of diflunisal were detected in the solid dispersions in comparison with the corresponding physical mixtures, which are always formed by polymorph II. At high concentrations of drug (75:25 and 80:20), x-ray diffraction patterns of solid dispersions showed the partial recrystallization of the drug, displaying the main diffraction peaks of polymorph I when ethanol was used as coprecipitation solvent, whereas diflunisal form IV was obtained in chloroform.