Phase Diagram and Dissolution-Rate Studies on Sulfathiazole-Urea Solid Dispersions

Abstract

The phase diagram of the sulfathiazole-urea binary system was determined from the physical mixtures of sulfathiazole Form I-urea and sulfathiazole Form II-urea through differential thermal analysis (DTA) and conventional capillary tube methods. The eutectic temperature and the final melting points were found to be different for the two polymorphic systems. The mutual solid solubilities in solid dispersion forms were confirmed by DTA and X-ray diffraction methods. The characteristics of DTA thermogram, X-ray diffraction spectrum, and in vitro dissolution rates of the eutectic mixture containing 52% (w/w) of sulfathiazole were found to change with aging. This may have a significant effect on the clinical application of sulfathiazole. The dissolution rates of sulfathiazole in solid solutions of urea were found to be extremely fast, over 700 times higher than the pure compound.