Phase transitions in secnidazole: Thermal stability and polymorphism studied by X-ray powder diffraction, thermal analysis and vibrational spectroscopy

Secnidazole is an old drug efficient as an antimicrobial agent used against some anaerobic bacteria and parasites, such as giardia, ameba and trichomonas that are causing some neglected diseases. This drug is commercialized as a hemihydrate, being stable at room temperature, but showing relevant phase transitions on heating. The understanding of the dehydration process on solvates provides valuable information for quality control and development of solid formulations. In order to study the thermal behavior of secnidazole, Raman spectroscopy, X-ray powder diffraction, hot-stage microscopy and thermal analysis were employed to verify the structural modifications involved with this drug. Raman spectroscopy was a key technique to monitor the dehydration process in situ, providing the spectral fingerprints for identifying the anhydrous form. In addition, the reported results suggest the existence of two anhydrous monotropic polymorphs.