The study of different solid forms of Emodepside

Emodepside is an endoparasiticide used in veterinary drugs. It exists in four different crystal forms which were characterised using DSC, TGA, evolved gas analysis (using FT-IR spectroscopy), hot-stage microscopy, FT-Raman, FT-IR and FT-NIR spectroscopy and powder X-ray diffraction. Thermal analysis showed that the forms II–IV contain considerable amounts of water being easily lost upon heating. Hot-stage microscopy indicated that the crystal structures do not change upon the loss of water. To examine whether the forms II–IV are hydrates or modifications of Emodepside having water adsorbed to the crystal surface and/or absorbed into disordered defect regions, the behaviour of the forms at different relative humidities was analysed using FT-Raman and FT-NIR spectroscopy and vapour sorption analysis. Changes in the CO stretching region in the Raman spectra upon the removal of water revealed that all three forms represent non-stoichiometric hydrates forming isomorphic dehydrates. Sorption analysis indicated the presence of localised water molecules in the structure of form IV. Several OH combination bands of water were found in the NIR spectrum of each form indicating differently bound water molecules. Crystal structure analysis of form IV at ambient humidity revealed four well-defined water positions in the asymmetric unit.