Polyethylene Glycol-Assisted Melt Crystallization of Two New Piroxicam Polymorphs Revealed by 3D Electron Diffraction

Two new polymorphs (forms VIII and IX) of piroxicam were discovered through poly(ethylene glycol) (PEG)-assisted melt crystallization, and their structures were revealed by 3D electron diffraction (3D ED). This discovery provides insight into the potential of PEG in pharmaceutical polymorph discovery and verifies the significance of 3D ED as an essential technique for structural determination of pharmaceuticals. Furthermore, the direct contribution of intermolecular hydrogen bonding to melting points was discussed based on the structural divergency between the newly solved form VIII and the previously reported form IV. Combining PEG-assisted melt crystallization and 3D ED not only accelerated the discovery of new polymorphs but also provided unique opportunities for understanding structure–property relationships in pharmaceutical crystals.