An Investigation into the Dehydration Behavior of Paroxetine HCl Form I Using a Combination of Thermal and Diffraction Methods: The Identification and Characterization of a New Anhydrous Form

Paroxetine HCl can exist as a non-stoichiometric hydrate (Form II) or as a stoichiometric hemihydrate (Form I); the latter is considered to be the stable form and its structure is well-known. However, little work has been performed to investigate its dehydration behavior, hence the generation of the anhydrous form via dehydration of Form I was investigated. A combination of thermal (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)) and diffraction (variable temperature X-ray powder diffraction (VT-XRPD)) techniques were used. Dehydrated Form I was prepared using ultra-dry conditions and the resulting product compared to dehydrated Form II. DSC indicated that the two dehydrated forms of Form I and II had distinct melting points. TGA experiments allowed the calculation of the activation energy for the dehydration of Form I, which varies between 86?114 kJ/mol. Pawley refinement of the VT-XRPD data suggested that Form I dehydrates to an isostructural anhydrate, since the unit cell parameters of this new form were very similar to those of Form I with only a smaller volume as consequence of dehydration. Comparison with dehydrated Form II indicated that these two forms represent different crystal entities, hence a new anhydrous form of paroxetine HCl has been identified. Paroxetine HCl can exist as a non-stoichiometric hydrate (Form II) or as a stoichiometric hemihydrate (Form I); the latter is considered to be the stable form and its structure is well-known. However, little work has been performed to investigate its dehydration behavior, hence the generation of the anhydrous form via dehydration of Form I was investigated. A combination of thermal (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)) and diffraction (variable temperature X-ray powder diffraction (VT-XRPD)) techniques were used. Dehydrated Form I was prepared using ultra-dry conditions and the resulting product compared to dehydrated Form II. DSC indicated that the two dehydrated forms of Form I and II had distinct melting points. TGA experiments allowed the calculation of the activation energy for the dehydration of Form I, which varies between 86?114 kJ/mol. Pawley refinement of the VT-XRPD data suggested that Form I dehydrates to an isostructural anhydrate, since the unit cell parameters of this new form were very similar to those of Form I with only a smaller volume as consequence of dehydration. Comparison with dehydrated Form II indicated that these two forms represent different crystal entities, hence a new anhydrous form of paroxetine HCl has been identified.