Solid-state characterization of non-stoichiometric hydrates of ester-type local anaesthetics: Part XI. Crystal polymorphism of local anaesthetic drugs

Abstract

Three structurally closely related local anaesthetic drugs, hydroxyprocaine hydrochloride (4-butylamino-2-hydroxybenzoic acid 2-dimethylaminoethyl ester hydrochloride, HPCHC), tetracaine hydrochloride (4-butylamino-2-hydroxybenzoic acid 2-dimethylaminoethyl ester hydrochloride, TCHC) and hydroxytetracaine hydrochloride (4-butylamino-2-hydroxybenzoic acid 2-dimethylaminoethyl ester hydrochloride, SLCHC) are found to form hydrated crystals. Those were characterized by thermal analysis (hot stage microscopy, differential scanning calorimetry, thermogravimetry), vibrational spectroscopic methods (FTIR-, FT-Raman-spectroscopy), powder X-ray diffractometry, solid-state NMR and water sorption/desorption analysis. The formation and the stability of the hydrated solid phases are evaluated by sorption isotherms derived from different sorption/desorption analytic methods. The three substances investigated show conformational polymorphism with the anhydrated phases including a high temperature form mod. I, which is highly hygroscopic and isostructural with the hydrate. The hydrated form is present in commercial products at various contents. These hemihydrates crystallize from water, whereas the anhydrates crystallize from all other tested organic solvents. Different methods of water sorption/desorption analysis indicate the formation of non-stoichiometric hydrates. Different methods of drying lead to the same results. Solid-state NMR spectra were used to obtain both structural and molecular level mobility information.