Crystal forms of the hydrogen oxalate salt of o-desmethylvenlafaxine

OBJECTIVES: To prepare new crystalline forms of the antidepressant o-desmethylvenlafaxine salt as potential new commercial forms and evaluate their physicochemical properties, in particular the dissolution rate. METHODS: A new hydrogen oxalate salt of o-desmethylvenlafaxine hydrogen oxalate (ODV-OX) was synthesized, and a polymorph screening was performed using different solvents and crystallization conditions. Crystalline forms were characterized by a combination of solid-state techniques: X-ray powder diffraction, differential scanning calorimetry, thermogravimetric analysis, FT-IR spectroscopy and single crystal X-ray diffraction. The stability of all crystalline phases was tested under International Conference on Harmonisation (ICH) conditions (40°C and 75% Relative Humidity (RH)) for 1 week. Dissolution tests were performed on the hydrogen oxalate salt ODV-OX Form 1 and compared with dissolution test on the commercial form of the succinate salt of o-desmethylvenlafaxine. KEY FINDINGS: Five crystalline forms of ODV-OX were isolated, namely three hydrated forms (Form 1, Form 2, Form 3) and two anhydrous forms (Form 4 and Form 5). CONCLUSIONS: Comparative solubility tests on ODV-OX Form 1 and o-desmethylvenlafaxine succinate evidenced a significant increase in solubility for the hydrogen oxalate salt (142g/l) with respect to the succinate salt (70g/l).