Solubility analysis of venlafaxine hydrochloride polymorphs by shake-flask method and real time monitoring

Aims: The aqueous solubility of two polymorphic forms of venlafaxine hydrochloride was investigated. Methods: The pH-dependent solubility (SpH) over a wide pH range was measured by saturation shake-flask (SSF) method at 25 °C. The solubility of the free base form was depicted by the intrinsic solubility (So). To identify the solid form present at the solubility equilibrium, X-ray powder diffraction (XRPD) and Raman spectroscopy was carried out. The dissolution was studied using real time concentration monitoring applying fiber optic UV probes. Results: No difference was found in the SpH values of Form I and Form II, in the pH range 7.5-12. Solid phase isolated from pH 10-12 suspensions was identified as free base by XRPD and Raman spectroscopy. Precipitates separated from pH 7-8 samples were also identical product. The transition of polymorphs to the free base was supported by the real time dissolution analysis. Conclusion: In this study we demonstrated a good agreement of equilibrium solubility measured by SSF method and in-situ UV fiber optic method. µDISS ProfilerTM has the advantage to provide much more information about dissolution process; with this approach the dissolution kinetic, the supersaturation and the time needed to reach the equilibrium can be easily monitored.