Solid state characterization, solubility, intrinsic dissolution and stability behavior of allopurinol hydrochloride salt

Since each solid form of an active pharmaceutical ingredient (API) can exhibit particular physicochemical properties, the objectives of this work were to characterize and study the solubility/stability properties of allopurinol hydrochloride salt (ALO-HCl) for the first time. ALO-HCl was obtained through an unreported recrystallization process and studied by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). All characterization techniques were effective for the differentiation of ALO-HCl from the preferred pharmaceutical form (ALO). DSC and TGA studies showed a solid-state conversion from ALO-HCl to ALO upon HCl loss. Solubility and dissolution tests showed that ALO-HCl converts to ALO in aqueous media. Moreover, the effect of the common ion decreased the amount of drug released from ALO-HCl during the intrinsic dissolution assay in HCl medium. The stability studies showed a partial conversion from ALO-HCl to ALO after 6 months of storage. The results indicate that comparative studies between crystalline forms of APIs are of great importance, as they contribute to the understanding of aspects related to the quality of medicines.