Solid-state characterisation of spironolactone 1/3 hydrate.

Spironolactone (SPR) is a poorly water-soluble drug widely used for the treatment of various diseases. The objective of this study was to carry out the preparation and solid-state characterization of SPR 1/3 hydrate. The solid form was generated by an unreported recrystallization process in acetone and characterized for the first time by a combination of X-ray powder diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), equilibrium solubility, and an accelerated stability study. XRD, DSC, and TGA studies revealed that SPR 1/3 hydrated converts completely to form II after heating to 180°C. Solubility studies at 37°C showed that SPR 1/3 hydrate was statistically less soluble than SPR form II in all tested media and that SPR form II partially converts to SPR 1/3 hydrate in aqueous media. Accelerated stability studies demonstrated that both forms were physically and chemically stable up to 6 months (40°C/75% RH). We concluded that contamination of SPR 1/3 hydrate in SPR raw materials is undesirable. Taking this into account we recommend its polymorphic monitoring either in active pharmaceutical ingredients or commercial tablets by solid-state identification/quantification methods (XRD, DSC, TGA, and FTIR). Of these, XRD proved to be the most conclusive and accurate.