Miniaturized assay for solubility and residual solid screening (SORESOS) in early drug development

Purpose. The aim was to develop a miniaturized method for solubility and residual solid screening of drug compounds in aqueous and non-aqueous vehicles in early drug development. Methods. Different crystal modifications of caffeine, carbamazepine, and piroxicam were added into 96-well filter plates and solubility was determined in 100 mu l of 17 pharmaceutical vehicles. After filtration, drug concentration was determined by Ultra Performance Liquid Chromatography (TM) (UPLC). Residual solid drug in the filter plates was analyzed by high-throughput (HT) transmission X-ray Powder Diffraction (XRPD). Results. HT XRPD analysis revealed solid form conversions of all compounds during solubility determination, e.g., formation of hydrates in aqueous vehicles (caffeine, carbamazepine, piroxicam) or conversion of a metastable crystal form to the stable form (caffeine). Drug solubility was strongly dependent on the crystal modifications formed during the solubility assay. Conclusions. The new assay allows the simultaneous, small scale screening of drug solubility in various pharmaceutical vehicles and identification of changes in solid form. It is useful for the identification of formulations and formulation options in non-clinical and clinical development.