Machine-Vision-Enabled Salt Dissolution Analysis

Salt formation is a well-established method to increase the solubility of ionizable drug candidates. However, possible conversion of salt to its original form of free acid or base-disproportionation-can have a drastic effect on the solubility and consequently the bioavailability of a drug. Therefore, during the salt selection process, the salt dissolution behavior should be well understood. Improved understanding could be achieved by a method that enables simultaneous screening of small sample amounts and detailed dissolution process analysis. Here, we use a machine-vision-based single-particle analysis (SPA) method to successfully determine the pH-solubility profile, intrinsic solubility, common-ion effect, pK_a, pH_max, and K_sp values of three model compounds in a fast and low sample consumption (<1 mg) manner. Moreover, the SPA method enables, with a particle-scale resolution, in situ observation of the disproportionation process and its immediate effect on the morphology and solubility of dissolving species. In this study, a potentially higher energy thermodynamic solid-state form of diclofenac free acid and an intriguing conversion to liquid verapamil free base were observed upon disproportionation of the respective salts. As such, the SPA method offers a low sample consumption platform for fast yet elaborate characterization of the salt dissolution behavior.