High-throughput analysis of aqueous drug solubility, supersaturation, and aggregation behaviour using second harmonic light scattering

Aqueous solubility is a crucial physicochemical property influencing drug absorption and bioavailability. Current solubility assays, whether assessing thermodynamic or kinetic solubility, involve trade-offs between accuracy, detection limit, speed, and resource consumption. Therefore, this study introduces a novel approach to drug solubility assessment based on non-resonant second harmonic scattering (SHS), which detects interfacial fluctuations of water molecules surrounding solutes. The apparent solubility of 14 poorly water-soluble model drugs was measured and compared to high pressure liquid chromatography (HPLC) data. Furthermore, the supersaturation propensity, defined as the ratio of solubility measured at one hour to that at 24 h, was evaluated for all 14 compounds. Lastly, the self-assembly behaviour was investigated, using sodium lauryl sulphate (SLS) as a reference system to benchmark micellization in the given forward-scattering SHS platform. The results showed a strong correlation between the SHS and HPLC solubility data (r = 0.9273). Supersaturation propensity was assessed and linked to the glass-forming ability and thermal properties of the drugs, whereby ketoconazole and tamoxifen exhibited the best supersaturation performance. Moreover, the critical micelle concentration of SLS appeared as a local minimum following a peak in SHS intensity, reflecting an increase in structural bulk centrosymmetry due to micelle formation. Similar micelle-like patterns were observed for five model drugs (i.e., amiodarone, felodipine, meclizine, tamoxifen, torcetrapib), suggesting the formation of self-assembled structures at concentrations above the solubility limit. These findings demonstrate the potential of non-resonant SHS as a promising analytical tool for solubility determination, offering a versatile, dynamic and high-throughput format with minimal compound and solvent consumption, while also providing insights into drug aggregation or self-assembly at the molecular level.