Evaluating Kinetic Solubility - A Comparative Study of Analytical Approaches Using UV-based Evaluation Methods

Supersaturation-based formulation strategies are widely used to enhance the oral bioavailability of poorly water-soluble drugs, yet the accurate identification of precipitation onset (kinetic solubility) remains method-dependent and poorly standardized. In this study, the kinetic solubility of eight model compounds was systematically investigated using in situ UV spectroscopy and dynamic light scattering (DLS), with UV data evaluated using baseline elevation, second-derivative analysis, and the zero-intercept method (ZIM). Baseline elevation and second-derivative approaches showed strong agreement, detecting precipitation at similar concentrations corresponding to the formation of optically detectable particles. In contrast, ZIM consistently identified transitions at lower concentrations, indicating higher sensitivity to early aggregation events. DLS measurements confirmed the presence of particles at concentrations corresponding to ZIM-detected endpoints, supporting its association with the initial stages of phase separation. In some cases, liquid-liquid phase separation (LLPS) was observed, with ZIM detecting early phase transitions prior to visible liquid droplets. Comparative statistical analysis proved that the second derivative and baseline methods provided the highest repeatability (within-unit %CV 3.5% and 4.8%, respectively), whereas ZIM-based approaches showed greater variability despite their higher sensitivity. The combination of ZIM and DLS provides a more comprehensive understanding of the onset of precipitation and supersaturation behaviour of poorly soluble drugs, supporting improved characterization and kinetic solubility evaluation in early formulation development.