Phase Behavior of Drug-Lipid-Surfactant Ternary Systems toward Understanding the Annealing-Induced Change

The present study systematically investigates the effect of annealing conditions and the Kolliphor P 407 content on the physicochemical and structural properties of Compritol (glyceryl behenate) and ternary systems prepared via melt cooling (Kolliphor P 407, Compritol, and a hydrophilic API) representing solid-lipid formulations. The physical properties of Compritol and the ternary systems with varying ratios of Compritol and Kolliphor P 407 were characterized using differential scanning calorimetry (DSC), small- A nd wide-angle X-ray scattering (SWAXS) and infrared (IR) spectroscopy, and hot-stage microscopy (HSM), before and after annealing. The change in the chemical profiles of different Compritol components as a function of annealing was evaluated using 1H NMR spectroscopy. While no change in the polymorphic form of API and Kolliphor P 407 occurred during annealing, a systematic conversion of the α-to β-form was observed in the case of Compritol. Furthermore, the polymorphic transformation of Compritol was found to be dependent on the Kolliphor P 407 content. As per the Flory-Huggins mixing theory, higher miscibility was observed in the case of monobehenin-Kolliphor P 407, monobehenin-dibehenin, and dibehenin-tribehenin binary mixtures. The miscibility of Kolliphor P 407 with monobehenin and 1,2-dibehenin was confirmed by 1H NMR analysis. The observed higher miscibility of Kolliphor P 407 with monobehenin and 1,2-dibehenin is proposed as the trigger for the physical separation from the 1,3-diglyceride and triglycerides during melt solidification of the formulations. The phase separation is postulated as the mechanism underlying the formation of a stable β-polymorphic form (a native form of 1,3-diglyceride) of Compritol upon annealing. This finding is expected to have an important implication for developing stable solid-lipid-surfactant-based drug formulations.