Improving the Solubility of Formononetin and Enabling Hydrogel-Based Wound-Oriented Applications Through the Effect of Hot-Melt Extrusion

Isoflavones are plant-derived polyphenols with broad biological activity; however, their application in topical formulations is limited by poor aqueous solubility. The aim of this study was to enhance the aqueous solubility of formononetin using a solvent-free hot-melt extrusion (HME) approach and to enable its incorporation into a hydrogel formulation suitable for skin delivery. Amorphous formononetin-based systems were prepared by HME using polymeric carriers and hydroxypropyl-β-cyclodextrin, with and without prior inclusion complex formation. The resulting formulations were characterized using XRPD, DSC, and FT-IR/ATR to assess amorphization and intermolecular interactions. Aqueous solubility and skin permeability were evaluated using solubility testing, PAMPA, and Franz diffusion cells. The optimized amorphous system exhibited a substantial increase in apparent aqueous solubility compared to crystalline formononetin while maintaining comparable permeability. Cyclodextrin–formononetin interactions were effectively generated during the extrusion process, rendering pre-inclusion unnecessary. The selected system was successfully incorporated into a hydrogel matrix. This study demonstrates that solvent-free HME combined with cyclodextrins is an effective strategy for improving formononetin solubility and enabling its application in hydrogel-based topical delivery systems.