Synergistic combination of quercetin and kaempferol via milk exosome-based emulsion for enhanced antioxidant and anti-skin aging efficacy.

The natural flavonoids quercetin (Que) and kaempferol (Kae) show great potential for anti-aging applications but suffer from poor solubility and skin permeability. This study aimed to develop a synergistic topical emulsion system to overcome the poor bioavailability of the Que and Kae for enhanced efficacy. The optimal synergistic molar ratio (Que:Kae = 1:2) was first identified via 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging and cellular assays. The drugs were then loaded into milk-derived exosomes (mExo) using a Box-Behnken design (BBD), achieving high encapsulation (>69%), a size of ∼120 nm, and a polydispersity index (PDI) of 0.180. This complex was incorporated into an oil-in-water emulsion, with its formulation also BBD-optimized for stability. In a D-galactose-induced skin aging mouse model, the final Que/Kae-mExo@Eml formulation significantly outperformed control groups (free drug mixture, blank exosome emulsion, and vitamin C). It most effectively reduced oxidative stress (malondialdehyde, MDA), enhanced antioxidant enzymes (superoxide dismutase, SOD and glutathione, GSH), suppressed matrix metalloproteinase-1 (MMP-1), promoted collagen I (Col I) synthesis, and improved skin histology, elasticity, and hydration. In conclusion, an optimized, stable emulsion leveraging a synergistic 1:2 drug ratio and an mExo delivery system was successfully developed, demonstrating superior comprehensive efficacy against skin aging.