Menthol–Fatty Acid HDES Boosts In Vitro Oral Bioavailability of Oleanolic Acid via Synergistic Digestive Release and Cellular Absorption

To improve the oral bioavailability of oleanolic acid (OA), this study developed a menthol-fatty acid-based hydrophobic deep eutectic solvent (HDES) system. Through a comprehensive evaluation using in vitro simulated digestion and Caco-2 cell transport models, the short-chain HDES was found to increase the apparent in vitro bioavailability index of OA by 9.3-fold compared to conventional ethanol systems, with efficacy showing clear fatty acid chain-length dependence. The mechanism was systematically investigated through spectral characterization and cellular studies, revealing a two-stage enhancement process: during the digestion phase, HDES significantly improved OA bioaccessibility to 14.30% compared to 4.90% with ethanol; during the absorption phase, it markedly increased cellular uptake to 25.79% versus 4.71% with ethanol. Molecular analysis indicated that the optimal hydrophobicity and diffusion properties of HDES contributed to this enhancement. This study reveals a fatty acid chain-length-dependent mechanism in HDES-facilitated OA delivery, providing a tunable strategy for enhancing the absorption of hydrophobic bioactive compounds.