Liposomal Delivery of Olea europaea L. Leaf Polyphenols: From Extraction to Functional Evaluation in a Hyperglycemia Cell Model

Background/Objectives: Olive leaf polyphenols exhibit strong antioxidant and antidiabetic potential. However, their application in nutraceutical and pharmaceutical products is hindered by limited stability, poor solubility, and susceptibility to gastrointestinal degradation. Liposomes offer a viable strategy to enhance their protection and functional performance. Methods: A formulation-driven strategy was employed to develop a gastro-resistant liposomal system for olive leaf polyphenols. Extraction conditions were optimized using a Box–Behnken response surface design to maximize phenolic recovery while ensuring compatibility with phospholipid-based systems. Antioxidant activity and key secoiridoids were assessed by spectrophotometric assays and LC–MS/MS. The optimized extract was incorporated into uncoated and Eudragit® L100-coated liposomes, which were physicochemically characterized. Antidiabetic effects were evaluated in intestinal STC-1 cells under glucose-induced hyperglycemic conditions. Results: The optimized extract (OE) exhibited high antioxidant activity (124.56 ± 9.57 mg GAE/g, 151.61 ± 2.77 mg TE/g, and 472.92 ± 26.14 mg TE/g in TPC, DPPH and FRAP assays, respectively). LC-HRMS metabolomic profiling confirmed a balanced phytochemical composition, with oleuropein as the main compound (143.144 ± 4.914 mg/g). The optimized Eudragit®-coated liposomes were spherical unilamellar vesicles with a mean diameter of 101 ± 6.1 nm, moderate polydispersity (0.46 ± 0.02), and a negative zeta potential (−17 ± 3.5 mV). High entrapment efficiency was achieved, reaching 65 ± 2.1% for oleuropein and 96 ± 0.3% for hydroxytyrosol. The structural integrity of the vesicles was maintained during storage and in the simulated gastrointestinal environment. The nanoformulation reduced intestinal glucose uptake and intracellular reactive oxygen species levels, and restored GLP-1 levels. Conclusions: The combination of optimized extraction and liposome-based formulation enabled the development of stable, delivery-ready olive leaf polyphenols for potential nutraceutical and pharmaceutical applications.