Evaluation of Eudragit® Retard polymers for the microencapsulation of alpha-lipoic acid.

Background: Microencapsulation of natural antioxidants in polymeric systems represent a possible strategy for improving the oral bioavailability of compounds that are otherwise poorly soluble. Objective: α-lipoic acid (ALA) was microencapsulated with polymethacrylate polymers (blends at various ratios of Eudragit® RS100 and RL100 resins). Method: Microspheres were produced by solvent displacement of an ethanol co-solution of ALA and polymers; themicrosuspensions were then freeze-dried, using trehalose as a cryoprotector. Microspheres were characterized in the solid state for micromeritic properties and drug loading, as well as by infrared spectroscopy, powder X-ray diffractometry and differential scanning calorimetry. The antioxidant activity of free and encapsulated ALA was assessed by the DPPH test. Results: In vitro release studies, performed in simulated gastric (pH 1.2) and intestinal fluid (pH 6.8), showed that, depending on polymer composition and drug-to-polymer ratio, ALA release can be slowed down, compared to the dissolution pattern of the free drug. Solid-state characterization confirmed the chemical stability of ALA in the microspheres, suggesting that ALA did not develop strong interactions with the polymer and was present in an amorphous or a disordered-crystalline state within the polymer network. As indicated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, the microencapsulation of ALA in Eudragit® Retard matrices did not alter its antioxidant activity. Conclusion: ALA was effectively encapsulated in Eudragit® Retard matrices, showing a chemical stability up to 6 months at room conditions and at 40°C. Moreover, since the drug maintained its antioxidant activity in vitro, the potential application of these microparticulate systems for oral administration would deserve further studies.