Protein loading capacity, effect of pH and storage temperature on the stability of a water/ethyl oleate/Span 80-Tween 80 ternary system

For many years, W/O microemulsions have been intensively studied as promising delivery systems for water-soluble proteins [1, 2], due to their prominent advantages, including thermodynamic stability, protective effect against intestinal enzymes and absorption enhancement. Although W/O microemulsions have been well formulated and characterized during the recent decades in term of their improved delivery properties, there is a lack of information about their protein solubilisation capacity, due to the fact that proteins and peptides were generally incorporated in them below their solubility level. Bovine serum albumin (BSA) was chosen as model protein and then loaded inside the selected microemulsion (ethyl oleate 62%; aqueous phase 8% and Tween 80/Span 80 30% (HLB 13) as mixture of surfactants) in order to determine the maximum protein loading amount. Then, the effect of the storage temperature (4 °C and 25 °C) on the prepared microemulsion in water or buffer solution at different pH values (pH 5, 7 and 9) and loaded with the maximum amount of the protein was investigated. From the results obtained by visual analysis, optical microscopy and dynamic light scattering (DLS), it was found that an amount of BSA up to 2 mg/ml can be loaded inside microemulsion. Moreover, all microemulsions resulted to be clear and stable over 6 months when stored both at 25°C and 4°. Although the loading capacity was not high, these findings indicated that such formulation could represent a promising delivery system both for oral and parenteral administration of hydrophilic and bioactive therapeutics (i.e. proteins and peptides), at very low concentrations.