Liposomal Formulation and Characterization of the Opioid Peptide Leucine Enkephalin

Abstract

The objective of this study was to encapsulate opioid peptide leucine enkephalin in liposomes of various lipid composition, and characterize in terms of particle size, stability and release characteristics. Multilamellar liposomes (MLV) were prepared by a thin-film hydration method and subsequently converted to the desired sized large unilamellar liposomes (LUV) by extruding through 0·1 μm polycarbonate membrane using positive pressure. The encapsulation was estimated using centrifugation for MLVs and protamine aggregation method for LUVs. Charge was induced on the liposome surface by adding stearylamine for positive charge and phosphotidylserine for negative charge. Effect of charge, lipid composition, and leucine enkephalin concentration on the amount of encapsulation in liposomes was studied. Effect of charge on the stability of liposome formulations was also investigated. Particle size of liposome preparations was characterized and stability of all three liposome formulations under various conditions was investigated. Encapsulation of leucine enkephalin in LUVs was influenced by the lipid concentration, fatty acid chain length, surface charge and addition of cholesterol. The encapsulation also increased as a function of drug concentration up to 1 mg mL −1. Stability studies indicated that aggregation of liposomes was minimized with liposomes bearing surface charge under various stress conditions. The release of drug from positively charged liposomes was significantly lower compared with neutral and negatively charged liposomes, indicating electrostatic interaction between the drug and liposome. It can be concluded that optimum encapsulation, stability and release characteristic can be achieved by choosing appropriate lipid composition and drug concentration.