Physico-chemical characterization of formulations containing endomorphin-2 derivatives

In this study semisolid formulations containing AcYPFF (N-acetyl-Tyr-Pro-Phe-Phe-NH₂) tetrapeptide were obtained and characterized in terms of rheology, stability by multiple light scattering and particle size distribution by laser diffraction. Additionally, the release studies of tetrapeptide from formulations obtained were performed. The influence of different factors such as semisolid and membrane type on tetrapeptide release rate was examined. The release experiments of tetrapeptide modified with palmitoyl group (PalmYPFF) were also carried out. The results proved that formulation type and its rheological properties strongly determined the permeation process of the tetrapeptide. The fastest release of tetrapeptide was observed from hydrogel that had the lowest viscosity. The kinetic data of tetrapeptide released from oil-in-water (o/w) and water-in-oil (w/o) emulsions prepared at elevated temperature showed good fit to the Higuchi equation, whereas when AcYPFF was released from oil-in-water (o/w) emulsion prepared with the addition of auto-emulsifier high linearity with Korsmeyer-Peppas model was observed. While when tetrapeptide was released from Hydrogel the most suitable model was the first-order kinetics. It was suggested that mechanism that led to the release of tetrapeptide from all formulations was non-Fickian diffusion transport. The presence of palmitoyl group changed the solubility of tetrapeptide both in formulation and receptor fluid and thus the release rate of active compound was modified.