Vesicular phospholipid gel-based depot formulations for pharmaceutical proteins: development and in vitro evaluation

The object of this study was to evaluate the potential of vesicular phospholipid gels (VPGs) as an alternative delivery system for therapeutic proteins. Therefore, the model protein erythropoietin (EPO) was incorporated into various VPG formulations by dual asymmetric centrifugation. In order to characterize and to quantify the incorporated protein, different extraction protocols were investigated. Among several detergents and organic solvents an extraction method applying chloroform was found most suitable. SDS-PAGE analysis of EPO extracted from VPGs revealed excellent protein stability which was maintained in the VPGs' matrix after an incorporation process with dual asymmetric centrifuge. Irrespective of the investigated formulation all VPGs delivered the protein over prolonged periods of time at close to linear kinetics without any burst effect. Both the lipid content and the lipid charge had a strong influence on the release behavior. For instance formulations based on 300 mg/g lipid delivered 83% EPO after 280 h while gels based on 550 mg/g lipid liberated 64% within 400 h. From the parallelism of release and erosion kinetics found for all formulations it was concluded that erosion rather than diffusion is the dominant release controlling mechanism for these macromolecule-loaded VPGs. For the first time the present study presents VPGs as promising alternative depot systems for protein drugs.