Confocal microscopy for the elucidation of mass transport mechanisms involved in protein release from lipid-based matrices

It was the aim of this study to identify the governing mechanisms during protein release from cylindrical lipid matrices by visualizing mass transport and correlating the data with in vitro dissolution testing. Glyceryl trimyristate cylinders of 2 mm diameter, 2.2 mm height and 7 mg weight were manufactured by compression of a protein-lipid powder mixture prepared by a polyethylene glycol (PEG) co-lyophilization technique. BSA was fluorescence-labeled and the distribution visualized and quantified at different stages of the release process by confocal microscopy in parallel to the quantification in the release buffer. The impact of matrix loading and protein molecular weight was assessed with the model proteins lysozyme, BSA, alcohol dehydrogenase and thyroglobulin. Buffer penetration and protein release occurred simultaneously from the outer regions of the cylinder progressing towards the center. Release from the top and bottom of the matrix was not negligible but much slower than penetration from the side, probably due to an oriented arrangement of lipid flakes during compression. The different quantification strategies were found to yield identical results. At 6% protein loading, buffer penetration was complete after 4 days, while only 60% of the protein was liberated in that time and release continued up to day 63. Protein release kinetics could be described using the power law equation M ( t ) /M ( infinity ) = kt ( n ) with an average time exponent n of 0.45 (+/-0.04) for loadings varying between 1 and 8%. A percolation threshold at 5% pure protein loading and 3-4% mixed loading (PEG and protein at a 1:1 mass ratio) could be identified. Release rate was found to decrease with increasing molecular weight. Protein release from lipid-based matrices is a purely diffusion controlled mechanism. Potential protein stabilization approaches should address the time span between complete buffer penetration of the matrix and 100% release of the remaining loading, which would be exposed to an aqueous environment before leaving the matrix.