Targeting liposomes with protein drugs to the blood-brain barrier in vitro

In this study, we aim to target pegylated liposomes loaded with horseradish peroxidase (HRP) and tagged with transferrin (Tf) to the BBB in vitro. Liposomes were prepared with the post-insertion technique: micelles of polyethylene glycol (PEG) and PEG-Tf were inserted into pre-formed liposomes containing HRP. Tf was measured indirectly by measuring iron via atomic absorption spectroscopy. All liposomes were around 100 nm in diameter, contained 5-13 microg HRP per mumol phospholipid and 63-74 Tf molecules per liposome (lipo Tf) or no Tf (lipo C). Brain capillary endothelial cells (BCEC) were incubated with liposomes at 4 degrees C (to determine binding) or at 37 degrees C (to determine association, i.e. binding+endocytosis) and the HRP activity, rather than the HRP amount was determined in cell lysates. Association of lipo Tf was two- to three-fold higher than association of lipo C. Surprisingly, the binding of lipo Tf at 4 degrees C was four-fold higher than the association of at 37 degrees C. Most likely this high binding and low endocytosis is explained by intracellular degradation of endocytosed HRP. In conclusion, we have shown targeting of liposomes loaded with protein or peptide drugs to the BCEC and more specifically to the lysosomes. This is an advantage for the treatment of lysosomal storage disease. However, drug targeting to other intracellular targets also results in intracellular degradation of the drug. Our experiments suggest that liposomes release some of their content within the BBB, making targeting of liposomes to the TfR on BCEC an attractive approach for brain drug delivery.