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.

Coupling of metal containing homing devices to liposomes via a maleimide linker: use of TCEP to stabilize thiol-groups without scavenging metals.

Liposomes for drug delivery are often prepared with maleimide groups on the distal end of PEG to enable coupling of homing devices, such as antibodies, or other proteins. EDTA is used to stabilize the thiol group in the homing device for attachment to the maleimide. However, when using a homing device that contains a metal, EDTA inactivates this by scavenging of the metal. Holo-transferrin (Tf) containing two iron atoms (Fe(3+)), has a much higher affinity for the Tf receptor than apo-Tf (which does not contain any Fe(3+)). To couple Tf to a liposome, the introduction of a thiol group is necessary. During this process, by using N-succinimidyl S-acetylthioacetate (SATA), followed by 2-3 h coupling to the liposomes, Fe(3+) is scavenged by EDTA. This causes a decreased affinity of Tf for its receptor, resulting in a decreased targeting efficiency of the liposomes. Tris(2-carboxyethyl)phosphine (TCEP) hydrochloride is a sulfhydryl reductant that is often used in protein biochemistry. We found that TCEP (0.01 mM) does not scavenge Fe(3+) from Tf and is able to protect thiol groups for the coupling to maleimide. Furthermore, TCEP does not interfere with the maleimide coupling itself. In this communication, we describe the preparation of liposomes, focussing on the coupling of Tf to the maleimide linker at the distal end of PEG, without loosing Fe(3+) from Tf. This method can be applied to other metal-containing homing devices as well.

Characterization and modulation of the transferrin receptor on brain capillary endothelial cells.

PURPOSE: The expression level of the transferrin receptor (TfR) on brain capillary endothelial cells (BCECs) and the endocytosis of 125I-transferrin (125I-Tf) by this receptor was investigated. Furthermore, the influence of iron, the iron scavenger deferoxamine mesylate (DFO), astrocytic factors, a GTP-ase inhibitor (tyrphostin-A8, T8), lipopolysaccharide (LPS), and the radical scavenger N-acetyl-L-cysteine (NAC) on the TfR expression was studied to gain insight in the use and optimization of the TfR for drug targeting to the brain. METHODS: Experiments were performed with primary cultured bovine BCECs that were incubated with 125I-Tf at 4 degrees C (to determine binding) or at 37 degrees C (to determine endocytosis) in the absence or presence of the modulators. For full saturation curves in the absence or presence of iron or DFO, analysis was performed with a population approach using NONMEM, allowing us to estimate a single value for affinity (Kd, concentration of 50% receptor occupancy) and separate values for maximum receptor occupancy (B(max). RESULTS: On BCECs, the TfR is expressed extracellularly (B(max) of 0.13 fmol/microg cell protein), but also has a large intracellular pool (total B(max) of 1.37 fmol/microg cell protein), and is actively endocytosing Tf via clathrin-coated vesicles. At 4 degrees C, a Kd of 2.38 microg/ml was found, whereas the Kd at 37 degrees C was 5.03 microg/ml. Furthermore, DFO is able to increase both the extracellular as well as the total binding capacity to 0.63 and 3.67 fmol/microg cell protein, respectively, whereas it had no influence on Kd. B(max) at 37 degrees C after DFO preincubation was also increased from 0.90 to 2.31 fmol/microg cell protein. Other modulators had no significant influence on the TfR expression levels, though LPS increased cellular protein concentrations after 2-h preincubation. CONCLUSIONS: The TfR is expressed on BCECs and actively endocytoses Tf, making it a suitable target for drug delivery to the bloodbrain barrier and the CNS. DFO up-regulates the TfR expression level, which may influence targeting efficiency.