Affinity Engineering of Anti-human Transferrin Receptor Antibody for Drug Delivery across the Blood-Brain Barrier

Abstract

Transferrin receptor (TfR) is transmembrane carrier protein for transferrin which binds iron. It is enriched in some therapeutic target cells, such as cancer cells and brain endothelial cells. Especially, in brain endothelial cells, TfR mediates transcytosis across the blood-brain barrier (BBB). Since the BBB hinders penetration of macromolecules into the brain, crossing BBB is one of the critical points of brain targeting drug delivery. Thus, anti-TfR antibody is modified with therapeutic molecules, which are difficult to diffuse across the BBB, and transports them into brain parenchyma. Nevertheless, few cases have reported that anti-TfR antibody engineering increased permeability for human BBB, and no structure-based engineering has been presented. The engineering for decreased affinity of anti-TfR antibody is regarded as one of the key factors, because high affinity of antibody hinders their release from the TfR. To induce the brain uptake using TfR mediated transcytosis, optimization of antibody binding affinity is required. In this study, anti-human TfR antibody is optimized for enhancing the brain penetration. Here I report X-ray crystal structure of Fab fragment of anti-human TfR antibody. X-ray diffraction data were collected to 2.74 Å resolution. The unit cell has the dimensions of a = 65.074 Å, b = 110.766 Å, c = 197.95 Å, α = 90˚, β = 90˚, γ = 90˚ and the space group is P212121. By using the solved structure, docking simulation was performed with TfR protein. On the basis of the structure and simulation results, antibody that has optimized lower binding affinity was characterized. As a result of measurement using in vitro BBB model, the optimized anti-human TfR antibody shows enhanced permeability in comparison to wild-type. This study can be used for optimization of anti-TfR antibody due to their delivery across the BBB, analyzing the interaction between anti-TfR antibody and TfR protein. It will be allow enhancing the brain uptake.