Delivery of β-galactosidase to mouse brain via the blood-brain barrier transferrin receptor

Enzyme replacement therapy of lysosomal storage disorders is complicated by the lack of enzyme transport across the blood-brain barrier (BBB). The present studies evaluate the delivery of a model enzyme across the BBB following enzyme conjugation to a BBB receptor-specific monoclonal antibody (mAb). Bacterial beta-galactosidase (116 kDa) was conjugated to the rat 8D3 mAb to the rat transferrin receptor (TfR) via a streptavidin-biotin linkage. The unconjugated beta-galactosidase or the beta-galactosidase-8D3 conjugate was injected intravenously in adult mice, and enzyme activity was measured at 1 and 4 h in brain and peripheral organs (liver, spleen, kidney, and heart). Unconjugated beta-galactosidase was rapidly removed from the blood compartment owing to avid uptake by liver and spleen. There was minimal uptake of the unconjugated beta-galactosidase by brain. Following conjugation of the enzyme to the 8D3 TfRmAb, there was a 10-fold increase in brain uptake of the enzyme based on measurement of enzyme activity. Histochemistry of brain showed localization of the enzyme in the intraendothelial compartment of brain following intravenous injection of the enzyme-mAb conjugate. The capillary depletion technique showed that more than 90% of the enzyme-8D3 conjugate that entered into the endothelial compartment of brain passed through the BBB to enter brain parenchyma. In conclusion, high molecular weight enzymes, such as bacterial beta-galactosidase, can be conjugated to BBB targeting antibodies for effective delivery across the BBB in vivo. Fusion proteins comprised of BBB targeting antibodies and recombinant enzymes could be therapeutic in the treatment of the brain in human lysosomal storage disorders.