Selective plasma pharmacokinetics and brain uptake in the mouse of enzyme fusion proteins derived from species-specific receptor-targeted antibodies.

Enzymes may be re-engineered for brain drug targeting as an IgG-enzyme fusion protein, where the IgG is a monoclonal antibody (MAb) against an endogenous blood-brain barrier (BBB) receptor transporter, such as the insulin receptor or transferrin receptor (TfR). Iduronate 2-sulfatase (IDS) is fused to the heavy chain of a genetically engineered MAb against the human insulin receptor (HIR). Neither the HIRMAb alone, nor the HIRMAb-IDS fusion protein, is delivered across the BBB in the mouse, owing to lack of cross-reactivity of the HIRMAb with the insulin receptor in the mouse. The uptake of the HIRMAb-IDS fusion protein in peripheral organs exceeds that of the HIRMAb, which is attributed to uptake mediated via the mannose-6 phosphate receptor in non-brain organs. In contrast to the lack of BBB transport of the HIRMAb-IDS fusion protein, there is high BBB penetration in the mouse of an IDS fusion protein and a chimeric MAb against the mouse TfR. The comparison of the brain distribution of two different IgG-IDS fusion proteins, with different reactivity for an endogenous BBB receptor, illustrates the difference in brain targeting of a biopharmaceutical caused by the targeting properties of the IgG domain of the fusion protein.