Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys.

Effective delivery of protein therapeutics to the central nervous system (CNS) has been greatly restricted by the blood-brain barrier (BBB). We describe the development of a BBB transport vehicle (TV) comprising an engineered Fc fragment that exploits receptor-mediated transcytosis for CNS delivery of biotherapeutics by binding a highly expressed brain endothelial cell target. TVs were engineered using directed evolution to bind the apical domain of the human transferrin receptor (hTfR) without the use of amino acid insertions, deletions, or unnatural appendages. A crystal structure of the TV-TfR complex revealed the TV binding site to be away from transferrin and FcRn binding sites, which was further confirmed experimentally in vitro and in vivo. Recombinant expression of TVs fused to anti-ß-secretase (BACE1) Fabs yielded antibody transport vehicle (ATV) molecules with native immunoglobulin G (IgG) structure and stability. Peripheral administration of anti-BACE1 ATVs to hTfR-engineered mice and cynomolgus monkeys resulted in substantially improved CNS uptake and sustained pharmacodynamic responses. The TV platform readily accommodates numerous additional configurations, including bispecific antibodies and protein fusions, yielding a highly modular CNS delivery platform.

Brain delivery and activity of a lysosomal enzyme using a blood-brain barrier transport vehicle in mice.

Most lysosomal storage diseases (LSDs) involve progressive central nervous system (CNS) impairment, resulting from deficiency of a lysosomal enzyme. Treatment of neuronopathic LSDs remains a considerable challenge, as approved intravenously administered enzyme therapies are ineffective in modifying CNS disease because they do not effectively cross the blood-brain barrier (BBB). We describe a therapeutic platform for increasing the brain exposure of enzyme replacement therapies. The enzyme transport vehicle (ETV) is a lysosomal enzyme fused to an Fc domain that has been engineered to bind to the transferrin receptor, which facilitates receptor-mediated transcytosis across the BBB. We demonstrate that ETV fusions containing iduronate 2-sulfatase (ETV:IDS), the lysosomal enzyme deficient in mucopolysaccharidosis type II, exhibited high intrinsic activity and degraded accumulated substrates in both IDS-deficient cell and in vivo models. ETV substantially improved brain delivery of IDS in a preclinical model of disease, enabling enhanced cellular distribution to neurons, astrocytes, and microglia throughout the brain. Improved brain exposure for ETV:IDS translated to a reduction in accumulated substrates in these CNS cell types and peripheral tissues and resulted in a complete correction of downstream disease-relevant pathologies in the brain, including secondary accumulation of lysosomal lipids, perturbed gene expression, neuroinflammation, and neuroaxonal damage. These data highlight the therapeutic potential of the ETV platform for LSDs and provide preclinical proof of concept for TV-enabled therapeutics to treat CNS diseases more broadly.

Identification of peptide ligands facilitating nanoparticle attachment to erythrocytes.

Peptide ligands capable of mediating nanoparticle adhesion to human red blood cells (RBCs) were identified from a large bacterial display peptide library. Peptides were displayed on the surface of fluorescent Escherichia coli, enabling quantitative measurement of RBC binding and high-throughput screening using fluorescence-activated cell sorting. One of the isolated clones remained attached to RBCs under high-shear stresses equivalent to those encountered in vivo. Furthermore, nanoparticles functionalized with the identified RBC-binding peptides exhibited nearly 100-fold increased RBC binding relative to nonfunctionalized particles in the presence of physiologically relevant concentrations of human serum albumin, indicating that peptides remained functional in the absence of the protein scaffold used for display. The RBC-binding peptides identified here provide new opportunities for sustained therapeutic delivery applications whereby nanoparticulate drug carriers can be attached to RBCs to achieve long-circulating carrier systems.

Site-specific targeting of antibody activity in vivo mediated by disease-associated proteases.

As a general strategy to selectively target antibody activity in vivo, a molecular architecture was designed to render binding activity dependent upon proteases in disease tissues. A protease-activated antibody (pro-antibody) targeting vascular cell adhesion molecule 1 (VCAM-1), a marker of atherosclerotic plaques, was constructed by tethering a binding site-masking peptide to the antibody via a matrix metalloprotease (MMP) susceptible linker. Pro-antibody activation in vitro by MMP-1 yielded a 200-fold increase in binding affinity and restored anti-VCAM-1 binding in tissue sections from ApoE⁻/⁻ mice ex vivo. The pro-antibody was efficiently activated by native proteases in aorta tissue extracts from ApoE⁻/⁻, but not from normal mice, and accumulated in aortic plaques in vivo with enhanced selectivity when compared to the unmodified antibody. Pro-antibody accumulation in aortic plaques was MMP-dependent, and significantly inhibited by a broad-spectrum MMP inhibitor. These results demonstrate that the activity of disease-associated proteases can be exploited to site-specifically target antibody activity in vivo.

Quantitative specificity-based display library screening identifies determinants of antibody-epitope binding specificity.

Despite the critical importance of molecular specificity in bimolecular systems, in vitro display technologies have been applied extensively for affinity maturation of peptides and antibodies without explicitly measuring the specificity of the desired interaction. We devised a general strategy to measure, screen, and evolve specificity of protein ligand interactions analogous to widely used affinity maturation strategies. The specificity of binding to target and nontarget antibodies labeled with spectrally distinct fluorophores was measured simultaneously in protein mixtures via multiparameter flow cytometry, thereby enabling screening for high target antibody specificity. Isolated antibody specific ligands exhibited varying specificity, revealing critical amino acid determinants for target recognition and nontarget avoidance in complex mixtures. Molecular specificity in the mixture was further enhanced by quantitative directed evolution, yielding a family of epitopes exhibiting improved specificities equivalent, or superior to, the native peptide antigen to which the antibody was raised. Specificity screening simultaneously favored affinity, yielding ligands with three-fold improved affinity relative to the parent epitope. Quantitative specificity screening will be useful to screen, evolve, and characterize the specificity of protein and peptide interactions for molecular recognition applications.