Antibody-Based Biologics for CNS Disorders

The blood-brain barrier (BBB) is permeable only to small lipophilic molecules, whereas hydrophilic nutrients, such as glucose and amino acids, cross the BBB via carrier-mediated transport systems. In contrast, circulating proteins do not cross the BBB. However, the BBB expresses receptors able to induce receptor-mediated transcytosis (RMT) of selective proteins across the BBB, as in the case of insulin, transferrin, and insulin-like growth factor 1 (IGF1). This led to the hypothesis that the transport of protein-based biologics to the brain may be possible by targeting BBB receptors that induce RMT. Monoclonal antibodies targeting these receptors penetrate the BBB via RMT and are distributed throughout the brain, acting as molecular Trojan horses and/or shuttle systems. A generation of brain-penetrating fusion proteins was produced targeting BBB insulin, transferrin, and IGF1 receptors, respectively, with full antibodies and/or antibody fragments acting as transport domains. These antibody-based fusion proteins were validated in various experimental models, including lysosomal storage disorders (LSDs), stroke, Parkinson's disease, and Alzheimer's disease (AD), respectively. Clinical trials with brain-penetrating antibody-based biologicals targeting BBB transferrin and insulin receptors have been completed or are in progress in LSDs and in AD. The aim of this review is to examine the progress made in antibody-based biologicals for CNS disorders from genetic engineering to clinical trials.