The Blood-Brain Barrier (BBB)—constituted by continuous brain capillary endothelial cells connected by complex tight junctions (claudin-5, occludin, and ZO-1), astrocytic end-feet, and pericytes—prevents over 98% of small-molecule drugs and virtually 100% of large-molecule biologics, antibody therapeutics, and gene-editing complexes from entering the central nervous system (CNS) parenchyma.
1. Receptor-Mediated Transcytosis (RMT) Using Molecular "Trojan Horses"
The most validated biochemical strategy for non-invasive BBB penetration leverages endogenous nutrient transport receptors expressed on the luminal capillary endothelium:
- Transferrin Receptor (TfR1) Bispecifics: Engineering monovalent, low-to-moderate affinity antibodies against TfR1. High-affinity antibodies remain trapped inside endothelial lysosomes; low-affinity monovalent binders engage TfR1 on the blood side, undergo endocytosis, and readily detach on the abluminal brain parenchyma side.
- Low-Density Lipoprotein Receptor-Related Protein 1 (LRP1) & Angiopep-2: Small peptide ligands that trigger rapid receptor-mediated transcytosis, ferrying conjugated CRISPR ribonucleoproteins (RNPs) or therapeutic enzymes into the brain.
2. MRI-Guided Focused Ultrasound (FUS) with Microbubbles
Focused Ultrasound represents an engineered biophysical approach. Intravenously injected lipid-shelled gas microbubbles circulate through brain capillaries. When exposed to low-frequency focused ultrasound pulses directed at specific anatomical regions (e.g., hippocampus or striatum), the microbubbles oscillate (stable cavitation), transiently stretching endothelial tight junctions and creating a safe, reversible 4-to-6 hour therapeutic window for large biologics and LNPs to enter the target brain tissue.