Permeability of the Blood-Brain Barrier and Transport of Nanobodies Across the Blood-Brain Barrier

Abstract

The presence of a blood-brain barrier (BBB) and a blood-cerebrospinal fluid barrier presents animmense challenge for effective delivery of therapeutics to the central nervous system. Many potential drugs, which are effective at their site of action, have failed due to the lack of distribution in sufficient quantity to the central nervous system (CNS). In consequence, many diseases of the central nervous system remain undertreated. Antibodies, IgG for example, are difficult to deliver to the CNS due to their size (~155 kDa), physico-chemical properties and the presence of Fc receptor on the blood-brain barrier. Smaller antibodies, like the recently developed nanobodies, may overcome the obstacle of the BBB and enter into the CNS. The nanobodies are the smallest available antigen-binding fragments harbouring the full antigenbinding capacity of conventional antibodies. They represent a new generation of therapeutics with exceptional properties, such as: recognition of unique epitopes, target specificity, high affinity, high solubility, high stability and high expression yields in cost-effective recombinant production. Their ability to permeate across the BBBmakes thema promising alternative for central nervous system disease therapeutics. In this review, we have systematically presented different aspects of the BBB, drug delivery mechanisms employed to cross the BBB, and finally nanobodies — a potential therapeutic molecule against neuroinfections.

Transportation of Single-Domain Antibodies through the Blood–Brain Barrier

“…Several nanobodies with a high pI (~9.5) have been described to spontaneously cross the BBB by adsorptive-mediated transcytosis [13,60,61,125]. CPPs and trojan peptides crossing the BBB can also be fused to nanobodies to favor entrance into the CNS by using this route [157].…”

“…The integrity of anatomical barriers is a hallmark of health [12]. The BBB protects the CNS from infections and toxic substances such as endogenous metabolites, xenobiotics, or exogenous neurotoxic substances in the bloodstream [13]. Lack of integrity and dysfunction of the BBB are associated with numerous neurological diseases and brain pathologies, including cancer, thus resulting in BBB leakiness [14][15][16][17].…”

“…Several nanobodies with a high pI (~9.5) have been described to spontaneously penetrate the BBB [13,60] (Figure 4). Basic nanobody targeting GFAP (mVHH E9) with pI = 9.4, and also its fusion with enhanced green fluorescent protein reporter (mVHH E9-GS-EGFP, pI = 9.3), have been shown to spontaneously cross the BBB and specifically label this intracellular target in brain astrocytes in vivo [60].…”

Diagnosis of Glioblastoma by Immuno-Positron Emission Tomography

“…The development of immuno-PET probes based on antibody subunits targeting glioblastoma biomarkers can overcome BBB selectivity emerging as promising probes for the non-invasive diagnosis of gliomas [ 131 ]. Among them, sdAbs such as nanobodies present multiple properties, including a smaller MW, enabling better BBB entrance, tumor uptake and biodistribution, and faster clearance than conventional antibodies [ 132 , 133 ].…”

“…The development of antibody subunits targeting glioblastoma biomarkers that overcome the BBB selectivity emerges as a promising tool that could contribute to glioblastoma diagnosis by immuno-PET [ 131 ]. Single-domain antibodies (sdAbs) such as nanobodies have a lower MW, enabling better BBB penetrance, tumor uptake, and faster blood clearance than monoclonal antibodies [ 132 , 133 ]. Nanobodies are the single variable domain of the heavy-chain-only antibodies of Camelidae (camel, dromedary, llama, alpaca, vicuñas, and guananos) [ 63 , 134 ].…”