Neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s remain a great challenge for today’s society and scientific community. The lack of neuropathological understanding combined with the difficulty of penetrating the blood brain barrier (BBB) constitute the greatest obstacles to the generation of effective therapeutics.
The BBB constitutes the initial obstacle in the delivery of therepautics into the brain. Therefore, molecules intended for use as CNS diagnostics or therapeutics must be delivered to the region of choice via administration routes that bypass the BBB. These include intrathecal/intraventricular, intracerebral administration, or in combination with delivery technologies that enhance their penetration across the BBB upon systemic administration.
Even when intracerebrally administered, mAb exhibit limited diffusion and brain penetration due to their large size and interactions with the Extra Cellular Matrix (ECM). The extracellular space (ECS) is packed with cells such as neurones and glial, which constitute a tightly packed and convoluted environment – modelling studies estimate the width of the ECS to be around 35-65nm.
In contrast, VHH have a distinct advantage as their unique physical properties (limited interactions with the ECM, small size and superior stability) achieve both deeper brain penetration and broader brain exposure through a higher diffusion rate.
In the case of delivery through the BBB via receptor-mediated transcytosis (RMT), VHH single domain antibodies present certain advantages over mAb as well due to their stability, solubility and size capabilities.
Figure 1 Representation of the receptor-mediated transcytosis (RMT) process. (a) Initially, an RMT ligand binds to a specific RMT receptor on the luminal cell membrane, which (b) leads to the internalization of both receptor and ligand in intracellular vesicles via endocytosis. (c) These vesicles then travel within the cell cytoplasm to reach the abluminal membrane where fusions of endosomes with the cell membrane releases the vesicular cargo inside the brain.
This has been confirmed in a recent study on the brain biodistribution of mAb versus VHH antibodies via perivascular transport following intrathecal infusion in rodents. Their findings revealed that the VHH was able to achieve greater brain penetration and distribution compared to the mAb. Thus, demonstrating that VHH are advantageous for their use as CNS therapeutic entities through intracerebral or intrathecal administration routes. This is particularly relevant in neurodegenerative CNS diseases, such as Parkinson’s, which originate from a known specific brain region.
In a vast amount of neurodegenerative diseases, such as Alzheimer’s, pathology follows the aggregation of proteins via a prion-like chain reaction mechanism. A pathological misfolded version of a protein, such as tau, will form a template for the aggregation and misfolding of another tau molecule, thus facilitating disease progression. It is yet unclear how these aggregates cause disease pathology and neurodegeneration, but their presence is strongly associated to the disease.
In a more generalised disease approach where protein aggregation occurs in a prion-like manner VHH antibodies’ ability to penetrate ‘inaccessible epitopes’ under the very harsh conditions in which pathological protein aggregation occurs, make them very promising agents in preventing disease progression and in possible future therapeutics. VHH could potentially act as a blockade binding on to aggregated protein preventing the recruitment of healthy protein and halting the chain-like aggregation and associated pathology.
To summarize, VHH are attractive candidates for use in neurodegenerative diseases due to their unique attributes which enable easier penetration through the BBB and distribution within the dense pathophysiological environment. In addition, their ability to access hidden and small epitopes can be utilized in targeting protein aggregates which are seen in most of these neurodegenerative diseases.
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