For a new way to shuttle drugs past the scrupulous shield of the blood brain barrier, we can thank a shark.
Scientists at King’s College London and the biotechnology company Ossianix have developed a new class of molecules based on shark antibodies that can deliver a neuroprotective therapeutic across the blood brain barrier. In a new bioRxiv preprint, the team reported that their bispecific antibody crosses the blood brain barrier with high efficiency, successfully binds to and activates its target, the TrkB receptor, and protects against neurodegeneration in a mouse model of Parkinson’s disease (1).
“There is huge interest in the industry, virtually in every company, about delivering a variety of different types of agents — antibodies, peptides, and RNAi — to the brain,” said Frank Walsh, the co-founder and CEO of Ossianix and co-senior author of the study. “The field has been curtailed just by a lack of shuttles that actually give therapeutic levels.”
In the 1980s and 90s, scientists were excited about the potential therapeutic benefits neurotrophins might offer for neurodegenerative diseases. Neurotrophins bind specific Trk receptors on neurons, which leads to the neurons’ protection from degeneration. The problem was that neurotrophins are not great drugs. For example, the neurotrophin BDNF, which binds to the TrkB receptor, is large and bulky, making it next to impossible to deliver across the blood brain barrier.
“Most of the young neuroscientists today don't know what a neurotrophin is,” said Patrick Doherty, a neuroscientist and professor emeritus at King’s College London and a co-senior author of the study. “The field fell away, simply because people were not able to deliver a biologically active molecule across the blood brain barrier.”
Over the years, however, scientists developed TrkB agonist antibodies that could bind to BDNF’s target receptor, TrkB, and mimic BDNF’s neuroprotective activity. But it was still difficult to get TrkB agonist antibodies across the blood brain barrier where they would be therapeutically useful.
To solve that problem, the team looked to shark blood.
Sharks and camels produce tiny antibodies called immunoglobulin new antigen receptors (IgNARs), which weight just 11-12 kDa — much smaller than human antibodies, which weigh around 150 kDa.
The variable domain of IgNARs, called VNARs, can function as single domain antibodies on their own. By modifying a VNAR to bind to human transferrin receptor 1, a protein found on the epithelial cells that line the blood brain barrier, the team found that their molecule could cross the blood brain barrier (2). They reasoned that if they could fuse the TrkB agonist antibody onto their new brain-penetrable VNAR to create a bispecific antibody, perhaps they could bring the TrkB agonist’s therapeutic effects to the neurons in the brain.
The team found that two versions of their bispecific antibody retained activity in vitro, and when injected into mice, the molecules accumulated in the brain, reaching nanomolar concentrations by 18 hours after treatment. Continuing with their more active compound, the researchers found that it localized to dopaminergic neurons in the brain and activated ERK1/2 signaling, a hallmark of neurotrophin receptor activation. Finally, using a classical mouse model of Parkinson’s disease, the researchers found that their bispecific antibody significantly protected against neuronal cell loss.
“We were more than singing and dancing. We were really over the moon,” said Doherty about seeing the results. The team is hoping to move this research forward into large mammalian model systems and eventually into clinical trials for neurodegenerative diseases.
“[This] group has made a real breakthrough,” said William Pardridge, an expert in the blood brain barrier and a professor emeritus at the University of California, Los Angeles, who was not involved in the study. “This could be a blood brain barrier penetrating neuroprotective agent with a broad spectrum of applicability. It could be useful in the treatment of Parkinson's disease, Alzheimer’s disease, and it could be useful in the treatment of acute brain injury, such as stroke or brain trauma.”
Pardridge is interested in seeing additional functional data. For example, he would like the team to test if the new molecule improves motor symptoms associated with Parkinson’s disease in a mouse model. Walsh and Doherty plan to do those experiments soon.
Doherty is optimistic about the new molecule’s prospects for treating neurodegenerative diseases. “The beauty of the neurotrophins is they can protect neurons against a very, very wide variety of insults. All you have to do is keep neurons healthier for a little bit longer,” he said. “If you were to delay the onset of Alzheimer's disease by five years, you would halve the number of people with Alzheimer's disease,” he continued. “It would remarkably impact the number of people who actually develop the conditions.”
References
- Clarke, E. et al. A single domain shark antibody targeting the transferrin receptor 1 delivers a TrkB agonist antibody across the blood brain barrier to provide full neuroprotection in a mouse model of Parkinson’s Disease. Preprint at:https://www.biorxiv.org/content/10.1101/2020.03.12.987313v3.full.pdf (2021).
- Stocki, P. et al. Blood-brain barrier transport using a high affinity, brain-selective VNAR antibody targeting transferrin receptor 1. FASEB 35, e21172 (2020).
