The blood-brain barrier (BBB) prevents toxins and pathogens in the blood from entering brain tissue, but it also keeps out potential treatment for genetic diseases that affect the central nervous system (CNS).
Since starting his lab in 2017, Fengfeng Bei, PhD, of the Brigham and Women’s Hospital Department of Neurosurgery has been pursuing solutions to this problem. In a new paper published in Nature Biomedical Engineering, his research team reports on a novel adeno-associated virus (AAV) variant tested in preclinical models that is significantly more efficient than previously developed delivery vehicles.
“Our study is exciting because it shows that we are one step closer to delivering gene therapy across the blood-brain barrier in humans,” Dr. Bei says. “Our findings demonstrate that AAVs could provide a valuable tool for developing systemic gene therapies against certain diseases where CNS delivery is required.”
Combining AAVs With Cell-penetrating Peptides
Upon launching his lab, Dr. Bei set out to develop gene therapies to repair the damage done by brain trauma or spinal cord injuries. The challenge was finding an effective delivery tool for the genes.
Previous studies had demonstrated that AAVs—small, non-disease-causing viruses that can be engineered to carry and deliver DNA sequences to targeted cells—could penetrate the BBB in mouse models. However, they failed to do so in non-human primates.
To improve upon existing AAVs, Dr. Bei and his colleagues turned to cell-penetrating peptides (CPPs), a group of short peptides that can cross biological membranes like the BBB. The toxicity of many CPPs means they may not be a viable solution on their own. Dr. Bei had been contemplating a workaround.
“Early on, I had the idea to try combining these peptides with an AAV—specifically AAV9, which is the gold standard for crossing the BBB but not particularly efficient,” he says. “We hypothesized that we could combine these modalities to get a better capsid that would be more efficient in crossing the BBB.”
The team collected about 100 CPPs and planned to test them all but then got a “hit” at number 16. Further study of AAV.CPP.16 in preclinical models involving both mice and non-human primates showed a significant enhancement of delivery efficiency across the BBB compared with previously tested AAVs.
Lysosomal Storage Diseases Among Possible Clinical Targets
At this point, Dr. Bei notes, AAV.CPP.16 still falls short of being able to target the majority of brain cells (although it can target a considerably higher percentage than AAV9). For this reason, he believes the most likely initial target for the vector would be a rare genetic disease in which “you only have to rescue a small percentage of cells to reach a clinical threshold to see some behavioral or functional improvement in patients.”
Lysosomal storage diseases are one possibility. Dr. Bei is collaborating with Yulia Grishchuk, PhD, who leads a lab in the Center for Genomic Medicine at Massachusetts General Hospital, to explore applications of AAV.CPP.16 for these diseases.
“We have had some promising data for one such therapy,” Dr. Bei says. “If we reliably observe benefit in the mouse model, then we could be talking about IND-enabling studies within the next two years.”
Study co-author E. Antonio Chiocca, MD, PhD, chair of the Department of Neurosurgery, is also optimistic about the potential of AAV.CPP.16 to treat more common brain diseases.
“There are several neurodegenerative diseases that affect the entirety of the brain, such as Alzheimer’s, where intravenous delivery would allow this new AAV to distribute to the entire organ,” Dr. Chiocca says. “In addition, more focal disorders such as stroke could benefit by locoregional intravascular delivery of this new vector to allow for neuroregeneration or neurorepair.”
Dr. Bei plans to explore further refinements to AAV.CPP.16. “CPP16 is about fivefold to tenfold better than AAV9 in monkeys,” he says. “If we can increase efficiency another fivefold to tenfold [in humans], that would make CPP16 an even more appealing tool.”