Delivery of genetic cargo to specific cells is one of the big challenges facing the gene therapy field. A particular challenge is delivery to the brain. Now, researchers have developed a family of AAVs that are more than three times better at delivering their cargo to the primate brain than the current leading AAV delivery vehicle, AAV9.

To do it, they used an mRNA-based directed-evolution strategy in multiple strains of mice. In addition, they used a de novo selection in cynomolgus macaques to identify engineered vectors with increased potency in the brain.

The new AAVs can cross the blood-brain barrier, which typically keeps many drugs from getting into the brain. The engineered AAV capsids also accumulate much less in the liver than AAV9, potentially reducing the risk of liver side effects that have been seen in other AAV9-based gene therapies. This family of AAVs, called the PAL family, could be a safer and more efficient way to deliver gene therapies to the brain.

“We generated a massive pool of randomly generated AAV capsids and from there narrowed down to ones able to get into the brain of both mice and macaques, deliver genetic cargo, and actually transcribe it into mRNA,” said Allie Stanton, a graduate student in the lab of Pardis Sabeti, PhD, a professor at Harvard University, an Institute Member of the Broad Institute of Harvard and MIT, and an HHMI investigator.

This work is published in Med in the article, “Systemic administration of novel engineered AAV capsids facilitates enhanced transgene expression in the macaque CNS.

“AAVs are a really good gene therapy vector because you can put whatever you want inside of its shell, which will protect it and get it into a wide variety of cell types,” said Stanton.

Pardis Sabeti, PhD [the Broad Institute of MIT and Harvard]

However, the majority of an injected AAV dose typically ends up in the liver, meaning that high doses of AAV are required to get even a fraction into a different target tissue, such as the brain. In some cases, these high doses result in liver damage and death in clinical trials.

Stanton and colleagues focused on pinning down AAVs that cross the blood-brain barrier. They turned to a method developed in the Sabeti lab called DELIVER, in which scientists generate millions of capsids and look for AAVs that successfully deliver their payload to certain target cells. Using DELIVER, the team developed the PAL family of AAVs that cross the blood-brain barrier more effectively than AAV9—the only FDA-approved viral vector for use in the nervous system.

They found that the PAL AAVs were three times more effective at producing therapeutic mRNA in the macaque brain compared to AAV9.

The team also found that the engineered viruses had a unique pull to the brain. PAL-treated macaques had one-fourth of the viral material in their livers as AAV9-treated primates did, suggesting that the new capsids could help limit the liver toxicity of other gene therapies.

The authors say PAL AAVs could potentially work in humans given how similar macaques are to humans but added that the AAVs didn’t work well in mice, making it difficult to test these vectors in mouse models of disease. The authors noted that this work highlights “the critical need for using appropriate animal models to both identify and evaluate novel AAVs intended for delivery to the human central nervous system.”

Moving forward, the team hopes that their work will provide a starting point for even more effective viral vectors. “We are encouraged by the early results of the PAL family AAVs and can see several promising lines of investigation using directed evolution and engineering to further increase their efficiency,” Sabeti said.

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