Researchers refine experimental gene therapy for herpes

The experimental drug, a form of gene therapy, consists of an injection of millions of laboratory-modified viruses that have been hollowed out to carry inside them two different enzymes — tiny proteins that work like molecular scissors. The therapy employs three different species of these transporter viruses. Known as adeno-associated viruses, or AAVs, they are used routinely in gene therapies to ferry genes or gene editing proteins to target sites.

In the Hutch herpes experiments, the AAVs home in on clusters of nerves harboring inactive, or latent, herpesvirus, and then the scissors go to work, targeting and cutting up certain segments of viral genes. For their latest experiments, the Hutch team selected a different mix of AAVs, and improved their results.

Previously, Jerome and Aubert reported that the drug can eliminate more than 90% of the latent herpesvirus in nerve clusters near the faces of the mice injected with the enzyme-carrying AAVs. In this new study, they describe how they have tested the therapy for the first time to treat infections in a cluster of nerves called dorsal root ganglia, near the genital tract of mice. They found the experimental therapy reduced latent virus there by 97%.

Jerome said that for technical reasons, measuring latent infections in dorsal root ganglia is extraordinarily difficult, but Aubert was able to achieve it.

“We didn’t know how well our therapy worked in those ganglia, and the answer was it worked there the best of all, which is very good news,” Jerome said.

Until this latest report, Jerome and Aubert did not have a good method to evaluate whether reducing latent virus also reduced the level of viral shedding. That is because the mice used in these studies do not naturally reactivate latent herpes infections — the source of the alarming, periodic outbreaks of painful sores that recur in humans.

The team solved that problem by using a drug that stimulates recurrences of herpes simplex type 1, or HSV-1, in mice. The results showed what might have been expected but had never been proved: Those mice whose latent virus had been reduced by the gene therapy shed far less virus than controls whose latent virus had not.

An analysis combining all five of the mouse experiments found a statistically significant reduction in both the frequency and amount of viral shedding among all the treated animals compared to controls. Importantly, the levels of reduction were dependent on both the dosage and duration of the treatments.

“Shedding is the critical thing, and we see a huge difference here,” Jerome said.

Concerns about potential toxicities

The Hutch scientists note, however, that the good news is tempered by recent concerns within the gene therapy field about the potential for therapies using AAVs to cause liver and nerve damage.

The toxicity issue has been raised by other gene therapy researchers based on their own experiments. It does not appear to involve the gene-cutting enzymes used in the Hutch herpes studies, where those molecular scissors are performing as expected. Instead, it focuses on the AAVs used to package and transport them.

“AAV vectors in general have been considered quite safe. … However, emerging work has suggested that AAV vectors may not be as fully innocuous as previously assumed,” the Hutch authors wrote in the bioRxiv paper.

They observed liver toxicity in some of their mice, but said it seems to be the result of very high concentrations of AAV, well above what is now known to be needed.

“We know what the toxic dose is, and we get good results below that,” Jerome said.

The neurotoxicity is potentially more concerning. The Hutch team reported “subtle evidence of neuronal injury” in mice when tissue samples were examined under a powerful microscope. However, they also saw no evidence in those animals of changes in behavior or mobility — such as gait or balance disturbances — after the gene therapy.

“We saw some toxicity in slides in the early experiments, and maybe it will be something we can handle very easily,” Jerome said. “But we just don’t know. We want to know what exactly is causing this.”

A priority in the next round of research will be to solve that mystery and, if necessary, find a way around it. The researchers remain confident the problem is solvable.

“Just like any scientific project, we always stumble on some things. It’s actually how we make discoveries,” said Aubert.

“It rarely goes smoothly. There is always something that you did not expect to show up,” she said. “It’s like working with a recipe. ‘Oh, I put in too much salt! Let’s try to step back and put in less and see how it turns out.’ Or it could be that a missing little ingredient, and I just need to figure out what it’s missing to make it more flavorful.” 

Assuring safety before human trials can begin

The researchers are performing additional preclinical studies of the therapy in guinea pigs, which, unlike mice, have naturally recurring outbreaks from latent herpes infections. As with the mice, the initial focus of this research is in HSV-1, which is primarily associated with cold sores. However, Fred Hutch researcher Dr. Anna Wald points out that recent studies, including one she published with colleagues at the University of Washington, are hinting at a shift.

HSV-1 is becoming — particularly in first infections for adults under 30 — the leading cause of genital herpes, the more feared condition that has been traditionally associated with HSV-2. Regardless, if the experimental therapy works for HSV-1, the researchers are confident it can be relatively easily adapted to target HSV-2.

Wald, who also heads the Division of Allergy and Infectious Diseases at UW, has been researching vaccines to prevent or treat HSV infection since 1991. She said that Jerome and Aubert’s work is a paradigm shift in the field, because the assumption has been that once a person is infected with a herpesvirus — and there are eight members of that family — it is impossible to get rid of that particular strain.

“It is a completely novel approach, in which you can take a member of the herpesvirus group and eradicate it from the host. As a proof of principle, the fact that it could be done on even one of them is kind of mind-boggling,” she said.

Jerome and Aubert are still hopeful that they can get FDA approval to test the therapy in humans in an early-stage clinical trial — designed primarily to ensure that it is safe — before the end of 2023. If the toxicity issues are more daunting, the hoped-for human studies will need to wait until those questions are resolved.

The work was funded by the National Institutes of Health, the Caladan Foundation and more than 1,600 individual donors. Cellectis developed the meganucleases used in this research. 

Note: Scientists at Fred Hutch played a role in developing these discoveries, and Fred Hutch and certain of its scientists may benefit financially from this work in the future.