A cancer-fighting version of herpes shows promise in early human trials

An illustration of a herpes simplex virus.

An illustration of a herpes simplex virus.
Illustration: Shutterstock (Shutterstock)

Scientists may be able to turn a longtime bacterial enemy into a cancer-fighting ally, new research suggests this week. In preliminary data from a phase I trial, a genetically modified version of the herpes virus has shown promise in treating difficult-to-eradicate tumors, with one patient experiencing complete remission for 15 months so far. However, much more research will be needed to confirm the treatment’s early success.

The viral treatment is known as RP2 and is a genetically engineered strain of herpes simplex 1, the virus responsible for most cases of oral herpes in humans, as well as some cases of genital herpes. RP2 is developed by the company Replimune the design to work on two fronts. Injected directly into the tumor, the virus is supposed to selectively infect and kill certain cancer cells. But it also blocks the expression of a protein known as CTLA-4 produced by these cells, and it hijacks their machinery to produce another molecule called GM-CSF. The net result of these cellular changes is to weaken the cancer’s ability to hide from and fend off the immune system.

In a phase I trial conducted by researchers at the Institute of Cancer Research and The Royal Marsden NHS Foundation Trust in the UK, RP2 was given as the only treatment to nine patients with advanced cancer who had not responded to other treatments; it was also given in combination with another immunotherapy drug to 30 patients. Three patients on RP2 alone appeared to respond to the treatment, meaning their cancers shrank or stopped growing, and seven patients on the combination therapy also responded. In particular, one patient with a form of carcinoma along the salivary gland has shown no evidence of cancer for at least 15 months after treatment with RP2 alone. There were no life-threatening side effects reported in the trial, with the most common symptoms after treatment being fever, chills and other flu-like illness.

the findings, presented this week at the 2022 European Society for Medical Oncology Congress (ESMO), are preliminary as they have not yet been vetted through the formal peer review process. They are also based on a very small sample size, meaning any results should be taken with caution. But phase I trials aren’t meant to show that a treatment is effective, only that it’s safe enough for people to take. So the fact that some people with seemingly incurable cancers already seem to be responding to RP2, the team argues, is a very good sign that it can live up to its potential.

“Our study shows that a genetically engineered, cancer-killing virus can deliver a one-two punch against tumors — directly destroying cancer cells from the inside while also calling the immune system against them,” said lead author Kevin Harrington, professor of cancer biology. Therapies at the Institute for Cancer Research, in a announcement from the organization.

Scientists have been hopeful about cancer-fighting viruses for a long time. But it is only recently that this hope has finally begun to pay off. In 2015 was the first viral therapy approved in the United States for certain advanced cases of melanoma. In May, researchers in California launched a phase I clinical trial of their anti-cancer virus, called Vaccinia. Other things companies are developing their own candidates, either alone or in combination with other treatments. And Replimune is developing two other candidates based on their modified herpes virus.

While many experimental therapies may ultimately fail to cross the finish line and reach the public, it is possible that at least some of these viruses may one day become a new standard of cancer treatment.

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