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A common plant virus is an unlikely ally in the war on cancer

A cowpea plant flower.
Zoom in / A cowpea plant flower.

Jack Hoopes spends a lot of time with dying dogs. Hoopes, a veterinary radiation specialist at Dartmouth College, has spent his decade-long career treating canine cancers with the latest experimental therapies as a path to human care development. Recently, many of Hoopes̵

7; hairy patients have come to him with relatively common oral cancer that will almost certainly kill them within months if left untreated. Even if the cancer goes into remission after radiation treatment, there is a high probability that it will re-emerge soon.

For Hoopes, it’s a grim prognosis that’s all too familiar. But these puppies are in luck. They are patients in an experimental study that explores the effectiveness of a new cancer treatment derived from a common plant virus. After receiving viral therapy, many of the dogs had their tumors completely gone and lived to old age without recurrent cancer. Given that around 85% of dogs with oral cancer will develop a new tumor within a year of radiotherapy, the results were surprising. The treatment, according to Hoopes, had the potential to be a lifesaving breakthrough, both human and canine.

“If a treatment works in canine cancer, it has a very good chance of working, to some degree, in human patients,” Hoopes says.

The new cancer therapy is based on cowpea mosaic virus, or CPMV, a pathogen named after the mottled pattern it creates on the leaves of infected cowpea plants, which are perhaps better known as source of black-eyed peas. The virus doesn’t replicate in mammals as it does in plants, but as the researchers behind the therapy found, it still triggers an immune response that could be key to more effective treatments for a wide variety of cancers.

The idea is to use the virus to overcome one of the most serious problems in oncology: a doctor’s best ally, the patient’s immune system, does not always recognize a cancer cell when it sees one. It is not the fault of the body; cancer cells have properties that cause the immune system to think nothing is wrong. Oncologists have been wondering about this for nearly a century, and it is only in the last decade that researchers have really begun to control the immunosuppressive properties of cancer. Immunotherapy, which has emerged as one of the most promising types of cancer treatment, is about developing techniques to help the body’s immune system recognize cancerous cells so that it can react. It’s the medical equivalent of putting a large flashing neon sign over the tumor that reads “ATTACK HERE”. And this is where cowpea mosaic virus could help.

One dose for dogs

To treat his canine patients, Hoopes typically injects 200 micrograms of virus-like particles – about three times the dose of a typical flu shot – directly into their tumors. These particles are not live cowpea mosaic viruses; rather, they are viruses that have had their genetic material removed or have been inactivated so that they cannot replicate. Each puppy receives four doses of viral particles in two weeks while also taking standard radiation therapy. The dog’s immune system recognizes pathogens as foreign bodies and goes into attack mode. When the body chases the particles, it takes the cancerous cells with it.

While other viruses could theoretically be used as a decoy of the immune system, CPMV has proven far more effective at triggering a response than any other pathogen that researchers have tried so far. They are still not sure what makes this particular virus so effective, but the important thing is that it works. “It worked better than the radiation itself, which is a huge plus for us,” says Hoopes. “The immune system is more powerful than we thought.”

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