March 9, 2010 - A virus that destroys cancer cells but leaves normal cells unharmed works against prostate cancer, a human study shows.
The virus is called reovirus, and nearly everyone has been infected with it. But almost nobody notices, because at worst, the virus causes mild flu-like symptoms. But when it infects cancer cells, reovirus is a tiger.
For nearly a decade, researchers have been looking for ways to exploit reovirus as a nontoxic cancer treatment. Now a new study takes that search one step closer to reality.
Six prostate cancer patients at Canada's Tom Baker Cancer Center had the virus injected directly into their prostate tumors by Don G. Morris, MD, PhD, and colleagues. The patients then had their prostate glands removed by previously scheduled surgery.
"The beauty of the prostate study is that we gave one injection, and by three weeks later we had the entire prostate gland to look at. So we could inject the virus into a nest of tumor cells and see what it did," Morris tells WebMD.
What it did was trigger the cancer cells' self-destruct program. All around the injection site, the reovirus -- a product from Oncolytics Biotech Inc. called Reolysin -- made cancer cells go away. Normal cells were not harmed.
The downside was that the virus did not spread throughout the prostate. Cancer cells not in the immediate area of the injection were spared.
"I don't think it is a dead end. What these studies have done is give us enough ammunition to go to regulators and say, 'Here is data that prostate cancer is an attractive target for reovirus,'" Morris says. "And we have a lot of safety data showing it is safe to give intravenously."
Putting reovirus into the bloodstream would allow it to reach cancers throughout the body, not just at the site of injection. But there's a big hurdle to overcome: After the first injection, the body mounts immune responses that eliminate the virus.
One way of overcoming this problem is by using reovirus together with chemotherapy. Chemo kills cancer cells, but also dampens antiviral immune responses. A recent human study shows this strategy can benefit patients with advanced head and neck cancer.
Another way to overcome the problem is to harness anti-reovirus immune responses to attack cancer cells.
"The virus only sticks to tumor cells, so it sort of directs the immune response to the tumor area," Morris says. "We have actually taken reovirus and injected it in combination with tumor antigens, so the immune system kills tumors pasted with the virus."
There's still a lot of work to do before reovirus in general, or Reolysin in particular, becomes an approved cancer treatment, says cancer expert Rameen Beroukhim, MD, PhD. Beroukhim was not involved in the Morris study.
"There is a long way between this interesting study and showing that reovirus is going to be beneficial to patients," Beroukhim says. "But the hope is that this treatment could be tweaked in some way to have a systemic anti-cancer effect."
Morris is the first to acknowledge that there's a lot more work to do. One major issue, he says, is that not every patient's cancer is susceptible to reovirus.
"In the majority of cancers, if you take specimens from different patients, not all cancers are killed. Maybe seven or eight of 10 patients' cancers are sensitive. So we need to find out what makes a cancer cell susceptible to reovirus," he says.
The Morris study was partially funded by Oncolytics Biotech, and the company funds his laboratory. But Morris says he has not been compensated by any companies in the oncology-virus field for the last five years. Although he is a patent holder on some indications for reovirus, he has signed away rights to financial gain.
Morris and colleagues report their findings in the March 15 issue of Cancer Research.