TORONTO — Can the sunshine vitamin help prolong the lives of men suffering from advanced prostate cancer? An international team of researchers thinks the answer may be yes.

Researchers from Canada, the United States and Europe are launching a clinical trial to test whether specially formulated high-dose vitamin D, taken in combination with a chemotherapy drug called docetaxel, can increase survival time and improve quality of life in men for whom medical science currently has little to offer.

"It's quite exciting,'' said Dr. Kim Chi, one of three principle investigators in the trial and a medical oncologist at the B.C. Cancer Agency.

"The concept is interesting. The initial data from the first clinical trial . . . is very compelling.''

Kim said men who took the treatment in the safety and dosing studies that must precede the full-blown clinical trial experienced no significant side-effects. In fact, the treatment appeared to make the chemotherapy easier to tolerate -- adding to the sense of promise about its potential.

"If that all comes to fruition it would be a nice step forward for people with (advanced) prostate cancer,'' Chi said.

But he noted even if it works this isn't about a cure, only "extending life and making treatment better.''

The Canadian Cancer Society estimates that 20,700 men in this country will be diagnosed with prostate cancer this year and 4,200 will die from the disease.

About 150 sites in Canada, the U.S. and Europe will enrol men in the trial over the next year, then follow them for about a year after. The researchers hope to sign up about 1,000 participants.

Eligible for the trial are men whose cancer has ceased responding to hormone therapy, the first-line treatment for advanced prostate cancer.

Half of the men will received the current standard treatment for prostate cancer for which hormone treatment has failed -- docetaxel and prednisone, an oral steroid.

The other half will receive docetaxel and DN-101, a high-dose calcitriol pill. Calcitriol is a biologically active form of vitamin D, which itself is a hormone the body needs to help it use calcium to build bones and teeth. Vitamin D can be absorbed through a limited number of food products and by exposure to sunlight.

Previous research points to a possible role for vitamin D in fighting some forms of cancer. It's been observed, for instance, that death rates due to breast, colon, ovarian and prostate cancer are lower in sunnier climes. And in laboratory experiments vitamin D has been shown to kill some cancer cells.

Frank King, of North Vancouver, has enrolled in the study.

King, 79, has been fighting advanced prostate cancer since 2002. He was randomly placed in the standard treatment arm of the study, so he won't be getting the experimental therapy. He admitted that was a disappointment.

"I thought possibly this study would give the people on the study side . . . a better shot,'' King said from Vancouver.

" But then considering it all, I thought: Well somebody has to be in the control group or they wouldn't know whether the other one is working or not.''

"I have a son, two sons-in-law and three grandsons that this may help down the road.''

High doses of vitamin D can be toxic, producing a build-up that leads to the formation of kidney stones and even kidney failure. So participants in the study will need close supervision, said Reinhold Vieth, a vitamin D expert at Toronto's Mount Sinai Hospital who is not involved in the trial.

"Patients likely would not have obvious tolerability issues, but eventually they will probably exhibit undesired calcium deposits in blood vessels and kidneys,'' Vieth noted.

Chi said the study leaders want to stress that this treatment is not something men should try at home.

"We don't want people heading out there and taking boatloads of vitamin D. It can be very toxic and life-threateningly toxic,'' he said.

The trial is being sponsored by Novacea, Inc. of San Francisco, which is developing DN-101.