The results were so startling that researchers decided to release details of the two cases before the drug trial – in which the patients took part – was complete. Doctors said their progress had exceeded all expectations. The men were treated at the Mayo Clinic in Minnesota in the US, one of the top medical centres in the world.
Dr Eugene Kwon, the urologist who was in charge of their treatment, compared the results to the first pilot breaking the sound barrier.
"This is one of the Holy Grails of prostate cancer research. We have been looking for this for years," he said.
Ipilimumab is one of a class of drugs called monoclonal antibodies, which stimulate the body's own immune system to fight disease. The experimental treatment is being developed by Bristol-Myers Squibb and Medarex, a US biotech company. The drug is being trialled on malignant melanoma, the most serious form of skin cancer, Hodgkin's disease, lung cancer and prostate cancer. Studies are most advanced in melanoma, where it has been shown to prolong survival in patients with advanced forms of the disease. In the Mayo Clinic study of prostate cancer, researchers say that standard hormone treatment ignited the immune response, and adding ipilimumab was like "pouring gasoline on the pilot light".
Prostate cancer is the most common cancer in men – 34,000 new cases and more than 10,000 deaths are reported each year in Britain, where rates of its occurrence have tripled in the past 30 years, mainly due to improved detection. The US has the highest incidence of the disease.