Two patients with inoperable prostate cancer have made dramatic recoveries after receiving one dose of an experimental drug that is creating excitement among cancer specialists.

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.

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"This is one of the Holy Grails of prostate cancer research. We have been looking for this for years," he said.

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.

Rodger Nelson and Fructuoso Solano-Revuelta were diagnosed with advanced prostate cancer and sought treatment at the Mayo Clinic.

They were told the disease had spread beyond the prostate. Mr Nelson's cancer was encroaching on the abdomen and Mr Solano-Revuelta's tumour was the size of a golf ball. Patients in such condition are told they may have only months to live, and are normally only offered palliative care. But after one infusion of the drug ipilimumab, a monoclonal antibody that stimulates the immune system, given with conventional hormone therapy, their tumours shrank enough to be surgically removed. Both men have since made a full recovery and returned to their businesses.

The pair were part of a trial involving 108 patients, half of whom received the experimental drug. The trial is ongoing but the improvement of the two patients was so dramatic and unexpected that they were removed from the study so they could undergo curative surgery.

Dr Kwon said yesterday: "Halfway through the trial we began seeing remarkable responses. Some patients had dramatic shrinkage of their tumours so practically all traces had disappeared. We had thought we might get some incremental delay in the progression of the cancer. It had not dawned on us that we might go from an inoperable tumour to an operable one. That just doesn't happen."

The surgery would not have gone ahead if Carol Nelson, Mr Nelson's wife and a former nurse, had not challenged the surgeons to attempt it.

"The idea of surgery on an 'inoperable' tumour had not dawned on us. It is often disappointing and not advisable. But she is a tough lady – and having been a nurse she knew how to control the doctors. We said we didn't think surgery would work, but we would try," Dr Kwon said.

The outcome was better than any of them could have anticipated. From having an enlarged prostate gland riddled with cancer which had spread, Mr Nelson was found to have a shrunk prostate gland with tiny pockets of cancer that were "very hard to find".

Michael Blute, the surgeon who operated on Mr Nelson, said at one point doctors feared they might have the wrong patient: "I was cutting away scar tissue trying to find cancer cells. The pathologist was checking samples as we proceeded and sent word back asking if we had the right patient. He had a hard time finding any cancer. I have never seen anything like this before. The pathologists were floored."

The procedure was repeated, with the same result, on Mr Solano-Revuelta. A third "inoperable" patient underwent surgery last week.

Dr Kwon said: "These were patients for whom there was no hope. The course of their disease has been altered in a dramatic fashion. We have a major finding which we never expected to stumble across but we have to complete our studies."

He said the findings had to be confirmed in further studies and the results published in a peer-reviewed journal. A larger trial is due to begin in the autumn. The cases are described in the Mayo Clinic's research publication, Discovery's Edge.

Professor Malcolm Mason, a Cancer Research UK prostate cancer specialist, said: "These case reports are extremely interesting and encouraging. Ipilimumab might potentially be a strong stimulator of the immune system, and it seems logical that it might also be effective in prostate cancer.

"But caution is needed, as earlier trials with this drug in other types of cancer were less successful than reported here, and its true value can only become clear through large-scale, randomised clinical trials, two of which are already under way. The other cautionary note is that both men received hormone therapy, which in some instances causes dramatic reductions in tumour size by itself."

Ipilimumab: How it works

*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".