Two men with advanced and inoperable prostate cancer have recovered after being treated with an experimental antibody drug.

The patients, who are now free of cancer, were taking part in the trial of a drug called ipilimumab that boosts the immune system. Before treatment at the Mayo Clinic in Rochester, Minnesota, each of them had aggressive tumours that had grown into abdominal areas.

Eugene Kwon, the trial leader, said: "The goal of the study was to see if we could modestly improve upon current treatments for advanced prostate cancer. The candidates for this study were people who didn't have a lot of other options. However, we were startled to see responses that far exceeded any of our expectations."

First, the patients received traditional hormone therapy to remove testosterone, which fuels prostate cancer. Researchers then introduced a single dose of ipilimumab. The drug boosts the immune system's response to the cancer.

When the surgeons made their incisions, they had a surprise. Dr Michael Blute, a Mayo clinic urologist, said: "The tumours had shrunk dramatically. I had never seen anything like this before. I had a hard time finding the cancer."

Dr Kwon said: "This is one of the holy grails of prostate cancer research. We've been looking for this for years."

John Neate, chief executive of The Prostate Cancer Charity, said: "If these early and small-scale results are replicated in larger trials, this represents a potentially very exciting development.