Oestrogen patches worn by women to relieve the symptoms of menopause could also help men fight prostate cancer, scientists have found.
Hormone Replacement Therapy (HRT) may offer an effective alternative treatment for the cancer, without the side-effects of other therapies.
Oestrogen patches, which work by replacing the hormone that women cease to produce in middle age, are currently the most commonly prescribed treatment for the symptoms of menopause, such as hot flushes and loss of libido.
The new research, funded by the charity Cancer Research UK, shows that the patches are also effective in lowering levels of the male hormone testosterone – which fuels the spread of prostate cancer in the early stages of the disease.
Prostate cancer is the most common cancer in men, with 40,000 diagnosed in Britain each year. It is the second biggest cause of cancer death in men after lung cancer.
Currently the most effective known treatments include LHRH agonists such as Zoladex, which are injected and interfere with testosterone production, and oestrogen taken in pill form. But both of these have aggressive side effects. Injected drugs are known to increase the risk of osteoporosis and diabetes, while the oestrogen pill has been linked to higher risks of blood clots and stroke.
By comparison, HRT is a relatively low-risk treatment, with breast tenderness and skin irritation the most common adverse side-effects during trials.
The results of the Phase II trial, which involved 250 men with locally advanced or spreading prostate cancer, suggest that HRT patches can lower testosterone as effectively as LHRH agonists.
Dr Ruth Langley, at the Medical Research Council Clinical Trials Unit in London, who led the study published today in The Lancet Oncology, said: "These promising new findings suggest that we might be able to use oestrogen patches or an oestrogen gel to treat prostate cancer without significantly increasing the risk of heart disease and stroke.
"We think the reason oral oestrogen caused these side effects is because the oestrogen reached the liver in high concentrations straight from the stomach, whereas if the oestrogen can be absorbed through the skin the effect on the liver is avoided."
The report's co-author, Professor Paul Abel, at Imperial College London, said: "The next step is to test if the oestrogen patches are as effective at stopping the growth of prostate cancer as the current hormone treatments.
"We're now testing this in over 600 patients and some early results could be available later this year."
Kate Law, the director of clinical and population research at Cancer Research UK, added: "More men than ever are surviving prostate cancer thanks to advances in research, but we still urgently need to find more effective treatments and reduce side-effects.
"This trial is an important step towards better and kinder treatments that could bring big benefits to men with prostate cancer in the future."
Critics of HRT in women have raised concerns over the long-term use of the drug. Two studies, published in 2002 and 2003, linking HRT to an increased risk of breast cancer and heart disease in women, raised concerns in the medical community as well as the general public, with doctors advised to prescribe the lowest possible dosage.