The research relied on analysis of tumour DNA circulating in patients' blood, rather than repeated biopsies. The findings need to be checked in larger studies but the researchers say that monitoring circulating tumour DNA could eventually help doctors check when prostate cancer treatments are no longer working and change the treatment course accordingly.
Professor Paul Workman, interim chief executive at The Institute of Cancer Research, UK, which collaborated in the study, said: 'This important discovery reveals how some cancer treatments can actually favour the survival of the nastiest cancer cells, and sets out the rationale for repeated monitoring of patients using blood tests, in order to track and intervene in the evolution of their cancers'.
One of the key findings was that, after an initial period of effectiveness, drugs called gluticocorticoids appeared to activate dangerous mutations in the tumour which contributed to the progression of the disease. Gluticocorticoids are steroidal drugs frequently prescribed to prostate cancer patients alongside hormone therapy.
Speaking to the Daily Mail, study leader Dr Gerhardt Attard said: 'Glucocorticoids are initially effective, they are great treatments which we have been using for more than a decade now. They make people feel better and their tumours shrink'.
'But in time - it could be a couple of years or less, depending on the patient - the cancer manages to adapt and start growing again. In one in five people the treatment might start driving the disease, activating harmful mutations'.
The researchers stressed that the study was an early piece of research and needed to be confirmed in more patients. Dr Matthew Hobbs, deputy director of research at Prostate Cancer UK, which provided the majority of funding for the study, said: 'Anyone currently taking medication for advanced prostate cancer should not stop doing so as a result of these findings, but should speak to their clinician if they have any concerns'.
The study also suggested that measuring levels of circulating tumour DNA might give a reliable indication of the emergence of treatment-resistant prostate cancer. Currently doctors rely on repeated biopsies, which are more invasive and, the researchers say, more expensive.
In the UK, prostate cancer is the commonest cancer in men with more than 40,000 new cases diagnosed each year.
The study is published in Science Translational Medicine.