A commonly prescribed diabetes drug has the potential to "revolutionise" cancer treatment, it was claimed today.
The drug, metformin, boosts an important immune system defence against cancer, scientists have learned.
They believe the medicine could be used in conjunction with vaccines to provide promising new cancer therapies.
Metformin may also enhance the effectiveness of anti-viral vaccines.
The Canadian and US researchers stumbled on the breakthrough while investigating immune system mechanisms in mice.
They found that the ability of immune system cells to "remember" biological agents they have targeted before depends on them burning fatty acids for energy instead of sugar.
"Immunological memory" has been studied for years but not been well understood until now. It is the key process behind vaccines, which trigger an immune reaction by offering the immune system a recognisable target.
One of the biggest obstacles in the way of developing successful cancer vaccines is that the immune system's memory often fails when it comes to cancer.
Tumour proteins go unrecognised, despite memory "jogging" by a vaccine, so the cancer is allowed to grow unchecked.
The new research suggests that metformin, which increases fatty acid metabolism to combat diabetes, could boost immunological memory.
This in turn could lead to exciting new anti-cancer therapies, say the scientists.
Study leader Professor Yongwon Choi, from the University of Pennsylvania in Philadelphia, said: "Our findings were unanticipated, but are extremely important, and could revolutionise current strategies for both therapeutic and protective vaccines."
The research is published today in the journal Nature.
Prof Choi's team looked at CD8 T-cells, specialised white blood cells of the immune system. Under the right conditions these cells become long-lived "memory" cells that recognise targets encountered before from previous infections or vaccinations.
Dr Erika Pearce, also from the University of Pennsylvania, said: "We serendipitously discovered that the metabolising, or burning, of fatty acids by T-cells following the peak of infection is critical to establishing immunological memory.
"We used metformin, which is known to operate on fatty-acid metabolism, to enhance this process, and have shown experimentally in mice that metformin increases T-cell memory as well as the ensuing protective immunity of an experimental anti-cancer vaccine."
Co-author Dr Russell Jones, from McGill University in Montreal, Canada, said many genes involved in diabetes regulation were also known to play a role in cancer progression.
He added: "There is also a significant body of data suggesting that diabetics are more prone to certain cancers. However, our study is the first to suggest that by targeting the same metabolic pathways that play a role in diabetes, you can alter how well your immune system functions."
Metformin is one of the most widely prescribed drugs for Type 2 diabetes and helps patients to control their blood sugar levels.
Dr Kat Arney, senior science information officer at Cancer Research UK, said: "This is a fascinating piece of research, which sheds light on the complex links between the immune system, cell metabolism and cancer.
"At the moment, this research has only been done in mice and there is a long way to go before it can be applied to cancer patients, but it certainly holds promise for the future."