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Scientists have discovered how cancerous cells can "elbow" their way out of tumours, offering clues for new drugs to prevent cancers spreading.

They say they have identified a protein called JAK which helps cancerous cells generate the force needed to move.

Writing in Cancer Cell, they say the cells contract like muscle to force their way out and around the body.

Cancer Research UK said the study provided fresh understanding of ways to stop cancer spreading.

When cancers spread, a process known as metastasis, they become more difficult to treat, as secondary tumours tend to be more aggressive.

It is thought that 90% of cancer-related deaths occur after metastasis.

JAK attack

Scientists at the Institute of Cancer Research, who investigated the chemicals involved in cell migration in melanoma - skin cancer - say cancerous cells can move in two ways.

They can "elbow" their way out of a tumour or the tumour itself can form corridors down which the cells can escape.

Lead researcher Prof Chris Marshall said both processes were being controlled by the same chemical.

"There is a common theme of using force, force generated by the same mechanism - the same molecule, called JAK," he said.

JAK is not a new culprit in cancer. It has been linked to leukaemia, so some drugs are already being developed which target the protein.

"Our new study suggests that such drugs may also stop the spread of cancer," Prof Marshall said.

"The test will be when we start to see whether any of these agents will stop the spread. We're thinking of clinical trials in the next few years."

Dr Lesley Walker, Cancer Research UK's director of cancer information, said: "A huge challenge in successfully treating cancer is stopping it from spreading around the body, and keeping cancer that has already spread at bay.

"Discovering how cancer cells can funnel grooves though tissues, to squeeze away from primary tumours and spread to new sites, gives scientists fresh understanding of ways to stop cancer spread - literally in its tracks."