A PILL which stops cancer spreading could be developed after scientists discovered how to prompt the immune system to kill tumour cells.Metastasis - the spread of cancer from one organ or part of the body to another - is the primary cause of death for those suffering from the disease.Researchers have now identified a molecule - called Cbl-b - which blocks the spread, bringing scientists one step closer to meeting the "holy grail" of cancer therapy challenges. If successful in future trials, the discovery could transform the disease from deadly to controllable.Yesterday Professor Caetano Reis e Sousa, Cancer Research UK scientist, described the findings as "tantalising" but cautioned that further research was needed to see if the research would benefit cancer patients."The immune system has the ability to kill tumours but it can be held back by molecular 'brakes'," he said."The authors of this study have been able to take the foot off one of these brakes, potentially giving the green light to the immune system to kill the tumour cells. This is a tantalising finding but it's early stage research, and whether this will ever work in patients will need to be looked at in further studies."In experiments on mice, deletion or targeted inactivation of the molecule boosted the anti-tumour function of cells - dramatically inhibiting the spread of breast cancer and melanoma - the deadliest form of skin cancer.In the experiments the treatment was administered orally or through a cavity in the stomach.Professor Josef Penninger, of the Austrian Academy of Sciences in Vienna, whose results are published in the journal Nature, said: "Metastases are the main reason why cancer patients die."Our results hold promise that it might be possible to develop Cbl-b or TAM inhibitors, which act as a 'brake' on the immune system, that empowers the innate immune system to kill cancer metastases."This would be indeed opening the Holy Grail for cancer therapy. But more research needs to be done ... to test for possible side-effects."Around 331,000 people in the UK were diagnosed with cancer last year. In Scotland, the number of people diagnosed with the disease has now reached 30,200 cases each year. Approximately 159,000 people died from cancer in the UK in 2011. North of the Border the number was just over 8,000.The researchers also found that the blood-thinning drug warfarin exerted anti-metastatic activity in mice. They said this provided a molecular explanation for the long-standing conundrum on how the blood-thinning drug warfarin can also inhibit tumour metastases.