Cancer drugs that are designed to shrink tumours by cutting off the supply to their blood may be doing the opposite and helping them spread to other parts of the body, a study has warned.

Researchers from Harvard Medical School in Boston investigated the drugs in question, Glivec and Sutent, and discovered that although it’s proven that they reduce the size of the cancer tumour – it could also make them more aggressive and mobile as a result.

Experts looked at a little-studied group of cells called pericytes, which provide structural support to the blood vessels and act as a ‘gatekeeper’ that stops tumours growing.

However, these pericyte cells are wiped out by advanced cancer drug treatments that are designed to prevent the growth, meaning the tumour has more freedom to ‘metastasise’, or spread, around the body.

Glivec, the brand name of the drug imatinib, and Sutent, also known as sunitinib, have both shown to significantly increase patient survival rates. However, researchers argue that ultimately, these drugs could be making the cancers more deadly, as metastasis to vital organs, such as the liver or brain, are two chief causes of cancer deaths.

The study, published in the Cancer Cell journal, came to this conclusion after removing pericytes from breast cancer tumours in genetically engineered mice. Throughout the following 25 days, researchers saw a 30% decrease in tumour growth but a three-fold increase in the number of secondary tumours growing in the animals’ lungs.

Professor Raghu Kalluri, from the study, says: “If you just looked at tumour growth, the results were good. But when you looked at the whole picture, inhibiting tumour vessels was not controlling cancer progession. The cancer was, in fact, spreading.”

To add to this, the study also found a five-fold increase in cancer tumours spreading in oxygen-starved areas, which had little pericytes present.

When cancer cells are deprived of oxygen, they become more mobile, passing through leaky blood vessel walls and behave like treatment-resistant stem cells. They discovered that smaller tumours shredded more cancer cells into the blood system than larger tumours with a good supply of pericytes.

"We must go back and audit the tumour and find out which cells play a protective role versus which cells promote growth and aggression," he said. "Not everything is black and white. There are some cells inside a tumour that are actually good in certain contexts."

Dr Joanna Owens, Cancer Research UK’s science information manager, told The Huffington Post: “Drugs that block blood vessel formation – so called anti-angiogenic therapies – are a relatively new type of treatment with enormous promise for treating cancer.

"This research helps us understand more about how they work in different patients, and refine clinical practice. These types of drugs are only used after extensive trials that demonstrate a clear benefit for some patients. Cancer is a complex disease and we need research like this to help us learn how to tackle it more effectively.”