The study, in mice and tissue samples, used a protein called TNF that can track down sites in the brain where cancer has spread by recognising a marker found only on tumour blood vessels.
The researchers found that TNF can home in on these sites and temporarily open the blood-brain barrier (BBB) allowing drugs to pass from the blood system into the tumour.
The BBB acts as a shield that prevents potentially dangerous particles such as bacteria entering the brain. But it's this same shield that stops cancer drugs reaching tumours that have spread to the brain.
The TNF protein only broke down the BBB in the blood vessels that pass through the tumour, leaving the healthy parts of the brain undamaged by potentially toxic drugs.
The research showed that when TNF is injected into the bloodstream, the breast cancer drug herceptin (trastuzumab) – which is not normally able to cross the BBB – can reach cancer cells in the brain.
The research is reported in the Journal of the National Cancer Institute.
Dr Nicola Sibson of the Department of Oncology at the University of Oxford said: 'Treatments that work very well against the original site of the cancer lose their effectiveness when the cancer spreads to the brain – as these drugs are prevented from getting to the tumour because of the blood-brain barrier.
'A number of attempts have been made to open up the BBB but they've all struggled because they're either not specific enough to open the BBB only at the site of the tumour or not effective enough to allow the drug across to kill the cancer.'
Dr Kat Arney, science information manager at Cancer Research UK, which funded the work, said: 'Getting treatments through the blood-brain barrier remains one of the greatest challenges for cancer researchers. This exciting result points the way to a potentially game-changing moment in finding ways to treat cancers that have spread to the brain. We now need to test this approach in cancer patients to see if it will have the same effect.'