Scientists have identified a chemical that can sneak through the blood-brain barrier to treat tumours.
A layer of cells around the brain keeps out toxic substances
The barrier exists to prevent toxic substances getting into the brain, which makes it hard to deliver drugs.
Researchers found enough of the chemical, JV-1-36, could bypass the guard to block tumour growth.
The University of Saint Louis study, in Proceedings of the National Academy of Science, suggests the compound may also be useful in treating other cancers.
The team carried out tests on mice who had had malignant glioblastomas, the most common form of brain tumour, implanted.
They then gave an intravenous injection of JV-1-36, which inhibits the effect of the hypothalamic growth hormone-releasing hormone (GHRH).
GHRH's role should be to trigger the hormone that makes children grow, but it has also been found to fuel the growth of cancerous tumours.
Receptors for the hormone have been found in other cancer cells including breast, ovary, prostate, pancreas and colon.
The researchers found that the P-gp system, which acts as an extra "security guard" at the blood brain barrier and usually keeps anti-cancer drugs out of the brain, blocked some of the JV-1-36, but let much of it pass into the brain.
The researchers say the compounds gets into the brain by dissolving into the cell membranes which comprise the blood-brain barrier, and not being picked up by P-gp.
They say this appears to be because it is not recognised as being a "foreign" substance.
Professor William Banks, who led the research at the Saint Louis University School of Medicine, said: "The blood-brain barrier is set up to very carefully patrol what it lets into the brain and what it keeps out.
"It makes these decisions based on the physicochemical properties.
"Most of our drugs that fight cancers are toxic to cancer cells and to other cells too. That's why the blood-brain barrier locks them out of the brain."
He added: "There are times when there's a big difference between an animal model and the human condition.
"In terms of getting drugs across the blood brain barrier to fight cancer, there's not such a big difference.
"There's pretty much the same rules in any blood-brain barrier - be it mouse or human."
Ed Yong, Science Information Officer at Cancer Research UK, said: "Glioblastomas are a common type of brain cancer in adults.
"Finding innovative new treatments is crucial but extremely complex, since most drugs are excluded by the blood-brain-barrier."
He added: "By finding a drug that successfully passes this barrier, these researchers have made a promising first step.
"But this drug must be developed further and pass rigorous clinical trials in humans before it can be used as a treatment for brain cancer."