A drug used to treat schizophrenia may provide an effective weapon against treatment-resistant cancers, research suggests.
The anti-psychotic drug thioridazine appears to "tame" deadly cancer stem cells, turning them into ordinary non-cancerous cells.
Stem cells are thought to be the source of many cancers and are often impervious to traditional chemotherapy.
First identified in certain types of leukaemia in 1997, they are now known to play a role in breast, brain, lung, gastrointestinal, prostate and ovarian cancers.
Scientists discovered thioridazine's surprising properties after screening hundreds of compounds looking for ways of attacking cancer stem cells.
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In tests, the drug turned leukaemia stem cells into stable cells that no longer divided uncontrollably.
It latched onto specific proteins found on the surface of the cancer stem cells, but not normal blood stem cells.
The same proteins, called dopamine receptors, are present on some breast cancer stem cells. They are also targeted by thioridazine in the brain, where they play a role in schizophrenia.
Plans are now under way for a clinical trial using thioridazine in combination with standard anti-cancer drugs to treat adult acute myeloid leukaemia.
Dr Mick Bhatia, from McMaster University in Canada, who led the research reported in the journal Cell, said: "You have to find something that's truly selective for cancer stem cells. We've been working for some time and it's hard to find that exact formula.
"We're excited about bringing this drug to patients. We also hope our platform can now be a pipeline for other cancer stem cells drugs."
Dopamine receptors may serve as biomarkers allowing the early detection of aggressive breast cancers and leukaemia, the researchers believe.
The receptors activate dopamine, a neurotransmitter chemical that, in the brain, relays signals between nerve cells. Excess dopamine activity is associated with schizophrenia.
Any use of thioridazine will have to take account of its side effects.
At higher doses, the drug can trigger involuntary tremors, restlessness, and in some cases a potentially fatal heart condition.
For this reason it is no longer a first choice of treatment for schizophrenia.
