Doctors may have found a way of harnessing the destructive power of ricin to fight cancer without potentially lethal side-effects.
Ricin is extracted from castor beans
Ricin, extracted from castor beans, is a powerful natural toxin and considered a potential biological weapon.
Scientists want to use it to kill cancer cells, by joining it to a protein that binds onto lymphoma cells.
However, some patients given the experimental treatment developed a symptom called "vascular leak syndrome", which affects blood vessels, particularly in the lungs.
There is no doubt that there is real potential for immunotoxins
Dr David Flavell, University of Southampton
However, a team from the University of Texas Southwestern Medical Center believe they have found a way to genetically modify the toxin so that it is less dangerous to humans - but still as lethal to cancer cells.
They looked at several other proteins known to cause vascular leak syndrome in humans, examining their genetic structure.
They found certain chemical features were similar in several of these proteins - suggesting that they might be the reason.
They then engineered ricin toxin which missed out these particular features, and tested their new toxin on mice.
They found that the GM toxin was just as good at killing tumour cells - but induced significantly less vascular leak syndrome in the mice.
This in theory should allow the dose of toxin in any future cancer drug to be increased, making it much more effective against the disease.
Their research was published in the journal Nature Biotechnology.
One team of UK scientists at the University of Southampton is carrying out similar work in the UK, using a different toxin
Dr David Flavell, a senior lecturer in cancer sciences at the university, is helping coordinate early clinical trials giving the drug to children whose leukaemia has relapsed.
He said: "This is a very interesting piece of research.
"There is no doubt that there is real potential for immunotoxins, probably as a way of either mopping up cancer cells that may have survived initial treatment, or allowing doctors to use a lower dose of other chemotherapy drugs which might be more damaging.
"We're hopeful about our toxin, and we have had no cases of vascular leak syndrome so far."
Dr Flavell is scientific director of the charity Leukaemia Busters, which funds the project in the UK.