The sting of the Middle Eastern scorpion Leiurus quinquestriatus unleashes a cocktail of neurotoxins containing a peptide that is non-toxic to humans but binds to tumour cells.
In tests, the peptide has invaded tumours in breast, skin, brain and lung tissue while leaving healthy cells untouched.
"It's as if the tumours collect it," New Scientist quoted Michael Egan of the company TransMolecular in Cambridge, Massachusetts, as saying.
To determine if the peptide could deliver lethal doses of radioactivity to cancer cells, researchers have now attached radioactive iodine isotopes to it.
In a trial last year, the researchers injected this agent directly into the tumours of 59 people suffering from inoperable brain cancer.
All the patients have now died, but those receiving a higher dose lived for three months longer, on average.
In recent weeks, researchers at the University of Chicago in Illinois have begun injecting TM601 into the bloodstream of people with different types of malignant brain cancer.
This latest trial will allow the company to test whether it can seek out and kill secondary tumours throughout the body, as well as known primary ones.