The protein, extracted from the venom of the Copperhead viper, slowed the growth of tumours in mice implanted with human breast cancer cells by up to 70 per cent. But its most dramatic effect was in controlling metastasis - the frequently lethal spread of cancer to different parts of the body. Here, the protein reduced the spread of the cancerous cells to the lungs by 90 per cent compared to a control group.
Dr Francis Markland, a biochemist heading a team at the University of Southern California School of Medicine, said yesterday that the treatment could be tested on human patients in the "not too distant future".
That will be an important test, since many "cures" for cancer which have shown promise in laboratory animals have been disappointing in human trials.
But the Californian team appears to be confident in the possibilities. In March an unnamed group filed a US patent for the use of the protein, called contortrostatin, in drugs.
The Copperhead, Agkistrodon contortrix, grows to about three feet long and has a distinctive copper head and reddish brown bands. Although poisonous, its bite is rarely fatal.
Contortrostatin, or "CN", is classed as a "disintegrin" and tends to stop other proteins sticking together. Its effect is to prevent blood clotting, which can lead to haemorrhages in the snake's victims.
When applied to tumour cells, it does not kill them but appears to put them into a "suspended animation" that prevents both metastasis and tumour growth, said Dr Markland.
Because the protein does not attack cells directly, it would not have the side effects, such as nausea, of chemotherapy drugs.