Scientists have used gene therapy to reduce the size of rat tumours, and there is hope that a similar treatment could help humans.

The treatment, devised by researchers from Nice in France, involves injecting modified tumour cells under the skin of rats with advanced colon cancer.

Within 30 days, the tumours themselves - even at far distant locations - had shrunk by an average 70% compared with those not given the therapy.

The finding reinforces the promise of such treatments, even though UK experts say that a different approach is likely to yield the best results.

Spreading cancers

The study, published in the journal Gut, involved aggressive rat colon cancer cells.

A gene from the food poisoning bacteria E.coli was inserted into these cells, which were then injected back just under the skins of cancer-stricken rats.

The gene is responsible for making a body chemical which converts a normally harmless chemical into a lethal chemotherapy drug.

So when the harmless chemical comes in the vicinity of the modified tumour cells, it is converted into the cell-killing chemical.

In this way the chemotherapy is delivered only to the tumour cells and not elsewhere in the body, reducing the potential for unpleasant side-effects.

However, what the French team also noticed was that these modified tumour cells seemed to prime the immune system to launch an attack on tumour cells, regardless of where they were in the body.

Bystander effect

This "bystander" effect is key to the success of such a therapy in many cancers where the cancer is unlikely to be confined to the original tumour, perhaps spreading to the liver and lungs.

Colon cancer patients are particularly likely to be in this position, as their tumours are often not diagnosed until this stage.

The immune response generated by the French trial even managed to tackle metastatic tumour cells on the liver of the rats, if the modified tumour cells were injected directly into the liver.

The researchers believe that their formula holds promise as an anti-cancer "vaccine".

However, a UK expert suggested that while "suicide gene" therapy was promising, it was more likely to involve modified viruses provoking an immune response.

Professor Nick Lemoine, from Imperial College, London, told BBC News Online: "What is happening at the moment is that this gene therapy is being combined with radiotherapy and chemotherapy.

"Because the virus replicates inside the tumour cells, you get more bangs for your buck."