LONDON (Reuters) - Scientists are close to testing a genetically engineered "magic bullet" that could treat half of the most common cancers, New Scientist magazine said. Early laboratory tests have shown that the "bullets", which destroy the tumours by injecting them with a deadly toxin, were effective in treating adenocarcinomas which are found in lung, ovary, prostate, colon and breast cancers. Clinical trials with colon cancer sufferers could begin within the next 15 months. A magic bullet is a treatment that targets the cancerous cells without harming any of the healthy cells around them -- unlike chemotherapy which can harm healthy cells and result in serious side-effects. Earlier attempts at the approach failed because although the antibodies on the bullets found the cancerous cells they could not penetrate and destroy them without harming healthy cells as well. A medical center in Jerusalem developed fusion proteins that isolate the dangerous cells and inject them with a bacterial toxin while leaving healthy cells alone. One half of the re-engineered protein binds to the receptor on adenocarcinoma cells and the second half fires a fatal dose of the toxin that kills them by preventing them from making proteins. "The Israelis modified the natural toxin so that immune cells previously exposed to the bacteria would not recognise and attack it," the magazine explained.