US scientists have developed a new compound based on a chemical in the sweet wormwood plant, which appears to be effective at killing certain kinds of cancer cells in laboratory tests.

The plant, which is used in salads in some Asian countries, is also the source of a common anti-malarial drug called artemisinin.

Now, scientists at the University of Washington have modified the compound so that it targets cancer cells and is more than 1,200 times more specific in killing these cells than existing drugs.

The researchers tested the compound on human leukaemia cells and found it to be highly selective, killing 12,000 cancer cells for every healthy cell. Artemisinin itself kills around 100 cancer cells for every healthy cell, showing that the modified version of the drug is far more selective.

This could help to reduce the side-effects associated with chemotherapy and could also enable much higher doses of the drug to be safely tolerated.

Senior author Dr Tomikazu Sasaki, chemistry professor at the University of Washington, commented: "Side-effects are a major limitation to current chemotherapies. Some patients even die from them.

"The compound is like a special agent planting a bomb inside the cell."

According to the researchers, the chemical tag which they added to artemisinin attaches to proteins on the outside of cells which are designed to attract iron.

Cancer cells need particularly high iron levels for tumour growth so they tend to have large numbers of these proteins on their cell surfaces.

However, instead of only swallowing up iron, the affected cancer cell also swallows up the artemisinin attached to its surface - enabling the drug to enter the cell where it reacts with iron to release poisonous free radical molecules which destroy the cell.