Scientists said on Wednesday they were developing new experimental drugs that block the blood supply to tumors in a novel way which could be effective for treating difficult types of cancer.

Unlike other drugs that starve tumors of blood by preventing the growth of blood vessels, or angiogenesis, the new treatment takes a different approach.

It increases the formation of blood vessels but they do not work well so the tumor cannot grow and survive.

"You seem to end up with more blood vessels but they are less functional," Dr Gavin Thurston, of Regeneron Pharmaceuticals in Tarrytown, New York, said in an interview.

"This approach of blocking tumor vessels is still in its early stages. There are drugs out there that are already helping patients but there is still more to be learned in this particular area. This is another step along that path," he added.

Avastin, developed by San Francisco-based Genentech and sold by Roche is a leading anti-angiogenic drug. It targets a protein called vascular endothelia growth factor (VEGF) which tells cells to grow new blood vessels.

Several other anti-VEGF drugs have been developed but they are not effective against all tumors.

In two separate reports in the science journal Nature, Thurston and his team and scientists at Genentech, led by Minhong Yan, describe how the new approach targets a different molecule called Delta-like ligand 4 (Dll4).

"Contrary to conventional wisdom, the extra blood vessels formed through the blockade of Dll4 served to choke the tumors rather than feed them," Thurston explained.

He added that having both approaches, which may be mixed and combined, could be more powerful than just one.