A discarded anti-cancer drug might be more useful in the fight against cancer than previously thought, say researchers from the University of Missouri.

Doctors used 6-Thioguanine, or 6-TG, as a chemotherapy treatment to kill cancer cells in patients with leukaemia for decades.

However, in recent years, many doctors have shelved 6-TG in favour of newer drugs.

Now, researchers found that 6-TG can not only kill cancer cells, but also works to change how certain cancer cells function, weakening those cells so they can be killed by other drugs.

"While 6-TG is no longer one of the more powerful cancer-killing drugs doctors have at their disposal, we found that it could still be useful to fight cancer in conjunction with other drugs," said lead researcher Jeffrey Bryan.

Cancer cells often have epigenetic markers that cause genes to be either turned off or out of control. This causes those cells to grow rapidly, become difficult to kill and ultimately damage the body.

When testing the drug on cells from dogs with cancer, the MU researchers found that 6-TG can affect these epigenetic markers in cancer cells through a chemical process called demethylation.

This process works to turn off damaging epigenetic markers and turn on markers that make the cells act in a healthy manner. Bryan says this discovery could lead to future cancer treatments using multiple drugs to fight the disease from different sides.

"If we can use 6-TG to turn off dangerous markers in cancer cells so that those cells become easier to kill, we then can use more powerful cancer-killing drugs to eliminate the cells for good," Bryan explained.

Bryan says this research could potentially open doors for future research on other old cancer drugs that are no longer used by doctors.

"Epigenetic markers work similarly in dogs and humans, so we expect to see similar results with these drugs in humans as we do in dogs," Bryan said.