Australian researchers are claiming a major breakthrough in boosting the effectiveness of chemotherapy drug 5-Fluorouracil.

The drug has been a major weapon fighting cancers including colorectal, breast, head, neck and aero-digestive.

But in several cases, patients developed a resistance to the drug.

La Trobe University researcher Hamsa Puthalakath said the drug's resistance was linked to the absence of a protein, BOK.

The protein binds with an enzyme which causes cells to multiply and treatment to work.

Without the protein, the enzyme converts less of the drug into a toxic form to fight the cancer. The cancer then becomes dormant, is resistant to the treatment and is more aggressive.

"This shows that without (the protein) present, there is no point attempting to use 5-FU as an effective chemotherapy treatment," Assoc Prof Puthalakath said on Tuesday.

"We think that we have found cancer's 'Achilles heel' and this has significant implications for future drug development."

Armed with the knowledge that protein must be present for 5-FU to be effective, researchers can now develop a drug to overcome the resistance.

There's also scope to develop test for the drug resistance, meaning patients don't have to undergo chemotherapy to learn if the drug will be effective.

The research has been published in the Proceedings of the National Academy.

Australian Associated Press