Scientists discover how resistance to the chemotherapy drug 5-fluorouracil arises

By Sally Robertson, B.Sc.Jul 17 2019Reviewed by Kate Anderton, B.Sc. (Editor)

Australian researchers have unlocked the mystery as to why the chemotherapy drug 5-Fluorouracil (5-FU) is not effective at treating certain cancers and have claimed a breakthrough in boosting its efficacy. The research could potentially lead to a laboratory test that could flag up resistance to the drug and thereby reduce unnecessary chemotherapy treatments.

"We are now working to develop a drug that will make 5-FU effective in cancers previously resistant to treatment," says Associate Professor Puthalakath from La Trobe University.

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What is 5-Fluorouracil?

Fluorouracil is one of the most widely used chemotherapy treatments for many types of cancer including breast cancer, head and neck cancer, colon cancer, stomach cancer, and some skin cancers.

It is one of a group of drugs referred to as anti-metabolites, molecules that are similar to those naturally occurring in the body, but slightly different in structure. This difference enables anti-metabolites to disrupt the function of cancer cells by preventing them from making and repairing the DNA that they need to grow and proliferate.

Fluorouracil is administered into the bloodstream by a drip applied to the arm or hand, or by using a pump that a person carries on them for several days. The drug is also available in the form of a topical ointment that can be applied for the treatment of skin cancer.

Common side effects of the treatment when it is administered by drip or pump include:

• Loss of appetite
• Fatigue and weakness
• Bruising, nosebleeds, bleeding gums
• Nausea and/or vomiting
• Diarrhea
• Thinning hair
• Breathlessness
• Increased risk of infection
• Increased blood uric acid concentration
• Arrhythmia
• Mouth sores and ulcers

When used as a topical ointment for skin cancer, fluorouracil does not trigger the usual side effects, but it can temporarily irritate and inflame the areas of skin it is applied to.

The resistance problem

Although 5-Fluorouracil has been a major weapon in fighting many cancers and is on the World Health Organization’s (WHO) Model List of Essential Medicines, resistance is common. This acquired resistance represents a major problem – it occurs in up to half of patients with colorectal cancer that has spread, for example.

Uncovering the mechanism underlying resistance

Now, for the first time, Puthalakath has explained the exact mechanism by which this resistance to the drug develops. He says the resistance is linked to the inactivation of a little-understood protein called “BOK” (Bcl-2 related ovarian killer).

According to Puthalakath, researchers have been curious about the function of this protein for decades, with many convinced that its role is redundant. However, Puthalakath says he has always believed that nature retains proteins for a reason:

Our research shows BOK binds with an enzyme called UMPS, enhancing cells' ability to proliferate. Without BOK, cells struggle to synthesize DNA, and they can't proliferate. The same enzyme is also responsible for converting 5-FU into its toxic form in cancer. Therefore, to avoid 5-FU's toxicity, cancer cells turn off BOK."

Puthalakath

Through inactivating BOK, cancer cells then become dormant, absorb less fluorouracil and become resistant to the chemotherapy. This enables them to survive the treatment before they mutate to become even more aggressive.

Growing organoids to test for 5-FU resistance/sensitivity

For the study, which was funded by La Trobe University and the Swiss National Science Foundation, Puthalakath and colleagues developed organoid cultures using samples taken from patients with rectal and peritoneal cancer who were undergoing surgery at the Peter MacCallum Cancer Center. The organoids were tested for 5-FU resistance and sensitivity as soon as they had been grown in culture.

The presence or absence of BOK determines resistance

As recently reported in the Proceedings of the National Academy of Sciences (PNAS), the researchers found that the BOK protein was present among patients who responded to 5-FU, but not present among those who failed to respond.  

Bok-deficient cell lines were resistant to 5-FU and Bok down-regulation was a key feature of cell lines and primary colorectal tumor tissues that were resistant to 5-FU.

The results also showed that Bok affected nucleotide metabolism in a way that regulated levels of the tumor suppressor p53 and cellular proliferation.

This shows that without BOK present, there is no point attempting to use 5-FU as an effective chemotherapy treatment. In understanding the science behind chemotherapy resistance, we think that we have found cancer’s 'Achilles heel' and this has significant implications for future drug development."

Now researchers are equipped with the knowledge that BOK is needed for 5-FU to be effective, they can start to work on developing a drug that will overcome the resistance problem.

There is also the possibility of developing a test that will flag up resistance so that patients do not have to be administered the drug before finding out whether or not the drug will be effective, thereby avoiding unnecessary chemotherapy treatments.

Our results have implications for developing Bok as a biomarker for 5-FU resistance and for the development of BOK mimetics for sensitizing 5-FU-resistant cancers.”