Physicians have relied upon 6-thioguanine, or 6-TG, to combat leukemia for many years. A fresh study has revealed that the medication’s actions hinge upon a protein named NUDT5, a discovery that came entirely without warning.
A group at the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, joined by researchers from the University of Oxford, the Weizmann Institute of Science and the University of Dundee, made the finding. It shows that NUDT5 affects how cells react to 6-TG, yet it does so without following the common enzymatic action that most drugs depend on.
The Protein That Outlived Expectations
The Kubicek and Huber laboratories previously demonstrated that NUDT5, an enzyme, carries out another task beyond its role in chemical reactions. It functions instead as a scaffold that helps organize cellular metabolism. This finding was published in Science in 2025.
The fresh study looked into whether NUDT5’s enzymatic activity held any weight for 6-TG. It found no such link. Instead, what proved to matter was simply whether the protein existed at all.
“We initially expected that NUDT5 would influence 6-TG through its enzymatic activity,” says co-first author Tuan-Anh Nguyen from CeMM. “Instead, we found that inhibiting the enzyme had little effect. What mattered was whether the protein itself was present.”
Degradation Over Inhibition
The researchers took a strategy known as targeted protein degradation, which prompts cells to get rid of a protein rather than merely stopping its activity, and applied it to NUDT5. Through a cell-based system built for discovering NUDT5 degraders, they arrived at dNUDT5, their strongest degrader.
Anne-Sophie Marques, a first author of the paper and someone whose work at Oxford contributed to the findings, said “We developed a cell-based platform to accelerate the discovery of NUDT5 degraders. This platform helped guide the medicinal chemistry efforts that ultimately produced dNUDT5, our most active degrader,”.
The comparison was stark. Blocking NUDT5’s enzyme activity changed nothing about how cells responded to 6-TG. Removing NUDT5, however, protected cells from the drug’s toxic effects. Genetic experiments confirmed the same conclusion.
A Scaffold With Teeth
The findings show that NUDT5 influences how sensitive someone is to thiopurine medications without involving its chemical activity. Put another way, simply looking at what the enzyme chemically does fails to fully account for its effect on 6-TG.
“As the results came in, it became immediately clear that the dNUDT5 was protecting cells from 6-thioguanine toxicity in a dose-dependent manner. That was an incredibly exciting moment,” said Ludwig Bauer, a first author of the paper.
The Other Protein
Another discovery made by the researchers involved a connection between NUDT5 and NUDT15, a protein already established as influencing how patients react to thiopurine medications. The two proteins work in opposite ways.
The loss of NUDT15 increases cell sensitivity to 6-TG, while reducing NUDT5 has the opposite effect, making cells more resistant to the treatment. Together, these findings point to distinct mechanisms through which the two proteins influence thiopurine response, working in opposite directions.
“Our results show that proteins can have important biological functions that are completely independent of their enzymatic activity,” says corresponding author Stefan Kubicek, Principal Investigator at CeMM. “By removing NUDT5 rather than simply inhibiting it, we were able to uncover a hidden layer of biology that helps determine how cells respond to a clinically important drug.”
What This Means for Patients
A long-used leukemia drug’s effects are controlled by an unexpected mechanism, though the findings do not point toward a new treatment.
Researchers now have a clearer idea of what causes people to respond differently to thiopurine treatment. Their work demonstrates that targeted protein degradation can reveal biological functions that would otherwise stay hidden when scientists depend solely on conventional enzyme inhibitors.
The project received funding from a variety of sources. It was backed by the European Research Council (ERC) through the European Union’s Horizon 2020 research and innovation program. Other contributors included the Austrian Science Fund (FWF) and the Vienna Science and Technology Fund (WWTF). The Marie Skłodowska-Curie Actions Postdoctoral Fellowships program also provided support. The Innovative Medicines Initiative 2 Joint Undertaking (IMI2 JU), the Wellcome Trust, Merck Sharp & Dohme Corp., and Janssen Pharmaceutica NV rounded out the list of sponsors.
The Discovery That Outlived Expectations
What caught everyone off guard was the discovery itself. Researchers believed they understood exactly how this 70-year-old leukemia medication functioned. They were wrong.
Source material: “Scientists thought they knew how this 70-year-old leukemia drug worked,” ScienceDaily.
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