A 70-year-old leukemia drug has uncovered a hidden side of how cancer cells survive chemotherapy.
For more than 70 years, 6-thioguanine (6-TG) has been used to treat leukemia, yet scientists are still uncovering what determines whether cancer cells are killed by the drug or survive it. New research suggests that part of the answer lies with NUDT5, a protein with a previously overlooked role in shaping the drug’s effects.
Researchers at CeMM and the University of Oxford, working with collaborators at the Weizmann Institute of Science and the University of Dundee, discovered that this role does not depend on NUDT5’s usual enzymatic activity. The findings were published in Nature Communications.
NUDT5 Has a Hidden Role
Enzymes are usually understood by the chemical reactions they carry out, and drugs often work by blocking those reactions. But NUDT5 appears to do more than act as an enzyme, giving it a second role that conventional inhibitors can miss.
Earlier work from the Kubicek and Huber laboratories, published in Science in 2025, showed that NUDT5 can also behave as a molecular scaffold. It physically interacts with PPAT, an enzyme involved in producing purines, which are essential components of DNA and RNA. By restraining PPAT, NUDT5 helps limit the cell’s production of these molecular building blocks.
This provided an important clue because 6-TG is an antimetabolite. The drug must be metabolically activated inside cells, producing molecules that interfere with normal cellular processes. The response is also influenced by proteins that handle thiopurine metabolites, including NUDT15.
Removing NUDT5 Reveals the Difference
“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.”
To separate NUDT5’s catalytic activity from its other functions, the scientists did something more drastic than inhibiting it. They removed the protein.
The researchers used targeted protein degradation, an approach that allows scientists to eliminate selected proteins from cells rather than merely switching off their active sites. This can expose functions that conventional inhibitors leave intact.
At Oxford, the team developed a collection of highly selective molecules capable of degrading NUDT5, along with matched compounds that could bind the protein without destroying it. They also compared these molecules with conventional NUDT5 inhibitors.
Protein Degradation Exposes Hidden Biology
“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,” said Anne-Sophie Marques, a first author of the paper whose work at Oxford contributed to the findings.
The experiments produced a clear contrast. Inhibiting NUDT5’s enzymatic activity did little to alter the effects of 6-TG. Eliminating the NUDT5 protein, however, made cells more resistant to the drug. Genetic experiments in which NUDT5 was removed independently produced the same pattern.
In other words, NUDT5’s influence over 6-TG sensitivity appears to depend on the protein being physically present, not simply on the chemistry it performs.
NUDT5 and NUDT15 Act in Opposite Ways
“Chemical degraders give us a way to separate what a protein does as an enzyme from what it does as a physical presence in the cell,” says Professor Kilian Huber, Centre for Medicines Discovery at the University of Oxford and co-corresponding author of the study. “In this case, that distinction was decisive: removing NUDT5 revealed biology that conventional inhibitors missed.”
Drugs designed to inhibit enzymes may leave structural, organizational, or signaling roles untouched. Protein degradation gives researchers a way to test what happens when every function of a protein disappears at once.
“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 study also revealed a striking contrast between NUDT5 and another protein with a similar name, NUDT15.
NUDT15 is already known to influence responses to thiopurine drugs. It helps break down active metabolites of 6-TG, and losing NUDT15 makes cells more sensitive to the drug. NUDT5 behaved in the opposite way. When researchers depleted NUDT5, cells became more resistant.
What This Could Mean for Cancer Treatment
The result suggests that the two proteins affect thiopurine treatment through distinct biological routes rather than acting as interchangeable members of the same pathway.
“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.”
The earlier Science study offers a possible framework for understanding this effect. NUDT5 helps restrain purine production by interacting with PPAT. Removing that restraint can increase purine synthesis, potentially changing the metabolic environment in which thiopurine drugs operate. The new work strengthens the case that NUDT5’s physical role in the cell can matter as much as, or more than, its catalytic activity.
For patients, the findings are not yet a reason to change treatment. The researchers have not demonstrated a new therapeutic strategy, and the work primarily establishes a biological mechanism. But understanding that mechanism could eventually help explain why thiopurine responses vary and may provide researchers with additional features to examine when studying drug sensitivity.
Reference: “Targeted Protein Degradation of NUDT5 Dissociates Catalytic Inhibition from Protein Loss in 6-Thioguanine Response” by Anne-Sophie M. C. Marques, Ludwig G. Bauer, Tuan-Anh Nguyen, Alejandro Gonzalez Orta, Jan-Lennart Venne, Carol Cheng, Esra Balıkçı, Yusi Liu, Barr Tivon, Alena Kroupova, Alessio Ciulli, Nir London, Stefan Kubicek and Kilian V. M. Huber, 30 June 2026, Nature Communications.
DOI: 10.1038/s41467-026-74489-9
This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, the Austrian Science Fund (FWF), the Vienna Science and Technology Fund (WWTF), the Marie Skłodowska-Curie Actions Postdoctoral Fellowships programme, the Innovative Medicines Initiative 2 Joint Undertaking (IMI2 JU), the Wellcome Trust, Merck Sharp & Dohme Corp. and Janssen Pharmaceutica NV.
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