Uncovering a Novel Mechanism Behind a Long-Standing Leukemia Drug

For over seventy years, the medication 6-thioguanine (6-TG) has been a cornerstone in leukemia treatment. While its clinical effects are well-established, the precise molecular mechanisms that dictate why certain cells succumb to the drug while others survive remain incompletely understood.

Recently, a collaborative team from the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, alongside researchers from the University of Oxford, the Weizmann Institute of Science, and the University of Dundee, identified an unexpected variable in this cellular response: a protein known as NUDT5.

This discovery builds upon recent findings from the Kubicek and Huber laboratories (Science, 2025). That prior research revealed that NUDT5 plays a crucial role within cells that extends beyond its traditional enzymatic function. Instead of acting solely as a catalyst for chemical reactions, NUDT5 also serves as a molecular scaffold that aids in organizing cellular metabolism.

This unexpected function appears to be critical in determining how cells react to 6-TG.

“We initially hypothesized that NUDT5 would modulate 6-TG through its enzymatic activity,” explains co-first author Tuan-Anh Nguyen of CeMM. “Instead, we discovered that inhibiting the enzyme yielded negligible effects. What proved decisive was the mere presence of the protein itself.”

Removing NUDT5 Alters the Drug Response

While many enzyme-targeting drugs are designed to inhibit the chemical reactions those enzymes facilitate, the researchers sought to determine whether suppressing NUDT5 in this manner would likewise alter the effects of 6-TG.

To investigate this hypothesis, they employed an emerging strategy known as targeted protein degradation. Rather than simply blocking a protein’s activity, this approach triggers the cell to completely eliminate the protein.

“We engineered a cell-based platform to expedite the discovery of NUDT5 degraders. This platform guided the medicinal chemistry efforts that ultimately yielded dNUDT5, our most potent degrader,” stated Anne-Sophie Marques, a first author of the study whose research at Oxford contributed to these findings.

Under the leadership of the Huber laboratory at the University of Oxford, a medicinal chemistry program generated a suite of highly selective NUDT5 degraders. The team also developed matched control compounds capable of binding to NUDT5 without triggering its destruction.

The researchers then compared the effects of these molecules against conventional NUDT5 inhibitors.

The distinction was striking. Blocking NUDT5’s enzymatic activity did not significantly alter the cellular response to 6-TG. However, physically removing NUDT5 from the cells shielded them from the drug’s toxic effects. Genetic experiments corroborated this conclusion.

“Chemical degraders provide a method to distinguish between what a protein does as an enzyme and what it does as a physical entity within the cell,” says Professor Kilian Huber of the Centre for Medicines Discovery at the University of Oxford and co-corresponding author of the study. “In this instance, that distinction was crucial: eliminating NUDT5 unveiled biological insights that traditional inhibitors overlooked.”

A Hidden Function Beyond Enzyme Activity

The findings demonstrate that NUDT5 influences sensitivity to thiopurine drugs through a mechanism independent of its catalytic function. Consequently, merely studying the chemical actions of the enzyme is insufficient to explain its impact on 6-TG.

“As the data emerged, it became immediately apparent that dNUDT5 was safeguarding cells from 6-thioguanine toxicity in a dose-dependent fashion. This was an incredibly thrilling moment,” recalled Ludwig Bauer, a first author of the paper.

The team also uncovered a fascinating link between NUDT5 and another protein, NUDT15, which is already recognized for its role in patient responses to thiopurine drugs.

The two proteins appear to exert opposing effects.

The loss of NUDT15 increases cellular sensitivity to 6-TG, whereas reducing NUDT5 renders cells more resistant to the treatment. These findings suggest that the proteins influence thiopurine responses through distinct mechanisms that push cells in opposite directions.

“Our results illustrate that proteins can fulfill vital biological functions entirely independent of their enzymatic activity,” states corresponding author Stefan Kubicek, Principal Investigator at CeMM. “By removing NUDT5 rather than simply inhibiting it, we uncovered a hidden layer of biology that determines how cells respond to a clinically significant drug.”

A New Window Into Leukemia Drug Response

While these findings do not immediately translate into a new treatment, they reveal an unexpected mechanism that governs the effects of a long-utilized leukemia drug.

By demonstrating that NUDT5 influences 6-TG through a non-catalytic function, this research may help scientists better understand the variability in thiopurine treatment responses. It also highlights how targeted protein degradation can expose biological functions that might remain invisible when researchers depend solely on traditional enzyme inhibitors.

This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program, the Austrian Science Fund (FWF), the Vienna Science and Technology Fund (WWTF), the Marie Skłodowska-Curie Actions Postdoctoral Fellowships program, the Innovative Medicines Initiative 2 Joint Undertaking (IMI2 JU), the Wellcome Trust, Merck Sharp & Dohme Corp. and Janssen Pharmaceutica NV.

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