Scientists at the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, alongside international collaborators at Oxford, the Weizmann Institute, and Dundee, have discovered that the 70-year-old leukemia drug 6-thioguanine (6-TG) relies on an unexpected cellular mechanism. Rather than depending on enzyme catalytic activity, the drug’s therapeutic response is governed by the structural protein NUDT5 acting as a molecular scaffold inside human cells.
In Plain English: The Clinical Takeaway
- The Drug: 6-thioguanine (6-TG) is a decades-old medication used primarily to treat leukemia by inducing toxicity in cancer cells.
- The Discovery: Researchers found that a protein called NUDT5 controls how cells react to 6-TG, but not by performing chemical reactions as an enzyme.
- The Future Impact: Using targeted protein degradation to completely remove NUDT5 protected cells from 6-TG toxicity, opening new avenues for understanding patient treatment variability.
Unraveling a Decades-Old Thiopurine Mechanism
For more than seven decades, clinicians have administered 6-thioguanine (6-TG) to manage leukemia. While its clinical outcomes are thoroughly documented, the precise molecular pathways dictating why specific cells succumb to the drug while others survive have remained poorly understood. A collaborative European research initiative has now illuminated this blind spot by isolating an unexpected factor: the protein NUDT5.
This breakthrough builds directly upon foundational work published in Science in 2025 by the Kubicek and Huber laboratories. That earlier investigation established that NUDT5 possesses a vital secondary role inside cells, functioning not merely as a catalyst for chemical reactions, but as a physical molecular scaffold that organizes cellular metabolism.
Targeted Protein Degradation Versus Conventional Inhibition
Traditionally, pharmaceutical compounds designed to target enzymes function by blocking their chemical activities. To test whether suppressing NUDT5 would alter cellular sensitivity to 6-TG, the research team deployed an emerging pharmacological strategy known as targeted protein degradation. This approach forces the cell to entirely eliminate the protein rather than just inhibiting its catalytic functions.
Anne-Sophie Marques and colleagues at the University of Oxford developed a specialized cell-based screening platform that accelerated the discovery of NUDT5 degraders. This effort yielded dNUDT5, a highly active chemical degrader. Concurrently, the Huber laboratory created matched control compounds capable of binding NUDT5 without inducing its destruction.
When researchers contrasted these molecules against conventional enzyme inhibitors, the results revealed a stark biological divide. Suppressing NUDT5’s catalytic activity caused virtually no change in how cells responded to 6-TG. However, completely removing the NUDT5 protein from the cellular environment shielded those cells from the drug’s cytotoxic (cell-killing) effects.
Co-first author Tuan-Anh Nguyen from CeMM noted that initial expectations centered entirely on enzymatic activity, but experimental data proved otherwise. “Instead, we found that inhibiting the enzyme had little effect. What mattered was whether the protein itself was present,” Nguyen explained.
Competing Cellular Signals: NUDT5 Versus NUDT15
The investigation also uncovered a complex interplay between NUDT5 and another well-characterized protein in thiopurine pharmacology: NUDT15. While genetic loss of NUDT15 renders cells significantly more sensitive to 6-TG, reducing or eliminating NUDT5 produces the exact opposite effect, driving cellular resistance.
This opposing dynamic demonstrates that these related proteins steer thiopurine drug responses through distinct, competing cellular mechanisms. Professor Kilian Huber of the Centre for Medicines Discovery at the University of Oxford emphasized the clarity provided by chemical degraders. “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. In this case, that distinction was decisive: removing NUDT5 revealed biology that conventional inhibitors missed,” Huber stated.
| Protein Target | Intervention Type | Cellular Effect on 6-TG Response | Underlying Mechanism |
|---|---|---|---|
| NUDT5 | Enzyme Inhibition | No meaningful change | Catalytic blocking fails to alter toxicity |
| NUDT5 | Targeted Degradation (dNUDT5) | Increased resistance (Cell protection) | Removal of structural molecular scaffold |
| NUDT15 | Genetic Loss / Reduction | Increased sensitivity | Altered thiopurine drug metabolism |
Contraindications & When to Consult a Doctor
While this study represents a foundational leap in molecular biology, it does not alter current clinical prescribing guidelines or immediate leukemia treatment protocols. Patients undergoing chemotherapy regimens involving thiopurines must strictly follow their hematologist’s directives. Discontinuing or altering prescribed doses based on experimental laboratory findings carries severe risks of disease progression. Individuals experiencing adverse side effects—such as persistent fatigue, unexplained bruising, or signs of infection—should immediately consult their treating oncologist or clinical care team.
A New Foundation for Precision Oncology
Although these findings do not immediately introduce a novel clinical therapy, they fundamentally expand our understanding of intracellular drug metabolism. By proving that NUDT5 governs 6-TG sensitivity through non-catalytic structural functions, the research establishes a framework for investigating why patient responses to thiopurine treatments vary so widely.
As principal investigator Stefan Kubicek of CeMM summarized, “Our results show that proteins can have important biological functions that are completely independent of their enzymatic activity. 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.”
References
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences. Laboratory findings on NUDT5 and thiopurine drug responses.
- Science. Research publications regarding non-enzymatic functions of NUDT5 from the Kubicek and Huber laboratories (2025).
- University of Oxford, Centre for Medicines Discovery. Translational medicinal chemistry reports on targeted protein degraders (dNUDT5).
