The story begins with glioblastoma, the most aggressive form of adult brain cancer. Despite surgery, radiation and chemotherapy, median survival remains roughly 14.6 months, and a major reason is the persistence of glioblastoma stem cells, which drive tumor initiation, progression and resistance to therapy. Because aberrant epigenetic programs help these cells maintain their plasticity, the researchers had previously screened a library of epigenetic chemical probes and found that QC6352 suppressed glioblastoma stem cell viability at nanomolar concentrations. In the new study, the team validated that potency across six patient-derived glioblastoma stem cell lines, including RKI1, MMK1, JK2, FPW1, HW1 and GBM6, with growth-inhibition potency values ranging from 3 to 133 nanomolar. Live-cell imaging showed that nearly 80 percent of treated cells arrested in the G0/G1 phase of the cell cycle, and the drug left normal human astrocytes unharmed up to 1 micromolar, a tenfold therapeutic window.

The cellular phenotype was striking. Treated cells ramped up expression of senescence and apoptosis genes, accumulated senescence-associated beta-galactosidase and Annexin V staining, and lost expression of stemness markers such as NES, SOX2 and SOX9, shifting toward a nonproliferative, differentiated state. In mice bearing orthotopic GBM6 xenografts, QC6352 at 100 milligrams per kilogram slowed tumor growth and extended median survival from 28.5 to 34.5 days without affecting body weight or blood counts. On the surface, everything pointed to a spectacular epigenetic vulnerability. But the team, led by Lenka Munoz of the University of Sydney together with Paul Workman, applied a discipline that is recommended far more often than it is practiced: testing multiple, structurally distinct inhibitors against the presumed target.

That discipline exposed the crack. Alongside QC6352, the researchers evaluated the orthogonal KDM4 inhibitor ML324, which they confirmed inhibits KDM4A with essentially the same biochemical potency, an IC50 of 326 nanomolar versus 355 nanomolar for QC6352. Yet the two compounds behaved nothing alike in cells. While QC6352 at 100 nanomolar completely repressed glioblastoma growth and triggered apoptosis, ML324 at 500 nanomolar had no impact whatsoever, and even at 5 micromolar it failed to block colony formation. Both compounds raised the levels of the histone marks H3K9me3 and H3K36me3 that KDM4 enzymes normally remove, but the team cautioned that such changes can also reflect broader heterochromatin and senescence responses, so they went further, directly testing whether either drug actually engaged KDM4 proteins inside living cells.

It did not. Cellular thermal shift assays detected no stabilization of overexpressed KDM4A even at 100 micromolar of either compound, and single-molecule tracking of SNAP-tagged KDM4A in live HeLa cells revealed no change in the mobility or chromatin-binding behavior of the enzyme after drug treatment. Expression analysis deepened the skepticism: KDM4A and KDM4B transcripts were present at low levels in only some glioblastoma stem cell lines, KDM4C and KDM4D were largely undetectable across twelve lines, and single-cell RNA sequencing of biopsies from 62 patients showed consistently low KDM4 expression in malignant cells. Proteomics found no KDM4 protein at all in A172 glioblastoma cells. Crucially, knocking out KDM4A, KDM4B or KDM4C individually with CRISPR-Cas9 left cells just as sensitive to QC6352 as before, and genome-wide CRISPR dropout screens using the 71,090-guide TKOv3 library identified 3,334 fitness genes in RKI1 cells without a single KDM4 family member among them.

The chemogenomic screens added another layer. Treating TKOv3-transduced cells with QC6352 at its 90 percent effective concentration of 70 nanomolar and at its 20 percent effective concentration of 25 nanomolar revealed 609 sensitizing and 278 resistance genes, with RNA splicing and mRNA and tRNA processing emerging as key biological processes. Again, no KDM4 enzyme met the threshold. The evidence was converging on an uncomfortable conclusion: the celebrated KDM4 probe was doing something else entirely in glioblastoma cells.

To find the real mechanism, the team turned QC6352’s transcriptional fingerprint into a search query against the Connectivity Map, comparing its signature with more than 20,000 pharmacologic perturbations across 71 cell lines. The top matches were brequinar, a known DHODH inhibitor, and floxuridine and raltitrexed, both inhibitors of thymidylate synthase. All three converge on a single metabolic pathway: de novo pyrimidine synthesis, the assembly line that produces the nucleotide building blocks of DNA, RNA and lipids. The logic of the pathway also explained a curious secondary hit, the cardiac glycosides digoxin, strophanthidin and bufalin, which indirectly block the uridine salvage route by suppressing sodium-dependent nucleoside uptake. When the researchers simply added uridine to the culture medium, it completely rescued glioblastoma cells from QC6352-induced growth arrest, the classic signature of a blocked pyrimidine supply.

Biochemistry then delivered the decisive numbers. In a recombinant enzyme assay, QC6352 inhibited human DHODH with an IC50 of 0.3 nanomolar, making it more potent than the established DHODH inhibitors BAY2402234 at 1.1 nanomolar and brequinar at 2.6 nanomolar. ML324, by contrast, showed no DHODH inhibition at all. Metabolomics sealed the case: within 24 hours of treatment, both QC6352 and BAY2402234 drove a roughly 10,000-fold accumulation of the upstream DHODH substrates dihydroorotate and carbamoylaspartate while depleting the downstream nucleotides UMP, UDP and UTP. A 1.95-angstrom cocrystal structure of QC6352 bound to human DHODH showed the compound occupying the hydrophobic channel leading to the active site, blocking ubiquinone cofactor binding, with its isonicotinic acid head forming polar contacts with R136, Q47 and Y356 and its tail packing into a hydrophobic tunnel. The same scaffold, in the KDM4A structure, chelates the catalytic nickel ion, explaining how one molecule can moonlight in two entirely different enzyme families.

The clinical implications came into sharp focus with zavondemstat, a structural analog of QC6352 now in trials for advanced and metastatic cancers. The R-enantiomer inhibited DHODH at 0.6 nanomolar, outperforming brequinar, farudodstat and BAY2402234, while its S-enantiomer was 40-fold weaker against DHODH, a chirality-dependent potency gap that tracked far better with the compounds’ cellular efficacy than their modest twofold difference in KDM4A inhibition. Uridine fully reversed the effects of both enantiomers, and expressing an inhibitor-resistant DHODH A58T mutant rescued cells from QC6352, zavondemstat and BAY2402234 alike, while uridine could not rescue the unrelated drugs GSK-J4 or doxorubicin. Guided by the crystal structures, the team even designed three new isonicotinic acid analogs that retain potent KDM4A inhibition, with IC50 values between 41 and 208 nanomolar, while showing no DHODH activity above 20 micromolar, providing the field with genuine KDM4-selective probes for the first time in this chemical series.

Beyond glioblastoma, the study is a cautionary tale for chemical biology as a whole. Despite long-standing guidelines urging the use of at least two structurally distinct probes plus inactive controls, reliance on a single compound remains common, and QC6352 shows exactly how that shortcut can produce confident but wrong conclusions about drug mechanism. DHODH itself is a validated therapeutic target, with the inhibitors leflunomide and teriflunomide already approved for autoimmune disease, though no DHODH inhibitor has yet been approved for cancer. The authors note that zavondemstat’s KDM4-independent mechanism does not diminish its clinical promise, and may even explain it, but they urge that ongoing trials be benchmarked against prior DHODH inhibitor studies. They also flag that DHODH should now be considered a frequent hidden target in phenotypic drug discovery, since multiple unrelated chemotypes keep landing on it. For glioblastoma, a disease desperate for options, the message is paradoxically hopeful: a drug already in the clinic may work through a metabolic vulnerability that is increasingly recognized as one of cancer’s most druggable Achilles heels.

Subject of Research: Target deconvolution of the KDM4 inhibitor QC6352 revealing DHODH as the driver of antiglioblastoma activity

Article Title: DHODH is the key target of the KDM4 inhibitor QC6352 in glioblastoma stem cells

Article References: Sterling, J. R., Holtsmark, J. R., Joun, G. L., Graus, M. S., Du, T. Y., Kuchibhotla, M., Johns, T. G., deWeck, A., Loo, L., Neely, G. G., Tran, K., Shard, C., Gomez, G. A., Patil, A., Shi, D. D., Losman, J.-A., Saxton, A. J., Baker, J. R., Lei, Z., … Munoz, L. (2026). DHODH is the key target of the KDM4 inhibitor QC6352 in glioblastoma stem cells. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02306-x

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02306-x

Keywords: glioblastoma, DHODH, QC6352, zavondemstat, KDM4, pyrimidine biosynthesis, chemical probes, target deconvolution, epigenetics, drug mechanism, cancer metabolism, CRISPR screens

The compounds in question were originally developed as inhibitors of KDM4, a family of proteins that can drive the growth and spread of cancer cells when they become overactive. KDM4 enzymes belong to a broader class of epigenetic regulators that modify histones, the protein scaffolds around which DNA is wound, and their overactivity has been linked to several aggressive tumour types. Zavondemstat, built on the same chemical principles as the research tool QC6352, progressed into clinical testing on the assumption that blocking KDM4 was the key to its therapeutic effect. The new study upends that assumption, showing that both compounds instead predominantly inhibit DHODH, an enzyme that cancer cells depend on to manufacture the molecular building blocks required for rapid proliferation.

Lead author Professor Lenka Munoz of the University of Sydney School of Medical Sciences and the Charles Perkins Centre offered a vivid analogy for what DHODH inhibition means inside a tumour cell. She described DHODH as a machine producing the bricks needed to build new DNA. If the machine is switched off, the cell starts running out of bricks and can no longer efficiently copy its DNA or keep dividing. In other words, the compounds are not adjusting the epigenetic programming of cancer cells through KDM4; they are starving them of the pyrimidine nucleotides essential for DNA synthesis, a fundamentally different form of metabolic stress.

The discovery emerged, somewhat unexpectedly, from research with a different goal altogether. The team set out to investigate whether zavondemstat and QC6352 could be repurposed for glioblastoma, the most common and most aggressive form of brain cancer, a disease for which effective treatments remain desperately limited. Munoz explained that when the researchers tested other KDM4 inhibitors, those compounds failed to reproduce the anti-cancer effects observed with QC6352. If blocking KDM4 were truly driving the observed activity, other inhibitors of the same target would have been expected to behave similarly. Instead, the results pointed to QC6352 acting through an entirely different mechanism, prompting the team to dig deeper.

Using patient-derived glioblastoma stem cells, tumour models and a series of genetic, mechanistic and molecular experiments, the researchers systematically traced the compounds’ anti-cancer activity to DHODH rather than KDM4. Patient-derived stem cells are considered a particularly demanding model system because they preserve many of the characteristics of the original tumour, including the cellular subpopulations thought to drive recurrence and resistance. The convergence of genetic and pharmacological evidence across these models gave the team confidence that DHODH, not KDM4, is the key target of QC6352 in glioblastoma stem cells, as the paper’s title states.

The implications extend well beyond glioblastoma. Researchers around the world have used QC6352 as a leading tool compound to study KDM4 biology, and Munoz noted that the related drug zavondemstat progressed into clinical trials based on the same understanding. Her team’s study found that much of the anti-cancer activity of both compounds is driven by blocking DHODH rather than KDM4. That means a substantial body of published research may have drawn conclusions about KDM4 function from experiments in which the compound was, in fact, hitting a different target entirely. Scientific conclusions built on such misattributed mechanisms can propagate through the literature, shaping hypotheses, grant funding and drug discovery programmes in ways that are difficult to unwind.

For the clinical programme, the stakes are equally high. Munoz emphasised that getting a drug’s mechanism wrong can lead to poorly designed clinical trials, inappropriate patient selection and years of research focused on the wrong biological target. Clinical trials for targeted cancer therapies typically enrol patients whose tumours show evidence of the presumed target, on the logic that those patients are most likely to benefit. If the true target is different, the biomarker strategy may select patients who are unlikely to respond, while patients who could benefit from DHODH inhibition are excluded. There are well-known historical examples of cancer drugs advancing through large clinical trials before researchers realised they were not working through the mechanism originally proposed, and Munoz stressed that the new study shows this is not merely a historical problem but one still happening today.

Beyond the immediate implications for zavondemstat and QC6352, the findings open a potentially promising new avenue for glioblastoma treatment. DHODH inhibition has attracted growing interest in oncology, and several drugs targeting the enzyme are already being investigated for other cancers. The possibility that such agents could eventually be tested in patients with brain cancer adds a therapeutic silver lining to what is otherwise a cautionary tale. Glioblastoma remains one of the deadliest human cancers, and any mechanism-based strategy that can be pursued with compounds already in clinical development offers a faster route to the clinic than starting from scratch.

The researchers also took a constructive step to repair the damaged research toolchain. Having established that QC6352 confounds KDM4 biology through off-target DHODH inhibition, they developed new compounds that inhibit KDM4 without affecting DHODH. These improved chemical probes should allow scientists to study the genuine role of KDM4 enzymes in cancer with far greater accuracy, replacing a compromised tool with ones whose selectivity has been rigorously characterised. The episode underscores a principle increasingly emphasised in chemical biology: the value of a research compound depends on knowing precisely what it does, and validation of target engagement should be a prerequisite for both laboratory use and clinical translation.

Munoz argued that greater rigour is needed to ensure researchers understand exactly how potential treatments work before they move into clinical testing. Establishing a drug’s true mechanism early, she said, can protect patients, prevent wasted research effort and help ensure that limited funding is directed towards genuinely promising treatments. The study, an experimental investigation conducted in cells and tumour models, was funded by the National Health and Medical Research Council, Tour de Cure and the Tim Trigg Scholarship into Glioblastoma Research, and the authors declared no competing interests. As zavondemstat continues its journey through clinical development, the work stands as a striking reminder that in modern drug discovery, the question of how a drug works can be as consequential as whether it works at all.

Subject of Research: Mechanism of action of the KDM4 inhibitor QC6352 and the clinical drug zavondemstat in glioblastoma stem cells

Article Title: Cancer trial drug may work differently than scientists believed, raising concerns for clinical research

Article References: Cancer trial drug may work differently than scientists believed, raising concerns for clinical research. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: zavondemstat, QC6352, KDM4, DHODH, glioblastoma, cancer drug development, drug mechanism, clinical trials, chemical biology, University of Sydney, Nature Chemical Biology, targeted therapy