The research focused on a family of cellular survival strategies known collectively as DNA damage tolerance. When chemotherapeutic drugs such as anthracyclines and platinum compounds inflict lesions on DNA, they do their most damage during replication, the process by which a cell copies its entire genome before dividing. Stalled replication forks, the Y-shaped structures where the two DNA strands are prised apart and copied, are a lethal hazard. Cells respond with several alternative maneuvers: translesion synthesis, in which specialized DNA polymerases such as POLη, POLι and POLζ copy directly across the damaged stretch; fork reversal, in which DNA translocases such as ZRANB3, HLTF and SMARCAL1 remodel the fork so the lesion can be repaired; and repriming, in which the enzyme PRIMPOL simply restarts copying downstream of the block, leaving a gap that POLζ later fills in. Because these pathways allow cancer cells to tolerate the very damage that chemotherapy is designed to inflict, they have emerged as prime suspects behind chemoresistance.

What makes triple-negative breast cancer especially tricky is that its cells are not static targets. They frequently undergo epithelial-to-mesenchymal transition, a developmental program that makes them more invasive and more stem-like, and this plasticity is thought to change how they handle DNA damage. To capture that dynamic behavior, the researchers grew ten breast cancer cell lines, four triple-negative and six non-triple-negative, in two different configurations: flat two-dimensional monolayers and three-dimensional spheroids, which better mimic the architecture of a real tumor. They then measured a panel of markers, including the epithelial genes KRT18 and CDH1, the mesenchymal markers ZEB1 and Vimentin, and the stemness indicators CD44, CD24 and Nestin, both with and without exposure to the clinically relevant drugs Olaparib, Carboplatin and Doxorubicin.

The results were striking. Triple-negative cell lines, particularly in 3D culture, showed elevated Vimentin, reduced epithelial markers, a higher CD44-to-CD24 ratio and increased Nestin, hallmarks of a mesenchymal, stem-like state. More importantly, these phenotypic changes came with a measurable change in replication behavior. Using the DNA fiber spreading assay, a technique that labels newly synthesized DNA with two different nucleotide analogs and measures the length of the resulting tracks under a microscope, the team found that replication slowed dramatically and consistently in all four triple-negative lines when they were grown as spheroids, while the six non-triple-negative lines showed no such pattern. Adding N-acetylcysteine, a scavenger of reactive oxygen species, did not abolish the slowdown, and the ratio of long to short tracks revealed no excess fork stalling. In other words, the triple-negative cells were not failing to replicate; they were deliberately slowing down.

The molecular signature behind that slowdown pointed squarely at DNA damage tolerance. Western blotting revealed higher levels of monoubiquitinated PCNA, the sliding clamp that coordinates replication and whose ubiquitination marks perturbed forks, in the triple-negative lines across treatments and culture conditions. Immunofluorescence microscopy then showed that nuclear foci of the translocase ZRANB3, and of POLι foci colocalizing with it, accumulated more frequently in 3D than in 2D. Only in the 3D-cultured triple-negative cells did the combination of PCNA ubiquitination and ZRANB3 recruitment coincide with the shortened replication tracks, linking the stem-like state to a shift toward fork reversal as the dominant tolerance strategy.

To dissect which pathways each cell line actually used, the researchers silenced key components with siRNA and repeated the fiber assays in three triple-negative lines: MDA-MB-231, Hs578t and MDA-MB-468. The picture was heterogeneous but revealing. In 2D, all three lines relied on repriming driven by the checkpoint kinase CHK1, with contributions from translesion polymerases that varied by cell line. In 3D, the balance tipped toward fork reversal, mediated by ZRANB3 in MDA-MB-231, SMARCAL1 in Hs578t and MDA-MB-468, and HLTF in a context-dependent fashion, with HLTF’s dual roles as a translocase and as the ligase that polyubiquitinates PCNA producing opposite effects in the two culture formats. Despite the heterogeneity, the underlying dependence on fast, flexible lesion bypass was a common thread.

That common thread became a therapeutic opportunity. The team screened ten small-molecule inhibitors targeting different parts of the DNA damage response, from translesion polymerases and CHK1 to RAD51, RAD52, ATR, MRE11, DNA2 and the origin-firing kinase CDC7. Two stood out. The POLζ inhibitor JH-RE-06 showed half-maximal inhibitory concentrations of 7 micromolar or below in all three cell lines under both culture conditions, and the CHK1 inhibitor PF477736 followed at 20 micromolar or below. Mechanistically, both drugs target multiple tolerance routes at once: CHK1 supports both repriming and fork stability, while POLζ fills the single-stranded gaps left by repriming and extends translesion synthesis. Fiber assays confirmed that CHK1 inhibition slowed replication in every cell line regardless of culture format, nearly erasing the 2D-versus-3D difference, while POLζ inhibition decelerated forks in specific contexts. By contrast, inhibitors of homologous recombination and break-induced replication had little or no effect on viability, arguing that the cells’ survival hinged on tolerance rather than repair of broken forks.

The most clinically consequential finding concerned Olaparib, a PARP inhibitor approved for tumors with BRCA1 or BRCA2 defects. The triple-negative lines used in the study carry wild-type BRCA genes, and accordingly they responded poorly to Olaparib alone, becoming almost fully resistant in 3D culture. But when the researchers combined Olaparib with low doses of the POLζ, CHK1 or even RAD51 inhibitor, all three cell lines were resensitized in 2D, and two of the three, Hs578t and MDA-MB-468, were resensitized in 3D as well. The logic is a form of synthetic lethality: PARP inhibition blocks the restart of reversed forks, forcing cells onto the very translesion and repriming routes that the partner drug simultaneously disables. The lone exception was 3D-cultured MDA-MB-231, which resisted every combination. That line stood out for its heavy reliance on ZRANB3-mediated fork reversal, its high CD44 levels and its low burden of DNA damage markers, reinforcing the idea that stemness, ZRANB3-dependent fork protection and chemoresistance are mechanistically intertwined.

The study has limits the authors acknowledge: it examined ten cell lines with deep mechanistic work in only three, no direct inhibitors of PRIMPOL or the fork-remodeling enzymes were available, and no patient data were included. Even so, the translational momentum is real. CHK1 inhibitors such as Prexasertib and MK-8776 have already been tested in Phase I and II trials across solid tumors including triple-negative breast cancer, and the USP1 inhibitor KSQ-4279, which prevents PCNA de-ubiquitination, has entered Phase I testing with early signs of synergy with Olaparib. POLζ inhibitors have not yet reached the clinic, but their tolerability profile appears acceptable. If the pattern holds in patients, the message of this work is that the shifting disguises of triple-negative breast cancer may matter less than the replication machinery the cells can never abandon, and that attacking the common denominators of DNA damage tolerance, alone or alongside PARP inhibition, could finally turn the tumor’s adaptability against it.

Subject of Research: DNA damage tolerance pathway targeting as a therapeutic strategy against chemoresistance in triple-negative breast cancer

Article Title: Targeting common denominators of DNA damage tolerance mechanisms as therapeutic strategy to cope with dynamic phenotypic changes in triple-negative breast cancer cells

Article References: Niedermayer, K., Greco, S., Oechsle, A., Köhler, T., Pantel, K., Friedl, T. W. P., Janni, W., Rack, B., Pfister, K., Gottifredi, V., & Wiesmüller, L. (2026). Targeting common denominators of DNA damage tolerance mechanisms as therapeutic strategy to cope with dynamic phenotypic changes in triple-negative breast cancer cells. Cell Death Discovery, 12(1), Article 405. https://doi.org/10.1038/s41420-026-03377-4

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03377-4

Keywords: triple-negative breast cancer, DNA damage tolerance, translesion synthesis, fork reversal, PCNA ubiquitination, POLζ, CHK1, Olaparib, PARP inhibitor, chemoresistance, epithelial-to-mesenchymal transition, cancer stem cells

Replication stress arises from many sources, and the review methodically maps them. Oncogene activation is a principal culprit. When growth-promoting signals such as MYC or cyclin E are unleashed, they push cells through the cell cycle faster than replication machinery can be properly licensed, forcing replication to begin in inappropriate places. Shortened G1 phases leave origin licensing incomplete, and transcription programs fire at full blast while forks attempt to copy the same sequences. The resulting collisions between replication and transcription complexes are among the most potent sources of chromosome breakage, particularly at long, highly transcribed genes known as common fragile sites, which have been recognized as breakage hotspots since experiments with the polymerase inhibitor aphidicolin in the 1980s.

Transcription-replication conflicts come in mechanistically distinct flavors. Head-on conflicts, where the replication fork meets an oncoming RNA polymerase, and co-directional conflicts produce different R-loop levels and activate different DNA damage responses. R-loops, three-stranded structures containing an RNA-DNA hybrid and a displaced single strand, have emerged as pivotal intermediates that can either protect or poison genomes depending on how they are resolved. Studies of BRCA1 and BRCA2 have shown that these breast and ovarian cancer tumor suppressors do more than repair double-strand breaks; they actively suppress R-loop accumulation by coordinating transcription elongation and RNA processing. Similarly, chromatin remodelers such as the SWI/SNF complex and spliceosome factors help clear conflicts, explaining why mutations in these complexes, common in myelodysplastic syndromes and Ewing sarcoma, produce cells exquisitely sensitive to ATR inhibition.

Beyond collisions, the review highlights metabolic and structural pressures. Nucleotide pools are a striking example: early work demonstrated that deoxyribonucleotide deficiency distorts origin choice and spacing, while later studies showed that oncogene-induced senescence is partly a consequence of nucleotide starvation. Cancer cells also accumulate unprocessed uracil and ribonucleotides in DNA, both of which impede fork progression. DNA secondary structures add another layer. Unstable inverted repeats fold into hairpins and cruciforms, and expanded TA-dinucleotide repeats in microsatellite-unstable colorectal cancers form structures that grind forks to a halt. These repetitive obstacles, rather than mismatch repair failure alone, are now understood to be why such tumours become fatally dependent on the WRN helicase, which unwinds these structures and prevents catastrophic chromosome shattering.

The consequences of unresolved replication stress reverberate through the cancer genome. Stalled forks are vulnerable to nuclease degradation unless BRCA2 protects the nascent DNA, and collapse generates breaks that drive chromosomal instability, linking structural and numerical chromosome chaos. Under-replicated regions persist into mitosis, producing anaphase bridges, micronuclei and copy-number changes that mirror the polymorphic and pathogenic variants seen in human populations. Several avenues of rescue have been characterized, including fork reversal, repriming by the PRIMPOL primase-polymerase and specialized translesion polymerases that replicate past damaged bases, but these repair routes are intrinsically error-prone. Translesion synthesis underlies acquired chemotherapy resistance, and repriming leaves single-stranded DNA gaps that, recent work shows, are a key determinant of PARP inhibitor sensitivity in BRCA-deficient cells.

The ATR-CHK1 checkpoint pathway is the master guardian that keeps replication stress survivable. ATR is recruited to RPA-coated single-stranded DNA through its partner ATRIP and activated by TopBP1 or ETAA1, with the MRN complex and the 9-1-1 clamp sharpening its activation. Once active, ATR suppresses excess origin firing, stabilizes stalled forks, enforces an intrinsic S/G2 checkpoint that prevents cells from entering mitosis with unfinished replication, and prevents global exhaustion of RPA, a failure mode that culminates in replication catastrophe and genome fragmentation. Without ATR, forks collapse and single-stranded DNA accumulates beyond rescue, a finding that transformed the enzyme from a basic research curiosity into a top-priority drug target.

That targeting has now reached the clinic. ATR inhibitors such as ceralasertib and elimusertib have produced durable responses in tumours with high replication stress, including small cell lung cancer, and synthetic lethal combinations with ATM loss, ERCC1 deficiency, CCNE1 amplification and APOBEC3 activity are being pursued. WEE1 inhibitors override the G2 checkpoint and force stressed cells into lethal mitosis, with combination strategies including sequential PARP and WEE1 dosing designed to minimize toxicity. The clearest success story remains PARP inhibition, introduced in landmark 2005 studies demonstrating synthetic lethality in BRCA-mutant cells. Mechanistic work has since refined the model: PARP inhibitors trap PARP1 and PARP2 on DNA and act through persistent single-stranded gaps that BRCA-deficient cells cannot fill, and next-generation PARP1-selective agents such as AZD5305 promise better tolerability. WRN inhibition in microsatellite-unstable cancers represents the newest synthetic lethal frontier, with resistance mechanisms beginning to be mapped by CRISPR screens.

The review also reframes replication stress as a chronic, sublethal condition that sculpts tumour evolution over years rather than an acute lethal event. This slow burn generates mutational signatures, including those of APOBEC3 cytidine deaminases, that fuel heterogeneity, therapy resistance and metastasis, and therapy itself can amplify APOBEC3A-driven evolution in persistent cancer cells. Importantly, replication stress intersects with the immune system. Under-replicated DNA in micronuclei activates the cGAS-STING pathway, and chromosomal instability promotes metastasis through cytosolic DNA sensing, while SAMHD1 and MYC normally suppress inflammatory signaling from replication-derived nucleic acids. ATR inhibition can potentiate the inflammatory tumour microenvironment, opening rational combinations with immune checkpoint blockade, an approach already validated in microsatellite-unstable and BRCA-mutated tumour settings.

The authors position replication stress not as a single weakness but as a dynamic, context-dependent vulnerability landscape shaped by each tumour’s oncogenic, metabolic and chromatin state. Biomarkers such as replication stress signatures, gap formation and fork protection status may soon guide which patients receive ATR, WEE1, PARP or WRN inhibitors, and in which combinations. The therapeutic opportunity, they argue, lies in recognizing that tumours pay a permanent price for their genomic chaos, and that the rescue pathways they depend upon are now precisely druggable targets.

Subject of Research: Replication stress in cancer: origins, consequences and therapeutic opportunities

Article Title: Replication stress in cancer: origins, consequences and therapeutic opportunities

Article References: Chen, J., & Zou, L. (2026). Replication stress in cancer: origins, consequences and therapeutic opportunities. Nature Reviews Cancer. https://doi.org/10.1038/s41568-026-00979-z

Image Credits: AI Generated

DOI: 10.1038/s41568-026-00979-z

Keywords: replication stress, cancer, ATR, CHK1, PARP inhibitors, synthetic lethality, R-loops, genomic instability, WRN helicase, WEE1 inhibitors, precision oncology, DNA replication

ATR, short for ataxia-telangiectasia and Rad3-related, belongs to the PIKK family of giant kinases that also includes ATM, DNA-PKcs, mTOR and SMG1. Like its siblings, ATR is an enormous HEAT-repeat scaffold carrying a kinase domain buried deep within, and it operates as a dimer paired with its obligate partner ATRIP. Since the first cryo-EM structures of human ATR–ATRIP appeared in 2018, researchers have known the complex’s overall architecture but not the physical choreography of its activation. The central puzzle has been phosphorylation: ATR autophosphorylates itself, and this modification is considered a hallmark of the active kinase, yet nobody had seen structurally what phosphorylation does to the machine or how the activator protein TopBP1 drives the process. The new study addresses that gap directly by trapping the complex at successive stages of its catalytic cycle.

The team began with precatalytic snapshots. In two structures determined with the non-hydrolysable ATP analogue ATPγS wedged in the active site, they found that nucleotide binding alone accomplishes something remarkable: it primes the catalytic center for chemistry without wholesale rearrangement of the kinase domain. When the researchers then added a fragment of Chk1, the key downstream effector of the ATR pathway, the substrate docked neatly onto the primed enzyme. In other words, ATP binding and substrate positioning can proceed as relatively independent, modest steps rather than requiring a dramatic conformational detonation. This finding revises the textbook expectation that PIKK kinases need massive global movements to become catalytically competent, and it suggests that the rate-limiting event in ATR activation lies elsewhere — specifically, in the regulatory rearrangements triggered by TopBP1.

That TopBP1-dependent rearrangement is the structural centerpiece of the paper. In the activated structure, two molecules of TopBP1 bind simultaneously to the ATR–ATRIP dimer, one engaging each protomer from opposite faces. TopBP1 latches onto the HEAT-repeat scaffold far from the active site, and in doing so it locks the N-HEAT and M-HEAT domains of ATR together in both protomers. In the precatalytic and substrate-bound states, these scaffold elements make intimate contact in one protomer while hovering apart in the other — an inherent asymmetry. TopBP1 binding abolishes that asymmetry, snapping the dimer into a near-symmetric conformation that promotes ATR–ATRIP autophosphorylation and drives efficient phosphorylation of Chk1. Detailed atomic contacts reveal how this is achieved: an intensive network of hydrogen bonds welds a TopBP1 beta strand to the M-HEAT domain, the tryptophan residue W1145 of TopBP1 buries into a hydrophobic pocket at the N-HEAT/M-HEAT junction, and salt-bridge interactions fasten a TopBP1 helix onto the N-HEAT superhelix.

Perhaps the most provocative observation concerns what happens after activation. The researchers purified ATR–ATRIP complexes that had been fully autophosphorylated in the presence of TopBP1, then stripped the activator away and determined the structure of the phosphorylated kinase on its own. Remarkably, the phosphorylated complex remained fully competent to phosphorylate Chk1, and it did so regardless of how much TopBP1 was added back. Structurally, the released enzyme adopted an asymmetric conformation, yet its kinase domain was essentially indistinguishable from that of the TopBP1-bound, near-symmetric activated state. The authors conclude that catalytic activity is compatible with both near-symmetric and asymmetric architectures, meaning that phosphorylation — not a rigid, TopBP1-imposed symmetry — is the true durable switch. Once ATR has phosphorylated itself, the checkpoint engine keeps running even if its ignition key is removed, a property with clear implications for how sustained checkpoint signaling is maintained at damaged replication forks.

The autophosphorylation story was reinforced biochemically. Mass spectrometry mapped dozens of newly phosphorylated sites on ATR and ATRIP, and phosphatase treatment confirmed that the mobility shifts seen in gels were genuinely phosphorylation-dependent. Activity assays showed a clean dose-response: non-phosphorylated ATR–ATRIP phosphorylated Chk1 only when TopBP1 was titrated in, whereas the pre-phosphorylated complex catalyzed Chk1 modification at TopBP1 concentrations approaching zero. Interestingly, an isolated ATR head domain displayed only basal activity with or without TopBP1, underscoring that the full dimeric scaffold — and its TopBP1-driven reorganization — is required to reach full catalytic power. Together these experiments elevate ATR autophosphorylation from a correlative biomarker to a mechanistically demonstrated molecular switch for checkpoint activation.

The paper’s second act concerns therapy. ATR inhibitors such as berzosertib (VX-970/M6620), ceralasertib (AZD6738), gartisertib (VX-803/M4344) and elimusertib (BAY 1895344) are in clinical development as sensitizers that push tumor cells with pre-existing DNA repair defects over the edge. The team determined four separate inhibitor-bound structures of ATR–ATRIP, one for each compound, providing the first side-by-side atomic views of how this pharmacological class occupies the ATR active site. The nucleotide-binding pocket turns out to be divisible into four sub-compartments: an adenine-binding pocket lined by residues including M2325, Y2365, I2377, W2379 and V2380; an inner hydrophobic pocket; a ribose-binding site; and a triphosphate-binding site. All four inhibitors anchor themselves in the adenine pocket, but each then exploits the pocket’s flexibility differently, extending flanking chemical groups into the hydrophobic cavity, the ribose site, or the triphosphate region.

These structures explain both potency and selectivity. ATR’s adenine and hydrophobic sub-pockets are shifted toward the C-lobe relative to the corresponding sites in DNA-PKcs, mTOR and ATM, a displacement caused by the bulky side chains of M2325 and V2493 in ATR. Inhibitors can therefore nestle into an ATR-specific geometry that the closely related PIKK kinases cannot accommodate. Superimposition with structures of DNA-PKcs, mTOR and ATM further showed that a key hydrogen bond formed by berzosertib with glycine 2385 of ATR would be sterically blocked by the larger residues occupying the equivalent positions in the sibling kinases. In effect, the drugs exploit the conformational plasticity of the ATR active site — the very same flexibility that allows ATPγS to prime the kinase — to achieve multipocket engagement that favors ATR over its family members. For medicinal chemists, the four structures amount to a comparative map of which sub-pockets each clinical candidate touches, a starting grid for designing next-generation inhibitors with improved selectivity and resistance profiles.

Taken together, the study delivers a structural framework in which ATR regulation can finally be read as a sequence of physically defined events. ATP binding primes the catalytic cleft and positions Chk1; TopBP1 binding to the HEAT scaffold reorganizes the dimer and licenses autophosphorylation; phosphorylation converts the enzyme into a durably active state that survives TopBP1 dissociation; and small molecules shut the whole system down by wedging into the plastic ATP pocket. The work also resonates with parallel findings on ATM, where asymmetric activation of the dimeric kinase has recently been visualized, hinting at shared regulatory logic across the PIKK family. For cancer biology, the implications are tangible: tumors rely obsessively on ATR to survive their own replication chaos, and a structure-level understanding of both the activation switch and the drug-binding pocket should accelerate the design of inhibitors that hit ATR harder and more selectively. As ATR inhibitors advance through clinical trials, the molecular movie assembled by Wang, Cheng, Qiao and colleagues provides the kind of mechanistic blueprint on which the next generation of checkpoint-targeted therapies can be drawn.

Subject of Research: Cryo-EM structural mechanisms of phosphorylation-driven activation and inhibitor binding of the human ATR–ATRIP checkpoint kinase complex

Article Title: Mechanistic insights into phosphorylation-driven activation and therapeutic inhibition of human ATR–ATRIP

Article References: Wang, L., Wang, M., Zhao, L., Rao, Q., Wu, H., Ma, B., Wang, J., Zheng, J., Li, Y., Xu, Y., Guo, J., Cheng, J., & Qiao, S. (2026). Mechanistic insights into phosphorylation-driven activation and therapeutic inhibition of human ATR–ATRIP. Nature Structural & Molecular Biology. https://doi.org/10.1038/s41594-026-01887-4

Image Credits: AI Generated

DOI: 10.1038/s41594-026-01887-4

Keywords: ATR, ATRIP, TopBP1, Chk1, cryo-electron microscopy, DNA damage response, replication stress, autophosphorylation, kinase activation, ATR inhibitors, berzosertib, cancer therapy

The logic of the new study rests on a vulnerability embedded in the biology of cancer itself: the DNA damage checkpoint. When anticancer drugs inflict lesions on DNA, healthy cells respond by halting the cell cycle through checkpoint pathways, buying time for repair before replication resumes. Two major checkpoints govern this process. The G1 checkpoint depends on the ATM–p53–p21 axis, while the G2/M checkpoint is controlled by the ataxia telangiectasia and Rad3-related kinase, or ATR, acting through Chk1 and Cdc25. Because malignant cells frequently carry p53 mutations and cannot arrest in G1, they become unusually dependent on the ATR-driven G2/M checkpoint to survive genotoxic stress. Normal cells, by contrast, retain a functional ATM–p53–p21 pathway. This asymmetry creates a therapeutic window: blocking ATR should selectively cripple cancer cells while sparing healthy tissue.

AZD6738, developed as an oral ATR inhibitor, does exactly that. By suppressing ATR-dependent phosphorylation—most notably Chk1 phosphorylation at serine 345—it disables the G2/M checkpoint that resistant tumors rely upon. The drug has already attracted attention across oncology, with Phase II trials suggesting clinical activity in combination with the immunotherapy durvalumab in previously treated advanced gastric cancer, particularly in patients lacking ATM expression, and in non-small cell lung cancer. Emerging data also support combinations with the PARP inhibitor olaparib in triple-negative breast cancer, with durvalumab in melanoma, and with radiotherapy. But in colorectal cancer, preclinical work on ATR inhibition had remained sparse. The Nagoya team had previously shown that AZD6738 potentiates both 5-FU and trifluridine in treatment-naïve colorectal cancer cells. The critical open question was whether that synergy survives the acquisition of 5-FU resistance—the setting in which trifluridine is actually given to patients.

To answer it, the researchers built a rigorous experimental system using two genetically defined human colorectal cancer cell lines, HCT116 and DLD-1, alongside 5-FU–resistant derivatives established through more than 100 passages of stepwise drug escalation. Both resistant sublines, HCT116/5FUR and DLD-1/5FUR, exhibited roughly 100-fold higher IC50 values for 5-FU than their parental counterparts, confirming profound acquired resistance. All cell lines were authenticated by short tandem repeat profiling and confirmed free of mycoplasma contamination. The team then designed experiments around a deliberately conservative dosing strategy: AZD6738 was used at 0.5 micromolar, a concentration shown to have no effect on proliferation after 72 hours in any of the four cell lines, so that any enhancement of cell killing could be attributed to genuine potentiation rather than simple additive toxicity.

The in vitro results were striking. As expected, 5-FU and its combination with AZD6738 lost all effectiveness in the resistant sublines—the combination that worked in parental cells was completely abrogated by acquired resistance, echoing earlier findings with Chk1 inhibitors. Trifluridine, however, told a different story. Although trifluridine alone was modestly weakened in the resistant cells, adding noncytotoxic AZD6738 significantly reduced cell viability across a wide dose range, from 1 to 1000 micromolar. Combination index analysis confirmed true synergy, with values of 0.735 in HCT116/5FUR and 0.418 in DLD-1/5FUR, where values below 1 indicate synergistic interaction. In time-course assays, the combination suppressed proliferation significantly at 72 hours in both resistant lines—p = 6.0 × 10⁻⁵ for HCT116/5FUR and p = 4.0 × 10⁻⁷ for DLD-1/5FUR—and restored a level of growth inhibition comparable to what trifluridine achieved in the parental, drug-sensitive cells. DLD-1/5FUR, the most resistant line, saw its trifluridine IC50 collapse from 155 micromolar to just 3.5 micromolar in the presence of AZD6738.

The molecular fingerprints of the combination revealed its mechanism. Western blotting showed that trifluridine alone produced little induction of γH2AX, a marker of DNA double-strand break signaling, but cotreatment with AZD6738 triggered a marked surge. Cleaved caspase-3, the executioner enzyme of apoptosis, rose far more strongly with the combination than with trifluridine alone, indicating that stalled checkpoint signaling had collapsed into programmed cell death. Meanwhile, the trifluridine-induced phosphorylation of Chk1 at Ser345—the very signal AZD6738 is designed to block—was suppressed by the drug, confirming on-target activity. Flow cytometry added a cell-cycle dimension: trifluridine alone drove a pronounced G2/M arrest, effectively locking damaged cells in a state of suspended repair, while the combination relieved that arrest, pushing cells past the checkpoint and into catastrophic division with unrepaired DNA damage.

Crucially, the synergy translated into living tumors. In a mouse xenograft model bearing DLD-1/5FUR tumors, neither TAS-102 alone nor AZD6738 alone significantly shrank tumors compared with controls. But the combination significantly suppressed tumor growth over the two-week treatment period, and at necropsy only the combination group showed a significant reduction in tumor weight relative to all other groups, including TAS-102 monotherapy (p = 0.039). Immunohistochemistry on excised tumors corroborated the mechanism in vivo: 25.8 percent of tumor cell nuclei stained positive for γH2AX in the combination group, compared with only 4.1 percent in the TAS-102 group. Equally important, safety signals were reassuring. Body weight did not differ among groups, and organ weights and blood counts showed no exacerbation of the hematologic toxicity that TAS-102 alone can induce, suggesting the addition of AZD6738 did not compound harm in this preclinical model.

The pharmacological reasoning behind the finding illuminates why trifluridine, unlike 5-FU, retains its potency in resistant tumors. Both drugs inhibit thymidylate synthase, an enzyme essential for DNA synthesis, and both can be incorporated into DNA and RNA. However, overexpression of thymidylate synthase is a major driver of 5-FU resistance, neutralizing the TS-inhibitory arm of both drugs. Trifluridine’s distinguishing feature is its exceptionally high rate of direct incorporation into DNA during S phase in place of thymidine, and earlier work by Emura and colleagues demonstrated that trifluridine retains substantial cytotoxicity in 5-FU–resistant colorectal cancer cells precisely because its activity depends largely on DNA incorporation rather than TS inhibition. That massive misincorporation generates replication stress, which the ATR–Chk1 axis manages at the G2/M checkpoint—making trifluridine and ATR inhibition a mechanistically rational pairing even after 5-FU resistance has taken hold.

The authors are candid about the study’s limitations. Mechanistic conclusions rest solely on pharmacological ATR inhibition without genetic validation; normal colon epithelial cells were not evaluated, leaving the therapeutic window uncertain; and only a single concentration of AZD6738 was tested, so broader dose–response studies will be needed. The roles of p53 and ATM also remain unresolved, as synergy appeared in both p53 wild-type and p53-mutant models, and no direct experiments interrogated these determinants. Nevertheless, the clinical relevance of the setting is hard to overstate: trifluridine is administered as TAS-102, known commercially as Lonsurf, almost exclusively after failure of fluoropyrimidine-based first- and second-line regimens, meaning most patients receiving it already harbor 5-FU resistance. By providing proof of concept that pharmacological ATR inhibition can restore—and even rescue—the antitumor activity of trifluridine in that refractory context, the study lays the groundwork for mechanistic follow-up and, ultimately, translational trials of a ceralasertib–TAS-102 combination in patients whose options are running out.

Subject of Research: ATR inhibition with AZD6738 to enhance trifluridine efficacy in 5-FU–resistant colorectal cancer

Article Title: AZD6738 (Ceralasertib) Enhances Trifluridine's Antitumor Effect in 5‐FU–Resistant Colorectal Cancer Cells

Article References: Uehara, S., Suzuki, T., Kato, J., Asai, H., Kato, A., Harata, S., Ushigome, H., Yamakawa, Y., Hirokawa, T., Takahashi, H., Matsuo, Y., & Takiguchi, S. (2026). AZD6738 (Ceralasertib) Enhances Trifluridine's Antitumor Effect in 5‐ FU –Resistant Colorectal Cancer Cells. Cancer Reports, 9(9), Article e70668. https://doi.org/10.1002/cnr2.70668

Image Credits: AI Generated

DOI: 10.1002/cnr2.70668

Keywords: AZD6738, ceralasertib, trifluridine, TAS-102, colorectal cancer, 5-FU resistance, ATR inhibitor, DNA damage checkpoint, Chk1, G2/M checkpoint, drug resistance, preclinical study