The biological rationale behind the combination rests on how cancer cells cope with DNA damage. ATR, or ataxia telangiectasia and Rad3-related protein kinase, is a central regulator of the DNA damage response. When replication stress causes DNA replication forks to stall, ATR stabilises those forks, delays the firing of late replication origins and activates S-phase and G2-M checkpoints, buying the cell time to repair damage before it divides. Blocking ATR therefore allows double-strand DNA breaks to accumulate, driving genomic instability and ultimately cell death. PARP1, meanwhile, is involved in repairing single-strand breaks during base excision repair, and PARP inhibitors such as olaparib work partly by trapping PARP1 on DNA, causing stalled replication forks to collapse into double-strand breaks.

Those double-strand breaks are normally repaired by the homologous recombination repair pathway, in which the tumour suppressor proteins BRCA1 and BRCA2 play key roles. In tumours carrying BRCA1/2 mutations, homologous recombination is defective, so PARP inhibitors kill the cells through synthetic lethality: two independent weaknesses that are individually survivable but lethal together. PARP inhibition also increases replication stress, making tumour cells increasingly dependent on ATR for survival. Preclinical work had shown selective synergy between ceralasertib and olaparib in BRCA-deficient and ATM-deficient models, and in most models of acquired PARP inhibitor resistance, raising hopes that the pairing could both deepen responses and overcome resistance in the clinic.

The trial, registered as NCT02264678, was a modular, open-label, multicentre study. In the dose-escalation phase, Part A1 tested ceralasertib alone at 80 mg twice daily or 160 mg once daily in 13 patients, while Part A2 escalated ceralasertib in combination with olaparib across 67 patients with advanced solid tumours. Dose expansion then followed in two breast cancer cohorts: Part B3 enrolled 37 patients with BRCA1/2-mutated, HER2-negative breast cancer who were PARP inhibitor-naive, and Part B4 enrolled 25 patients with BRCA1/2 wild-type triple-negative breast cancer. All expansion patients received the recommended Phase 2 dose until disease progression or discontinuation criteria were met.

The safety findings shaped the final dosing regimen. No dose-limiting toxicities occurred with ceralasertib monotherapy, but haematological toxicity emerged as the limiting factor when the two drugs were combined. Dose-limiting toxicities appeared in several escalation cohorts, predominantly involving neutropenia and thrombocytopenia, and attempts to push the ceralasertib dose higher or extend the number of dosing days proved intolerable. The recommended Phase 2 dose was set at ceralasertib 160 mg once daily on days 1 to 7 of each 28-day cycle, plus olaparib 300 mg twice daily given continuously. At this schedule, tolerability was illustrated by limited dose reductions and discontinuations, with a median relative dose intensity of 100 percent for ceralasertib in the expansion cohorts.

Across the expansion cohorts, treatment-emergent adverse events were nearly universal, affecting 97.3 percent of patients in Part B3 and 96.0 percent in Part B4, but grade 3 or worse events were less frequent, at 43.2 percent and 36.0 percent respectively. The most common severe toxicities were haematological: anaemia and neutropenia predominated in the BRCA-mutant cohort, while anaemia, thrombocytopenia and neutropenia appeared in the triple-negative cohort. Adverse events led to discontinuation in only a small minority of patients, two of 37 in Part B3 and one of 25 in Part B4, and no deaths from adverse events occurred in Part B3. The investigators concluded that no new safety signals were observed beyond those already known for each drug class.

The efficacy results revealed a striking split between the two expansion cohorts. In Part B3, 14 of 37 patients, or 37.8 percent, achieved a confirmed objective response, comprising one complete response and 13 partial responses, with a median duration of response of 9.8 months. Among the 30 patients whose BRCA1/2 mutation was centrally confirmed by next-generation sequencing, the response rate rose to 40.0 percent, the median duration of response reached 11.0 months, and median progression-free survival was 9.0 months. Median overall survival in this centrally confirmed group was 26.2 months. In sharp contrast, Part B4 produced no objective responses at all among patients with BRCA wild-type triple-negative breast cancer lacking homologous recombination repair mutations, with median progression-free survival of just 3.1 months.

Pharmacokinetic and pharmacodynamic analyses added mechanistic texture to the clinical findings. Ceralasertib was rapidly absorbed, reaching peak plasma concentrations within about an hour and a half, and exposures at both 160 mg once daily and 80 mg twice daily provided 24-hour coverage over the concentration required for 90 percent ATR inhibition. When the drugs were combined, single-dose ceralasertib exposure was modestly reduced compared with monotherapy, and ceralasertib did not accumulate at steady state. In paired tumour biopsies, levels of phosphorylated RAD50, a pharmacodynamic biomarker of DNA damage response activation, increased on treatment in all three evaluable samples, alongside rises in the DNA damage markers gamma-H2AX and phosphorylated CHK1, confirming target engagement, although the small sample size limited interpretation.

Exploratory biomarker work examined whether ATM status or expression of Schlafen-11, an emerging predictor of sensitivity to DNA-damaging therapy, could identify responders. Responders showed a higher proportion of ATM-low tumours than non-responders, and median SLFN11 H-scores were lower in patients achieving complete or partial responses than in those with stable or progressive disease, with 85.7 percent of responders classified as SLFN11-low. However, neither marker was strongly associated with clinical outcome, and the authors note that predictive biomarkers for ceralasertib remain to be elucidated, partly because replication stress, the key hypothesised predictor of ATR inhibitor sensitivity, still cannot be measured accurately in clinical tissue samples.

The authors place their findings in context by comparing them with landmark PARP inhibitor monotherapy trials in BRCA-mutated metastatic breast cancer, where median progression-free survival has ranged from 4.0 to 8.6 months and response rates have varied widely depending on prior platinum exposure. Without a monotherapy comparator arm, the contribution of ceralasertib to the observed activity cannot be conclusively determined, and the parallel randomised VIOLETTE study found no significant progression-free survival benefit from adding ceralasertib to olaparib in triple-negative breast cancer, possibly because haematological toxicity forced a relatively low ceralasertib dose. Nevertheless, the combination demonstrated preliminary activity with manageable safety in PARP inhibitor-naive patients with BRCA1/2-mutated HER2-negative breast cancer, and the authors argue that further work should identify which patient groups, defined by prior PARP exposure, specific mutations or biomarkers, are most likely to benefit from adding ATR inhibition to PARP therapy.

Subject of Research: Phase 1 trial of the ATR inhibitor ceralasertib combined with the PARP inhibitor olaparib in advanced solid tumours and breast cancer

Article Title: Safety and efficacy of ceralasertib, an ATR kinase inhibitor, combined with olaparib: dose escalation in advanced solid tumours and dose expansion in advanced breast cancer

Article References: Lopez, J. S., Im, S.-A., Postel-Vinay, S., Campone, M., El-Khoueiry, A. B., Abida, W., Arkenau, T., Lee, K.-W., Rha, S. Y., Jodrell, D. I., Roylance, R., Irurzun-Arana, I., Jones, G. N., Lukashchuk, N., Stephens, C., Norris, C., Loembé, A.-B., Dean, E., & Krebs, M. G. (2026). Safety and efficacy of ceralasertib, an ATR kinase inhibitor, combined with olaparib: dose escalation in advanced solid tumours and dose expansion in advanced breast cancer. British Journal of Cancer. https://doi.org/10.1038/s41416-026-03601-z

Image Credits: AI Generated

DOI: 10.1038/s41416-026-03601-z

Keywords: ceralasertib, olaparib, ATR inhibitor, PARP inhibitor, BRCA1/2, breast cancer, triple-negative breast cancer, DNA damage response, replication stress, Phase 1 trial, synthetic lethality, targeted therapy

To capture that transformation in motion, the team did something many earlier studies failed to do. Rather than relying on the human cell lines that dominate the radioresistance literature, lines that are poorly suited to immunocompetent systems and therefore blind researchers to the immune dimension of treatment failure, the investigators generated radioresistant isogenic variants of murine lung adenocarcinoma cell lines. Because these variants are genetically matched to their radiation-sensitive parents, any differences that emerge after repeated irradiation can be attributed to adaptive changes rather than pre-existing genetic background. The team then interrogated these paired models using transcriptomics, three-dimensional co-culture systems, and syngeneic mouse models that retain fully functioning immune systems.

The results paint a picture of tumors that have reorganized themselves at multiple levels simultaneously. The radioresistant clones displayed stable adaptations to DNA damage, including enhanced clonogenic survival after irradiation and constitutive activation of DNA damage response pathways, with the ATR pathway, named for the ataxia telangiectasia and Rad3-related kinase, standing out as particularly active. ATR functions as a master regulator of the cellular response to replication stress and DNA damage, and its persistent activation appears to give cancer cells a cushion against the strand breaks that ionizing radiation inflicts. Critically, the clinical relevance of this observation was not left to speculation: analyzing data from non-small cell lung cancer patients, the researchers found that high ATR levels were significantly associated with immunosuppression, linking a tumor-intrinsic survival mechanism to a hostile immune environment in human disease.

The transcriptomic analysis uncovered a second layer of the adaptation. Radioresistant tumors showed altered DNA repair programs alongside a marked downregulation of type-I interferon signaling and innate immune pathways. This is a crucial detail because radiation therapy, in addition to directly damaging DNA, is supposed to function as an in situ vaccine. When tumor cells die from irradiation, they are expected to undergo immunogenic cell death, releasing damage-associated molecular patterns that activate the cGAS-STING sensing axis and other innate pathways, which in turn recruit and activate cytotoxic T cells capable of finishing off survivors. By dampening these pathways, radioresistant tumors essentially mute the alarm system that radiation depends on to alert the immune system, reducing immunogenic cell death and converting what should be an inflammatory dying process into a quiet one.

The consequences of that muting were visible throughout the tumor microenvironment. In syngeneic mice, radioresistant tumors recruited a distinctly immunosuppressive cellular cast: increased numbers of T regulatory cells, M2-like macrophages, and myeloid-derived suppressor cells, along with reduced recruitment of cytotoxic T lymphocytes. These cell types are well-known accomplices of tumor survival, suppressing antitumor immunity through a variety of inhibitory signals and metabolic depletion. The three-dimensional co-culture experiments added a functional confirmation: when immunosuppressive cells were present alongside cancer cells in the same culture, they protected the malignant cells from radiation-induced cytotoxicity. In other words, the immune cells that radioresistant tumors attract are not bystanders. They are active participants in treatment failure, forming a shield that absorbs or suppresses the immune attack that radiation is meant to provoke.

Perhaps the most striking evidence for the centrality of immune suppression came from a transplant experiment of sorts. When the same radioresistant tumors were grown in immunodeficient mice, animals lacking the adaptive and innate immune machinery that normally shields the tumor, they regained their sensitivity to radiotherapy. Stripped of their immunosuppressive entourage, the tumors that had been so resilient in immunocompetent hosts became vulnerable again. This single observation reframes the entire problem of radioresistance: a substantial portion of it is not written into the cancer cell’s genome at all, but is instead conferred by the ecosystem the tumor has cultivated around itself.

That reframing immediately suggested therapeutic possibilities, and the team pursued them with encouraging results. The first approach involved artificially kick-starting innate immunity using agonists of Toll-like receptors, specifically the combination of poly(I:C), which activates TLR3, and R848, which targets TLR7 and TLR8. When these agonists were delivered to radioresistant tumors, they restored antitumor responses and improved the efficacy of radiotherapy, in part through a mechanism centered on the chemokines CXCL9 and CXCL10. These chemokines act as beacons for immune cells, and their induction re-established the recruitment of cytotoxic immune cells into the tumor, effectively rebuilding the alarm system that the radioresistant clones had dismantled. The findings give the study its title and its conceptual core: CXCL9-driven immune rewiring can restore the conditions under which radiation therapy works as intended.

The second approach attacked the problem from inside the cancer cell. Using ceralasertib, a pharmacological inhibitor of ATR, the researchers sensitized radioresistant tumors to radiation, directly undermining the constitutive DNA damage response activation that had given the cells their survival cushion. Together with the innate immune agonists, this establishes a rationale for combined strategies that hit both halves of the radioresistance program at once: stripping away the tumor’s molecular repair capacity with ATR inhibition while simultaneously reigniting the immune recruitment signals that allow radiation to trigger a lasting antitumor response. Neither approach alone represents a complete solution, but the logic of the combination, grounded in the mechanistic findings rather than empirical trial and error, is what distinguishes this work from earlier attempts to sensitize tumors to radiation.

The broader implications reach beyond lung cancer. The study demonstrates that the way radioresistance research is conducted matters: models lacking intact immune systems systematically obscure the contribution of the microenvironment, and this team’s decision to build isogenic immunocompetent models paid off with insights that would have been invisible in conventional systems. For clinicians treating non-small cell lung cancer, the work suggests that markers such as ATR expression and chemokine signatures could eventually help identify patients at risk of locoregional relapse before it occurs, and that combining radiotherapy with TLR agonists or ATR inhibitors deserves serious clinical evaluation. For now, the findings rest on preclinical evidence, and the usual caveats about translation from mice to humans apply. But they offer something that has been in short supply in this field: a mechanistically coherent account of how tumors learn to shrug off radiation, and a map of the vulnerabilities created along the way. As radiotherapy continues to anchor the treatment of lung cancer worldwide, strategies that restore rather than replace its power may prove to be the key to preventing the relapses that have long defied the beam.

Subject of Research: Mechanisms of radioresistance in non-small cell lung cancer involving ATR signaling and immune microenvironment rewiring

Article Title: CXCL9-driven immune rewiring and tumor-intrinsic ATR signaling inhibition restore radiotherapy response in lung cancer

Article References: Leon, S., Otegui, N., Redrado, M., Marchena-Perea, E. M., Guruceaga, E., Castro, F., Montuenga, L. M., Oliveira, M. J., Aristu, J. J., Serrano, D., & Calvo, A. (2026). CXCL9-driven immune rewiring and tumor-intrinsic ATR signaling inhibition restore radiotherapy response in lung cancer. Journal of Experimental & Clinical Cancer Research. https://doi.org/10.1186/s13046-026-03834-z

Image Credits: AI Generated

DOI: 10.1186/s13046-026-03834-z

Keywords: radiotherapy, non-small cell lung cancer, radioresistance, ATR inhibitor, ceralasertib, CXCL9, tumor microenvironment, immunosuppression, TLR agonists, innate immunity, DNA damage response, cancer immunotherapy

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