Pancreatic Cancers With ATM Defects May Respond to Irinotecan, Study Finds

Pancreatic ductal adenocarcinoma remains one of the most lethal human malignancies, and for the small fraction of patients whose tumors carry mutations in the ATM or CHEK2 genes, treatment decisions have long been guided by an assumption that now appears questionable. A new study published in Genome Medicine suggests that these patients, who make up roughly five to ten percent of those with the disease, may derive far greater benefit from topoisomerase I inhibitors such as irinotecan than from the platinum chemotherapy and PARP-inhibitor strategies that are typically offered when a DNA repair defect is suspected. The finding, drawn from a combination of real-world clinical data and precisely engineered laboratory models, points toward a new way of thinking about so-called BRCAness in pancreatic cancer and could reshape how oncologists match therapies to tumor genetics in this hard-to-treat population.

The concept of BRCAness has dominated precision oncology discussions of DNA damage response defects for more than a decade. Tumors with loss of homologous recombination repair, the high-fidelity pathway that accurately fixes double-stranded DNA breaks, become exquisitely vulnerable to agents that create or exploit such breaks, including platinum drugs like oxaliplatin and cisplatin and PARP inhibitors such as olaparib. This logic works well for tumors with BRCA1 or BRCA2 mutations. However, ATM and CHEK2 mutations tell a different story. ATM, the gene encoding ataxia-telangiectasia mutated kinase, sits at the apex of a signaling cascade that coordinates the cellular response to DNA damage, orchestrating cell cycle checkpoints and the recruitment of repair machinery. CHEK2, encoding the checkpoint kinase 2 that acts downstream of ATM, is part of the same pathway. Crucially, defects in this pathway do not necessarily abolish homologous recombination repair itself, which means that tumors carrying ATM or CHEK2 mutations may retain enough repair capacity to withstand platinum and PARP-inhibitor therapy.

The research team, led by investigators at the Medical College of Wisconsin’s LaBahn Pancreatic Cancer Program together with collaborators at Wayne State University’s Karmanos Cancer Institute, approached the problem from two complementary directions. First, they mined an institutional real-world database to identify patients with advanced or metastatic pancreatic ductal adenocarcinoma whose tumors harbored ATM or CHEK2 mutations, assembling a cohort of 16 patients whose treatment histories spanned 48 separate lines of chemotherapy. Second, they turned to the laboratory, where they used CRISPR gene editing to create a panel of isogenic PANC-1 pancreatic cancer cell lines carrying either homozygous or heterozygous knockout of ATM. The cell lines were allele-verified by whole-exome sequencing, ensuring that any differences in drug response could be attributed specifically to the degree of ATM loss rather than to background genetic variation between different cell models.

The clinical results were striking. When the researchers compared the outcomes of second-line treatments for these ATM or CHEK2-mutant patients, irinotecan-containing regimens outperformed every other therapeutic category, including the platinum-based and PARP-inhibitor-containing combinations that the BRCAness framework would have predicted to be most effective. The median progression-free survival on irinotecan-based second-line therapy was 11.5 months, compared with just 3 months for the other treatment types, a difference that reached statistical significance with a p value below 0.001. In a disease where median survival is typically measured in months and treatment options after first-line failure are limited, an eight-and-a-half-month difference in disease control represents a clinically meaningful advance for this molecularly defined subgroup.

The laboratory experiments provided a mechanistic explanation for this clinical observation. The researchers exposed their isogenic cell line panel to SN-38, the active metabolite of irinotecan, which the liver generates from the prodrug in patients. SN-38 works by poisoning topoisomerase I, an enzyme that relieves torsional stress in DNA by creating transient single-strand breaks during replication and transcription. When topoisomerase I is inhibited, the enzyme becomes trapped on DNA as covalent topoisomerase I-DNA adducts, and the collision of these adducts with advancing replication forks generates double-stranded breaks that require an intact DNA damage response to resolve. Preliminary work had suggested that certain forms of ATM pathway dysfunction might prevent the removal of these topoisomerase I-DNA adducts, allowing the lesions to persist and accumulate into lethal genomic instability.

The CRISPR-edited panel confirmed this dose-dependent relationship with remarkable precision. In colony formation assays, the homozygous ATM knockout cells showed a half-maximal inhibitory concentration, or IC50, of just 0.3 nanomolar for SN-38, while the heterozygous knockout cells showed an IC50 of 0.8 nanomolar and the wild-type controls an IC50 of 7 nanomolar. In other words, complete loss of ATM rendered the cells more than twenty-fold more sensitive to the drug than unedited cells, and even the loss of a single ATM copy, which better models the heterozygous mutations frequently seen in patient tumors, conferred nearly nine-fold sensitization. Proliferation and viability assays told the same story, with all comparisons reaching statistical significance at p values below 0.01. The graded response across the isogenic panel demonstrates that the degree of ATM loss directly determines the degree of topoisomerase I inhibitor sensitivity.

These findings carry substantial implications for the clinical management of pancreatic cancer. Currently, comprehensive genomic profiling of pancreatic tumors routinely identifies ATM and CHEK2 mutations, and many oncologists extrapolate from the BRCA literature to offer platinum chemotherapy or PARP inhibitors to these patients. The new data suggest that this extrapolation may be misguided for a meaningful proportion of the ATM pathway-mutant population, since these tumors may not exhibit homologous recombination repair deficiency and may therefore not respond consistently to those agents. Instead, the study proposes that topoisomerase I inhibition exploits a distinct vulnerability: the inability of ATM-defective cells to process the stalled topoisomerase I-DNA complexes that SN-38 generates. Irinotecan is already an established component of the liposomal irinotecan combination used in second-line metastatic pancreatic cancer, which makes the prospect of biomarker-guided deployment particularly practical.

The study’s design also highlights the growing value of pairing real-world evidence with engineered laboratory models. The clinical cohort, while modest in size at 16 patients and 48 chemotherapy lines, was drawn from institutional protocols approved by the Medical College of Wisconsin Institutional Review Board, including the MCW Master Predict observational program registered as NCT05802069, and reflects the actual treatment decisions and outcomes recorded in routine care. Because each patient served in part as their own comparison across different treatment lines, the analysis captures the heterogeneity of real clinical practice while still permitting a statistically robust comparison. The isogenic CRISPR-edited cell lines then provided the controlled experimental system that retrospective clinical data can never offer, isolating the effect of ATM dosage on drug sensitivity with allele-level verification.

Several caveats temper the enthusiasm. The clinical cohort was small, and retrospective real-world analyses are inherently susceptible to selection biases in how patients were assigned to different treatments. The laboratory work relied on a single pancreatic cancer cell line background, PANC-1, albeit engineered with rigor, and in vitro sensitivity to SN-38 does not guarantee that the pharmacokinetics, toxicity profile, and combination effects of irinotecan in patients will mirror the dish. The authors themselves are careful to frame their conclusions as hypothesis-generating, calling for prospective investigation of topoisomerase I inhibitors in ATM pathway-mutated pancreatic ductal adenocarcinoma rather than claiming an immediate change to standards of care. Nonetheless, the convergence of a large clinical signal, a statistically significant survival difference, and a clean dose-dependent molecular mechanism is exactly the kind of evidence that justifies launching biomarker-stratified clinical trials.

Looking forward, the study opens several avenues. Antibody-drug conjugates that deliver topoisomerase I inhibitors selectively to tumor cells are an increasingly prominent class of anticancer agents, and the mechanistic logic established here suggests that ATM pathway status could serve as a predictive biomarker for such drugs across multiple tumor types, not only pancreatic cancer. More broadly, the work challenges the field to move beyond the binary of homologous recombination proficiency and deficiency, recognizing that different DNA damage response defects create different, drug-specific vulnerabilities. For the five to ten percent of pancreatic cancer patients whose tumors carry ATM or CHEK2 mutations, the message is one of cautious optimism: a therapy already in the clinic may work far better in their specific molecular context than anyone had previously demonstrated, and prospective trials to confirm that promise may not be far behind.

Subject of Research: Sensitivity of ATM/CHEK2-mutant pancreatic ductal adenocarcinoma to topoisomerase I inhibition

Article Title: Beyond BRCAness: ATM pathway defects confer sensitivity to topoisomerase I inhibition

Article References: Kamgar, M., McFall, T., Mehdi, M., Thapa, B., Szabo, A., Ahmed, G., Davidson, R., Scheuber, G., Shreenivas, A., Thomas, J. P., Sriram, D., Evans, D. B., Tsai, S., Christians, K. K., Erickson, B., Hall, W. A., Chen, H.-Z., Lytle, N., Sarkar, N. D., … Kurzrock, R. (2026). Beyond BRCAness: ATM pathway defects confer sensitivity to topoisomerase I inhibition. Genome Medicine. https://doi.org/10.1186/s13073-026-01774-z

Image Credits: AI Generated

DOI: 10.1186/s13073-026-01774-z

Keywords: ATM, CHEK2, pancreatic cancer, irinotecan, SN-38, topoisomerase I inhibitor, DNA damage response, BRCAness, CRISPR, precision oncology, progression-free survival, homologous recombination repair