Hormone receptor-positive breast cancer is the most common form of the disease, accounting for roughly 70 percent of all breast cancer cases worldwide. These tumors depend on estrogen receptor signaling to grow, which is why the backbone of treatment for metastatic disease combines endocrine therapy, typically an aromatase inhibitor that deprives tumors of estrogen, with a cyclin-dependent kinase 4 and 6 inhibitor, a targeted drug that halts cancer cell division. Although this combination has transformed outcomes for many patients, resistance almost inevitably develops, and once the disease progresses, subsequent therapies tend to be less effective and harder to tolerate. Understanding the molecular seeds of that resistance has become one of the most urgent questions in breast cancer medicine.

One of the most important mechanisms of resistance involves the ESR1 gene itself, which encodes the estrogen receptor alpha protein, the principal molecular target of endocrine therapy. Activating mutations in ESR1 are rare at the time of metastatic diagnosis, but they emerge under the selective pressure of aromatase inhibitor treatment, appearing in roughly 40 percent of patients with recurrent disease who have been previously exposed to endocrine therapy. These mutations lock the estrogen receptor into a permanently active state, allowing cancer cells to proliferate even when estrogen is pharmacologically depleted. Crucially, previous research has shown that ESR1 mutations can be detected in circulating tumor DNA, fragments of tumor genetic material drifting in the bloodstream, a median of approximately six months before radiographic progression becomes apparent.

That lead time matters because two landmark clinical trials have demonstrated that acting on it can change patient outcomes. In the phase 3 PADA-1 study, French investigators showed that switching from an aromatase inhibitor to the selective estrogen receptor degrader fulvestrant, while continuing the CDK4/6 inhibitor palbociclib, upon detection of a rising ESR1 mutation in blood significantly improved progression-free survival, reducing the risk of progression by 39 percent. More recently, the phase 3 SERENA-6 trial demonstrated that switching to camizestrant, a next-generation oral selective estrogen receptor degrader, while maintaining a CDK4/6 inhibitor produced a statistically significant and clinically meaningful benefit in an interim analysis. Together, these trials suggest that ESR1 mutation emergence is not merely a biological curiosity but a genuine clinical decision point.

Yet clinical trials are conducted under idealized conditions, with protocol-mandated serial blood testing and standardized treatment algorithms. What actually happens in ordinary oncology practices across the United States, and what the emergence of ESR1 mutations means for patients treated there, has remained far less certain. To address this gap, a research team led by investigators from Emory University, AstraZeneca, and Flatiron Health conducted a retrospective cohort study of patients diagnosed with ER-positive, HER2-negative metastatic breast cancer between January 2018 and June 2024. The database draws deidentified records from both community and academic oncology practices, offering a portrait of care that closely resembles the average patient experience rather than the rarefied environment of a major cancer center.

The scale of the analysis was substantial. Of 8,581 eligible patients, 7,772, or 91 percent, initiated first-line therapy. Among these, only 17 percent had their tumors tested for ESR1 mutations during first-line treatment, revealing that systematic monitoring for this resistance mechanism is far from routine. The median age of patients was 63 years, about 63 percent were White, 75 percent received care in community rather than academic settings, and the median time from the start of first-line therapy to the first ESR1 test was 7.4 months. Treatment patterns were strikingly similar regardless of mutation status: the combination of an aromatase inhibitor with a CDK4/6 inhibitor was the most common first-line regimen overall, used in 55 percent of all tested patients, 58 percent of those with mutations, and 54 percent of those without.

The clinical consequences of mutation detection, however, were anything but similar. To avoid statistical biases that can distort retrospective analyses, the researchers anchored all outcome measurements to the time of the first ESR1 test rather than the start of therapy, and they used propensity score matching to balance the two groups on key covariates including liver metastases, estrogen receptor expression level, age, duration of prior aromatase inhibitor therapy, and menopausal status. After matching, patients with an ESR1 mutation detected at their first test had a median real-world progression-free survival of just 7.7 months, compared with 13.6 months for those without a detected mutation, a hazard ratio of 0.68. Overall survival told the same story: median overall survival was 32.2 months in the mutation group versus a median that had not yet been reached in the non-mutation group, with a hazard ratio of 0.58, corresponding to a substantially elevated risk of death.

The mutation also reshaped the trajectory of care. A dramatically higher share of patients with detected ESR1 mutations, 84 percent compared with 52 percent, went on to receive second-line therapy during follow-up, and those who did were more likely to receive a CDK4/6 inhibitor combined with fulvestrant, or the oral selective estrogen receptor degrader elacestrant, a drug specifically approved for ESR1-mutated disease. In the unmatched analysis, 23 percent of patients with mutations initiated second-line therapy versus 9 percent of those without. These patterns are consistent with the interpretation that ESR1 mutation emergence marks a biologically aggressive, endocrine-resistant phase of disease that shortens the time clinicians must change course, while also reflecting a growing awareness among oncologists that this mutation calls for mutation-directed treatments.

The study’s authors were careful to acknowledge its limitations. Only a minority of patients were tested at all, and the reasons for testing were unknown, meaning the tested population may not represent all patients with metastatic disease. Testing in routine practice often occurs as part of broader genomic panels searching for actionable alterations in genes such as BRCA1, BRCA2, PIK3CA, AKT1, and PTEN, rather than as longitudinal ESR1 monitoring, and the mix of tissue-based and blood-based assays from different commercial vendors introduces variability in sensitivity that could have caused misclassification. Additionally, real-world progression, defined by clinician documentation rather than standardized RECIST imaging criteria, may not perfectly align with trial endpoints, and residual confounding cannot be excluded despite careful matching. Only about 14 percent of all first-line patients could be included in the survival analyses, raising questions about generalizability.

Even with these caveats, the real-world progression-free survival of 7.7 months following mutation detection closely mirrors the 9.2 months observed in the control arm of SERENA-6, where patients with emerging mutations remained on their original aromatase inhibitor and CDK4/6 inhibitor combination. That convergence between randomized trial data and observational practice is striking: it suggests that patients whose ESR1 mutations are detected but left unaddressed fare just as poorly in the community as they did in trial control arms, while trial patients whose therapy was switched fared better. The message for oncology is becoming difficult to ignore. ESR1 mutation emergence defines a clinically vulnerable, pre-progression window of roughly six to nine months, and the tools to exploit that window, liquid biopsy monitoring and next-generation estrogen receptor degraders, now exist. The remaining challenge, the study’s authors conclude, is figuring out how to weave systematic ESR1 surveillance and timely treatment modification into the everyday fabric of cancer care, so that the warning sign carried in a patient’s bloodstream is never missed.

Subject of Research: Real-world treatment patterns and outcomes of emerging ESR1-mutated ER-positive metastatic breast cancer in the United States.

Article Title: Real-world treatment patterns and clinical outcomes in patients with emerging estrogen receptor 1 (ESR1)-mutated ER+ metastatic breast cancer in the U.S., 2018-2024

Article References: Meisel, J. L., Chen, C., Kris, A., Ru, M., Pham, T., & Roose, J. (2026). Real-world treatment patterns and clinical outcomes in patients with emerging estrogen receptor 1 (ESR1)-mutated ER+ metastatic breast cancer in the U.S., 2018-2024. Breast Cancer Research and Treatment, 219(2), Article 9. https://doi.org/10.1007/s10549-026-08061-w

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08061-w

Keywords: ESR1 mutations, metastatic breast cancer, endocrine resistance, ER-positive HER2-negative, circulating tumor DNA, CDK4/6 inhibitors, aromatase inhibitors, selective estrogen receptor degraders, real-world evidence, progression-free survival, SERENA-6, PADA-1

The stakes of this testing gap are considerable. ER-positive, HER2-negative disease accounts for roughly 70 percent of all breast cancer cases and, after relapse, is responsible for most breast cancer deaths. Endocrine therapies that disrupt estrogen receptor signaling remain the backbone of treatment, often combined with CDK4/6 inhibitors as first-line therapy. Yet most patients eventually progress, typically within two to three years, and mutations in the ligand-binding domain of ESR1 are a major mechanism by which tumors escape estrogen deprivation. These mutations are rare at initial metastatic diagnosis, detected in fewer than 5 percent of patients before first-line therapy, but they emerge over time under the selective pressure of aromatase inhibitors and other endocrine treatments, reaching appreciable prevalence in patients with recurrent or endocrine-resistant disease.

The clinical landscape shifted decisively in 2023 when the FDA approved elacestrant, an oral selective estrogen receptor degrader, for patients with ESR1-mutated, ER-positive/HER2-negative advanced disease that had progressed after at least one line of endocrine therapy, alongside the Guardant360 CDx assay as a companion diagnostic. A second oral SERD, imlunestrant, followed in 2025. More provocatively, the phase 3 SERENA-6 trial demonstrated that switching patients to camizestrant upon detection of an emergent ESR1 mutation during first-line treatment, while continuing the CDK4/6 inhibitor, significantly improved progression-free survival compared with continued aromatase inhibitor therapy, with a hazard ratio of 0.44. The earlier PADA-1 trial had already shown a similar benefit for mutation-guided switching to fulvestrant. Together, these results argue that serial monitoring for ESR1 mutations before radiologic progression could fundamentally change how first-line therapy is managed.

Against that backdrop, the new study set out to quantify how often ESR1 testing actually occurs, when mutations appear during first-line treatment, and which patients get tested. Among the 7,772 patients who initiated first-line therapy, only 222, or 3 percent, had an ESR1 test result reported in the 90 days before treatment began, and just 1,355, or 17 percent, were tested at any point during first-line therapy. Even among patients who moved on to a second line of treatment, fewer than a quarter had been tested on or before the start of that line. Testing rates did climb over the study period, rising from 11 percent of patients diagnosed in 2018 and 2019 to 19 percent in 2020 and 2021 and 22 percent in 2022 through 2024, but the authors emphasize that the vast majority of tumors were never interrogated for this actionable biomarker.

The mechanics of testing also revealed a heavy reliance on tissue rather than blood. Of the 1,595 tests performed during first-line therapy, 60 percent used tissue specimens while 40 percent used liquid biopsies that detect circulating tumor DNA. Nearly all tests, 96 percent, were next-generation sequencing assays, with Foundation Medicine and Guardant serving as the most common testing providers. The median turnaround time from specimen collection to result reporting was 28 days, although for a quarter of tests more than 12 weeks elapsed before results were available, a lag that matters when treatment decisions may hinge on timely mutation detection. Guideline recommendations from ASCO updated in 2023 favor ctDNA-based testing at progression precisely because it enables less invasive, serial monitoring.

Perhaps the most clinically consequential finding concerns the timing of mutation emergence. Using the date each specimen was collected, rather than the date results were reported, the researchers calculated that ESR1 test positivity was 6.7 percent among specimens provided at baseline, rising to 23 percent for specimens collected 9 to 12 months into first-line therapy, 38 percent at 15 to 18 months, and 40 percent at 18 to 24 months. Critically, mutations were detected in specimens collected across every time interval, demonstrating that ESR1 mutations can emerge at essentially any point during treatment rather than clustering predictably around a single window. This dynamic, rolling pattern of emergence echoes kinetics data from PADA-1, where cumulative incidence approached 40 percent before or at progression but instantaneous detection rates varied unevenly across the treatment course.

The spectrum of mutations observed reinforced known biology. In the secondary clinico-genomic cohort, the most common variants were D538G, found in 43 percent of mutated tumors, followed by Y537S at 30 percent and E380Q at 23 percent, all canonical ligand-binding domain alterations. Multiple ESR1 variants coexisted in 19 percent of mutated tumors. Among patients whose tumors acquired ESR1 mutations during first-line therapy, co-mutations were frequent: PIK3CA alterations appeared in 38 percent of those tested, BRCA1 or BRCA2 alterations in 10 percent, AKT1 in 4 percent, and PTEN in under 3 percent, underscoring that ESR1 is one of several resistance mechanisms at work in these tumors.

Who gets tested also proved informative. Patients tested during first-line therapy were demographically similar to those not tested, with comparable ages, sexes, races, and rates of liver metastases, though tested patients were somewhat more likely to receive care in community oncology practices, a pattern the authors caution may reflect documentation differences rather than true practice variation. Prior endocrine therapy exposure emerged as a meaningful correlate of baseline positivity: among patients with recurrent disease previously treated with aromatase inhibitor in the early-stage setting, 22 percent of those tested before first-line therapy harbored ESR1 mutations, and positivity rose to 21 percent among patients with two or more years of prior endocrine therapy, compared with 10 percent among those with less than a year of exposure.

The study’s limitations temper some interpretations. Because testing was not standard of care for much of the period studied, tested patients cannot be assumed to represent the broader population, and positivity rates may be overestimates if testing was prompted by unmeasured signals of disease worsening. Most patients were never tested, and those who were typically provided only a single specimen. The small number of specimens in some time intervals limited the precision of positivity estimates. Nonetheless, the consistency of the findings with randomized trial kinetics data lends credibility to the central conclusion: ESR1 mutations are a dynamic, time-dependent feature of first-line treatment, and one-time or infrequent testing is structurally incapable of catching them reliably. In SERENA-6, a single test detected only half of emergent mutations, while two to five serial tests detected up to 90 percent.

The authors argue that the observed testing shortfall represents a concrete opportunity. With mutation-guided switching now supported by randomized evidence linking early detection to prolonged progression-free survival, delayed deterioration in patient-reported outcomes, and better maintenance of health-related quality of life, frequent ctDNA-based surveillance during first-line therapy could allow oncologists to intercept endocrine resistance before scans confirm progression. As oral SERDs multiply and guidelines increasingly endorse molecular monitoring, this real-world dataset serves as both a benchmark and a warning: the science of anticipatory, adaptive oncology is ready, but routine practice has not yet caught up with it.

Subject of Research: Real-world ESR1 mutation testing patterns and emergence during first-line treatment of ER-positive/HER2-negative metastatic breast cancer in the United States

Article Title: Real-world estrogen receptor alpha 1 (ESR1) testing patterns and results for ER+/HER2- metastatic breast cancer in the United States, 2018–2024

Article References: Meisel, J., Pham, T., Chen, C., Kris, A., & Roose, J. (2026). Real-world estrogen receptor alpha 1 (ESR1) testing patterns and results for ER+/HER2- metastatic breast cancer in the United States, 2018–2024. Breast Cancer Research and Treatment, 219(2), Article 8. https://doi.org/10.1007/s10549-026-08060-x

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08060-x

Keywords: ESR1 mutation, metastatic breast cancer, endocrine therapy resistance, liquid biopsy, circulating tumor DNA, SERD, elacestrant, camizestrant, SERENA-6, next-generation sequencing, CDK4/6 inhibitor, biomarker testing