The study, published in Breast Cancer Research and Treatment, was a retrospective observational cohort analysis led by Joyce O’Shaughnessy of Texas Oncology and Baylor University Medical Center, together with investigators from Gilead Sciences, the University of Chicago Medicine, and the University of Kansas Medical Center. The team mined the Flatiron Health Research Database, a longitudinal platform that aggregates de-identified electronic health records from approximately 280 US cancer clinics spanning roughly 800 care sites, about three quarters of them community practices and one quarter academic centers. That composition matters, because it means the results reflect the care most American patients actually receive rather than the highly selected populations enrolled in clinical trials.

To build the cohort, the researchers screened 37,582 patients and identified 5,041 women aged 18 or older with HR-positive/HER2-negative metastatic breast cancer who began standard cytotoxic chemotherapy after endocrine therapy between January 1, 2015, and December 31, 2024. Patients were excluded if they had received antibody-drug conjugates, CDK4/6 inhibitor monotherapy, HER2-targeted treatment, or investigational agents, a design choice intended to isolate the outcomes of conventional chemotherapy in the endocrine-resistant setting. The median age at metastatic diagnosis was 63 years, 63 percent of patients were non-Hispanic White, and 82 percent were treated in community settings. Most had recurrent rather than de novo metastatic disease, 68 percent, and 65 percent had already received at least two prior lines of endocrine-based therapy for metastatic disease.

The biology underlying this treatment sequence is central to understanding the findings. HR-positive/HER2-negative disease accounts for roughly 70 percent of metastatic breast cancer cases and carries a five-year survival rate of only 37 percent. For patients whose metastatic disease remains sensitive to endocrine therapy, the preferred first-line approach combines endocrine therapy with a cyclin-dependent kinase 4/6 inhibitor, a class of drugs that halts tumor cell proliferation by blocking the transition from the G1 to the S phase of the cell cycle. Pivotal randomized trials have shown substantial survival gains from this combination: in MONALEESA-3, median overall survival reached 68 months with ribociclib plus fulvestrant compared with 52 months for endocrine therapy alone, while MONARCH 3 demonstrated 67 months with abemaciclib plus an aromatase inhibitor versus 54 months with endocrine therapy alone.

But resistance is inevitable for most patients. Once the disease escapes endocrine control, typically after progression on endocrine therapy with or without a CDK4/6 inhibitor, treatment options narrow sharply. Subsequent lines may include additional endocrine agents paired with targeted therapies such as PI3K inhibitors, AKT inhibitors, mTOR inhibitors, selective estrogen receptor degraders, or PARP inhibitors in patients with BRCA mutations. When those avenues are exhausted, cytotoxic chemotherapy becomes the default, and it is precisely this population that the new study illuminates. In the cohort, the median time from metastatic diagnosis to the first post-endocrine chemotherapy line was 19 months, and 97 percent of patients received a chemotherapy agent as that first post-endocrine treatment.

The treatment patterns revealed by the data are striking in their consistency. Capecitabine monotherapy, an oral fluoropyrimidine, was the most commonly prescribed first post-endocrine regimen at 34 percent, followed by chemotherapy combined with endocrine therapy at 22 percent. The most frequent chemo-endocrine combinations were capecitabine plus fulvestrant, capecitabine plus exemestane, and capecitabine plus letrozole. From the second line onward, some clinicians reintroduced endocrine monotherapy, at rates of 16 percent in second line, 13 percent in third, 11 percent in fourth, and 12 percent in fifth, reflecting the persistent, if diminished, hormonal sensitivity of many tumors and the enduring practice of endocrine re-challenge even after documented resistance.

The survival curves tell the grimmer part of the story. Median real-world overall survival from the start of first-line post-endocrine chemotherapy was 13.5 months, and it declined with each successive line: 10.7 months at second line, 8.5 months at third, 7.5 months at fourth, and 7.4 months at fifth. Time to next treatment or death, a validated real-world proxy for progression-free survival, followed the same downward staircase, falling from 5.1 months at first line to just 3.3 months at fifth line. Most sobering of all, 30 percent of patients who began first-line post-endocrine chemotherapy never survived to receive a second line, and comparable proportions, between 32 and 36 percent, failed to reach each subsequent line of therapy. Outcomes varied modestly by regimen: patients treated with capecitabine or chemotherapy plus endocrine therapy lived numerically longer, 16.7 and 15.0 months respectively, than those receiving taxanes at 10.5 months or other chemotherapy monotherapies at 9.8 months, though the authors caution that prior taxane exposure in the curative setting, which affected 68 percent of the recurrent-disease patients, may have blunted taxane efficacy in the metastatic phase.

The subgroup analyses exposed disparities that mirror broader inequities in American cancer care. Non-Hispanic Black patients had a numerically shorter median overall survival of 11.4 months compared with 13.8 months for non-Hispanic White patients, along with shorter time to next treatment, a gap the authors attribute in part to greater metastatic disease burden, with 46 percent of Black patients harboring three or more metastatic sites versus 40 percent of White patients. Conversely, Hispanic/Latino patients showed the longest median survival at 19 months, possibly reflecting their younger median age of 59 years. Patients who died after first-line post-endocrine therapy were more likely to have poorer performance status, liver or brain metastases, lower body mass index, a shorter interval from metastatic diagnosis to chemotherapy, and more frequent BRCA1/2 mutations, all statistically significant differences that underscore how aggressive disease biology drives early mortality in this setting.

The study’s authors and outside evidence point toward antibody-drug conjugates as the most promising path forward. These engineered molecules couple a cytotoxic payload to an antibody that delivers it selectively to tumor cells, and two of them have now demonstrated overall survival advantages over chemotherapy in randomized trials of post-endocrine metastatic disease. In the TROPiCS-02 trial, sacituzumab govitecan extended median overall survival to 14.4 months versus 11.2 months for chemotherapy in HR-positive/HER2-negative disease. In DESTINY-Breast04, trastuzumab deruxtecan achieved a median overall survival of 23.9 months versus 17.5 months in patients with HER2-low disease, a category that included 32 percent of the patients in the current cohort with known HER2 immunohistochemistry expression. Because the study deliberately excluded patients who had received these agents, and because most of the cohort was diagnosed before their approval, the analysis could not capture their real-world impact, a limitation the authors acknowledge alongside the retrospective design, the absence of data on earlier treatment lines, and the lack of recorded reasons for death or censoring.

What the study delivers, ultimately, is a rigorous baseline against which the next generation of therapies must be measured. For a disease subtype that touches the vast majority of metastatic breast cancer patients, the message is unambiguous: once endocrine therapy fails, the conventional chemotherapy pathway leaves patients with roughly a year of median survival, a figure that erodes with every successive line and that leaves nearly a third of patients without the chance to try again. The authors argue that the survival benefits demonstrated by antibody-drug conjugates should translate into earlier use in the post-endocrine metastatic course, and they call for real-world database studies to confirm how the trial results perform in community practice, where 82 percent of these patients were treated. Until then, the study stands as both a warning and a roadmap, quantifying the cost of therapeutic stagnation in one of oncology’s largest and most vulnerable populations.

Subject of Research: Real-world treatment patterns and survival outcomes among US patients with HR-positive/HER2-negative metastatic breast cancer receiving chemotherapy after endocrine therapy resistance

Article Title: A retrospective, observational cohort study of treatment patterns and outcomes among patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer in the United States following endocrine therapy

Article References: O’Shaughnessy, J., Rehnquist, M., Sadetsky, N., Verret, W., Cinar, P., Sjekloca, N., Nguyen, L., Nanda, R., & Sharma, P. (2026). A retrospective, observational cohort study of treatment patterns and outcomes among patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer in the United States following endocrine therapy. Breast Cancer Research and Treatment, 219(3), Article 19. https://doi.org/10.1007/s10549-026-08075-4

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08075-4

Keywords: metastatic breast cancer, HR-positive HER2-negative, endocrine therapy resistance, chemotherapy, CDK4/6 inhibitors, antibody-drug conjugates, overall survival, real-world evidence, Flatiron Health database, capecitabine, health disparities, oncology