Acquired ESR1 Mutations in Plasma: Reading Endocrine Resistance in Real Time

The clinical question

Most estrogen receptor, positive, HER2-negative (ER+/HER2−) breast cancers respond well, at first, to endocrine therapy. The problem is durability. Under the sustained selective pressure of an aromatase inhibitor, subclones carrying mutations in the estrogen receptor gene, ESR1, can expand. These mutations, clustered at ligand-binding-domain hotspots such as Y537S and D538G, stabilize the agonist conformation of the receptor, locking it into a constitutively active, estrogen-independent state [5]. The tumor keeps signaling even when estrogen production is suppressed. Clinically, that translates into a familiar pattern: an initial response, then progression on endocrine therapy.

The question a plasma assay is asked to answer is deceptively simple: does this patient's tumor now carry an acquired ESR1 mutation? Because these mutations are almost always acquired under treatment rather than present at diagnosis, the archived diagnostic biopsy is frequently uninformative. The relevant clone emerges later, its prevalence rising with successive lines of endocrine therapy, and it may be spatially heterogeneous across metastatic sites [6,7]. This is precisely the setting where a blood-based assay has a structural advantage, it samples shed tumor DNA from multiple deposits at once, at the moment the clinical decision is being made.

What's measured, and how

The analyte is circulating tumor DNA (ctDNA): short fragments of tumor-derived cell-free DNA shed into the bloodstream, isolated from the plasma fraction of a peripheral blood draw. The target is an activating ESR1 ligand-binding-domain mutation, scored in a binary fashion, mutant versus wild-type.

Detection relies on either targeted next-generation sequencing (NGS) or allele-specific PCR platforms. Both must contend with the central technical challenge of plasma testing: the mutant allele fraction can be vanishingly low, often well under 1%, and ESR1-mutant subclones may be polyclonal, with several distinct mutations coexisting in one patient. Assays intended for this use are therefore engineered for low limits of detection, but no plasma assay is perfectly sensitive. A negative ctDNA result does not exclude an ESR1 mutation, it may reflect a tumor that sheds little DNA, a low-volume disease burden, or a mutation below the assay's detection floor. This asymmetry matters clinically: a positive result is actionable, while a negative result is best read as "not detected" rather than "absent", plasma- and tissue-only mutations each occur, and discordance between compartments is well documented [7]. That distinction should be communicated plainly on the report.

The evidence

The key data come from EMERALD, a phase III randomized trial comparing elacestrant, an oral selective estrogen receptor degrader (SERD), against standard-of-care endocrine therapy in previously treated ER+/HER2− advanced breast cancer. Bidard and colleagues showed that patients whose tumors carried ESR1 ligand-binding-domain mutations, detected in plasma ctDNA by the Guardant360 CDx, derived a differential progression-free survival benefit from elacestrant relative to standard endocrine therapy (PFS HR 0.55, 95% CI 0.39, 0.77; p=0.0005) [1]. Critically, the mutation status that stratified benefit was defined by a blood test, not tissue, which is what makes EMERALD a plasma-anchored indication rather than a tissue assay retrofitted to blood.

That evidence translated directly into regulatory action. In January 2023 the FDA approved elacestrant (marketed as ORSERDU) for ER+/HER2− advanced breast cancer with an ESR1 mutation, following disease progression on at least one prior line of endocrine therapy. The label mandates detection of an ESR1 mutation in plasma ctDNA as a companion-diagnostic gating criterion for treatment [2]. In other words, plasma ctDNA ESR1 testing is not merely permitted here — it's the required mechanism for identifying candidates for this drug class. This is one of the clearer examples in solid-tumor oncology of a plasma assay being written into an approval as the selection tool.

The story doesn't end at progression, though. The PADA-1 trial asked a forward-looking question: what if you monitor for rising ESR1-mutant clones during first-line therapy, before radiographic progression? Bidard and colleagues randomized patients with a rising plasma ESR1 mutation, detected while still on an aromatase inhibitor plus palbociclib, to either switch preemptively to fulvestrant plus palbociclib or continue unchanged. The trial demonstrated that serial ctDNA monitoring can flag emerging resistant clones early, and that acting on that signal is feasible in a prospective setting [3]. This extends the potential utility of plasma ESR1 testing from a one-time, post-progression selection assay toward a dynamic monitoring strategy.

More recent work continues to characterize how these mutations behave over time. A 2025 primary study of serial cell-free DNA analysis in HR+/HER2− metastatic disease during palliative endocrine therapy (indexed as PMC12815856) reported longitudinal ESR1 mutation dynamics consistent with the value of repeat sampling and with testing timed relative to lines of therapy [4]; the stronger support for serial-monitoring value rests on earlier prevalence and serial-polyclonality data [6,7] rather than on this single uningested study. The evidence increasingly suggests that a single snapshot may underuse the analyte, mutation fractions rise and fall under treatment pressure.

Validated uses and open questions

The validated uses separate cleanly from the investigational ones.

The post-progression companion-diagnostic use is validated and regulatory-grade: detecting an acquired ESR1 mutation in plasma to establish eligibility for elacestrant is an FDA-recognized indication with a defined intended use [1,2]. A clinician ordering this test after progression on endocrine therapy is operating within an approved framework. The therapy linkage is at the level of a drug class, an oral SERD, and eligibility, not a directive about how any individual should be treated.

The monitoring-and-preemptive-switch use is promising but investigational. PADA-1 provides prospective, randomized support for the concept [3], and the serial cfDNA data add mechanistic and temporal texture [4], but this strategy hasn't been established as a standard companion-diagnostic pathway in the way the post-progression setting has. Whether routine serial monitoring should drive treatment switches outside a trial is an active, fast-moving question, one where the evidence is still consolidating and reasonable experts differ.

For MRD-guided decisions specifically, using ctDNA to detect minimal residual disease and guide adjuvant choices, the ESR1 mutation context does not yet supply a validated indication. Acquired ESR1 mutations are a phenomenon of the treated, advanced setting; they're not a native minimal-residual-disease marker in early disease. That application remains outside what the current evidence supports for this biomarker.

The practical takeaway: plasma ctDNA ESR1 testing has earned a validated, label-defined role at progression, gives us a scientifically grounded but still-investigational monitoring rationale, and should be interpreted with the humility the analyte demands, a detected mutation is actionable, but "not detected" is not the same as "absent."

References

1. Bidard F-C, Kaklamani VG, Neven P, et al. Elacestrant (oral selective estrogen receptor degrader) versus standard endocrine therapy for estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer: results from the randomized phase III EMERALD trial. Journal of Clinical Oncology. 2022. DOI:10.1200/JCO.22.00338. PMID 35584336.
2. U.S. Food and Drug Administration. ORSERDU (elacestrant) Prescribing Information / FDA Approval. FDA Drug Label, NDA 216564. 2023. FDA-NDA-216564.
3. Bidard F-C, Mayaud C, de Luca V, et al. Switch to fulvestrant and palbociclib versus no switch in advanced breast cancer with rising ESR1 mutation during aromatase inhibitor and palbociclib therapy (PADA-1): a randomised, open-label, multicentre, phase 3 trial. Lancet Oncology. 2022;23(11):1367, 1377. DOI:10.1016/S1470-2045(22)00555-1. PMID 36183733.
4. Authors as indexed in PMC12815856. Serial analysis of ESR1 mutations in cell-free DNA from hormone receptor-positive, HER2-negative metastatic breast cancer during palliative endocrine therapy. PubMed Central (PMC12815856). 2025. PMCID:PMC12815856.
5. Toy W, Shen Y, Won H, et al. ESR1 ligand-binding domain mutations in hormone-resistant breast cancer. Nature Genetics. 2013;45(12):1439, 1445. PMID 24185512.
6. Fribbens C, O'Leary B, Kilburn L, et al. Plasma ESR1 mutations and the treatment of estrogen receptor-positive advanced breast cancer. Journal of Clinical Oncology. 2016;34(25):2961, 2968. PMID 27269946.
7. Takeshita T, Yamamoto Y, Yamamoto-Ibusuki M, et al. Comparison of ESR1 mutations in tumor tissue and matched plasma samples from metastatic breast cancer patients. Translational Oncology. 2017;10(5):766, 771. PMID 28778025.