Biomarkers Beyond ER, PR, and HER2 in Advanced Breast Cancer

Key Takeaways

• HER2-low is not an established biologic entity. Both ASCO-CAP and an ESMO consensus panel declined to treat it as a distinct molecular category.
• ESR1 mutations arise under treatment pressure and are typically undetectable in the primary tumor.
• ASCO now recommends routine ESR1 testing at recurrence or progression, ctDNA preferred – a strong recommendation backed by high-quality evidence.

Standard pathology establishes estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status, and those markers still anchor first-line strategy. In advanced disease, the useful question changes: not what the tumor was at diagnosis, but what it has become under treatment. The alterations that matter most in later lines are largely acquired – which means they typically are only found by testing again.¹

HER2 in the lower range of the assay

Low-level HER2 expression — immunohistochemistry (IHC) 1+, or 2+ with negative in situ hybridization (ISH) — is present in roughly two-thirds of hormone receptor-positive tumors, and now confers potential eligibility for HER2-directed antibody-drug conjugate therapy after progression on endocrine therapy.^2,3

Its status as a category, however, is contested. The 2023 ASCO-College of American Pathologists guideline update declined to create HER2-low as a separate result category, finding no evidence it is a reproducibly defined subtype with distinct prognostic or predictive implications, and describing the IHC 0 versus 1+ boundary as a threshold artifactually created to determine drug access based on trial eligibility.¹

An ESMO consensus panel voted to the same effect: HER2-low is not a distinct molecular entity, and its biology is driven primarily by hormone receptor status.²

The practical consequence is instability. Close to 40 percent of cases switch between IHC 0 and HER2-low when paired primary and metastatic samples are compared, and ASCO-CAP accordingly affirms consideration for testing HER2 on a metastatic site where tissue is available.¹

ESR1 mutations and acquired endocrine resistance

Ligand-binding domain mutations in ESR1 are the best-characterized mechanism of acquired endocrine resistance, producing a constitutively active receptor that drives transcription without ligand. Reported prevalence ranges from roughly 1 percent in primary tumors to 10 to 50 percent in metastatic, endocrine-resistant disease – a spread governed by assay sensitivity and by how much endocrine therapy preceded testing.^4,5

The selective pressure is not aromatase inhibitor (AI)-specific. In one series, 4 of 5 mutations detected at newly diagnosed metastasis arose after tamoxifen alone, and 1 emerged during neoadjuvant endocrine therapy. Progression-free survival (PFS) on AI treatment was significantly shorter in mutation carriers – a median of 3 versus 15 months – including in patients never previously exposed to an AI.⁵

Testing: Specimen and timing

ASCO reversed its position in 2023. Where 2 prior guidelines found evidence insufficient for routine ESR1 testing, a rapid recommendation update now advises routine testing of HER2-negative, ER-positive metastatic breast cancers for emergent mutations at recurrence or progression on endocrine therapy – graded strong, with evidence quality rated high.^6,7

Testing should use a CLIA-certified assay on blood or tissue obtained at progression, because these mutations develop under selection pressure and are typically undetectable in the primary tumor. Blood-based circulating tumor DNA (ctDNA) is preferred for its greater sensitivity. A wild-type ESR1 result may warrant retesting at subsequent progressions.^6,7

European recommendations concur, adding that a not-detected result should prompt reflex tissue testing rather than being read as definitive. ASCO also advises testing PIK3CA at the same point if not already done.^6,7

Options once ESR1 is detected

Detecting an ESR1 mutation opens a therapeutic class rather than a single agent. Selective estrogen receptor degraders (SERDs) bind the receptor, destabilize it, and promote its degradation. Fulvestrant, the first-generation intramuscular agent, has been available for 2 decades. Oral SERD and proteolysis-targeting chimera (PROTAC) therapies are now FDA-approved specifically for ER-positive, HER2-negative, ESR1-mutated advanced disease progressing after at least 1 line of endocrine therapy, with plasma-based companion diagnostic testing establishing eligibility.^8-10

The registrational trials behind these approvals are not interchangeable, which matters when applying their results: They differ in whether prior cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) exposure was required of all participants. Adverse event profiles and monitoring requirements differ between agents as well, so prescribing information should guide selection.^9,11-13

Across this work, the biomarker itself is the consistent signal. In EMERALD, patients without detectable ESR1 mutations derived no significant PFS benefit (hazard ratio 0.86; 95% CI, 0.63 to 1.19), while those with mutations did (hazard ratio 0.55; 95% CI, 0.39 to 0.77). ESR1 status selects for benefit rather than merely describing prognosis.^7,12

Other alterations worth capturing

Roughly 40 to 50 percent of hormone receptor-positive advanced tumors carry a PIK3CA, AKT1, or PTEN alteration, and an AKT inhibitor is approved with fulvestrant in that population (if also HER2-negative) after progression on an endocrine-based regimen.^14,15

FDA restricted the label to biomarker-altered patients despite a significant PFS benefit in the overall trial, because benefit-risk was unfavorable without the alteration. Germline BRCA1 and BRCA2 testing remains separately indicated.^6,15

Turning a panel into a sequencing decision

The through-line is that these findings are acquired. HER2 shifts between samples; ESR1 emerges under treatment; both are missed by relying on the diagnostic block. Retesting at progression, with attention to specimen type, is what keeps the panel decision-useful rather than historical.^1,5,7