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FAQ on Endocrine-Resistant ER Positive HER2 Negative MBC
Practical Questions and Answers on the Current Management of Endocrine-Resistant ER+/HER2- Metastatic Breast Cancer

Released: July 14, 2026

Mabel Mardones
Mabel Mardones, MD
Joyce O'Shaughnessy
Joyce O'Shaughnessy, MD
Michelle Shiller
Michelle Shiller, DO
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Key Takeaways
  • ESR1 mutations are acquired resistance mutations that commonly emerge after AI-based therapy and are best assessed at progression, often with blood-based ctDNA testing.
  • Next-generation oral SERDs and ER degraders work by targeting the receptor itself, which is critical when the mutant ER is active independent of estrogen.
  • Baseline safety assessments are essential when initiating oral ER-directed therapies, with lipid monitoring for elacestrant and evaluation of QT risk and potential drug interactions for vepdegestrant.

The treatment landscape for estrogen receptor (ER)–positive/HER2-negative metastatic breast cancer continues to evolve with the emergence of oral selective estrogen receptor degraders (SERDs) and other ER-targeted therapies designed to address acquired endocrine resistance. In this commentary, Joyce O’Shaughnessy, MD, Michelle Shiller, DO, and Mabel Mardones, MD, address key questions raised during a recent live symposium titled “Beyond Resistance: Integrating Next-Generation SERDs into Community Management of ER-Positive/HER2-Negative Advanced Breast Cancer.” Questions explore the clinical significance of ESR1 mutations, the role of molecular testing in treatment selection, strategies for sequencing and combining ER-directed therapies, and practical considerations for optimizing safety, adherence, and long-term patient outcomes.

What is the clinical significance of ESR1 mutations after progression on an aromatase inhibitor (AI) plus a CDK4/6 inhibitor?

Joyce O’Shaughnessy, MD:
ESR1 mutations are truly acquired resistance mutations. They are not commonly found in the primary breast cancer specimen. They emerge under the selective pressure of estrogen deprivation, particularly after AI exposure. The mutant ER is constitutively active; it does not need estrogen to drive transcription, which is why AIs become ineffective in this context. Nevertheless, the cancer may continue to be driven by ER signaling, underscoring the mutant ER as a critical therapeutic target for overcoming endocrine resistance.

How do next-generation SERDs remain active when AIs are no longer effective?

Joyce O’Shaughnessy, MD:
The key distinction is that next-generation SERDs bind directly to the ER and lead to ERα degradation, blocking its activity independent of estrogen. This mechanism of action is very different from that of AIs, which is simply to lower estrogen levels. In ESR1-mutated disease, the receptor is active without a ligand, so therapies that target the receptor itself are needed.

Which patients are most appropriate for single-agent oral SERD or ER degrader therapy?

Joyce O’Shaughnessy, MD:
For single-agent therapy, we want to select patients whose disease remains endocrine sensitive. In the phase III EMERALD trial, for example, patients with ESR1-mutated disease who had been on a prior CDK4/6 inhibitor for at least 12 months appeared to derive more durable progression-free survival (PFS) benefit from the oral SERD elacestrant compared with standard-of-care therapy (PFS: 8.6 vs 1.9 months, HR: 0.410; 95% CI: 0.26-0.63). The ER degrader vepdegestrant is approved for adults with ER-positive/HER2-negative advanced or metastatic breast cancer with an ESR1 mutation with disease progression after at least 1 line of endocrine therapy. In practice, the ESR1 mutation helps identify the population where these agents are most effective.

How should we think about combination therapy after progression on first-line endocrine therapy plus a CDK4/6 inhibitor?

Joyce O’Shaughnessy, MD:
Although ESR1 mutations are an important mechanism of acquired endocrine resistance, they frequently coexist with other genomic alterations, including PIK3CA mutations, AKT pathway alterations, and PTEN loss, underscoring the importance of combination strategies. In the phase III EMBER-3 trial, imlunestrant plus abemaciclib significantly improved PFS compared with imlunestrant alone (P <.001), even in patients with ESR1-mutant disease (PFS: 11.1 vs 5.5 months; HR: 0.49; 95% CI: 0.33-0.71). Similarly, in the phase III evERA study, giredestrant plus everolimus reduced the risk of disease progression or death by 62% in the ESR1-mutant population (PFS: 5.72 vs 5.52 months, HR: 0.84; 95% CI: 0.59-1.18). The future is not just targeting ESR1 alone; it is understanding the biology of resistance and building rational therapeutic combinations.

When should ESR1 testing be performed?

Michelle Shiller, DO:
In HR-positive/HER2-negative metastatic breast cancer, ESR1 testing should be performed at progression on endocrine-based therapy, particularly after exposure to an AI plus a CDK4/6 inhibitor. ESR1 mutations are uncommon de novo, so reviewing the diagnostic biopsy is usually not the most informative approach. The goal is to understand the tumor at the time of resistance.

Should healthcare professionals (HCPs) use tissue testing or blood-based ctDNA testing for ESR1?

Michelle Shiller, DO:
Blood-based ctDNA testing is often a more practical and informative approach, especially when biopsy is difficult or when acquired resistance is suspected. This approach is minimally invasive, can be repeated, and may better capture heterogeneity across disease sites. That said, if ctDNA is negative, clinical suspicion remains high, and there is a lesion amenable to biopsy, tissue testing should be considered. It is important to remember that a negative plasma test may reflect low shedding rather than absence of a mutation.

What if ctDNA testing shows no ESR1 mutation and no other actionable alteration?

Michelle Shiller, DO:
First, I would make sure the assay was able to address the question being asked. Was it a comprehensive panel, or was it specific to ESR1? If the patient has a lesion that can be biopsied, I encourage biopsy if the patient is willing. Testing the treatment-naive tumor would be a last resort because the tumor has evolved under selective pressure. The specimen closest in time to the current clinical problem is usually more informative.

Joyce O’Shaughnessy, MD:
Clinically, I would generally repeat ctDNA testing at progression, as ESR1 mutations and other actionable alterations can emerge over the course of treatment. A negative ctDNA result does not necessarily mean that a mutation is absent; assay sensitivity and the degree of tumor DNA shedding can affect detection.

How should HCPs interpret multiple mutations, such as ESR1 plus PIK3CA or AKT alterations?

Michelle Shiller, DO:
The variant allele fraction (VAF) can provide some insight into how much of a given mutation is present, but we are still learning how to best apply this information over time. If ESR1 is stable and the VAF of AKT1 is increasing over time, it may suggest that the phosphoinositide kinase pathway is playing a greater role in disease progression. Since dynamics are so important, serial testing may help identify the resistance clone that is emerging.

How do you choose among elacestrant, imlunestrant, and vepdegestrant when no head-to-head comparisons are available?

Mabel Mardones, MD:
In the absence of head-to-head data, I generally consider factors such as familiarity, adverse event profile, drug–drug interactions, access, and patient-specific considerations when selecting therapy. We have more experience with elacestrant, but imlunestrant has shown promising efficacy and tolerability in clinical trials. Vepdegestrant is newer in routine practice, so we will learn more over time. Ultimately, it is helpful to have multiple options available, particularly when a patient does not tolerate a specific therapy well.

Joyce O’Shaughnessy, MD:
I suspect we may ultimately use more than 1 of these agents over the natural history of a patient’s disease course, sometimes as monotherapy and other times in combination. Important questions remain regarding how distinct ESR1-mutant subclones respond to different ER-targeted therapies and whether resistance mechanisms are shared across agents.

What practical safety issues should HCPs consider when starting these agents?

Mabel Mardones, MD:
These drugs are generally well tolerated, but several practice safety considerations are important. With elacestrant, HCPs should obtain baseline lipid testing, monitor for nausea, and evaluate for possible CYP3A-mediated interactions. With imlunestrant, GI toxicities, particularly nausea, as well as liver function, should be monitored. With vepdegestrant, HCPs should be aware of the potential for QTc prolongation, and a baseline ECG is needed while monitoring electrolytes and concomitant medications that may affect cardiac function. Food requirements also play a role: elacestrant and vepdegestrant are taken with food, whereas imlunestrant is taken on an empty stomach. These considerations highlight the important role of pharmacists and the broader care team in optimizing the safe and effective use of these therapies.

How can HCPs support adherence and persistence with oral ER-directed therapy?

Mabel Mardones, MD:
Patients cannot benefit from therapies they are unable to tolerate or do not take consistently, so setting expectations early is critical. HCPs should provide written educational materials, review possible drug–drug interactions and supplements, address financial barriers, and involve caregivers when appropriate. Because patients may not fully absorb information when learning that their disease has progressed, ongoing education and reinforcement are often necessary. Building trust, maintaining open communication, and proactively managing adverse effects are essential for supporting adherence and long-term treatment persistence.

Your Thoughts
In your practice, what is the biggest barrier to integrating ESR1 testing and next-generation ER-directed therapy: timely molecular results, interpreting multiple actionable mutations, treatment access, or supporting patients through oral therapy adherence?

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