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Optimizing Oral SERD Therapy in Metastatic Breast Cancer: Translating Biomarker Insights Into Patient-Centered Care

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Released: October 29, 2026

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In this commentary, advanced practice providers who are experts in breast cancer discuss the evolving role of oral estrogen receptor–directed therapies in HR-positive/HER2-negative metastatic breast cancer, with a focus on overcoming endocrine resistance and individualizing treatment. The discussion examines the clinical significance of ESR1 mutations, optimal timing and interpretation of ctDNA testing, and phase III efficacy data from EMERALD, EMBER-3, VERITAC-2, and SERENA-6. Particular attention is given to biomarker-guided treatment selection, including the potential for early therapeutic intervention following detection of emerging ESR1 mutations before clinical disease progression, and sequencing strategies when multiple actionable alterations are present. The experts also address agent-specific safety considerations, adverse event monitoring and management, patient counseling, adherence, and practical barriers to care, emphasizing the importance of balancing treatment efficacy, tolerability, and quality of life.

Jamie L. Carroll
Jamie L. Carroll, MSN, APRN, CNP
Cierra K. Ryan
Cierra K. Ryan, PA-C
Oral SERDs in MBC

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The treatment landscape for hormone receptor (HR)–positive/HER2-negative metastatic breast cancer (MBC) continues to evolve rapidly. With the emergence of oral selective estrogen receptor degraders (SERDs) and other targeted therapies, there are more opportunities than ever to individualize treatment and address endocrine resistance. However, these advances also introduce new challenges related to biomarker testing, treatment sequencing, adverse event management, and patient adherence.

As advanced practice providers, our responsibility extends beyond selecting an effective therapy. We must also consider the person in front of us, their prior treatment experience, goals, and how we can help them maintain quality of life on long-term treatment.

Recognizing Endocrine Resistance and the Importance of Biomarker Testing
Jamie L. Carroll, MSN, APRN, CNP: Despite significant advances with CDK4/6 inhibitors and endocrine therapy, disease progression remains inevitable for many patients with MBC. Median progression-free survival (PFS) with first-line CDK4/6 inhibitor plus endocrine therapy is approximately 24 to 28 months, but the duration of benefit with subsequent therapies is generally shorter.

One important mechanism of acquired endocrine resistance involves mutations in the ESR1 gene. These mutations are uncommon at initial diagnosis but may develop in approximately 20% to 40% of patients following exposure to aromatase inhibitors (AIs) in the metastatic setting. This is why the timing of biomarker testing matters.

We recommend assessing for ESR1 mutations and other actionable alterations when patients experience progression on endocrine therapy. The American Society of Clinical Oncology recommends routine testing for emerging ESR1 mutations at recurrence or progression on endocrine therapy, with or without CDK4/6 inhibition. Plasma circulating tumor DNA (ctDNA) testing is preferred because of its greater sensitivity for detecting acquired ESR1 mutations.

Blood-based testing offers important practical advantages. It is minimally invasive, can often be coordinated with routine laboratory appointments, and may eliminate the need for additional tissue biopsies.

A negative ctDNA result, however, must be interpreted within the clinical context, particularly when ctDNA levels may be insufficient for reliable mutation detection. If clinical suspicion remains high, repeat plasma testing or tissue-based testing may be considered. Retesting at subsequent disease progression may also be appropriate.

Integrating Oral ER-Directed Therapies Into Treatment Decisions
Cierra K. Ryan, PA-C: The expanding availability of oral estrogen receptor (ER)–directed therapies represents an important advancement for patients whose disease has progressed on prior endocrine therapy.

Several phase III clinical trials have demonstrated meaningful improvements in PFS in selected populations. For example, EMERALD demonstrated improved PFS with elacestrant compared with standard endocrine therapy, including among patients with ESR1-mutated disease following prior CDK4/6 inhibition. Similarly, EMBER-3 demonstrated improved median PFS with imlunestrant vs standard endocrine therapy in patients with ESR1 mutations (5.5 vs 3.8 months), while VERITAC-2 demonstrated a PFS benefit with vepdegestrant compared with fulvestrant in the ESR1-mutated subgroup (5.0 vs 2.1 months).

Particularly interesting is the SERENA-6 trial, which evaluated switching the endocrine backbone to camizestrant after detecting an emerging ESR1 mutation in ctDNA, before clinical disease progression, while maintaining CDK4/6 inhibition. The study reported median PFS of 16.0 months with the early switch compared with 9.2 months with continued AI therapy (HR: 0.44).

This introduces an important question: Should we wait for radiographic progression when molecular testing suggests endocrine resistance is emerging? For patients who meet the criteria for this newly approved treatment strategy, detection of an ESR1 mutation during first-line AI plus CDK4/6 inhibitor therapy may prompt a switch to camizestrant while continuing CDK4/6 inhibition, before clinical disease progression. Understanding patient selection, regulatory indications, and monitoring requirements is essential as this strategy enters clinical practice.

Choosing Therapy When Multiple Actionable Mutations Are Present
Jamie L. Carroll, MSN, APRN, CNP: Treatment selection becomes more complex when ctDNA testing identifies both an ESR1 mutation and another actionable alteration, such as a PIK3CA or AKT1 mutation. In these situations, we do not make decisions based solely on what appears on a molecular report. Rather, we consider how long the patient benefited from first-line therapy, the location and extent of progression, performance status, anticipated toxicities, and the patient's preferences.

For someone who has received 2 years of benefit from first-line treatment and develops limited progression, targeting ESR1 with an oral SERD may be reasonable. Conversely, a patient developing multiple new liver lesions or more extensive progression may warrant a different targeted treatment approach. We may ultimately need to address multiple resistance pathways over the course of treatment. The challenge is determining which intervention is most appropriate at a particular point in each patient's disease trajectory.

Managing Adverse Events While Preserving Quality of Life
Cierra K. Ryan, PA-C: One of our most important responsibilities is helping patients remain on effective therapy without compromising their quality of life. Oral SERDs and related ER-directed therapies can cause gastrointestinal symptoms, fatigue, arthralgias, and vasomotor symptoms. Certain agents also have distinctive safety considerations, including visual disturbances, bradycardia, QTc prolongation, or laboratory abnormalities.

We emphasize proactive counseling and open communication. Patients should understand which symptoms might occur, which warrant prompt reporting, and how we can help manage them. For patients receiving camizestrant, we emphasize prompt reporting and assessment of new visual disturbances, dizziness, palpitations, or other symptoms suggestive of cardiac dysfunction. Depending on the severity and nature of the symptoms, management may include ophthalmologic evaluation, ECG monitoring, and treatment interruption in accordance with prescribing recommendations. We also consider other contributing factors, including dehydration, hypotension, electrolyte abnormalities, and concomitant medications.

For less urgent symptoms, regular follow-up is critical. We typically assess patients at least monthly during treatment, with more frequent follow-up when warranted by the treatment regimen, emerging adverse events, or individual patient needs. When symptoms persist despite supportive interventions, we favor bringing patients in for evaluation rather than continuing telephone management indefinitely.

The National Cancer Institute's Common Terminology Criteria for Adverse Events provides a useful framework for grading toxicities, and agent-specific prescribing information guides decisions about treatment interruption or dose reduction.

Our goal is not simply to maintain patients on treatment but to achieve the best possible balance between therapeutic benefit, tolerability, and quality of life.

Addressing Adherence and Practical Barriers to Care 
Cierra K. Ryan, PA-C: Patient education is fundamental to successful oral therapy. This includes counseling patients on medication schedules, food requirements, missed doses, potential drug interactions, contraception when appropriate, and the importance of consistent follow-up. We also recognize that practical barriers, including financial concerns, travel distance, and access to testing, may interfere with care. Coordinating laboratory appointments with routine visits, considering mobile phlebotomy when available, and identifying other accessible testing options can help reduce these challenges.

When these barriers arise, we involve social work, collaborate with local primary care providers, and explore accessible monitoring options. A treatment strategy can be successful only if it is realistic for the person receiving it.

Summary
The expanding role of oral SERDs is changing how we approach endocrine resistance in HR-positive/HER2-negative MBC. Biomarker testing provides valuable direction, but effective treatment requires thoughtful sequencing, individualized toxicity management, and attention to the practical challenges patients face. Our regular interactions with patients, even when imaging is obtained only every few months, provide opportunities to identify subtle changes, build trust, and support safe treatment continuation. Ultimately, we are treating both the disease and the person living with it.

Your Thoughts
How is ESR1 mutation testing influencing your treatment decisions in HR-positive/HER2-negative MBC? When selecting or supporting oral SERD therapy, what practical strategies have helped you balance disease control, tolerability, and patients' priorities? Share these considerations with colleagues as treatment approaches continue to evolve.

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For patients with HR-positive/HER2-negative MBC who experience disease progression on endocrine therapy, how often do you obtain or confirm ESR1 mutation testing to guide subsequent treatment selection?

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