Blood Biopsy Trial — Getting Ahead of Treatment Resistance

How ctDNA testing and early ESR1 detection may help guide earlier treatment switching in HR-positive, HER2-negative metastatic breast cancer

How?

Using ctDNA blood testing to detect endocrine resistance before progression is obvious on a scan.

The subtype setting is HR-positive, HER2-negative metastatic breast cancer, especially in people receiving an aromatase inhibitor plus a CDK4/6 inhibitor.

Why?

For many patients, treatment changes only happen after scans show clear progression.

This take-and-wait pattern can leave a gap between when resistance begins and when treatment is changed.

The newer idea is to look for early molecular signs of resistance in the blood and switch treatment sooner.

The main study idea

The SERENA-6 trial tested a proactive strategy.

Instead of waiting for visible tumour growth, the study used circulating tumour DNA (ctDNA) to detect ESR1 mutations, which are among the earliest warning signs of developing endocrine resistance.

When an ESR1 mutation appeared, patients switched from an aromatase inhibitor to a SERD while staying within the same broader treatment logic.

The investigational drug used in the trial was camizestrant.

Why ESR1 matters here

ESR1 mutations can help cancer cells keep signalling through the estrogen receptor despite aromatase inhibition.

That makes them an important early marker of resistance in ER-driven disease.

SERDs work differently from aromatase inhibitors.

They bind directly to the estrogen receptor and drive its breakdown.

That is why this approach is so important in HR-positive disease, not breast cancer in general.

Main findings highlighted in the summary

The core message is that earlier switching appeared to greatly improve both:

time before progression
time before quality of life worsened

The research highlights these findings:

16 months median progression-free survival after switching early to camizestrant (SERD)
9.2 months median progression-free survival when staying on the earlier treatment
21 months before quality-of-life deterioration in the early-switch group
6.4 months before quality-of-life deterioration in the comparison group

Practical takeaway

This understanding points toward a more proactive model of care:

monitor for resistance earlier
detect ESR1 changes before radiographic progression
switch treatment sooner when the biology suggests escape is beginning

The wider idea is simple: do not wait for resistance to fully declare itself on a scan if a reliable molecular warning sign appears first.

Key questions from the SERENA-6 summary

How often should ctDNA be checked?

Q: How often should ctDNA be tested in patients with HR-positive, HER2-negative metastatic breast cancer on aromatase inhibitor plus CDK4/6 inhibitor therapy?

The SERENA-6 trial tested patients once every 2 to 3 months, aligning blood draws with routine clinical visits from the 6-month treatment mark onward.

This approach detected ESR1 mutations in the blood before visible progression appeared on imaging.

The first meaningful detection window opened at about 8 weeks into treatment.

Earlier cohort studies found that ESR1 mutations could appear in ctDNA around 3 to 6 months before radiographic progression became clear on scans.

Once an ESR1 mutation appeared, it often persisted across later samples.

That makes a single positive result clinically meaningful rather than background noise.

The practical takeaway is to begin ctDNA monitoring at the 2- to 3-month mark after starting first-line AI plus CDK4/6 inhibitor therapy.

Monitoring can then continue at each later scan cycle while that treatment line remains in place.

Which patients are the best fit for early ESR1 surveillance?

Q: Which patients are most likely to benefit from proactive ESR1 monitoring via ctDNA?

The clearest candidate group is patients with HR-positive, HER2-negative metastatic breast cancer who are receiving an aromatase inhibitor plus a CDK4/6 inhibitor as first-line treatment.

ESR1 mutations are usually an acquired resistance mechanism driven by aromatase inhibitor exposure.

They are not typically present at diagnosis.

That makes this treatment setting the most relevant place for surveillance.

Several clinical features may raise the chance of developing an ESR1 mutation:

prior AI exposure in the adjuvant or early-stage setting
higher baseline tumour burden or multiple metastatic sites
visceral metastases
first-line AI-based therapy already in place for 6 months or longer

Patients already on a SERD or on second-line therapy sit outside the main SERENA-6 window.

That strategy specifically targets the pre-progression phase of first-line treatment.

Will earlier switching improve overall survival?

Q: Is there evidence that the early-switch approach based on ESR1 detection will translate into an overall survival benefit, rather than just extending progression-free survival?

This remains the biggest open question.

In SERENA-6, overall survival data were still immature at the time of the primary analysis.

Only 34.3% of survival events had occurred.

That means firm OS conclusions cannot yet be drawn.

Updated survival data will matter a lot in deciding whether this becomes a standard approach.

There are still reasons for cautious optimism.

Earlier research linked ctDNA-detected ESR1 mutations with worse overall survival when patients stayed on aromatase inhibitor-based therapy.

Removing the resistance driver earlier could, in theory, slow clonal expansion and preserve later treatment options.

The trial also showed a large improvement in time to quality-of-life deterioration.

That was 21 months in the early-switch group versus 6.4 months in the control group.

In metastatic disease, that quality-of-life gain matters in its own right, even before survival data fully mature.

For cross-subtype context, see ctDNA and cfDNA in Breast Cancer: What “Low Shedding” Means. It explains low tumour shedding, tumour fraction, and negative liquid-biopsy results.

Key References

First-Line Camizestrant for Emerging ESR1-Mutated Advanced Breast Cancer
https://pubmed.ncbi.nlm.nih.gov/40454637/

Plasma ESR1 Mutations and the Treatment of Estrogen Receptor-Positive Advanced Breast Cancer
https://pmc.ncbi.nlm.nih.gov/articles/PMC4998737/

Risk of Early Progression According to Circulating ESR1 Mutation, Tumor Burden and Prior Treatment at the Time of First-Line Endocrine Therapy in Advanced Breast Cancer
https://pmc.ncbi.nlm.nih.gov/articles/PMC7254698/

The Genomic Landscape of Endocrine-Resistant Advanced Breast Cancers
https://www.sciencedirect.com/science/article/pii/S1936523319303948

ESR1 Gene Mutation in Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Implications for Current and Future Treatment Strategies
https://www.frontiersin.org/journals/oncology/articles/10.3389/fonc.2021.625636/full

Assessing the Clinical Readiness of the SERENA-6 Strategy
https://pmc.ncbi.nlm.nih.gov/articles/PMC12422312/

Clinical Value of Circulating ESR1 Mutations for Patients with Metastatic Breast Cancer
https://pmc.ncbi.nlm.nih.gov/articles/PMC6097501/

Patient-Reported Outcomes in the SERENA-6 Trial of Camizestrant
https://pubmed.ncbi.nlm.nih.gov/41125211/