Circulating tumor DNA (ctDNA) assays show high concordance with tissue biopsies and may yield a higher rate of identifying ESR1 mutations. This is because ctDNA captures tumor heterogeneity from multiple metastatic sites, which a single tissue sample can miss, providing a more comprehensive genomic picture.

ctDNA testing (liquid biopsy) is more effective than tissue biopsy for identifying ESR1 mutations. It samples DNA from all metastatic sites, capturing the disease's genetic heterogeneity and reflecting the most active resistance mechanisms, unlike a single-site needle biopsy which can miss them.

The SERENA-6 trial showed improved survival by switching therapy upon ctDNA detection of ESR1 mutations. However, it required screening over 3,300 patients to randomize just 315, highlighting the immense scale, cost, and patient drop-off of applying this serial monitoring strategy in standard clinical practice.

The Lidara study showed SERD benefit in patients without pre-existing ESR1 mutations. Success is likely multifactorial: SERDs are more effective and better tolerated than AIs. Critically, they also prevent the most common resistance mechanism—the acquisition of ESR1 mutations—from developing in the first place, altering the disease's future trajectory.

A study switching therapy based on ctDNA-detected ESR1 mutations revealed patients felt significantly better after the switch, even without visible tumor progression on scans. This counterintuitive finding suggests molecular progression has a subclinical impact on quality of life, supporting proactive, biomarker-driven treatment changes before patients clinically deteriorate.

ESR1 mutations in breast cancer are acquired alterations, meaning they can be missed by a single test. The speaker advocates for serial testing, especially after disease progression, using blood-based ctDNA analysis. This dynamic monitoring approach is essential for identifying patients who become eligible for targeted therapies over time.

Dr. Bardia emphasizes that ESR1 is an 'acquired alteration,' meaning the mutation can develop during treatment. This necessitates a shift from one-time diagnostic testing to a dynamic, serial testing model. Repeat testing is critical to identify these actionable mutations as they arise, allowing patients to access newly approved targeted therapies.

The SERINA-6 trial suggests a paradigm shift: proactively switching from an AI to an oral SERD upon detecting an ESR1 mutation in ctDNA—before clinical or radiographic progression—significantly improves progression-free survival and patient quality of life.

Clinicians must recognize that liquid and solid biopsies show significant discordance. ESR1 mutations are more frequently detected in liquid assays, while PIK3CA mutations are more often found in solid tissue. This variability by gene directly impacts the optimal testing strategy for patients.