The ELEGANT trial enrolls all high-risk ER-positive patients, not just those with ESR1 mutations. The rationale is that unlike in metastatic disease, early breast cancer is fundamentally ER-driven. Elicestrin targets both wild-type and mutant ER, making the mutation status less critical for efficacy in this earlier setting.

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.

Comparing elacestrant (EMERALD) and imlunestrant (EMBER-3) is flawed because the patient populations were fundamentally different. EMERALD's patients were more heavily pretreated, a fact starkly illustrated by the standard-of-care arms' median Progression-Free Survival of 1.9 months versus 3.8 months in EMBER-3.

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.

Despite the promise of liquid biopsies for monitoring, the SERENA-6 trial revealed a significant challenge: fewer than 10% of screened patients developed a detectable ESR1 mutation. This low yield questions the efficiency and broad applicability of this serial screening strategy to guide treatment changes.

Post-approval studies of the oral SERD elacestrant confirm its clinical benefit in ESR1-mutant breast cancer. However, this real-world evidence also reveals a new insight: patients who have both an ESR1 and a PIK3CA mutation tend to have a shorter time on treatment, suggesting that the PIK3CA mutation may drive resistance to this therapy.

An ESR1 mutation locks the estrogen receptor in a permanently "on" state, independent of estrogen. This renders aromatase inhibitors (AIs) ineffective but means therapies that degrade the receptor itself, like SERDs, can still be effective treatment options.

The SERINA-6 trial supports a paradigm shift: proactively screening for ESR1 mutations via blood test and switching to camisestrant upon detection, even without radiological progression. This early switch based on molecular signals nearly doubled median progression-free survival from 9 to 16 months.