Even without formal trial data on sequencing, clinicians are beginning to prescribe one oral SERD after progression on another (e.g., vepdegestrant after imlunestrant). This practice is driven by a belief in targeting the driving ESR1 mutation and by patient-specific factors like toxicity aversion, highlighting a real-world clinical need.

When patients present with both ESR1 and PI3K mutations, treatment selection isn't based on a definitive molecular test. Instead, oncologists make a clinical judgment, inferring the dominant resistance pathway from factors like the duration of prior therapy to guide their choice of targeted agent.

A key distinction for oncologists is that PIK3CA mutations are typically "truncal" (present from baseline), whereas ESR1 mutations are "acquired" after exposure to aromatase inhibitors. This biological difference dictates when and how to test for each biomarker throughout a patient's treatment journey.

For the complex scenario of patients with both ESR1 and PIK3CA mutations, a combination approach shows significant promise. The AVERA trial, combining an oral SERD (gerodestrant) with an mTOR inhibitor (everolimus), addressed both pathways and demonstrated a median progression-free survival of over 10 months, suggesting a superior strategy to targeting a single mutation.

In patients with both ESR1 and PIK3CA mutations, cross-trial data shows a similar median PFS of about 5.5 months regardless of strategy (oral SERD vs. PI3K/AKT inhibitor combos). Given this comparable efficacy, experts recommend starting with the better-tolerated single-agent oral SERD, especially in asymptomatic patients.

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 currently struggle to decide between an oral SERD or a PAM inhibitor when both ESR1 and PAM pathway mutations are present. Dr. Wander frames this as a temporary problem that will be solved within five years by the arrival of combination therapies featuring next-generation versions of both drug classes, making the choice unnecessary.

While PI3K pathway alterations are linked to a poor prognosis, real-world data provides reassurance for a common clinical dilemma. Patients with co-mutations in both ESR1 and PI3K pathways still achieve significant benefit from elacestrant monotherapy, with progression-free survival (5.2-6.3 months) comparable to the overall population in the pivotal EMERALD trial.

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.