Approaches to Mutation Testing for Patients With HR+/HER2-low mBC and ET-Resistant Disease

Spring and Bhave discuss their approach to obtaining genomic profiling in metastatic HR+/HER2-low breast cancer. Both prefer comprehensive sequencing upfront on metastatic biopsies, even if not immediately actionable, as it can inform future treatment options. For example, knowing PIK3CA mutation status can guide considering alpelisib later, and pre-diabetes can be managed proactively.

Spring often sends ctDNA first for faster results, then decides if tissue testing is needed, acknowledging PIK3CA detection is more sensitive in tissue. At each progression, she typically sends ctDNA and considers tissue sequencing if concerns arise about biology changes.

Bhave agrees plasma testing is preferred for progressing patients as it's quicker and less invasive. However, she rebiopsies in cases of unexpected responses or discordant results to recheck receptor status and rule out changing disease biology. For HER2-0 patients progressing through multiple therapies, she may rebiopsy before considering trastuzumab deruxtecan to confirm some level of HER2 expression.

They emphasize the importance of testing for ESR1 mutations given the recent FDA approval of elacestrant, a novel oral SERD, based on the EMERALD trial. Spring notes the real-world challenge of identifying optimal candidates at varying points in their treatment journey. Bhave highlights a subgroup analysis showing longer PFS with elacestrant in ESR1-mutant patients who responded to prior CDK4/6 inhibition for >12 months, supporting its use post-progression on first-line AI/CDK4/6i in ESR1-mutant cases.

This summary was AI-generated and edited for clarity.