What changed
On 4 September 2026, the FDA granted accelerated approval to camizestrant (Etcamah) plus a continued CDK4/6 inhibitor for adults with HR-positive/HER2-negative locally advanced or metastatic breast cancer when an ESR1 resistance mutation is detected by an FDA-authorized blood ctDNA test during aromatase-inhibitor + CDK4/6 therapy.
The causal shift
The previous clinical trigger was usually radiographic progression. SERENA-6 instead screened blood every 2–3 months and randomized 315 patients whose ESR1 mutation emerged after at least 6 months of first-line therapy while imaging still showed no progression. Switching the endocrine component to camizestrant produced median PFS 16.0 months versus 9.2 months for continuing the aromatase inhibitor (HR 0.44; 95% CI 0.31–0.60). Guardant360 CDx is the companion diagnostic.
What this proves — and what it does not
This is the first FDA cancer-therapy approval guided by a resistance mutation detected in ctDNA before imaging progression. It supports molecular interception of one defined resistance mechanism, not universal ctDNA-guided switching. Overall-survival data were immature, and FDA explicitly says clinical benefit from intervening this early is not yet confirmed; confirmatory studies are required. The label also carries important cardiac and embryo-fetal safety warnings.