AstraZeneca announced that the SERENA-4 Phase III trial of camizestrant (Etcamah) in combination with Pfizer's Ibrance (palbociclib) has failed to meet its primary endpoint of progression-free survival (PFS) in the upfront first-line treatment of ER-positive, HER2-negative advanced breast cancer, AstraZeneca (LSE/STO/NYSE: AZN) reported on September 14. A numerical PFS improvement was observed but did not reach statistical significance versus anastrozole plus palbociclib.
The removes one potential path to broad first-line use for camizestrant. The drug received accelerated FDA approval on September 4 for a more restricted indication — use in combination with a CDK4/6 inhibitor upon detection of an ESR1 mutation during first-line aromatase inhibitor therapy, before radiographic progression — based on the Phase III SERENA-6 trial. That label is explicitly pre-progression and biomarker-guided, not a broad first-line replacement for aromatase inhibitors. SERENA-4 was testing the broader proposition: whether substituting camizestrant for anastrozole at the outset, before any ESR1 mutation emerges, would delay disease progression.
Camizestrant is an oral selective estrogen receptor degrader (SERD) that binds and degrades estrogen receptor alpha, including ESR1-mutant forms that confer resistance to aromatase inhibitors, while providing complete ER antagonism without partial agonist activity. The rationale for first-line use was that deeper, earlier ER suppression might delay the emergence of resistance — a hypothesis the SERENA-4 data do not support at the level of statistical significance.
The result places camizestrant alongside Roche's giredestrant, which similarly produced only a numerical 11% improvement over letrozole plus palbociclib in the first-line Phase III persevERA trial. The two failures raise questions about whether replacing an aromatase inhibitor with an oral SERD from the outset provides enough additional benefit in endocrine-sensitive disease, where ESR1 mutations are uncommon at baseline.