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SERENA-6 — Camizestrant + CDK4/6i, ctDNA-guided switch in emergent ESR1-mutant HR+/HER2− advanced breast cancer, 1L

Abstract: LBA1007 · Session: Oral Abstract Session, Breast Cancer—Metastatic, Tue Jun 2, 2026, 11:57 AM–12:09 PM CT · Presenter: François-Clément Bidard, MD, PhD (Institut Curie, Paris/Saint-Cloud) · Embargo: ASCO 2026

At a glance

  • Sponsor: AstraZeneca
  • PI / presenter: François-Clément Bidard (Institut Curie); co-authors incl. N.C. Turner (Royal Marsden), E.L. Mayer, Y.H. Park
  • NCT ID: NCT04964934
  • Phase: III, randomized, double-blind, placebo-controlled
  • Design: ctDNA-guided switching trial. Patients on 1L aromatase inhibitor (AI) + CDK4/6i were serially monitored by ctDNA (every 2–3 mo). On emergent ESR1 mutation without clinical/radiographic progression, randomized to switch the AI to camizestrant 75 mg (continuing the same CDK4/6i) vs continue AI + CDK4/6i. ~3,256 screened; 315 with emergent ESR1m randomized.
  • N: 315 randomized (camizestrant + CDK4/6i n=157; AI + CDK4/6i n=158)
  • Primary endpoint(s): Investigator-assessed PFS (met at primary readout, ASCO 2025). ASCO 2026 readout = final PFS2 (time to second progression / death on next line) + ctDNA-clearance analyses
  • Comparator: Continued aromatase inhibitor + CDK4/6i (palbociclib / ribociclib / abemaciclib)
  • Indication: HR-positive, HER2-negative advanced/metastatic breast cancer with emergent ESR1 mutation
  • Line: 1L (switch within first-line endocrine-based therapy, ahead of progression)

Headline result

At the primary readout (ASCO 2025), the ctDNA-guided early switch to camizestrant roughly doubled investigator-assessed PFS — median 16.0 vs 9.2 months, HR 0.44 (95% CI 0.31–0.60; P<.00001). At ASCO 2026, SERENA-6 reported its final PFS2 (time to second progression) plus ctDNA-clearance analyses, the key secondary endpoint: the early switch sustained benefit through the next line of therapy — PFS2 HR 0.52 (95% CI 0.33–0.81; P=.0038) (per ASCO LBA1007). ctDNA clearance on the camizestrant arm correlated with longer-term efficacy outcomes, reinforcing the molecular-guided early-switch concept.

Mechanism / class

Camizestrant is a next-generation oral selective estrogen receptor degrader (SERD) and pure ER antagonist with activity against wild-type and ESR1-mutant ER. ESR1 mutations are a dominant acquired-resistance mechanism to aromatase inhibitors in metastatic HR+ breast cancer. SERENA-6 pioneers ctDNA-guided intervention — switching the endocrine backbone (replacing the AI, continuing the CDK4/6i) at molecular relapse before radiographic progression, a "treat the mutation, not the scan" paradigm.

Discussant takeaway

Interpretation remains contested. In April 2026 the FDA ODAC voted 6–3 that the early-switch strategy had not shown clinically meaningful benefit (PFS2/OS immaturity, lack of post-progression and quality-of-life context), and the FDA decision date was extended to review additional ctDNA-clearance data — the analyses now presented at ASCO 2026. Trade-press framing (CancerNetwork) centers the unresolved question of whether switching at ESR1-mutation detection is genuinely better than waiting for radiographic progression, which the trial design (no progression-triggered crossover comparator) does not fully answer. The sustained PFS2 benefit is the key counter-argument that the early switch is not merely borrowing time.

Practice-changing?

Pending. The PFS and PFS2 signal is strong and the ctDNA-guided paradigm is novel and provocative, and the EMA CHMP has issued a positive opinion for camizestrant + CDK4/6i in 1L ER+ advanced breast cancer. However, the FDA ODAC vote against clinical benefit of the early switch (OS immaturity, the value of pre-progression switching, monitoring burden) keeps this short of established US practice until the FDA decision and mature OS land. Adoption also hinges on operationalizing serial ctDNA surveillance in routine 1L care.

Sources

  • ASCO Meeting Library (abstract LBA1007): https://www.asco.org/abstracts-presentations/259337/abstract
  • ASCO press release (SERENA-6): https://www.asco.org/about-asco/press-center/news-releases/switching-camizestrant-after-detection-esr1-mutation-serena-6
  • ASCO Daily News: https://dailynews.ascopubs.org/do/serena-6-ctdna-guided-treatment-switch-camizestrant-before-clinical-progression
  • ASCO abstract LBA1007 (Larvol Clin mirror): https://clin.larvol.com/abstract-detail/ASCO%202026/81908847
  • AstraZeneca — ASCO 2026 data showcase: https://www.astrazeneca.com/media-centre/press-releases/2026/astrazeneca-to-showcase-phase-iii-data-at-asco-2026.html
  • AstraZeneca — US FDA decision date extended (PFS2 topline): https://www.astrazeneca.com/media-centre/press-releases/2026/us-fda-decision-date-camizestrant-extended.html
  • OncLive — camizestrant switch + CDK4/6i: https://www.onclive.com/view/camizestrant-switch-plus-cdk4-6-inhibition-continues-to-outperform-soc-in-hr-her2-breast-cancer
  • Targeted Oncology: https://www.targetedonc.com/view/camizestrant-plus-cdk4-6-inhibitors-shows-pfs-benefit-in-hr-her2--breast-cancer
  • CancerNetwork (ASCO 2026 breast preview): https://www.cancernetwork.com/view/anticipated-data-and-practice-changing-trials-an-asco-2026-breast-cancer-preview
  • AJMC: https://www.ajmc.com/view/serena-6-trial-validates-molecular-guided-therapy-switch-to-camizestrant
  • Primary readout (JCO, LBA4 2025): https://ascopubs.org/doi/10.1200/JCO.2025.43.17_suppl.LBA4