Editor's Note: At a recent international oncology symposium, Professor François-Clément Bidard from Institut Curie shared the updated results from the third data cut-off (DCO3) of the Phase III SERENA-6 study. The study focuses on patients with HR+/HER2- advanced breast cancer (ABC). During first-line treatment with an aromatase inhibitor (AI) combined with a CDK4/6 inhibitor (CDK4/6i), once a new-onset ESR1 mutation is detected via circulating tumor DNA (ctDNA) monitoring, the AI is immediately switched to Camizestrant, a novel oral selective estrogen receptor degrader (SERD), to assess clinical benefit. This report highlights key secondary endpoints—the final progression-free survival 2 (PFS2) and core data such as ctDNA kinetics analysis.01 Research Background: Challenges of Resistance Driven by ESR1 Mutations and Proactive Intervention Strategies
The first-line standard of care for HR+/HER2- advanced breast cancer is AI combined with CDK4/6i. However, under long-term exposure to an AI environment, approximately 30% to 40% of patients will develop acquired resistance due to ESR1 gene mutations. ESR1 mutations often lead to ligand-independent activation of the estrogen receptor (ER), thereby weakening the efficacy of AIs. In clinical practice, treatment regimens are often adjusted only after radiographic progression (PD), but by then, the tumor burden may have already increased and the resistance mechanisms become more complex, limiting the benefit of subsequent treatments. Camizestrant (CAMI) is a potent, next-generation oral SERD that has demonstrated superior efficacy compared to fulvestrant in the SERENA-2 study. The SERENA-6 study adopted an innovative “Switch” model: through dynamic ctDNA monitoring, it captures “molecular progression” before radiographic progression occurs. Patients are then randomized to either continue AI or switch to Camizestrant (both maintaining the original CDK4/6i), aiming to explore whether early intervention can improve long-term prognosis.
02 Study Design: A Precision Randomized Controlled Trial Based on ctDNA Monitoring
SERENA-6 is a multicenter, randomized, double-blind, placebo-controlled Phase III clinical trial. The study enrolled patients with HR+/HER2- ABC who had been receiving first-line treatment with an AI (anastrozole or letrozole) combined with a CDK4/6i (palbociclib, ribociclib, or abemaciclib) for at least 6 months. The core logic of the study lies in prospective ctDNA screening. Using Guardant360 sequencing, once a new-onset ESR1 mutation is detected in the plasma (and while radiographic evaluation has not yet reached PD), patients are randomized in a 1:1 ratio into the following two groups: • Camizestrant Group: Camizestrant (75mg/d) + maintenance of original CDK4/6i + AI placebo. • AI Group: Maintenance of original AI + maintenance of original CDK4/6i + Camizestrant placebo. The primary endpoint of the study was investigator-assessed PFS. Key secondary endpoints included PFS2 (defined as the time from randomization to second progression or death), time to start of chemotherapy, overall survival (OS), and patient-reported outcomes (PRO).
03 Final PFS2 Results: Significant Reduction in Risk of Subsequent Progression, Sustained Benefit
In the third data cut-off (DCO3) analysis released this time, the median follow-up time reached 23.5 months. Professor François-Clément Bidard pointed out that the study successfully achieved a statistically significant improvement in its key secondary endpoint, PFS2. • PFS Data Update: Following the preliminary results released last year, the DCO3 data once again confirmed the significant PFS benefit brought by switching to Camizestrant. The median PFS for the Camizestrant group was 16.8 months, compared to only 9.2 months for the AI group. The hazard ratio (HR) was 0.45 (95% CI: 0.35-0.59), with an absolute extension of median PFS by 7.6 months. At 24 months, 35% of patients in the Camizestrant group remained free of disease progression. • Core PFS2 Data: The median PFS2 for the Camizestrant group reached 25.1 months, significantly superior to the 18.5 months for the AI group. The HR value was 0.63 (95% CI: 0.48-0.81), with a highly statistically significant P-value. The absolute gain in median PFS2 between the two groups was 6.6 months. • Clinical Significance: The improvement in PFS2 means that switching to Camizestrant immediately upon detecting an ESR1 mutation in the first line provides benefits that are not offset by second-line therapy. This “early intervention” strategy builds a longer-lasting “moat” of disease control for patients. Additionally, the “Time to next subsequent treatment” analysis also showed consistent results, further validating the value of Camizestrant in extending the duration of benefit from endocrine therapy.
04 Subgroup Analysis and Co-mutation Characteristics: Camizestrant Demonstrates Broad-spectrum Benefit
The SERENA-6 study also explored the stability of Camizestrant across different molecular backgrounds. • Co-mutation Subgroups: Regardless of whether patients had PIK3CA or TP53 mutations at baseline, the Camizestrant group showed consistent PFS benefit. In the PIK3CA mutation subgroup, Camizestrant demonstrated strong control capabilities, suggesting the drug’s ability to overcome complex resistance environments. • ESR1 Mutation Types: Exploratory analysis for patients with single ESR1 mutations versus polyclonal (Multiple) ESR1 mutations showed that both groups benefited from switching to Camizestrant. This indicates that Camizestrant has potent degradation and inhibitory effects on various ESR1 resistance sites (such as D538G, Y537S, etc.).
05 Secondary Endpoints and Molecular Dynamics: Simultaneous Improvement in ctDNA Clearance and Quality of Life
Professor François-Clément Bidard emphasized the “dual superiority” of Camizestrant at both the molecular level and the level of patient subjective experience in his report. • ctDNA Kinetics Analysis: Guardant360 was used to evaluate total ctDNA levels. At weeks 4 and 8, total ctDNA levels in the Camizestrant group dropped significantly by 94% and 99%, respectively, while levels in the AI group rose by 16% and 64%. Notably, 51% of patients in the Camizestrant group achieved complete ctDNA clearance (i.e., undetectable), compared to only 1.9% in the AI group. Analysis confirmed that patients achieving ctDNA clearance were closely associated with longer PFS. • Chemotherapy-free Survival: Camizestrant significantly delayed the time to chemotherapy for patients, with a difference of 3.9 months between the two groups (HR 0.64), which is crucial for maintaining the quality of life for patients with advanced disease. • Patient-Reported Outcomes (PRO): Compared to maintaining AI therapy, patients switching to Camizestrant had a significantly lower risk of worsening cancer-related symptoms and functional decline, with particularly significant statistical differences in improvements in emotional functioning, pain relief, and shortness of breath. • Overall Survival (OS): Currently, OS data maturity is only 30%, which is still in the early stages. The current HR value is 0.87 (95% CI: 0.62-1.23), and the curve trend leans toward the Camizestrant group; longer follow-up is needed to observe the final benefit.
06 Expert Evaluation and Conclusion: Potential Reshaping of the Endocrine Therapy Paradigm
Professor François-Clément Bidard concluded that the SERENA-6 study is one of the most successful clinical trials exploring “intervention after molecular progression” in the field of breast cancer to date. The study not only confirms the potent efficacy of Camizestrant as a next-generation oral SERD but also validates the feasibility of guiding clinical decisions based on dynamic ctDNA monitoring. In terms of safety, Camizestrant demonstrated good tolerability with no new safety signals identified. Its most common adverse reactions (such as photosensitivity and bradycardia) were mostly Grade 1-2 and clinically manageable.
Concluding Recommendation:
For patients with HR+/HER2- advanced breast cancer receiving first-line AI + CDK4/6i treatment, regular ctDNA monitoring for ESR1 mutations holds significant clinical value. Once an ESR1 mutation is detected before radiographic progression, switching to Camizestrant combined with CDK4/6i therapy can significantly extend the patient’s PFS, PFS2, and chemotherapy-free survival, while better maintaining quality of life. The results of this study have not only been accepted for publication in The Lancet Oncology but are also expected to change clinical guidelines in the future, driving advanced breast cancer into an era of “precision switching guided by molecular monitoring.”
