The updated results showed the camizestrant combination reduced the risk of disease progression or death by 55% versus an AI plus a CDK4/6 inhibitor (based on a hazard ratio [HR] of 0.45; 95% confidence interval [CI] 0.34-0.59; p<0.00001). Median PFS was 16.8 months for the camizestrant combination compared with 9.2 months for the AI combination, representing a median improvement of 7.6 months.

Importantly, the PFS benefit observed with the camizestrant combination was sustained beyond initial progression. For the key secondary endpoint of PFS2, a measure of treatment durability beyond first progression, the final PFS2 analysis showed that the camizestrant combination reduced the risk of second disease progression or death by 37% versus the comparator arm (HR 0.63; 95% CI 0.46-0.86; p=0.00373), indicating that the benefit of switching to camizestrant plus a CDK4/6 inhibitor was maintained even after patients received subsequent therapies.​ Median PFS2 was 25.7 months for the camizestrant combination compared with 19.1 months for the AI combination.

The camizestrant combination also demonstrated substantially greater reductions in total ctDNA in blood than continued treatment with an AI plus a CDK4/6 inhibitor at week 4 and/or week 8 after randomization. Patients who switched to the camizestrant combination had a median 99% reduction in total ctDNA by week 8, with 51% achieving total ctDNA clearance, compared with a median 64% increase in total ctDNA by week 8, and 1.9% total ctDNA clearance among patients who remained on the AI combination. These results show the early effect of switching to camizestrant on ctDNA which is linked to tumor burden reduction.

Clearance of total ctDNA during treatment has been associated with long-term clinical benefit including improved overall survival (OS) across tumor types, including in patients with HR-positive, HER2-negative advanced breast cancer receiving endocrine-based therapy plus a CDK4/6 inhibitor.2,3 In an exploratory analysis pooled across both arms in SERENA-6, total ctDNA clearance was associated with OS benefit (HR 0.39; 95% CI 0.19-0.73), consistent with other studies.

Data for the key secondary endpoint of OS showed a numerical trend favoring the camizestrant combination (HR 0.87; CI 0.57-1.30) at 30% maturity. ​The trial will continue to final analysis to assess OS.