Everolimus plus endocrine therapy beyond CDK4/6 inhibitors progression for HR+ /HER2- advanced breast cancer: a real-world evidence cohort.

Purpose: Everolimus in combination with endocrine therapy (ET) was formerly approved as 2nd-line therapy in HR(+)/HER2(-) advanced breast cancer (aBC) patients (pts) progressing during or after a non-steroidal aromatase inhibitor (NSAI). Since this approval, the treatment landscape of aBC has changed dramatically, particularly with the arrival of CDK 4-6 inhibitors. Endocrine monotherapy after progression to CDK4/6 inhibitors has shown a limited progression-free survival (PFS), below 3 months. Evidence of the efficacy of everolimus plus ET after CDK4/6 inhibitors is scarce.

Methods: A retrospective observational study of patients with aBC treated with everolimus and ET beyond CDK4/6-i progression compiled from February 2015 to December 2022 in 4 Spanish hospitals was performed. Clinical and demographic data were collected from medical records. The main objective was to estimate the median progression-free survival (mPFS). Everolimus adverse events (AE) were registered. Quantitative variables were summarized with medians; qualitative variables with proportions and the Kaplan-Meier method were used for survival estimates.

Results: One hundred sixty-one patients received everolimus plus ET (exemestane: 96, fulvestrant: 54, tamoxifen: 10, unknown: 1) after progressing on a CDK4/6 inhibitor. The median follow-up time was 15 months (interquartile range: 1-56 months). The median age at diagnosis was 49 years (range: 35-90 years). The estimated mPFS was 6.0 months (95%CI 5.3-7.8 months). PFS was longer in patients with previous CDK4/6 inhibitor therapy lasting for > 18 months (8.7 months, 95%CI 6.6-11.3 months), in patients w/o visceral metastases (8.0 months, 95%CI 5.8-10.5 months), and chemotherapy-naïve in the metastatic setting (7.2 months, 95%CI 5.9-8.4 months).

Conclusion: This retrospective analysis cohort of everolimus plus ET in mBC patients previously treated with a CDK4/6 inhibitor suggests a longer estimated mPFS when compared with the mPFS with ET monotherapy obtained from current randomized clinical data. Everolimus plus ET may be considered as a valid control arm in novel clinical trial designs.