Abstract
OBJECTIVE: CDK4/6 inhibitors (CDK4/6is) in combination with endocrine therapy have secured a central role in the treatment of hormone receptor (HR)-positive advanced breast cancer (ABC) and have transformed the therapeutic landscape. Cross-line CDK4/6i therapy in which another CDK4/6i is continued after progression on a prior CDK4/6i may still offer advantageous therapeutic effects. Cross-line CDK4/6i therapy is an area of active investigation in the ongoing pursuit to improve outcomes for patients with HR+/human epidermal growth factor receptor 2 (HER2)- ABC. METHODS: This retrospective study enrolled 82 patients with HR+/HER2- ABC who were treated with cross-line CDK4/6is (abemaciclib, palbociclib, ribociclib, and dalpiciclib) after progression with another CDK4/6i. The primary endpoint was progression-free survival (PFS) according to version 1.1 of the Response Evaluation Criteria in Solid Tumors. Secondary endpoints included toxicity, objective response rate, disease control rate, and overall survival. Adverse events (AEs) were graded according to version 5.0 of the Common Terminology Criteria for Adverse Events, as promulgated by the U.S. Department of Health and Human Services. RESULTS: Eighty-two HR+/HER2- ABC patients who received cross-line CDK4/6i therapy from January 2022 to February 2024 were enrolled. The median age of the patients was 60 years. The median PFS of all patients was 7.6 months (95% CI, 5.9-9.2). Cox regression analysis identified lung metastasis and a switch to endocrine therapy following prior CDK4/6i therapy as independent predictive factors for PFS. Notably, patients who previously received abemaciclib and switched to palbociclib upon disease progression had a median PFS of 10.7 months. The strategy of transitioning to chemotherapy after progression on a prior CDK4/6i, then to a subsequent CDK4/6i merits further investigation. Hematologic toxicity was the most common grade ≥ 3 AEs. No instances of fatal safety events were observed. CONCLUSIONS: Cross-line CDK4/6i therapy is associated with significant clinical benefits and manageable safety profiles in patients with HR+/HER2- ABC, which underscores cross-line CDK4/6i therapy potential as an effective treatment strategy.