Elective Discontinuation of CDK4/6 Inhibitors in Patients with Metastatic Hormone Receptor-Positive, Her-2-Negative Breast Cancer: A Retrospective Single-Center Experience.

Introduction Cyclin-dependent 4/6 kinase (CDK4/6) inhibitors given with endocrine therapy until disease progression are standard of care in the treatment of women with advanced HR-positive Her-2-negative breast cancer. No data are available if therapy can be safely de-escalated to endocrine monotherapy in patients with long-lasting disease control. Methods We performed a retrospective analysis on the clinical course of 22 patients at our center who received CDK4/6 inhibitors with aromatase inhibitors or fulvestrant. All patients had at least stable disease for >6 months and made a joint decision with their provider to electively discontinue CDK4/6 inhibitors. Best objective response (BOR) at treatment discontinuation, progression-free survival, and re-treatment characteristics were recorded. Results Of 138 patients who received CDK4/6 inhibitors as first- or second-line therapy at our center, 22 met the inclusion criteria. Median duration of CDK4/6 treatment was 18 months (range 6-45). BOR was complete response in 1, partial response in 8, and stable disease in 13 patients. After a median duration of endocrine monotherapy of 9.5 months (range 5-44 months), 6 of 22 patients had progressive disease (1 local relapse and 5 systemic progression). All patients with disease progression had at least stable disease to chemotherapy (N = 1) or re-treatment with CDK4/6 inhibitors (N = 4). Conclusion Elective discontinuation of CDK4/6 inhibitors is feasible in patients with long-lasting disease stabilization. This strategy should be evaluated in prospective trials.