Changing road map
Invited discussant Kevin Kalinsky, MD, MS, from the Winship Cancer Institute at Emory University, Atlanta, commented that CDK4/6 inhibitors “have changed the road map for treating hormone receptor–positive, HER2-negative disease.”
To put the monarchE results in context, he compared them with those of the NATALEE trial, in which patients were randomized to endocrine therapy with or without the CDK4/6 inhibitor ribociclib (Kisqali). That combination was previously shown to provide a significant survival advantage for women with metastatic breast cancer.
In NATALEE, which included both high-risk and intermediate-risk patients with early breast cancer, the absolute difference in 3-year IDFS rates between the combination group and endocrine monotherapy groups was 3.3%.
To determine the ultimate value of combining a CDK4/6 inhibitor with endocrine therapy in early breast cancer, longer follow-up of both trials will be necessary, Dr. Kalinsky said.
“The reason that follow-up is critical for both of these studies is that for this subtype of breast cancer, based upon data including from the Early Breast Cancer Trialists Group, we can see approximately 50% of recurrences after the first 5 years, and we think of cytotoxic chemotherapy as benefiting patients within those first 5 years. And while we think of CDK4/6 inhibitors as being cytostatic drugs, we are seeing a carryover effect in which 2 years of abemaciclib is improving outcome at the 5-year landmark,” he said.
Questions that still need to be answered include the optimal duration of CDK4/6 inhibitor therapy, whether adjuvant therapy should be resumed when there are signs of renewed proliferation, and whether there would be a benefit to restarting CDK4/6 inhibitors when metastasis occurs.
The monarchE trial was sponsored by Eli Lilly and Co. Dr. Harbeck disclosed research funding and speaker’s bureau activity for Lilly and others, and a consulting or advisory role with Gilead, Roche, Sanofi, Sandoz, and Seagen. Dr. Kalinsky disclosed a consulting or advisory role with multiple companies, not including Lilly.