Improving Survival in Advanced, ER-Positive Breast Cancer

Use of CDK4/6 inhibitors has changed the landscape for treatment of ER-positive, HER2-negative, metastatic breast cancer. Large randomized clinical trials comparing an endocrine agent with or without one of the three currently available CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) in the first-line and second-line setting of ER-positive metastatic breast cancer have consistently shown a significant improvement in progression-free survival with the addition of a CDK4/6 inhibitor. The overall survival advantage has been slower to become evident; clinical trials in different populations of patients with ER-positive metastatic breast cancer have shown statistically significant improvement in overall survival with the addition of abemaciclib and ribociclib, but not palbociclib.

Updated results from the industry-sponsored, phase 3 MONALEESA-2 trial now confirm a significant overall survival benefit with ribociclib. In the trial, postmenopausal patients with ER-positive HER2-negative advanced breast cancer were randomized to receive either placebo or ribociclib (21 days on,7 days off) in combination with letrozole as first-line therapy. At a median of 6.6 years of follow-up, mortality was 54.2% in the ribociclib group versus 65.6% in the placebo group. Median overall survival was improved by 12.5 months with the addition of ribociclib (63.9 vs. 51.4 months). The survival advantage with ribociclib occurred even though later use of a CDK4/6 inhibitor following disease progression was more common in the placebo group than the ribociclib group (34.4% vs. 21.7%). No new toxicity concerns were identified with longer follow-up.