PURPOSE: Many patients with HR+, HER2− early breast cancer (EBC) will not experience recurrence or have distant recurrence with currently available standard therapies. However, up to 30% of patients with high-risk clinical and/or pathologic features may experience distant recurrence, many in the first few years. Superior treatment options are needed to prevent early recurrence and development of metastases for this group of patients. Abemaciclib is an oral, continuously dosed, CDK4/6 inhibitor approved for HR+, HER2− advanced breast cancer (ABC). Efficacy and safety of abemaciclib in ABC supported evaluation in the adjuvant setting. METHODS: This open-label, phase III study included patients with HR+, HER2−, high-risk EBC, who had surgery and, as indicated, radiotherapy and/or adjuvant/neoadjuvant chemotherapy. Patients with four or more positive nodes, or one to three nodes and either tumor size ≥ 5 cm, histologic grade 3, or central Ki-67 ≥ 20%, were eligible and randomly assigned (1:1) to standard-of-care adjuvant endocrine therapy (ET) with or without abemaciclib (150 mg twice daily for 2 years). The primary end point was invasive disease-free survival (IDFS), and secondary end points included distant relapse–free survival, overall survival, and safety. RESULTS: At a preplanned efficacy interim analysis, among 5,637 randomly assigned patients, 323 IDFS events were observed in the intent-to-treat population. Abemaciclib plus ET demonstrated superior IDFS versus ET alone ( P = .01; hazard ratio, 0.75; 95% CI, 0.60 to 0.93), with 2-year IDFS rates of 92.2% versus 88.7%, respectively. Safety data were consistent with the known safety profile of abemaciclib. CONCLUSION: Abemaciclib when combined with ET is the first CDK4/6 inhibitor to demonstrate a significant improvement in IDFS in patients with HR+, HER2− node-positive EBC at high risk of early recurrence.

Adjuvant abemaciclib combined with endocrine therapy (ET) previously demonstrated clinically meaningful improvement in invasive disease-free survival (IDFS) and distant relapse-free survival (DRFS) in hormone receptor-positive, human epidermal growth factor receptor 2-negative, node-positive, high-risk early breast cancer at the second interim analysis, however follow-up was limited. Here, we present results of the prespecified primary outcome analysis and an additional follow-up analysis. This global, phase III, open-label trial randomized (1 : 1) 5637 patients to adjuvant ET for ≥5 years ± abemaciclib for 2 years. Cohort 1 enrolled patients with ≥4 positive axillary lymph nodes (ALNs), or 1-3 positive ALNs and either grade 3 disease or tumor ≥5 cm. Cohort 2 enrolled patients with 1-3 positive ALNs and centrally determined high Ki-67 index (≥20%). The primary endpoint was IDFS in the intent-to-treat population (cohorts 1 and 2). Secondary endpoints were IDFS in patients with high Ki-67, DRFS, overall survival, and safety. At the primary outcome analysis, with 19 months median follow-up time, abemaciclib + ET resulted in a 29% reduction in the risk of developing an IDFS event [hazard ratio (HR) = 0.71, 95% confidence interval (CI) 0.58-0.87; nominal P = 0.0009]. At the additional follow-up analysis, with 27 months median follow-up and 90% of patients off treatment, IDFS (HR = 0.70, 95% CI 0.59-0.82; nominal P < 0.0001) and DRFS (HR = 0.69, 95% CI 0.57-0.83; nominal P < 0.0001) benefit was maintained. The absolute improvements in 3-year IDFS and DRFS rates were 5.4% and 4.2%, respectively. Whereas Ki-67 index was prognostic, abemaciclib benefit was consistent regardless of Ki-67 index. Safety data were consistent with the known abemaciclib risk profile. Abemaciclib + ET significantly improved IDFS in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative, node-positive, high-risk early breast cancer, with an acceptable safety profile. Ki-67 index was prognostic, but abemaciclib benefit was observed regardless of Ki-67 index. Overall, the robust treatment benefit of abemaciclib extended beyond the 2-year treatment period.

Purpose: Ribociclib plus endocrine therapy (ET) demonstrated a statistically significant progression-free survival and overall survival (OS) benefit in the phase III MONALEESA-7 trial of pre-/perimenopausal patients with hormone receptor (HR)-positive (HR+), HER2-negative (HER2−) advanced breast cancer (ABC). The median OS was not reached in the ribociclib arm in the protocol-specified final analysis; we hence performed an exploratory OS and additional outcomes analysis with an extended follow-up (median, 53.5 months). Patients and Methods: Patients were randomized to receive ET [goserelin plus nonsteroidal aromatase inhibitor (NSAI) or tamoxifen] with ribociclib or placebo. OS was evaluated with a stratified Cox proportional hazard model and summarized with Kaplan–Meier methods. Results: The intent-to-treat population included 672 patients. Median OS was 58.7 months with ribociclib versus 48.0 months with placebo [hazard ratio = 0.76; 95% confidence interval (CI), 0.61–0.96]. Kaplan–Meier estimated OS at 48 months was 60% and 50% with ribociclib and placebo, respectively. Subgroup analyses were generally consistent with the OS benefit, including patients who received NSAI and patients aged less than 40 years. Subsequent antineoplastic therapies following discontinuation were balanced between the ribociclib (77%) and placebo (78%) groups. Use of cyclin-dependent kinase 4/6 inhibitors after discontinuation was higher with placebo (26%) versus ribociclib (13%). Time to first chemotherapy was significantly delayed with ribociclib versus placebo. No drug–drug interactions were observed between ribociclib and either NSAI. Conclusions: Ribociclib plus ET continued to show significantly longer OS than ET alone in pre-/perimenopausal patients, including patients aged less than 40 years, with HR+/HER2− ABC with 53.5 months of median follow-up (ClinicalTrials.gov, NCT02278120).