Real-World Analysis of Breast Cancer Patients Qualifying for Adjuvant CDK4/6 Inhibitors

Adjuvant CDK4/6 inhibitors abemaciclib and ribociclib improved disease-free survival (DFS) added to endocrine therapy in hormone receptor (HR)-positive HER2-negative early breast cancer (EBC), in monarchE (NCT03155997) and NATALEE (NCT03701334) trials respectively. We assessed the proportion and outcome of EBC patients qualifying for adjuvant CDK4/6 inhibitors in the real-world. Consecutive female patients with HR-positive HER2-negative EBC between 1997 and 2017 from the Australian Capital Territory and South-East New South Wales Breast Cancer Treatment Group registry were analyzed. Patients eligible for abemaciclib had ≥4 axillary nodes involved or 1-3 nodes plus primary >5 cm or grade 3. Ribociclib eligibility was defined as node-positive and node-negative with primary >5 cm or >2 cm grade 3. Of 3840 patients, 671 (17.5%) were abemaciclib-eligible and 1587 (41.3%) ribociclib-eligible . The 5-year DFS was 77% and 94% in abemaciclib-eligible and noneligible registry patients respectively (HR 2.6, 95% CI 2.26-3.05, < .001). The 5-year DFS was 86% and 97% in ribociclib-eligible and noneligible registry patients respectively (HR 1.92, 95% CI 1.67-2.19, < .001). Compared with monarchE trial patients, abemaciclib-eligible registry patients were older (median 55 vs. 51 years), with lower nodal burden (≥4 nodes in 44% vs. 60%). There were more stage III cancers in NATALEE trial patients (60%) than ribociclib-eligible registry patients (24%). Many women with EBC will qualify for adjuvant CDK4/6 inhibitors (17.5% abemaciclib, 41.3% ribociclib) with resource and workforce implications. In the real-world setting, a greater proportion of adjuvant CDK4/6-eligible patients have lower stage disease, therefore the absolute benefit from treatment may be smaller than estimated by the trials.