Background: phase III RCT included 5,637 patients with high risk HR+ Her 2-, LN+, high risk   breast cancer (>=4 axillary lymph nodes or 1-3+LN with T>5cm or histologic grade 3, or Ki-67 >20%). Exclusions: metastatic, inflammatory and LN- breast cancer.

Arm A: Abemaciclib 150mg BID on a continuous dosing schedule for 2 years plus endocrine therapy (ET) for 5-10 years
Arm B: ET alone for 5-10 years

Primary end point: Invasive disease-free survival (IDFS)
mFollow up: 15.5mo

2- yr IDFS rate: 92.2% vs 88.7%, arm A vs B respectively; HR 0.75, 95%CI 0.60-0.93, P=.01

Main adverse events: Grade >=3 A vs B: 12.3% vs 7.2%, VTE 2.3% vs 0.5%, interstitial lung disease 2.7% vs 1.2%. Most common AEs in abemaciclib arm were diarrhea, neutropenia, and fatigue. Treatment discontinuation: Abemaciclib 16.6%.  Abemaciclib and ET 6.6%, ET 0.8%.

Conclusions: Abemaciclib when combined with ET is the first CDK4/6 inhibitor to demonstrate a significant improvement in IDFS in patients with HR+, HER2-, LN+, high-risk EBC.

Summarized by Thejus Jayakrishnan, MD

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