In the open-label, phase 3 monarchE trial, 5,637 patients with early, high-risk ER-positive/HER2-negative breast cancer were 1:1 randomised to abemaciclib (150 mg twice daily for 2 years) plus endocrine therapy or endocrine therapy alone. Abemaciclib plus endocrine therapy demonstrated a significant improvement in invasive disease-free survival (iDFS) versus endocrine therapy alone (HR 0.747; P=0.0096), corresponding to a 25.3% reduction in the risk of an IDFS event (see Figure). The 2-year iDFS rates were 92.2% versus 88.7%, respectively. Consistent benefit was seen in all prespecified subgroups.
Likewise, in the phase 3 PALLAS trial, 5,760 patients were randomised to receive either 2 years of palbociclib with adjuvant endocrine therapy or endocrine therapy alone. After a median follow-up of 23.7 months (351 events), iDFS was similar between the two arms, with 3-year iDFS of 88.2% for palbociclib plus endocrine therapy, and 88.5% for endocrine therapy alone (HR 0.93).
Figure: Invasive disease-free survival results of monarchE [4]

- Turner NC, et al. N Engl J Med 2018;379:1926-1936.
- Sledge GW, et al. JAMA Oncol. 2019;6:116-124.
- Slamon DJ. et al. N Engl J Med 2020;382:514-524.
- Johnston SRD, et al. Abemaciclib in high risk early breast cancer. ESMO 2020 Virtual Meeting, abstract LBA5.
- Mayer EL, et al. PALLAS: A randomized phase III trial of adjuvant palbociclib with endocrine therapy versus endocrine therapy alone for HR+/HER2- early breast cancer. ESMO 2020 Virtual Meeting, abstract LBA12.
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