Retrospective Observational Study of the Safety and Toxicity Management of Abemaciclib in Combination with Adjuvant Hormone Therapy in Patients with RH+ ,HER2-nonoveramplified Breast Cancer, Real-life Data (MONARCHE29)

Overall survival at 8 years under treatment for localized hormone-dependent breast cancer is 93.3% (1). Adjuvant therapy, especially hormone therapy, helps reduce the risk of recurrence. However, the risk of relapse remains significant, estimated at around 20% according to studies. The SOFT study, which compares the type of hormone therapy used in premenopausal patients, estimates a relapse risk of 21.1% at 8 years (1), especially when there is initial lymph node involvement. In fact, in cases of lymph node involvement, the cumulative relapse rate at 10 years after stopping hormone therapy ranges between 19% and 36% (2), and the risk of death from breast cancer 20 years after stopping hormone therapy is estimated at 28% to 49% (2). CDK4/6 inhibitors first demonstrated their efficacy at the metastatic stage. Abemaciclib improved median survival to 46.7 months compared to a median of 37.3 months with hormone therapy alone (Monarch 2 (3) and Monarch 3 (4)). Palbociclib showed in PALOMA-2 (5) an improvement in progression-free survival (24.8 months versus 14.5 months) without an improvement in overall survival. Ribociclib, in turn, demonstrated in MONALEESA 2 (6) an improvement in PFS (25.3 months versus 16 months) and in overall survival (63.9 months versus 51.4 months). These treatments have become the standard first-line treatment for patients with RH+ HER2 non-amplified breast cancer. Given the results in advanced lines, CDK4/6 inhibitors have been the subject of studies in localized breast cancer, particularly in this high-risk population where the recurrence rate remains significant. The MONARCH-E study, published on September 20, 2020 (7), led to the approval of Abemaciclib by European authorities at the time of the initial publication (median follow-up of 15.4 months) and to reimbursement starting in May 2023 after a second interim analysis (8) in this at-risk population, with a 5.6% reduction in relapse risk after 42 months of follow-up compared to hormone therapy alone. It is crucial to clearly define the at-risk population in order to offer them treatment intensification while maintaining a satisfactory quality of life. The group benefiting from Abemaciclib presented grade III toxicity in 43% of cases and grade IV toxicity in 2.5%. Real-world data are needed to better understand the management and toxicity of this treatment.