Pharmacokinetic Model of Abemaciclib: Correlation With Severe Diarrhea as the Primary Toxicity Endpoint in Patients With Localized Hormone Receptor-positive Breast Cancer

Inclusion Criteria: * Women ≥ 18 years old * Having breast cancer of all histologies combined * Type Luminal A or B with positive hormone receptors (\>10% expression for estrogen receptor and/or progesterone receptor) and HER2 negative or low epidermal growth receptor (according to GEFPICS1 definition) * Stage 2 or stage 3 according to the international classification, translated into the SENORIF recommendation * Having undergone complete excision surgery (R0 on the invasive tumor and/or on the ductal entity in situ) after neoadjuvant chemotherapy or not * Defined as high risk of recurrence according to the Monarch-E study, at initial diagnosis of the disease: either ≥ 4 affected axillary lymph nodes (≥N2 involvement), or 1-3 affected axillary lymph nodes (≥N1 involvement) associated with an Elston Ellis grade 3 or a tumor ≥ 5 cm * Initiation of adjuvant treatment with abemaciclib in combination with hormone therapy * Patient ECOG performance status between 0 and 2 * Patients with a neutrophil count (NCC) defined as normal prior to the first dose of abemaciclib, i.e., an absolute NCC ≥ 1500/ mm3 (≥ 1.5 x 109/L) without granulocyte colony-stimulating factor (GCSF) injection within 15 days prior to laboratory testing, as well as a platelet count ≥ 100,000/mm3 and a hemoglobin level ≥ 8g/dL. * Patient with the psychological and mental capacity to understand the protocol and sign the consent form independently * Must be affiliated with the social security system or receive benefits through a third party * Have signed the study consent form after reading the information sheet Exclusion Criteria: * Hypersensitivity to any of the excipients listed in section 6.1 of the abemaciclib (Verzenios) SPC * History of treatment with an anti-CDK4/6 (palbociclib, ribociclib, abemaciclib) for any indication * History of invasive cancer of any histology within the last 2 years, except for superficial skin tumors, not considered to be in complete remission * Presence of functional or inflammatory colorectal disease (Crohn's disease, ulcerative colitis) causing chronic diarrhea (as defined by the WHO as at least 3 bowel movements per day and/or liquid stools for at least 1 month) * Patient who has undergone total gastrectomy or suffers from short bowel syndrome * Patient unable to sign the consent form for societal reasons (illiteracy) or somatic reasons (central nervous system disease). * Persons benefiting from enhanced protection, namely minors, persons deprived of their liberty by judicial or administrative decision, persons staying in a health or social care institution, adults under legal protection, and finally patients in emergency situations. * Pregnant or breastfeeding women, women of childbearing age who are not using highly effective contraceptive methods (e.g., double-barrier contraception) during treatment and for at least 3 weeks after stopping treatment (The duration of contraception required for concomitant treatments, if any, should also be taken into account.)