CDK4/6 inhibitors for primary endocrine resistant HR-positive/HER2-negative metastatic breast cancer: a case report

Background: We report a case of hormone receptor (HR) positive, human epidermal growth factor receptor-2 (HER2) negative breast cancer with multiple liver metastases who achieved good clinical benefit across cyclin-dependent kinase 4/6 (CDK4/6) inhibitors in combination with endocrine therapy. Prior to this, the patient underwent neoadjuvant therapy and surgery as well as adjuvant endocrine therapy. We present and discuss three important treatment decision nodes associated with it. Case Description: A 60-year-old woman was diagnosed with invasive ductal carcinoma of the left breast (cT4bN2M0, stage IIIB, Luminal B HER2-negative type) at the West China Hospital of Sichuan University in 2020, and received neoadjuvant chemotherapy and modified radical mastectomy of the left breast from 2020 to 2021. Postoperative radiotherapy was performed in the left chest wall and left upper and lower clavicular region. Adjuvant therapy is anastrozole endocrine therapy. Multiple liver metastases developed in 2022. The pathological molecular typing of liver metastases was confirmed to be consistent with the primary lesion. In the context of primary endocrine resistance, the first-line treatment of choice was fulvestrant in combination with a targeted CDK4/6 inhibitor (abemaciclib). This combination regimen made the liver metastases visibly shrunk, leading to partial response (PR) and has achieved a progression-free survival of 12 months. And there were no serious drug-related adverse events during first-line treatment. Conclusions: CDK4/6 inhibitor is a promising antineoplastic agent for HR positive, HER2 negative breast cancer patients. With the development of research, the application scope of CDK4/6 inhibitors is gradually expanding, and the precision treatment of breast cancer requires more rational drug selection and combination.