[Recurrent Breast Cancer with Bone Metastasis Effectively Treated with Abemaciclib plus Endocrine Therapy-A Case Report].

We report a case of recurrent breast cancer with bone metastasis in a premenopausal woman. A 46-year-old woman underwent mastectomy for right breast cancer 6 years ago. Histopathological diagnosis was invasive ductal carcinoma, T2N3aM0, stage ⅢC. She received adjuvant chemotherapy and irradiation followed by tamoxifen. Four and a half years after surgery, serum tumor marker levels elevated, and bone metastasis in the sacral region was revealed by PET-CT scan. After suppressing ovarian function with LH-RH agonist, we switched the endocrine therapy from tamoxifen to letrozole with a CDK4/6 inhibitor. Five months after starting administration of abemaciclib, the bone metastasis disappeared on PET-CT. The elevated tumor markers normalized and have continued to decrease. Abemaciclib combined with endocrine therapy was significantly effective as first-line treatment for premenopausal women with metastatic breast cancer.