Response to Abemaciclib After 10 Lines of Therapy Including Palbociclib in Metastatic Breast Cancer: A Case Report With Literature Review

Metastatic breast cancer (BC) is considered incurable, and it is generally treated with sequential single-agent therapies to control it with palliative intent. Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) are used in the front-line setting of hormone receptor (HR)-positive, HER2-negative BC, and guidelines discourage the use of a second-line CDK4/6i after failure of first-line use of this class of drugs due to lack of data supporting this practice. We report a case of a postmenopausal woman with HR-positive and HER2-negative advanced BC who was treated with four lines of hormonal therapy and more than five chemotherapy regimens, with progression. Palbociclib was used in the sixth-line therapy and discontinued after 5 months. We then tried abemaciclib in the 11th-line setting, where it induced a response that lasted 16 months.