[A Case of Palbociclib Plus Fulvestrant‒Resistant Metastatic Breast Cancer That Responded to Abemaciclib Plus Fulvestrant].

We report the case of a 72‒year‒old woman with ER(+), PgR(+), HER2(-)metastatic breast cancer with liver involvement. The patient received palbociclib(125 mg daily po 3 weeks on/1 week off)and fulvestrant(500 mg im on days 1, 15, and 29, and once monthly thereafter)as second‒line endocrine therapy. Her metastatic liver lesions initially decreased in size and the tumor‒marker CA15‒3 level decreased. However, they progressed after seven months of therapy. She was subsequently switched to abemaciclib(150 mg twice/daily po)with fulvestrant as third‒line therapy, resulting in a decrease in the same liver lesions. She has continued treatment for 12 months. Based on this case, abemaciclib may be clinically useful for breast cancer that is not responsive or has become resistant to palbociclib, another selective CDK4/6 inhibitor.