Cyclin-dependent kinase (CDK) 4/6 inhibitors have changed the treatment pattern of patients with hormone receptor-positive HER2-negative advanced breast cancer significantly prolongling patients'PFS and OS,but there is no optimal therapy after its progression.Tucidinostat (formerly known as chidamide) is an oral subtype-selective histone deacetylase inhibitor, ACE trial demonstrated that tucidinostat plus exemestane improved PFS compared with placebo plus exemestane in HR+,MBC patients that progressed after previous endocrine therapy,only 7 patients enrolled in the trial had previously received CDK4/6 inhibitors.The aim of this trial is to evaluate the efficacy of tucidinostat-based therapy in hormone receptor-positive metastatic breast cancer patients previously treated with CDK4/6 inhibitor therapy in the real world.
