Efficacy and toxicity of palbociclib in heavily treated patients with metastatic breast cancer

Background: Palbociclib is reported to improve progression-free survival (PFS) in patients with estrogen-receptor (ER)-positive, human epidermal growth factor 2 (HER2)-negative metastatic breast cancer (MBC). However, evidence of the effects of palbociclib in heavily treated patients is limited. The present study aimed to determine the effects and toxicity of palbociclib in heavily pretreated patients and to find the differences between the patients treated with palbociclib in upfront line and those with in later-line.Methods: This retrospective, single-center, observational study enrolled 26 patients with ER-positive, HER2-negative MBC who started palbociclib (125 mg) plus endocrine therapy between December 2017 and July 2018. These patients were divided into 2 groups, upfront (