• To compare the efficacy of fulvestrant in combination with palbociclib versus fulvestrant plus placebo in terms of the rate of Progression-Free Survival (PFS) at 1 year in postmenopausal women with HR-positive/HER2-negative metastatic breast cancer previously treated with endocrine therapy for at least 5 years and remaining disease free for more than 12 months following its completion or have "de novo" metastatic disease