This is a combination Phase I and Phase II study, with an aim to evaluate the combination of GSK525762 and fulvestrant in women with HR+/HER2- advanced or metastatic breast cancer, who have disease that has progressed after prior treatment with at least one line of endocrine therapy. The objectives of the study are to first identify, in open-label single-arm Phase I, a recommended Phase II dose of GSK525762 that may be combined safely with fulvestrant. Phase I will follow a modified toxicity probability interval (mTPI) design, and a sentinel group will be evaluated first for dose-limiting toxicity and further expanded to collect additional safety data. This will be followed by a double-blind, randomized controlled Phase II, to identify the clinical activity of the two study treatments when given in combination. The composition of Phase II will be selected at the end of Phase I. Approximately, up to 140 subjects and 154 subjects will receive study treatment in Phase I and Phase II respectively. A completed subject will be one who is followed until death.