This phase II trial is evaluating two drugs (oral GSK525762 tablets and a Fulvestrant injection) in patients with a breast cancer that has spread to other parts of the body and progressed after a previous treatmentA Phase I/II Dose Escalation and Expansion Study to Investigate the Safety, Pharmacokinetics, Pharmacodynamics and Clinical Activity of GSK525762 in Combination With Fulvestrant in Subjects With ER+ Breast Cancer

Exclusion

• Prior therapy with any Bromodomain and extra-terminal (BET) inhibitor, any selective estrogen receptor degrader (SERD) including fulvestrant, or inhibitors of the Phosphoinositide-3-kinase (PI3K)/ serine/threonine-specific protein kinase (AKT)/Mammalian Target of Rapamycin (mTOR) pathway.
• Prior therapy with more than one line of cytotoxic chemotherapy following diagnosis of advanced/metastatic disease.
• More than or equal to 3 lines of systemic anti-cancer therapy in the advanced or metastatic setting.

• Recent prior therapy, defined as: a. Any investigational or approved non-biologic anti-cancer drug within 14 days or five half-life (whichever is greater) prior to the first dose of GSK525762 and fulvestrant. b. Any nitrosoureas or mitomycin C within 42 days prior to the first dose of GSK525762 and fulvestrant c. Any anti-cancer biologic agents within 42 days prior to the first dose of GSK525762 and fulvestrant. d. Any radiotherapy within 14 days prior to the first dose of GSK525762 and fulvestrant. If the participant received radiotherapy <90 days prior to study treatment, the irradiated lesion cannot be the only lesion used for evaluating response.

  e. Any major surgery within 28 days prior to the first dose of GSK525762 and fulvestrant

• Concomitant active malignancy other than HR+/HER2- breast cancer
• Therapeutic-dose anticoagulation (e.g., warfarin, low-molecular weight heparin [LMWH], or novel oral anticoagulants) must be discontinued and coagulation parameters must be normalized prior to the first dose of GSK525762 and fulvestrant. Prophylactic anticoagulation, with low doses (per standard practice) of agents such as LMWH, direct thrombin inhibitors, or factor Xa inhibitors is permitted.
• Current use of a prohibited medication or planned use of any forbidden medications during treatment with GSK525762 and fulvestrant. This includes medications with significant risk of Torsades de pointes as well as those that are potent inducers or inhibitors of Cytochrome P3A4 (CYP3A4) enzymes.
• Evidence of severe or uncontrolled systemic diseases (e.g., unstable or uncompensated respiratory, hepatic, renal, cardiac disease, or clinically significant bleeding episodes). Any serious and/or unstable pre-existing medical (aside from malignancy), psychiatric disorder, or other conditions that could interfere with participant's safety, obtaining informed consent or compliance to the study procedures, in the opinion of the Investigator. a) Systolic blood pressure higher than 150 millimeters of mercury (mmHg) or diastolic blood pressure higher than 90 mmHg found on 2 separate occasions separated by 1 week, despite adequate therapy, will be defined as uncontrolled hypertension. b) Uncontrolled diabetes mellitus (despite therapeutic; compliance to intervention) as defined by a hemoglobin A1c (HbA1c) level more than 8% and/or occurrence of more than two episodes of ketoacidosis in the 12 months prior to the first dose of study drug.
• Participants with advanced/metastatic, symptomatic, visceral spread, that are at risk of life-threatening complications in the short term including participants with massive uncontrolled effusions (pleural, pericardial, peritoneal), pulmonary lymphangitis, and over 50 percent (%) of liver involvement in metastases.
• Symptomatic or untreated leptomeningeal or brain metastases or spinal cord compression.
• Cardiac abnormalities as evidenced by any of the following: Baseline QT interval corrected by Fridericia's formula (QTcF) interval >=480 milliseconds (msec); Clinically significant conduction abnormalities or arrhythmias; Presence of cardiac pacemaker or defibrillator with a paced ventricular rhythm limiting electrocardiogram analysis; History or evidence of current >=Class II congestive heart failure as defined by New York Heart Association (NYHA); History of acute coronary syndromes (including unstable angina and myocardial infarction), coronary angioplasty, or stenting within the past 3 months. Participants with a history of stent placement requiring ongoing antithrombotic therapy (e.g., clopidogrel, prasugrel) will not be permitted to enroll; Clinically significant cardiomegaly, ventricular hypertrophy, or cardiomyopathy.
• Current active liver or biliary disease (with the exception of Gilbert's syndrome or asymptomatic gallstones, liver metastases or otherwise stable chronic liver disease per investigator assessment).
• Presence of hepatitis B surface antigen (HBsAg) or positive hepatitis C antibody test result at screening.
• History of known human immunodeficiency virus (HIV) infection.
• Any serious known immediate or delayed hypersensitivity reaction(s) to GSK525762 or fulvestrant, or idiosyncrasy to drugs chemically related to the investigational drugs.
• Hemoptysis >1 teaspoon in 24 hours within the last 28 days.
• Concurrent use of non-steroidal anti-inflammatory drugs (NSAIDs) (except for cases where NSAIDs provide benefit over other analgesics and in these cases, consideration should be given to the prophylactic administration of a proton pump inhibitor) and high dose aspirin (allowed up to <=100 milligrams orally daily).
• Participants with history of known bleeding disorder(s) including clinically significant hemorrhage (e.g., gastrointestinal, neurologic), within the past 6 months.
• Any clinically significant gastrointestinal abnormalities that may alter absorption, such as malabsorption syndrome, chronic gastrointestinal disease, or major resection of the stomach and/or bowels that could preclude adequate absorption of the study medication.