A 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.

Abstract

TPS1114 Background: Advanced or metastatic ER+BC (estrogen receptor positive breast cancer) is an incurable illness that will prove fatal for most afflicted women. Current standards of care include endocrine, targeted, and chemotherapy. Preclinical data suggest that reducing expression of the estrogen receptor (ER) as well as other ER-responsive genes may provide therapeutic benefit for women for whom endocrine therapy alone has proven inadequate. The bromodomain (BRD) and extra-terminal (BET) family of proteins (BRD2, BRD3, BRD4 and BRDT) bind to acetyl-histone residues and epigenetically control transcription of genes driving cell survival and proliferation. In preclinical models, BET proteins reduce ER expression and down regulate ER-dependent gene expression and may be a novel therapeutic target in multiple tumors e.g. ER+BC. GSK525762 is a pan-BET inhibitor showing strong synergistic activity with fulvestrant in killing ER+BC cells in vitro and in xenograft models. The combination of BET agents with endocrine therapy may provide therapeutic benefit and restore sensitivity to ER targeting agents like fulvestrant. Methods: The study is a Phase I/II dose-escalation, expansion (Phase I) and randomized control (Phase II) study with oral administration of GSK525762 in combination with fulvestrant in advanced or metastatic ER+BC subjects, whose disease has progressed on prior treatment with ≥ 1 line of endocrine therapy. Phase I of the study is designed as parallel single arms to determine the recommended Phase 2 dose based on safety, tolerability, PK, and efficacy profiles in two distinct populations of ER+ BC: 1. Subjects with disease that progressed on anti-estrogen and/or ≥ 1 AIs, OR. 2. Subjects with disease that progressed on CDK4/6 inhibitor plus letrozole. Phase I will employ a Bayesian predictive adaptive design. Phase II of the study is a randomized, double-blind, placebo-controlled study, the composition of which will be selected at the end of Phase I. Patients must have received < 3 lines of anti-cancer therapy (≤1 line of chemo), measurable disease, and PS 0-1. Funding: GSK. Clinical trial information: NCT02964507.