Abstract S4-01: A novel BRD4 inhibitor enhances endocrine therapy efficacy and circumvents endocrine-resistance in estrogen receptor-positive breast cancer models

Background: The selective estrogen receptor (ER) down-regulator (SERD) fulvestrant (Ful) antagonizes ER activity and degrades ER protein in a dose-dependent manner in the preclinical and clinical settings, though its efficacy is limited by an incomplete abolition of ER protein levels. Therefore additional strategies are needed to achieve a more complete suppression of ER level and activity. The bromodomain-containing protein 4 (BRD4), a member of the BET family that is required both for ESR1 gene expression and for ER-mediated gene transcription, represents an attractive therapeutic target for ER+ breast cancer (BC). Here, we investigated the efficacy of the novel BRD4 inhibitor GS-626510 (GS-6510) in a panel of ER+ BC parental and endocrine-resistant (EndoR) cell lines and in a patient-derived xenograft (PDX) model . Materials and Methods: The effects of GS-6510 (25nM – 290nM) alone or in combination with endocrine therapies were tested in ER+ MCF7, T47D, and ZR75-1 cell lines, as well as in their derivatives made resistant to estrogen (E2) deprivation (EDR), tamoxifen (TamR), or Ful (FulR). Cell growth (by methylene blue) after 6 days of GS-6510 and protein levels (by Western blot) after 2 days of GS-6510 were assessed. The in vivo efficacy of GS-6510, Ful, and the combination was tested in the ER+ HCBx34 PDX model. The mRNA levels of genes associated with cell cycle, ER signaling, and endocrine-resistance were assessed using the NanoString platform. Results: A dose-dependent inhibitory effect of GS-6510 was observed in all of the experimental settings. At a clinically relevant dose, GS-6510 reduced E2-stimulated cell growth and enhanced the efficacy of endocrine therapies in all parental cell lines. In the endocrine-sensitive HCBx34 PDX model, GS-6510 reduced tumor growth and, in combination with Ful, induced tumor regression and inhibited the expression of ER-dependent and cell cycle related genes including CCND1, MYC, and BCL2. Notably, the addition of GS-6510 to EndoR cell models that continue to rely on and express ER (MCF7 EDR and TamR) led to a substantial inhibition in cell growth (85%; 98%, respectively). Though the combination of GS-6510 and Ful was not associated with a significantly greater cell growth inhibition compared to GS-6510 alone in these models, a better suppression of ER levels was observed. Interestingly, GS-6510 also remained effective in EndoR models that exhibited an ER-independent growth, including all FulR lines, though its efficacy varied among the different cell lines and resistant derivatives. Conclusion: Our findings suggest that the epigenetic regulator BRD4 is a suitable target for therapeutic intervention in ER+ BC. The anti-tumor efficacy of GS-6510 in endocrine sensitive and especially in ER-dependent EndoR models is worthy of further clinical investigation. The growth inhibitory effects observed in some of the ER-independent EndoR models suggests that additional genes/pathways involved in endocrine resistance could be affected by GS-6510. Identifying these pathways and determining their predictive role are needed to guide patient selection for future clinical trials. Citation Format: De Angelis C, Nardone A, Cataldo ML, Fu X, Trivedi M, Yi S, Breckenridge D, Chamnsess GC, Vitorino P, Osborne CK, Schiff R. A novel BRD4 inhibitor enhances endocrine therapy efficacy and circumvents endocrine-resistance in estrogen receptor-positive breast cancer models [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr S4-01.