(A) MCF-7 Y537S cells were implanted s.c. into ovariectomized athymic, nude mice without exogenous β-estradiol supplementation. When tumors reached 150–200 mm³, mice were randomized (n = 8 tumors/arm) and received vehicle, OTX015 (100 mg daily by oral gavage, 6 days/week), fulvestrant (1 mg s.c. injection weekly), or OTX015 + fulvestrant as indicated. (B) WHIM20 cells harboring Y537S mutation were implanted s.c. into ovariectomized, NOD-SCID mice without exogenous β-estradiol supplementation. When tumors reached 150–200 mm³, mice were randomized (n = 6 tumors/arm) and received vehicle or OTX015 (100 mg daily by oral gavage, 6 days/week). The results were plotted as average tumor volume measured for each group ± SEM. (C) Reverse transcription-PCR for GREB1 in WHIM20 xenografts (n = 6) from vehicle and OTX015-treated mice. (D) Schematic representation of patient-derived xenograft explant assay. (E and F) WHIM20 explants (n = 3) were treated with vehicle, OTX015, JQ1, or I-BET762 at a 10 μM concentration for 48 hours, and effect on proliferation was measured by Ki-67 IHC staining. The quantification of the images was performed by the image analysis software ImageJ (NIH), and the results are shown as the ratio between the Ki-67⁺ area and the total area of the image. P values were determined using an unpaired, 2-tailed t test for pairwise comparisons. To correct for multiple comparisons, ANOVA with Dunnett’s test was used to compare various experimental arms with the vehicle arm. Nonparametric Wilcoxon rank-sum test with Bonferroni correction was used for comparing treatment groups.*P ≤ 0.05; **P ≤ 0.005; ***P ≤ 0.0005.