Abstract P215: The MDM2 inhibitor milademetan induces synthetic lethality in GATA3 mutant, ER positive breast cancer

GATA3 is mutated, predominantly via frameshift (fs) mutations, in 15-18% of estrogen receptor (ER)-positive breast cancers and GATA3 mutations define a subset of patients with poor response to hormonal therapy. Importantly, GATA3 mutations are not currently targetable. Computational analysis of the data from the large-scale, deep RNAi screen project DRIVE led to the identification of MDM2 as a synthetic lethal partner in a GATA3 mutant ER+ breast cancer cell line. The objective of this study was to evaluate the activity of milademetan (RAIN-32), an orally available, small molecule inhibitor of MDM2 in GATA3 mutant ER+ breast cancer. Here, we evaluated the effect of MDM2 silencing in ER+, GATA3 mutant breast cancer cell line MCF-7 harboring a loss-of-function mutation (p.D335Gfs) that has been recurrently observed in breast cancer patients and tested the antitumoral activity of milademetan in vitro (MCF-7 cell line) ex vivo (patient derived organoids; PDO) and in vivo (cell line-derived xenograft and patient-derived xenograft (PDX) models). Additionally, we hypothesized that the synthetic lethality is p53 dependent and to this aim we explored the p53 dependent effects of milademetan inhibition by determining the effect of MDM2 loss in the absence of p53 and analyzing the expression of p53-target genes. Finally, we evaluated the impact of GATA3 mutations on survival in breast cancer using TCGA and AACR Genie datasets.MDM2 silencing significantly reduced cell proliferation in MCF-7. This effect was shown to be p53 dependent as p53 knockdown reversed the effect of MDM2 loss. MCF-7 cell line was also sensitive to milademetan in vitro. Finally, we tested milademetan in ER+ breast cancer models (PDO and a fulvestrant-resistant PDX) harboring a GATA3fs mutation and wild type TP53. Treatment of these models displayed dose-dependent anti-tumor activity to milademetan ex vivo and in vivo. Sequencing data analyses of the publicly available datasets revealed lower survival in GATA3 mutant breast cancer and mutual exclusivity with TP53 mutations. In conclusion, MDM2 deficiency leads to synthetic lethality in GATA3 mutant breast cancers. Milademetan shows evidence of antitumor activity in genetically selected ER+ breast tumors with GATA3 mutations. These data provide evidence for clinical exploration of milademetan in ER+, GATA3 mutant breast cancer refractory to current therapies. Citation Format: Vijaya G. Tirunagaru, Gaia Bianco, Charlotte K. Y. Ng, Elisabetta Marangoni, Salvatore Piscuoglio, François-Clément Bidard, Robert C. Doebele. The MDM2 inhibitor milademetan induces synthetic lethality in GATA3 mutant, ER positive breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P215.