Abstract P6-10-10: Genetic alterations in breast cancer associated with MDM2 dependency and sensitivity to the MDM2 inhibitor milademetan
Authors: Bidard Francois-Clement , Roufai Diana Bello , Medford Arielle J. , Tirunagaru Vijaya , Doebele Robert C. , Bardia Aditya 
Published: March 2023

Abstract
Background: Murine double minute 2 (MDM2), a potent negative regulator of p53, promotes tumorigenesis if dysregulated. MDM2 dysregulation occurs via different mechanisms, including MDM2 gene amplification, MDM2 overexpression, and loss of cyclin-dependent kinase inhibitor 2A (CDKN2A), which encodes the MDM2 regulator p14ARF. Combined inactivation of MDM2 and GATA3 in hormone receptor-positive (HR+) breast cancer is lethal to the cell. Pharmacologic inhibition of MDM2 is a rational therapeutic strategy for MDM2-dependent, TP53 wildtype (WT) tumors, including tumors with MDM2 amplification or CDKN2A loss, and GATA3-mutant HR+ breast cancers. We determined the frequency and associated characteristics of genetic alterations of MDM2-dependent breast cancers, and evaluated sensitivity of these tumors to the small-molecule MDM2 inhibitor milademetan (RAIN-32).
Methods: Genomic data were obtained from three datasets: METABRIC; TCGA PanCancer Atlas, GDC v23.0 (April 2020); AACR Genie, v11. Among TP53 WT breast cancer samples from each dataset, the frequency of GATA3 frameshift mutations, MDM2 amplification (copy number [CN] ≥12), and CDKN2A homozygous loss was determined individually and as co-alterations. The antitumoral activity of milademetan was evaluated in a GATA3-mutant, TP53 WT HR+ breast cancer cell line (MCF7 GATA3 G336fs*17), a breast xenograft model (MCF7 GATA3 G335fs), and ex vivo in MDM2-amplified patient-derived breast cancer organoids (CTG-2810, ER+/PR+/HER2–, MDM2 CN 8).
Results: Genetic alteration frequencies in TP53 WT breast cancers by dataset are shown in the Table. GATA3 frameshift mutations (7.3–11.7%), MDM2 amplification (0.3–1.1%), and CDKN2A loss (0.2–1.2%) occurred across breast tumors, but were found with highest frequencies in HR+ tumors. Co-alteration frequencies in TP53 WT breast cancers across the aforementioned datasets were &lt; 1%: GATA3 mutations/MDM2 CN ≥12 (0.2–0.3%); GATA3 mutations/CDKN2A loss (0.1–0.2%); MDM2 CN ≥12/CDKN2A loss (0%). Mean MDM2 expression (log2 (TPM+1)) in HR+ breast cancers (TCGA) were: GATA3 mutations, 5.12; CDKN2A loss, 5.88; MDM2 CN ≥12, 8.13, TP53 WT without these alterations, 4.78; mutant TP53, 4.35. A GATA3-mutant ER+ breast cancer cell line was sensitive to milademetan in vitro (IC50 126 nM). Milademetan 100 mg/kg displayed antitumor activity in GATA3-mutant HR+ breast xenograft and PDX models (p&lt; 0.05 vs. vehicle). Milademetan also displayed activity in MDM2-amplified HR+ breast cancer organoids (IC50 0.2 μM). In a phase I study (NCT01877382), a patient with heavily pretreated MDM2-amplified breast cancer (MDM2 CN 16.8) had tumor shrinkage (18.2%) and PFS of 7.3 months with milademetan (orally 260 mg 3/14 days).
Conclusions: The frequency of genetic alterations potentially targetable by MDM2 inhibition among TP53 WT breast cancers (i.e., GATA3 mutations, MDM2 amplification, and CDKN2A loss) is greatest in the HR+ subset, and these genetic biomarkers are associated with higher MDM2 expression. Preclinical data show that the MDM2 inhibitor milademetan has antitumor activity in GATA3-mutant and MDM2-amplified HR+ breast cancers, and support the clinical evaluation of milademetan in these tumors. Two clinical trials of milademetan – MANTRA-2 (Phase 2 basket study in solid tumors with TP53 WT and MDM2 CN ≥12; NCT05012397) and MANTRA-4 (Phase 1 study of milademetan + atezolizumab in solid tumors with CDKN2A loss) – are enrolling patients or in planning.
Table.
Citation Format: Francois-Clement Bidard, Diana Bello Roufai, Arielle J. Medford, Vijaya Tirunagaru, Robert C. Doebele, Aditya Bardia. Genetic alterations in breast cancer associated with MDM2 dependency and sensitivity to the MDM2 inhibitor milademetan [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-10-10.