Abstract PO4-15-04: Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA

Abstract Background: MBC is usually classified based on ER and HER2 status. Although the absence of PR expression is commonly associated with a poorer response to endocrine therapy (ET) and an unfavorable prognosis, data regarding the underlining molecular features that occur in this subgroup are still unclear. The aim of this study was to investigate genomic differences between ER/PRpos HER2neg (PRpos) and ERpos PR/HER2neg (PRneg) MBC through circulating tumor DNA (ctDNA) profiling in a large multicenter consortium. Methods: This retrospective study analyzed a cohort of 1089 patients at Weill Cornell Medicine, Northwestern University, Massachusetts General Hospital, and Washington University in St. Louis with HER2neg MBC and ctDNA testing by Guardant360. Oncogenic pathway status (i.e., RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, P53, cell cycle, notch, and PI3K) was defined based on previous research (Sanchez-Vega et al. Cell, 2018). Associations across single nucleotide variations (SNVs), copy number variations (CNVs), pathway classification, and ER/PR status were tested by multinomial logistic regression and corrected for significant clinical characteristics (i.e., lines of treatment, metastatic sites). Prognosis was analyzed through Cox regression for overall survival (OS) defined from time of ctDNA collection. Results: Among the 1089 analyzed patients, PRpos was the most represented subtype (N:580, 53%), followed by PRneg (N:300, 28%) and Triple Negative (TNBC) (N:209, 19%). In the hormone receptor positive subgroup, bone (N:663, 75%), liver (N:323, 37%) and lymph nodes (N:314, 36%) were the main sites of distant involvement. The TNBC group was exposed to significantly fewer lines of therapy (RRR:0.37, P= 0.005 for ≥5 lines). As compared to PRneg, TNBC had less frequent and PRpos had more frequent bone involvement (RRR:0.44, P 0.001 and RRR:1.80, P 0.001, respectively) and less frequent liver involvement (RRR:0.64, P=0.019 and RRR:0.71, P=0.020, respectively for TNBC and PRpos). After multivariable analysis, a significant difference with respect to PRneg was observed in SNVs for the PI3K and P53 pathways for TNBC (RRR 0.49 P = 0.013 and RRR 4.06 P 0.001, respectively) but not for PRpos. On a single gene level, a lower incidence in CDH1 SNVs (RRR 0.09 P = 0.036) and a higher incidence of ESR1 SNVs (RRR 0.09 P = 0.036) was present in PRpos compared to PRneg. TNBC, on the other hand, was characterized by a lower prevalence of KRAS SNVs (RRR 0.09 P = 0.036) and PIK3CA SNVs (RRR 0.25 P 0.001) and a higher prevalence of TP53 SNVs (RRR 0.09 P 0.001). No ESR1mutations were detected in TNBC. Median OS was 15 months (mo) for TNBC, 21 mo for PRneg and 31 mo for PRpos (P 0.001). The differential prognostic impact of ctDNA features on OS was then analyzed across the PRpos and PRneg subpopulations. ESR1 SNVs was prognostic for both subgroups in univariable analysis (HR 2.19 P 0.001 and HR 1.96 P = 0.001 for PRpos and PRneg, respectively). In multivariable analysis corrected for lines of therapy, ESR1 SNVs retained its significance only in the PRpos subgroup (HR 1.41 P = 0.021) together with NF1 SNVs (HR 2.04 P = 0.034). On the other hand, TP53 SNVs had a significant impact on OS in the PRneg subgroup (HR 2.02 P = 0.014). Conclusions: Our study confirmed the prognostic impact of PR status in ER positive, HER2 negative MBC. Moreover, it suggested a differential profile in terms of ctDNA-detectable genomic features and their impact on survival, including well-established markers such as ESR1 SNVs, with a potential relevance for disease subtyping and future algorithms for drug development trials. Citation Format: Lorenzo Gerratana, Andrew Davis, Arielle Medford, Carolina Reduzzi, Katherine Clifton, Emily Podany, Whitney Hensing, Marko Velimirovic, Ami N. Shah, Lorenzo Foffano, Laura Munoz Arcos, Eleonora Nicolò, Charles S. Dai, Jennifer Keenan, Amir Behdad, Seth Wander, William Gradishar, Cynthia Ma, Fabio Puglisi, Aditya Bardia, Massimo Cristofanilli. Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-15-04.