Abstract PO4-15-09: Unraveling the influence of progesterone receptor status on endocrine therapy sensitivity: insight from epigenetic and fragmentomics ctDNA profiling in estrogen receptor positive, HER2 negative metastatic breast cancer (MBC)

Abstract Background: In patients (pts) with estrogen receptor (ER) positive (pos), HER2 negative (neg) MBC, the absence of progesterone receptor (PR) expression is generally associated with a poorer endocrine therapy (ET) response and an unfavorable prognosis. However, real-time molecular changes during ET according to PR status are still not fully understood. The aim of this study was to examine the prognostic impact of PR and its association with the methylation levels of ESR1 promoters A (promA) and B (promB) at different timepoints in pts with ERpos HER2neg MBC who received first-line ET. Methods: Pts were enrolled in the prospective multicenter MAGNETIC.1 trial (CRO-2018-56) between January 2018 and January 2023. As first-line ET, pts received either fulvestrant or AIs with or without CDK4/6 inhibitors (CDK4/6i) as first-line ET. ctDNA samples were collected at baseline (T0) and at 3-month intervals (T3 and T6) and analyzed through methylation-specific (MS) droplet digital PCR (ddPCR), ddPCR fragmentomics and next generation sequencing (NGS). Clinico-pathological differences across the PR expression spectrum were analyzed using Fisher’s exact test. Matched pairs variations across timepoints (T0, T3, T6) of ACTBfragments distribution and ESR1 promA and promB were tested through Wilcoxon signed rank test. Results: Out of the 111 enrolled pts, 28 had PRneg MBC, of which 20 had an invasive ductal carcinoma (IDC) histotype, and 10 were diagnosed as de novo disease. Bone was the most common metastatic site both in the PRpos and PRneg subgroups (75% vs 73%), most pts received ET+CDK4/6i (96% vs 95%). ESR1 and PIK3CA mutations were respectively detected in 8% and 6%, and in 23% and 25% pts respectively for PRpos and PRneg. The prognostic impact of PR status was evident for both PFS (median 38 mos and 14 mos respectively for PRpos and PRneg, P = 0.0048) and OS (median not reached and median 33 mos respectively for PRpos and PRneg, P= 0.001). A significant increase in promB was observed at T3 vs T0 and at T6 vs T3 in the PRpos subgroup (respectively P=0.0158 and P 0.001) but not in the PRneg. When comparing T6 to T0, no significant differences were observed regardless of PR status. At T3, significant decreases were observed in both the PRpos and PRneg populations for ACTBshort (P 0.001 and P=0.03, respectively), ACTBmedium (P=0.0049 and P= 0.035, respectively) and ACTBlong(P 0.001 and P 0.001, respectively), compared to T0. In the comparison of T6 to T3, a significant variation in ACTBshort levels was observed in both subgroups (P 0.001 for PRpos and P=0.012 for PRneg). while a significant reduction in ACTBmedium levels was observed only in the PRpos population when comparing T6 to T0 (P 0.001). Conclusions: PRneg status was confirmed as an unfavorable prognostic factor with respect to PRpos both for PFS and OS, potentially due to different ET sensitivity. Epigenetic ctDNA profiling revealed different dynamics of ESR1 promB between PR-positive and PR-negative MBC, suggesting a distinct role of ESR1 in the onset of ET resistance within the two subgroups. Citation Format: Lorenzo Foffano, Elisabetta Molteni, Arianna Dri, Alessandra Franzoni, Linda Cucciniello, Lucia Da Ros, Silvia Buriolla, Silvia Bolzonello, Claudia Noto, Stefania Russo, Simon Spazzapan, Elena Nascimbeni, Brenno Pastò, Giada Targato, Serena Della Rossa, Marta Bonotto, Alessandro Marco Minisini, Giuseppe Damante, Barbara Belletti, Lorenzo Gerratana, Fabio Puglisi. Unraveling the influence of progesterone receptor status on endocrine therapy sensitivity: insight from epigenetic and fragmentomics ctDNA profiling in estrogen receptor positive, HER2 negative metastatic breast cancer (MBC) 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 PO4-15-09.