Abstract P3-05-16: Extreme ESR1 polyclonality, mutational dynamics and effects on treatment outcomes in patients with ER+ metastatic breast cancer

Abstract Background: Mutations in ESR1 are commonly acquired on endocrine therapy (ET) and confer variable resistance to estrogen-directed treatments. The impact of polyclonality, including the dynamics of ESR1-mutant clones, interactions with co-occurring mutations, and associations with treatment outcomes have not been fully explored. In this case series, we investigate the course of 6 patients (pts) treated for ER+ metastatic breast cancer (MBC) who demonstrate extreme ESR1 polyclonality. Methods: We conducted a retrospective case review by querying clinical databases across 3 academic cancer centers. We selected all pts with >= 10 concurrent ESR1 variants on any circulating tumor DNA (ctDNA) Guardant 360 test in MBC, defined as extreme ESR1 polyclonality. We explored prior lines of therapy, sites of metastases, frequency of ESR1 variants, commonly occurring co-mutations, and clonal changes in ESR1 in response to treatments. Results: In our preliminary cohort (n=6), 5/6 pts were post-menopausal at primary diagnosis (median age 53.3y; 59.5y at first metastatic recurrence). 5/6 pts were ER+/PR+/HER2-; 1pt was ER+/PR-/HER2-. 4/6 pts received adjuvant chemotherapy (3/4 ddACT, 1/4 TC); 4/6 received adjuvant AI for >= 4 years (1 pt declined therapy; 1 pt found to have de novo metastasis). All 6 pts had bone metastasis at first metastatic diagnosis; 2/6 pts also had lymph node and adrenal involvement. Pts had serial ctDNA testing (range 1-4). On initial ctDNA test, there was a range of 0-31 ESR1 clones with a median of 5.5. Pts received a median of 1 line of ET in the metastatic setting prior to first ctDNA test. The most frequent ESR1 clone was Y537N, followed by Y537S, L327R. Capecitabine use was associated with a decline in polyclonality from 10, 16 and 20 variants to 4, 8 and 9, respectively. In 1 pt, eribulin and abraxane did not have the same effect; ESR1 polyclonality was maintained. One pt with 1 ESR1 clone after 2.5 years of fulvestrant+palbociclib developed 15 additional clones after 1.5 years of an experimental SERD+everolimus. Another pt with 1 ESR1 clone after anastrozole+palbociclib developed 9 additional ESR1 clones after 10 months of AKT inhibition+anastrozole. This same pt had a reduction to 4 ESR1 clones on capecitabine with an expansion to 8 clones on a subsequent experimental ADC. The most frequent co-alterations were in FGFR1 and CCND1, both occurring in 3/6 pts. In 1 pt with 12 ESR1 variants, only TP53 was co-mutated; another pt with 16 ESR1 variants had only AKT1 co-mutated. 1/6 pts carried PIK3CA mutations; this pt had 31 ESR1 variants on initial ctDNA test with 3 PIK3CA clones. Tumor mutational burden will be reported at the conference. Conclusions: This is the first investigation of dynamic changes in extreme ESR1 polyclonality in association with treatment outcomes in pts with ER+ MBC. Capecitabine was associated with a decrease in detectable ESR1-mutant clones, while increased polyclonality was found at progression on SERD+everolimus, AKT-inhibitor+AI, and an ADC. Although extreme ESR1 polyclonality is rare, understanding mutational dynamics will improve our understanding of polyclonality more broadly and its impact on treatment resistance. Additional analyses of these and additional pts will be presented at the conference. Citation Format: Tess O'Meara, Emily Podany, Marla Lipsyc-Sharf, Annika Putur, Shannon McLaughlin, Teresa T. Ho, Maxwell R. Lloyd, Arielle J. Medford, Joyce A. O'Shaughnessy, Aditya Bardia, Cynthia X. Ma, Hyo Han, Seth A. Wander. Extreme ESR1 polyclonality, mutational dynamics and effects on treatment outcomes in patients with ER+ metastatic breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-05-16.