Abstract PS11-01: [18F]Fluoroestradiol (FES)-PET as a predictive measure for endocrine therapy in patients with newly diagnosed metastatic breast cancer: Results from EAI142

Hannah Linden; Fenghai Duan; Farrokh Dehdashti; Amy M Fowler; Amy S Clark; Jennifer M Specht; Jian Q Yu; Hsiaoj Lee; Lanell M Peterson; Mark Muzi; Jennifer S Winn; Hayley Knollman; Mark E Burkard; Foluso O Ademuyiwa; Rathan M Subramaniam; Lucy Hanna; Angela M DeMichele; Jonathan E McConathy; Antonio C Wolff; David A Mankoff

doi:10.1158/1557-3265.SABCS24-PS11-01

Background: Multiple single institution studies have shown that FES-PET predicts clinical benefit of endocrine based therapy for patients with metastatic breast cancer (MBC). ECOG-ACRIN trial EAI142 is a multicenter imaging biomarker study of FES-PET to assess baseline FES uptake qualitatively and quantitatively prior to initiation of first-line endocrine therapy with or without CDK4/6 inhibitors in MBC. The primary objective is to determine the negative-predictive value (NPV) of FES uptake for predicting clinical benefit in patients with estrogen-receptor positive (ER+) MBC at 6 months after treatment with endocrine based therapy. In this analysis, we present the primary analysis: the imaging results and 6-month follow-up data in the fully enrolled cohort. Methods: From 2016-2022, 14 centers enrolled 105 female patients; 98 completed study procedures and were treated with endocrine based therapy. [18F]Fluorodeoxyglucose (FDG)-PET at baseline was encouraged, but not required, and was obtained in 66 patients. SUVmax of up to 5 of the most prominent lesions were recorded and qualitatively assessed for FES uptake in all patients. A sub-set analysis was done using FDG to guide lesion selection in the 66 patients who had both FES and FDG scans with the goal to better identify FES-negative lesions and heterogeneity in FES uptake. Patients were followed for progression-free survival (PFS) at 6 months (primary endpoint) and at 2 years (secondary endpoint). To determine the reproducibility of quantitative assessment of tumor FES uptake, 10 patients underwent a second FES-PET scan within 12 days of the first scan and prior to initiation of treatment in a test-retest sub-study. Results: Of the 450 lesions identified at baseline, the majority were in bone or soft tissue (262 bone, 124 lymph node, 40 visceral). Most patients had 5 or more evaluable lesions (83/98, 85%). The majority of patients were postmenopausal (80/98, 82%) and treated with an aromatase inhibitor (69/98, 70%) and CDK4/6 inhibitor (73/98, 74%). We identified a wide range in uptake (SUVmax of 1.2-44.7), but only 19/98 (19%) patients had any lesions with qualitatively mild, minimal or absent FES uptake. No patient had an average SUVmax for the most prominent lesions less than the pre-specified cut-off of 1.5, therefore an NPV was unattainable. A total of 16/98 (16%) patients progressed at 6 months. Of 4 patients with heterogenous uptake, 1 (25%) progressed at 6 months. In the sub-set analysis, FDG -guided lesion location did not identify any additional FES-negative lesions. We observed excellent reproducibility in 79 test-retest lesions (SUVmax 1.5-20.2) with a lesion level intra-class correlation coefficient of 0.94. Two-year PFS will be updated at the time of presentation. Conclusions: In patients undergoing first-line therapy for ER+ MBC, we evaluated ER expression by both qualitative and quantitative FES-PET. NPV could not be assessed due to a lower than expected proportion of FES-negative scans and rate of progressive disease at 6 months needed to meet the assumptions used for power calculations. Test-retest studies confirmed quantitative overall and lesional reproducibility of FES-PET. Citation Format: Hannah Linden, Fenghai Duan, Farrokh Dehdashti, Amy M Fowler, Amy S Clark, Jennifer M Specht, Jian Q Yu, Hsiaoj Lee, Lanell M Peterson, Mark Muzi, Jennifer S Winn, Hayley Knollman, Mark E Burkard, Foluso O Ademuyiwa, Rathan M Subramaniam, Lucy Hanna, Angela M DeMichele, Jonathan E McConathy, Antonio C Wolff, David A Mankoff. [18F]Fluoroestradiol (FES)-PET as a predictive measure for endocrine therapy in patients with newly diagnosed metastatic breast cancer: Results from EAI142 [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 PS11-01.

Abstract PS11-01: [18F]Fluoroestradiol (FES)-PET as a predictive measure for endocrine therapy in patients with newly diagnosed metastatic breast cancer: Results from EAI142

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