Abstract PD4-08: Real-world biomarker testing patterns in US patients with HER2-negative metastatic breast cancer

Abstract Background: International guidelines recommend that all patients (pts) with metastatic breast cancer (mBC) are offered testing for germline BRCA1/BRCA2 mutations (gBRCAm) and other biomarkers to aid treatment decisions. This retrospective study describes real-world testing patterns for germline/tumor (g/t)BRCAm and other biomarkers among US pts with HER2-negative [HER2−] mBC. Methods: Data were captured from US pts aged ≥18 years with HER2− mBC from January 12, 2018, to March 31, 2024, in the deidentified, electronic health record-derived Flatiron Health Research Database. Demographics, clinical characteristics, and testing patterns for g/tBRCAm and other actionable biomarkers were examined. Testing rates, test timing, and biomarker status were described across subgroups defined by demographics (including age, race, and ethnicity), tumor subtype (hormone receptor-positive [HR+]/HER2− mBC or triple-negative mBC [mTNBC]), and disease stage at mBC diagnosis (recurrent/de novo). Results: The BRCA testing rate among pts with mBC increased from 2018 (48%) to 2023 (62%); the overall testing rate was 19,820/34,379 (58%): 5880 pts (17%) received a gBRCA test only, 7285 (21%) received a tBRCA test only, and 6655 (19%) received both gBRCA and tBRCA tests. BRCA testing rates were numerically lower in pts with HR+/HER2− mBC (56%) versus mTNBC (68%), and in pts aged >65 years (HR+/HER2− mBC, 48%; mTNBC, 62%) versus ≤65 years (HR+/HER2− mBC, 64%; mTNBC, 74%; Table). The proportion of pts who received a test before initiation of first-line (1L) therapy was lower among pts with HR+/HER2− mBC than with mTNBC (recurrent, 60% vs 81%; de novo, 37% vs 57%). Among tested pts, 1067/15,168 (7%) with HR+/HER2− mBC and 494/4539 (11%) with mTNBC had a positive result for g/tBRCAm; 365/1067 (34%) pts with g/tBRCAm HR+/HER2− mBC and 138/494 (28%) pts with g/tBRCAm mTNBC were aged >65 years. Among 528 pts with gBRCAm HR+/HER2− mBC, co-mutations in PIK3CA, ESR1, PTEN, and AKT1 were identified in 48 (9%), 38 (7%), 20 (4%), and 3 (<1%) pts, respectively. Among pts diagnosed with gBRCAm HR+/HER2− mBC in 2020-2023 (median follow-up: 15 months), 175/371 (47%) did not receive a test for PIK3CA mutations at any time, and 48/97 (49%) pts diagnosed in 2023 (median follow-up: 6 months) did not receive a test for ESR1 mutations. In 2021-2023, 72/256 (28%) pts with g/tBRCAm mTNBC had tumors with programmed cell death-ligand 1 (PD-L1) combined positive score ≥10; 88 (34%) pts did not receive a PD-L1 test. Conclusions: Although BRCA testing rates increased from 2018 to 2023, 38% of US pts diagnosed with HER2− mBC in 2023 did not receive any BRCA test. BRCA testing rates were low among pts with HR+/HER2− mBC and/or aged >65 years, suggesting that many g/tBRCAm remained undetected. Wider and timelier testing for germline and tumor biomarkers is warranted to identify pts who are eligible for targeted therapies. Citation Format: S. Yadav, Q. Li, Z. Tan, K. E. Mishkin, J. F. Hayes, X. Xu, F. J. Couch. Real-world biomarker testing patterns in US patients with HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD4-08.