Association of MammaPrint and clinical outcomes by race among 5000 individuals with HR+HER2- early stage breast cancer enrolled in FLEX.

11147 Background: Black women with hormone receptor-positive (HR+), HER2-negative (HER2-) early stage breast cancer (EBC) have a 38% higher mortality rate than White women, a disparity not fully explained by social determinants of disease. Previous research demonstrating unequal performance of gene expression (GE) assays across racial groups has raised concerns that GE assays may underestimate risk of recurrence in Black patients (pts). Compared to other GE assays, the MammaPrint (MP) 70 gene signature consistently classifies a higher proportion of Black pts as High Risk compared with White pts. This analysis examines real world data and survival stratified by MP and self-reported race in pts with HR+HER2- EBC enrolled in FLEX. Methods: The ongoing FLEX (NCT03053193) trial enrolls pts undergoing standard of care MP testing, classifying tumors as Low, High 1 (H1), or High 2 (H2) risk of recurrence. BluePrint defines molecular subtypes as Luminal, Basal, or HER2. Clinical differences were assessed with Chi-squared or Fisher’s exact tests. Distant recurrence-free interval (DRFI), defined per STEEP criteria, was compared by race and MP using Kaplan-Meier estimates and log rank tests. Results: Among 5142 pts analyzed, 9.6% were Black and 90.4% were White. Node positive (30.4% vs 21.9%; p < 0.001) and Grade 3 disease (25.3% vs 14.1%; p < 0.001) were more common among Black pts. Black pts had significantly higher incidences of H1 (43.1%), H2 (18.3%), and Basal (9.3%), and lower rates of Low (38.5%) Risk tumors, compared with White pts (H1: 36.6%; H2: 7.4%; Basal: 3.3%; Low: 56.6%; p < 0.001). Black pts had higher rates of neo/adjuvant chemotherapy (CT) (52.9% vs 40.3%; p = 0.003) and use of an anthracycline-taxane regimens (43.0% vs 35.4%: p = 0.001) compared with White pts. However, despite a 61.4% incidence of H1/H2 risk disease among Black pts, only 52.9% received CT. In contrast, White pts had a 44.0% H1/H2 incidence and 40.3% received CT. Among all pts, 4-year DRFI was lowest for H2 (90.7%), 96.7% for H1, and 98.8% for Low Risk (p < 0.001). DRFI for Black pts was 98.3% for Low, 95.5% for H1, and 90.2% for H2 (p = 0.005). For White pts, DRFI was 97.9% for Low, 96.9% for H1, and 89.8% for H2 (p < 0.001). Among CT treated pts, DRFI was comparable for Black (n = 186) and White (n = 1130) pts with H1 (96.0% vs 96.4%) and H2 (90.3% vs 90.6%) tumors. Conclusions: Black pts have a higher prevalence of H2/Basal and higher risk clinical features in the nationwide prospective FLEX Registry trial. Black pts with HR+HER2- MP H1/H2 EBC were less likely to receive neo/adjuvant CT than White pts with H1/H2 tumors. This highlights a critical gap in real-world practice where Black pts may be undertreated. However, clinical outcomes are equivalent among similarly treated Black and White pts across MP risk groups. MP classifies fewer EBCs as Low Risk in Black pts, and this classification is accurate as 4-year DRFI is excellent in these pts. Clinical trial information: NCT03053193 .