11P Influence of ctDNA variant allele frequency (VAF) on survival outcomes in metastatic breast cancer (MBC)

Liquid biopsy (LB) provides valuable molecular insights in metastatic breast cancer (MBC). This study examines the relationship between the highest VAF (hVAF) in ctDNA and patient survival outcomes. In this retrospective analysis, the Guardant360 genomic assay, encompassing 74 genes and microsatellite instability status, was used on plasma samples prospectively collected at tumor progression from patients with MBC treated at Hospital Clinic of Barcelona. The hVAF for each patient was identified for analysis. Cox proportional hazards models assessed hVAF both as a continuous variable and in tertiles, focusing on its association with overall survival (OS). Multivariable analyses adjusted for menopausal status, performance status, bone-only disease, visceral disease, treatment line and cancer subtype was performed. For hormone receptor-positive(HR+)/HER2-negative(-) MBC treated after LB with endocrine therapy+CDK4/6 inhibitors (ET+CDK4/6i), hVAF's association with progression-free survival (PFS) was also explored. Between February 2021 and April 2022, 99 plasma samples from 99 patients were analyzed (median age: 59.7; 75.8% HR+/HER2-, 16.2% triple negative, 8.1% HER2+ MBC). Overall, ctDNA was detected in 98.0% of cases, with 94.0% showing pathogenic variants (PVs). The median number of PVs was 3 per patient (range: 1-16) and the most frequently mutated genes were TP53 (50;50.5%), ESR1 (38;38.4%) and PIK3CA (28;28.3%). Median follow-up was 27.2 months (0.56-37.45). OS was significantly different across hVAF tertiles (T3, T2 and T1 median OS [mOS] in months 13.1, 25.1 and NR; p<0.001). Higher hVAF significantly correlated with poorer OS both as continuous (hazard ratio [HR] 1.02, p<0.001) and categoric (HR T1 vs. T3: 3.76, p<0.001) variable. Multivariable analysis confirmed hVAF as an adverse prognostic factor (HR: 1.02, p=0.002). Continuous hVAF was negatively associated with PFS (HR: 1.05, p=0.007) in 24 patients under ET+CDK4/6i. This study reveals hVAF significantly impacts OS in MBC patients, suggesting its potential for refining prognostic stratification in clinical trials and guiding the selection of which patients might benefit from more aggressive therapies in subsequent lines.