Independent prognostic value of ultra-sensitive quantification of tumor pretreatment T790M subclones in EGFR mutated non-small cell lung cancer (NSCLC) treated by first/second generation TKI, depends on variant allele frequency (VAF): Results of the Frenc

{Objectives: T790M mutations in EGFR-mutated non-small cell lung cancer (NSCLC) account for nearly 50% of acquired resistance mechanisms to EGFR-TKIs. Earlier studies suggested that tumor T790M could also be detected in TKI-naive EGFR-mutated NSCLC. The aim of the study is to assess the prevalence and clinical significance of quantification of tumor pre-treatment T790M subclones. Materials and methods: We analyzed 366 EGFR-mutated NSCLC patients of the real-life IFCT Biomarkers France study with available pre-treatment formalin-fixed paraffin-embedded (FFPE) tumor DNA before treatment by first/second-generation EGFR-TKI. We used ultra-sensitive Droplet Digital Polymerase Chain Reaction (ddPCR) QX200 (BIO-RAD (R), Hercules, CA, USA). All samples were tested in duplicate. Results: ddPCR identified T790M in 19/240 specimens (8%). T790M-positive and T790M-negative populations were not different for clinical baseline characteristics. T790M Variant Allele Frequency (VAF) was >= 0.01% < 0.1%, >= 0.1% < 1%, >= 1% < 10%, and >= 10% in five (26.3%), six (31.6%), six (31.6%), and two (10.5%) patients, respectively. T790M VAF was > 0.1% in 11/13 (84%) patients with rapid (< 3 months) or usual progression (3-20 months) compared to 0/3 with low progression (> 20 months) (p = 0.02). In a Cox model, T790M mutation positivity was correlated with overall survival (OS) and progression-free survival (PFS) for 10% > VAF >= 1% (hazard ratio [HR] = 2.83, 95% confidence interval [CI] 1.13-7.07