Analytic validation of an FGFR-focused cell-free DNA liquid biopsy assay (FGFR-Dx)

Commercial liquid biopsy assays are routinely used by oncologists to monitor disease response and resistance to therapy. Additionally, in cases where tumor tissue is not available, clinicians may rely on cell-free DNA (cfDNA) testing as a surrogate for comprehensive tumor testing. While some gene rearrangements are well detected, current commercial liquid biopsy assays exhibit low sensitivity for fibroblast growth factor receptor (FGFR) rearrangements. FGFRs are altered in ∼2.5% of all cancers, including FGFR2 rearrangements in 10% of intrahepatic cholangiocarcinoma and FGFR3 point mutations and rearrangements in 10-15% of urothelial carcinoma. Therefore, we developed and analytically validated FGFR-Dx, an FGFR-focused cfDNA assay with improved sensitivity for FGFR rearrangements. FGFR-Dx comprehensively targets the introns in FGFR1-3 previously shown to be involved in gene fusions as well as all coding exons. Custom FGFR synthetic reference standards representing both single nucleotide variants (SNVs) and gene rearrangements were utilized at a range of variant frequencies and revealed a detection limit of 0.5% with sensitivities of 97.2% and 92.9% for SNVs and rearrangements, respectively. Furthermore, FGFR-Dx detected rearrangements and identified the intronic breakpoints from cfDNA collected from 13 of 15 patients with known FGFR fusions.