Analysis of FGFR alterations from circulating tumor DNA (ctDNA) and Tissue in a phase II trial of erdafitinib in urothelial carcinoma (UC).

420Background: Plasma samples from a Phase 2 study of the pan-FGFR inhibitor erdafitinibin advanced UC patients (pts) with FGFR mutations (mut) or fusions, were tested using next generation sequencing (NGS) for circulating tumor DNA(ctDNA), and results compared to central FGFR status determination from tissue. Methods: FGFR alterationswere measured in archival tissue using an RNA-based RT-PCR test and compared with FGFR alterationsidentified in pre-treatment plasma specimens using the Guardant360 ctDNA test. Sensitivityof the ctDNA test to detect the FGFR alterationsidentified by RT-PCR from tissue, the proportion of pts with a study-eligible FGFR alterationin ctDNA, and the ability to detect a tissue FGFR alterationin ctDNA in relation to clinical response were assessed. Results: Samples from 155 pts with detectable baseline ctDNA were included. Overall, concordance between ctDNA and tissue-based FGFR results was 56% (87/155): 63% (77/122) for tissue mut-positive pts vs 24% (7/29) in fusion-posit...