Clinical Utility of Serial Circulating Tumor DNA Analysis as a Minimally Invasive Biomarker in Advanced Urothelial Cancer

PURPOSE Circulating tumor DNA (ctDNA) analysis is an alternative to tissue biopsy for genotyping in various cancers. We aimed to establish a plasma ctDNA sequencing assay, then evaluate its clinical utility in advanced urothelial cancer (UC). MATERIALS AND METHODS This study included 82 patients with muscle-invasive or metastatic UC. A total of 158 blood samples and 73 tumor tissue samples were sequenced using our designed panel covering 53 UC-relevant genes and TERT promoter. We investigated the association between the proportion of ctDNA in total cell-free DNA (ctDNA fraction) and response to pembrolizumab treatment. ctDNA dynamics were explored during systemic therapy. RESULTS In paired tissue-ctDNA samples with estimated tumor fraction, half (50.2%) of the somatic mutations were shared between both sources, while some (17.6%) mutations were found only in ctDNA. In the metastatic setting, on-treatment increase in ctDNA fraction during pembrolizumab treatment was significantly associated with a poor response rate (18.7% v 76.1%; P = .001) and progression-free survival (median 2.8 v 9.8 months; P < .001). A comparison of cancer cell fraction, the fraction of tumor cells carrying somatic mutations, between pembrolizumab initiation and on-treatment showed a strong correlation among patients where ctDNA fraction increased during treatment ( r = 0.73). By contrast, no correlation was observed in patients without ctDNA fraction increase ( r = 0.09). Most mutations newly detected at pembrolizumab resistance have already been identified in tumor tissues at earlier stages. CONCLUSION Our newly established assay is suitable for assessing the genomic profile of ctDNA in patients with advanced UC. Our data may support future analyses of prognostic or predictive biomarkers for UC.