Circulating tumor DNA (ctDNA) to track responses and to capture the genomic heterogeneity of metastatic melanoma.

9582 Background: While immunotherapy and MAPK-targeted therapies have improved patient outcome, the genomic heterogeneity of melanoma contributes to treatment resistance. Molecular approaches for monitoring tumor burden, treatment response and resistance has enormous potential. ctDNA may allow global representation of all disease sites, and serial ctDNA analysis can track responses and provide insights into the spatio-temporal profile of the disease. Methods: Fifty-two metastatic melanoma patients (pts) were serially monitored during sequential lines of therapy with FDG-PET scans, LDH and ctDNA. Serial tumor biopsies were collected. Five of these 52 pts consented to multiregional tumor sampling at autopsy. Allele specific digital PCR assays, customized targeted and whole exome sequencing was used to measure ctDNA levels and identify genomic alterations. Results: In BRAF and NRAS mutant patients, mutant ctDNA was detected in 72% of patients at baseline and encompassed between 0.4%-86% (median 8.2%) of total ctDNA levels. LDH and FDG-PET metabolic tumor volume correlated significantly with mutant BRAF and NRAS copies in plasma [(r=0.7818, p=0.0032) and (r=0.63, p=0.0001) respectively]. High ctDNA levels were associated with worse overall survival. ctDNA levels tracked with FDG-PET responses. Targeted sequencing of baseline ctDNA and tumor samples identified multiple mutations in clinically important genes such as MAP2K1, MAP2K2, PTEN, CDKN2A, RAC1, CTNNB1,BRAF and NRAS; high concordance was observed between ctDNA and tumor. ctDNA analysis also identified multiple mutations associated with primary and acquired resistance to MAPK-targeted therapy. Comprehensive exome sequencing of ctDNA in patients undergoing autopsy provided an accurate representation of spatial heterogeneity, when compared to multi-regional tumor sequencing. Conclusions: These findings highlight the potential clinical utility of ctDNA analysis to monitor tumor responses and disease progression in the management of metastatic melanoma patients.