Abstract 1974: Clonal and subclonal mutational landscapes in circulating tumor DNA in metastatic colorectal cancer: An exploratory analysis from the phase III PARADIGM study

Introduction: We performed an exploratory analysis of circulating tumor DNA (ctDNA) in patients (pts) with RAS wild-type (WT) chemotherapy-naïve stage IV metastatic colorectal cancer (mCRC) from the ongoing phase III PARADIGM study (NCT02394834). The clonal and subclonal mutational landscapes of RAS WT pts were analyzed using a mCRC-focused custom ctDNA sequencing panel. Methods: Randomized pts (1:1) received mFOLFOX6 plus either panitumumab or bevacizumab. Pre-treatment plasma samples (DNA yield >10ng/mL and >10nM) were sequenced using the custom ctDNA panel (PlasmaSELECTTM-R 91, PGDx) to detect mCRC mutations, amplifications, and rearrangements in 90, 26, and 3 genes, respectively, as well as microsatellite instability (MSI), in 250kb targeted regions using stringent quality criteria. Pre-treatment archival tissue samples from 590 pts were analyzed by the Broad Institute Solid Tumor panel, which covered 1,072 genes with 7.3Mb targeted regions. Results: Between May 29, 2015 and June 8, 2017, 823 pts were enrolled across 197 sites. Plasma samples were collected from 756 pts, with 747 (98.8%) samples meeting the quality criteria. The average total and distinct sequencing coverages were 34,200 and 5,865, respectively, with 49.1% of bases mapping to regions of interest. Mutation frequencies in ctDNA were generally consistent with those observed in RAS-WT TCGA CRC and rectal cancers (n=283), except for TET2 and DNMT3A potentially due to clonal hematopoiesis. Mutations in BRAF, APC, CTNNB1, PTEN, PIK3CA, and MSI-status observed in ctDNA were significantly different between left and right-sided tumors, consistent with known differences in tumor sidedness. In the 747 plasma samples, 4,072 mutations were observed; 1,871 had a mutant allele frequency (MAF)