Acquired Genomic Alterations on First-Line Chemotherapy With Cetuximab in Advanced Colorectal Cancer: Circulating Tumor DNA Analysis of the CALGB/SWOG-80405 Trial (Alliance).

JCO Acquired genomic alterations (Acq-GAs), specifically , , and -ectodomain mutations and and amplifications, are recognized as major mechanisms of resistance to later-line anti-EGFR-antibody therapy in metastatic colorectal cancer (mCRC). However, data regarding emergence of these Acq-GAs under the selective pressure of first-line anti-EGFR-chemotherapy are lacking. We performed next-generation sequencing (Guardant360) on circulating tumor DNA obtained from paired plasma samples (pretreatment and postprogression) from the CALGB/SWOG-80405 trial, which randomly assigned patients with mCRC between first-line chemotherapy with cetuximab (anti-EGFR-chemotherapy) or bevacizumab (anti-VEGF-chemotherapy). The primary objective was to determine the prevalence of Acq-GAs on anti-EGFR-chemotherapy and compare this to the prevalence with anti-VEGF-chemotherapy on trial and pooled estimates (N = 292) seen with later-line anti-EGFR-antibody therapy as reported in the literature. Among the 61 patients on anti-EGFR-chemotherapy, only four (6.6%) developed ≥ 1 Acq-GAs of interest compared with 10.1% (7) on anti-VEGF-chemotherapy (odds ratio, 0.62; 95% CI, 0.20 to 2.11) and 62.0% on anti-EGFR-antibody therapy in later lines (odds ratio, 0.09; 95% CI, 0.03 to 0.23). Acq-GAs, classically associated with anti-EGFR-antibody resistance in later lines (, , and -ectodomain mutations; and amplifications), were rare with up-front use of anti-EGFR-chemotherapy indicating divergent resistance mechanisms. These findings have critical translational relevance to timing and value of circulating tumor DNA-guided anti-EGFR rechallenge in patients with mCRC, especially those treated with anti-EGFR therapy upfront.