Emergence of RAS or EGFR extracellular domain mutations and duration of response to EGFR blockade in colorectal cancer

Blockade of the Epidermal Growth Factor Receptor (EGFR) with the monoclonal antibodies cetuximab or panitumumab is effective in a subset of colorectal cancers (CRC), but the emergence of resistance limits the efficacy of these therapeutic agents. At relapse, the majority of patients develop RAS mutations, while a subset acquires EGFR extracellular domain (ECD) mutations. How clonal dynamics impact the emergence of resistance during EGFR blockade is poorly understood. We find that patients who experience greater and longer responses to EGFR blockade preferentially develop EGFR ECD mutations, while RAS mutations emerge more frequently in patients with smaller tumor shrinkage and shorter progression-free survival. In circulating cell-free tumor DNA (ctDNA) of patients treated with anti-EGFR antibodies, RAS mutations emerge earlier than EGFR ECD variants. Subclonal RAS but not EGFR ECD mutations are present in CRC samples obtained before exposure to EGFR blockade. When CRC cell populations are challenged with stressful conditions, RAS, but not EGFR-ECD, mutant subclones appear. These data indicate that clonal evolution of drug-resistant cells is associated with the clinical outcome of CRC patients treated with anti-EGFR antibodies.