Abstract 2148: MM-151 elicits broad and unique inhibition of cells harboring EGFR extracellular domain mutations — results of multiscale experiments with genome-edited cell lines

Mutations in the extracellular domain (ECD) of EGFR have been reported in colorectal (CRC) patients as a mechanism of acquired resistance to the EGFR-directed antibodies cetuximab and panitumumab. Similar to EGFR intracellular domain mutations observed in patients treated with EGFR kinase inhibitors, these EGFR-ECD mutations inhibit the binding of the antibodies while maintaining EGFR signaling activity. MM-151, an investigational agent consisting of three anti-EGFR IgG1 antibodies, has been recently reported to overcome these mutations in over-expression and patient-derived cell line models. Here, we extend these studies to characterize the mechanisms of action that enable MM-151 to provide robust inhibition of EGFR-ECD mutations beyond that achievable by other monoclonal and oligoclonal EGFR antibodies. First, utilizing a cell-free binding assay with recombinant EGFR-ECD mutants, we found that MM-151, but not the Sym004 combination of two antibodies, maintained oligoclonal binding to all mutants. Second, we performed a panel of 2D and 3D in vitro experiments with over-expression and genome-edited (CRISPR-Cas9) cell lines harboring EGFR-ECD mutations to characterize target engagement, cell signaling, proliferation, and the impact of hetero- versus homozygous expression of EGFR wild type or mutant alleles. We observed that the three mechanisms of action for MM-151- antagonism of both low- and high-affinity EGFR ligands, EGFR down-regulation, and immune-effector activity— were preserved in these models. Third, we performed xenograft experiments to evaluate the behavior of MM-151 and found that MM-151 maintained inhibitory activity in an in vivo context. The results of our multiscale experiments identified the impact of combining three antibodies to overcome the high plasticity of the EGFR extracellular domain and thus reveal the potential for clinical evaluation of MM-151 in both EGFR-naïve and EGFR-refractory CRC populations.