Differential in vitro activity of amivantamab, cetuximab, and panitumumab against EGFR ECD resistance mutations

Epidermal growth factor receptor (EGFR) ectodomain (ECD) mutations are a known acquired resistance mechanism to EGFR monoclonal antibodies cetuximab and panitumumab in metastatic colorectal cancer (mCRC). Amivantamab, an EGFR-MET bispecific antibody with immune cell-directing activity, has demonstrated meaningful antitumor activity in several solid tumor types. Amivantamab, cetuximab, and panitumumab bind to EGFR domain III; however, the precise epitopes vary, which could lead to differential binding and activity against EGFR ECD resistance mutations. Here, we compare the activity of these EGFR-targeting agents against a panel of 29 EGFR ECD mutations historically associated with resistance to cetuximab and panitumumab. HEK293Ta cells expressing EGFR ECD-mutated variants were used for binding and functional assays of amivantamab, cetuximab, and panitumumab. Amivantamab demonstrated increased binding and functional activity against a wider range of EGFR ECD mutations, including the most prevalent V441, G465, and S492 mutations, relative to cetuximab and panitumumab. Furthermore, structural modeling of amivantamab bound to EGFR indicated that the amivantamab epitope is shifted away from EGFR residues 441-444 and 464-465, which are the regions that form more extensive interactions with cetuximab and panitumumab. Of note, amivantamab was inactive against ECD mutations occurring at K489 and I491, which were previously identified as a part of the amivantamab EGFR epitope. The preclinical activity of amivantamab against EGFR ECD mutations was corroborated in a clinical case study. In OrigAMI-1 (ClinicalTrials.gov Identifier: NCT05379595), a phase 1b/2 study evaluating amivantamab monotherapy among heavily pretreated RAS/BRAF wild-type mCRC, one participant was retrospectively found to carry an EGFR ECD S492R mutation by baseline tissue next-generation sequencing. This participant achieved a partial response, providing preliminary clinical evidence that amivantamab is active against a mutation demonstrated to confer resistance to cetuximab. In conclusion, amivantamab may be more active against a wider range of EGFR ECD resistance mutations compared with cetuximab and panitumumab.