Abstract B042: Preclinical evaluation of petosemtamab, an epidermal growth factor receptor (EGFR) and leucine-rich repeat-containing G protein-coupled receptor (LGR5) bispecific antibody, on cancer stem cells

Abstract EGFR is a known oncogenic driver, and LGR5 is a transmembrane receptor expressed on cancer stem cells. Petosemtamab, a human common light chain IgG1 bispecific antibody with ADCC-enhanced activity targeting EGFR and LGR5, has shown potent in vitro antitumor activity in patient-derived xenograft and patient-derived organoid models. Promising interim clinical data from a phase 2, single-arm trial (NCT03526835) of petosemtamab 1500 mg every 2 weeks (Q2W; 28-day cycles) in 2L+ recurrent/metastatic head and neck squamous cell carcinoma (r/m HNSCC) demonstrated a 36% overall response rate (ORR) and median overall survival of 11.4 months in 75 evaluable patients (pts) [Le Tourneau, ESMO Asia 2024]. Petosemtamab in combination with pembrolizumab (400 mg Q6W) as 1L treatment for PD-L1+ r/m HNSCC demonstrated a 63% ORR and 79%-overall survival rate at 12 months in 43 efficacy evaluable pts [van Herpen, 2025 ASCO]. Petosemtamab’s unique mechanism of action includes: 1) inhibition of EGFR ligand binding and downstream signaling; 2) degradation of EGFR via LGR5 internalization; 3) engagement of host immune cells via enhanced ADCC. We evaluated petosemtamab-induced EGFR internalization and degradation. EGFR and LGR5 protein expression were analyzed by western blot in EGFR+LGR5+ colorectal cancer organoid model P18T after treatment with petosemtamab, its single-Fab arm variants, and cetuximab. EGFR internalization and degradation were studied by immunofluorescence imaging over time (24 hours), in the presence or absence of proteosome inhibitor bortezomib. Evaluation of protein expression by western blot showed that petosemtamab consistently led to EGFR degradation at 72 hours. This effect was independent of the presence of the LGR5 ligand (RSPO) and required both the EGFR and LGR5 binding arms of petosemtamab, as neither the EGFR x respiratory syncytial virus glycoprotein (RSVG) nor LGR5 x RSVG single-Fab arm formats nor the anti-EGFR mAb cetuximab induced EGFR degradation. EGFR degradation was diminished by bortezomib, highlighting the role of proteasome-mediated degradation in this process. Petosemtamab treatment was associated with increased LGR5 expression to a greater extent than observed with the single EGFR x RSVG Fab arm or cetuximab alone, although each EGFR agent increased LGR5 expression. Inhibition of the EGFR–RAS/RAF/MAPK signaling pathway has previously been shown to lead to increased LGR5 expression [High, 2025 AACR] supporting petosemtamab evaluation in relevant targeted therapies. These results confirm petosemtamab mediates efficient EGFR degradation in colorectal cancer cells in an organoid model expressing both EGFR and LGR5, and supports the further clinical development of petosemtamab. Citation Format: Konstantin Slobodnyuk, Szabolcs Fatrai, Sally Hill, Sidney Walker, Daniël de Groot, Ante S. Lundberg, Cecile A. W. Geuijen, Jeroen J. Lammerts van Bueren. Preclinical evaluation of petosemtamab, an epidermal growth factor receptor (EGFR) and leucine-rich repeat-containing G protein-coupled receptor (LGR5) bispecific antibody, on cancer stem cells [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr B042.