Background. Epidermal growth factor receptor (EGFr) is a transmembrane tyrosine kinase expressed on many different tumor types. Recent evidence suggests that Non-Small Cell Lung Carcinoma (NSCLC) patients with mutations in the kinase domain of the EGFr may account for most of the clinically observed responses to the small molecule tyrosine kinase inhibitors (TKI’s) due to an increased binding affinity to the mutant EGFr ATP binding pocket. Panitumumab, a fully human antibody, binds to the EGFr with high affinity (5x10-11 M) preventing ligand-induced activation resulting in arrest of tumor cell proliferation and apoptosis in some cases. The objective of this study was to determine if panitumumab has a differential activity against the clinically relevant EGFr mutations versus the wild type (WT) EGFr in both an isogenic in vitro system and in EGFr mutant and WT NSCLC cell lines in vitro and in vivo. Methods. To determine the inhibition of ligand induced autophosphorylation in vitro, WT and mutant EGFr over expressing in Chinese Hamster Ovary (CHO) cells (WT, G719S, L858R, Δ(750-759) and WT (A549, SK MES) and mutant (NCI H1975 (L858R), NCI H1650 (Δ(746-752)) EGFr containing NSCLC cells were treated with 0.002 to 2000 nM panitumumab prior to EGF stimulation. To determine efficacy, mice bearing 200mm3 established A549, SK MES, NCI H1650 and NCI H1975 xenografts were treated twice per week with panitumumab at 100, 200 or 500μg/mouse. Results. Treatment of both mutant and WT EGFr containing CHO and NSCLC cells with panitumumab resulted in a reduction of ligand-induced autophosphorylation. The IC50 for the inhibition of ligand-induced autophosphorylation by panitumumab in the CHO lines WT, G719S, L858R, Δ750-759 was 0.23, 0.32, 0.18 and 0.17 nM, respectively and in the NSCLC lines A549, SK MES, NCI H1975, and NCI H1650 was 0.34, 0.44, 0.92, 0.25 nM respectively. Treatment with a small molecule TKI reproduced published results. Treatment of established A549, NCI H1975, and NCI H1650 xenograft tumors with panitumumab resulted in statistically significant growth inhibition as compared to control mice. Treatment of established SK MES xenograft tumors resulted in no growth inhibition in this experiment. Conclusions. Panitumumab inhibited ligand-induced EGFR autophosphorylation in vitro in both mutant and WT EGFr containing CHO and NSCLC cells. Unlike results reported with the small molecule TKI’s, there was no significant difference in the IC50 between the mutant and WT EGFr containing cells treated with panitumumab. Administration of panitumumab showed statistically significant inhibitory effects on the growth of established tumors containing clinically relevant EGFr kinase domain mutations. These data provide preclinical evidence for the potential clinical application of panitumumab for treatment NSCLC patients with both wild type and mutant EGFr.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]