Abstract LB-316: Pharmacological and immunological characterization of the therapeutic anti-EGFR antibodies cetuximab, panitumumab and matuzumab: The combination of cetuximab and matuzumab results in enhanced effector functions
Authors: Stroh Christopher , Reusch Christof , Schmidt Jürgen , Splittgerber Jörg , Blaukat Andree 
Published: February 2011

Abstract
Background: Therapeutic antibodies can attack tumor cells by functionally interfering with the target and/or by employing immune effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC). The objective of this study was to analyze the anti-EGFR therapeutic antibodies cetuximab, panitumumab and matuzumab with regard to both modes of action: Interference with EGFR signaling and immune effector functions.
Methods: EGFR binding and inhibition of EGFR signal transduction were characterized for the individual antibodies and antibody combinations in various tumor cell lines. The ability of single antibodies or antibody combinations to kill tumor cells via ADCC or CDC was analyzed in ex vivo assays using human immune effector cell isolates from blood or human serum, respectively.
Results: In cellular assays (FACS and binding assays with 125I-labeled antibodies) cetuximab, panitumumab and matuzumab showed similar EGFR binding affinities. All three antibodies were capable of blocking EGFR signal transduction (phospho-EGFR, phospho-ERK1/2, phospho-Akt, phospho-STAT3 and inhibition of VEGF production) with cetuximab and panitumumab being slightly more potent than matuzumab. Using peripheral blood mononuclear cells (PBMCs) as effector cells, cetuximab and matuzumab mediated a marked, dose-dependent ADCC response, while no or only marginal ADCC was observed with panitumumab. Natural killer (NK) cells were the main effector cells of the PBMC fraction. Comparable results were obtained in assays using whole blood indicating no or only marginal contribution of polymorphonuclear leukocytes to the ADCC activity. No CDC activity was detected with either antibody alone. Overlapping EGFR binding epitopes for cetuximab and panitumumab that are distinct from the binding site for matuzumab have been reported. However, in cellular binding assays only cetuximab and matuzumab (but not panitumumab and matuzumab) were able to bind simultaneously to EGFR. The combination of cetuximab and matuzumab led to a strong internalization and degradation of EGFR and to an increased inhibition of EGFR signaling. Interestingly, while the cetuximab-matuzumab combination did not result in enhanced ADCC we observed a strong increase in CDC activity and recruitment of complement factors.
Conclusions: Cetuximab, panitumumab and matuzumab have different characteristics with regard to EGFR binding and immune effector functions. The non-competitive, simultaneous binding of cetuximab and matuzumab resulted in superior interference with EGFR function and enhanced CDC activity of the combination.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-316.