Treatment of human tumor xenografts with monoclonal antibody 806 in combination with a prototypical epidermal growth factor receptor-specific antibody generates enhanced antitumor activity

Monoclonal antibody (mAb) 806 is a novel epidermal growth factor receptor (EGFR) antibody with significant antitumor activity that recognizes a mutant EGFR commonly expressed in glioma known as delta2-7 EGFR (de2-7 EGFR or EGFRvIII) and a subset of the wild-type (wt) EGFR found in cells that overexpress the receptor, We have used two human xenograft mouse models to examine the efficacy of mAb 806 in combination with mAb 528, a prototypical anti-EGFR antibody with similar specificity to cetuximab. Treatment of nude mice, bearing s.c. or i.c. tumor human xenografts expressing the wt or de2-7 EGFR, with mAbs 806 and 528 in combination resulted in additive and in some cases synergistic, antitumor activity. Interestingly, mAb 528 was also effective against xenografts expressing the ligand independent de2-7 EGFR when used as a single agent, showing that its antitumor activity is not merely mediated through inhibition of ligand binding. When used as single agents, neither mAbs 806 or 528 induced down-regulation of the de2-7 EGFR either in vitro or in vivo. In contrast, the combination of antibodies produced a rapid and dramatic decrease in the total cell surface de2-7 EGFR both in vitro and in xenografts. Consistent with this decrease in total cell surface de2-7 EGFR, we observed up-regulation of the cell cycle inhibitor p27(KIP1) and a decrease in tumor cell proliferation as measured by Ki-67 immunostaining when the antibodies were used in combination in vivo. Thus, mAb 806 can synergize with other EGFR-specific antibodies thereby providing a rationale for its translation into the clinic.