Treatment with panitumumab resulted in the saturation of EGFR on A431 epidermoid carcinoma cells in vitro and in vivo in xenograft tumors as determined by flow cytometry. (A) A431 cells were incubated in vitro with increasing concentrations of unlabeled panitumumab and phycoerythrin (PE)-labeled panitumumab to determine lowest concentration required to achieve cell-surface binding saturation. (B, C [inset]) FACS dot plots of PE-panitumumab vs Alexa-EGFR of A431 cells treated with 17 nM of either B, control IgG or C, panitumumab for 1 hour. (D) The percent of EGFR saturation following treatment was determined in vivo by measuring the level of bound panitumumab on the dissociated tumor xenograft cells by flow cytometry and plotting test results against the standard curve generated in Figure 4A. (E, F [inset]) FACS dot plots of PE-panitumumab vs Alexa-EGFR of dissociated A431 xenograft cells treated with 500 μg of either E, control IgG or F, panitumumab; tumor cells were dissociated and assayed 7-days post-treatment.