The total EGFR and HER3 binding sites on AsPC-1 and BxPC-3 cell lines were determined by homologous competitive saturation binding assay using radiolabeled 89Zr-MEHD7945A. Briefly, after detachment from cell culturing flasks by trypsinization, AsPC-1 and BxPC-3 were collected and re-suspended in PBS/1% BSA. Radiolabeled 89Zr-MEHD7945A was incubated with increasing concentration of MEDH7945A to AsPC-1 or BxPC-3 cells in PBS/1% BSA at room temperature. Sample incubation was terminated by vacuum filtration of samples through glass microfiber filters followed by triple PBS/1% BSA wash and counted on Wizard2 gamma counter. The IC50 was analyzed by sigmoidal dose response curve using a four-parameter logistic nonlinear regression. Two-site, non-linear saturation binding model was employed to determine KDHi and KDLo, the radioligand concentrations required to achieve a half-maximum binding at equilibrium. The BmaxHi and BmaxLo,, the maximum specific bindings to the two receptor sites were also analyzed.