Surface plasmon resonance (SPR) (Biacore 3000, GE Healthcare) evaluated the binding affinity expressed as the equilibrium binding constant, KD of the MEHD7945A-DFO and the unmodified mAb using previously published protocols57. Immobilization of HER3 and EGFR proteins on a CM5 sensor chip was performed at 25 °C by amine-coupling chemistry using Biacore 3000 (GE Healthcare). To determine kinetic rate constants, the processed data were fitted to 1:1 Langmuir binding model using BIAevaluation software.