Surface plasmon resonance (SPR) analysis of the dissociation constant (KD) of DFO-derivatized MEHD7945A vs. the unmodified mAb demonstrated relatively similar binding for HER3 (37 pM vs. 8.1 pM, respectively) and EGFR (3.4 nM vs. 1.9 nM respectively). Competitive binding of 89Zr-MEHD7945A with increasing concentrations of cold MEHD7945A was conducted in both BxPC-3 and AsPC-1 cells. In AsPC-1 (Fig. 1C, left), a KDHi ~ 0.31 nM and KDLo ~ 29.00 nM were achieved. Measured BmaxHi and BmaxLo values were ~1.94 × 105 and ~7.02 × 104 receptors sites were available. The IC50 was ~0.37 nM (Fig. 1C, right). From the plot in Fig. 1D (left), the dissociation constants KDHi and KDLo in BxPC-3 cells were determined to be 0.34 nM and 12.02 nM with a BmaxHi of ~7.25 × 104 and BmaxLo of ~1.10 × 105. The IC50 for 89Zr-MEHD7945A was observed to be ~0.48 nM in BxPC-3 (Fig. 1D, right).