Figure 5A showed that an addition of excess cold MEHD7945A (100 nM) blocks the binding of 89Zr-MEHD7945A (1 nM). This demonstrated a saturable, concentration-dependent binding of the tracer. From the Scatchard plot (Fig. 5B), the Bmax was found to be 336.0 ± 25.5 fmol/mg or 2 × 105 binding sites per cell (assuming 1 million cells per tumor) and the KD was found to be ~0.51 ± 0.12 nM.Figure 5Ex vivo competitive binding assay. A digital autoradiograph of AsPC-1 tumor sections displayed saturable, concentration-dependent binding of 89Zr-MEHD7945A upon addition of 100-fold excess cold MEHD7945A. (A) A non-linear regression analysis and Scatchard plot of 89Zr-MEHD7945A plotted against the amount of bound ligand shows the Bmax ~ 336 fmol/mg and a KD ~ 0.51 nM (B).