In BxPC-3 cells, normal uptake of the radiotracer was measured at 11.72 ± 3.54% (Fig. 1B and Supplementary Table S1). Binding was significantly reduced when co-administered with MEHD7945A (10-fold: 1.02 ± 0.34%, p = 0.0001; 25-fold: 0.29 ± 0.16%, p = 0.0001) and cetuximab (10-fold: 1.21 ± 0.46%, p = 0.0001; 25-fold: 1.02 ± 0.23%, p = 0.0001). Competition with 10-fold DL3.6b lowered radiotracer accumulation to 6.98 ± 0.69% (p = 0.0001). Interestingly, radiotracer uptake was not diminished at 25-fold excess DL3.6b with 16.36 ± 0.88% binding (p = 0.0001). From the flow cytometry analysis, control cells exhibited 307.7 ± 11.2 MFI and 127.0 ± 3.6 MFI for EGFR and HER3 respectively. MEHD7945A-blocked cells decreased EGFR expression (182.7 ± 28.4 MFI, p = 0.0001) and HER3 (95.4 ± 7.7 MFI, p = 0.0123). Exposure to cetuximab mitigated EGFR expression (3.3 ± 0.8 MFI, p = 0.0001) whereas HER3 abundance remained unchanged (130.3 ± 4.0 MFI). Incubation with DL3.6b decreased HER3 (63.3 ± 8.2 MFI, p = 0.0001) while EGFR levels remained similar to unblocked control (284.0 ± 13.1 MFI, p = 0.0739) (Supplementary Fig. S3 and Supplementary Table S1).