A companion diagnostic to MEHD7945A is critical for patient selection. In this study, we report the development of 89Zr (t1/2 = 3.27 d) labeled MEHD7945A (89Zr-MEHD7945A) and an evaluation of its pharmacological properties in PDAC by evaluating in vivo spatial distribution of the tracer against regional localization of EGFR and HER3 in Kras wild-type (BxPC-3) and mutant (AsPC-1) pancreatic cancer. We further investigated its specificity to EGFR and/or HER3 through in vitro, in vivo and ex vivo competitive blocking studies. Shifts in EGFR and HER3 expression during these blocking assays were measured by the radiotracer and further validated through immunoblots, flow cytometry and immunohistochemistry.