We interrogated the dual specificity of the probe for either EGFR or HER3 through competitive binding assays. Interestingly, blocking either EGFR or HER3 separately increased or sustained the binding levels of the probe compared to blocking both receptors simultaneously. The highest blocking doses administered can be considered a treatment dose (HER3 IC50 ~ 0.12 µg/mL and EGFR IC50 ~ 0.02 µg/mL for the in vitro study17, and 0.04–1 mg per mouse in vivo49,50), potentially eliciting an activated feedback loop. Results of the study seemingly imply an “EGFR-only” mechanism of probe uptake. Looking closely at both AsPC-1 and BxPC-3 control untreated cells, the flow cytometry data indicated very low HER3 expression compared to EGFR, which is five-fold higher (Supplementry Table S1). We believe that any marginal change in HER3 when blocked with DL3.6b combined with the magnitude of EGFR expression may be below the sensitivity threshold of the radiotracer. With the in vivo competition assays, it would seem like the blocks were unsuccessful with the tracer uptake between control and blocked groups rendered statistically insignificant. In this case, external validation with immunohistochemistry (Fig. 4C) and western blots (Fig. 4D) supported feedback compensation of either EGFR or HER3, albeit on a smaller scale. We must keep in mind that the tumor was exposed to the blocking antibodies acutely vs. chronic exposure in a therapeutic setting; thus, a sustained and higher HER3 induction may not have been achieved. Nevertheless, the tracer detected varied expression of either RTKs. Admittedly, one of the main limitations of the probe lies in identification of which receptor is upregulated.