HER3 (human
epidermal growth factor receptor type 3) is a challenging target for diagnostic
radionuclide molecular imaging due to the relatively modest overexpression in
tumors and substantial expression in healthy organs. In this study, we compared four HER3-targeting PET tracers based on different types of targeting molecules in a preclinical model: the 89Zr-labeled therapeutic antibody
seribantumab, a
seribantumab-derived F(ab)2-fragment labeled with 89Zr and 68Ga, and the 68Ga-labeled affibody molecule [68Ga]Ga-ZHER3. The novel conjugates were radiolabeled and characterized in vitro using HER3-expressing BxPC-3 and DU145 human
cancer cells. Biodistribution was studied using Balb/c nu/nu mice bearing BxPC-3 xenografts. HER3-negative RAMOS xenografts were used to demonstrate binding specificity in vivo. Autoradiography was conducted on the excised
tumors. nanoPET/CT imaging was performed. New conjugates specifically bound to HER3 in vitro and in vivo. [68Ga]Ga-DFO-
seribantumab-F(ab')2 was considered unsuitable for imaging due to the low stability and high uptake in normal organs. The highest
tumor-to-non-
tumor contrast with [89Zr]Zr-DFO-
seribantumab and [89Zr]Zr-DFO-
seribantumab-F(ab')2 was achieved at 96 h and 48 h pi, respectively. Despite lower
tumor uptake, [68Ga]Ga-ZHER3 provided the best imaging contrast due to the fastest clearance from blood and normal organs. The results of our study suggest that affibody-based tracers are more suitable for PET imaging of HER3 expression than antibody- and
antibody-fragment-based tracers.