HER3 PET Imaging: (68)Ga-Labeled Affibody Molecules Provide Superior HER3 Contrast to (89)Zr-Labeled Antibody and Antibody-Fragment-Based Tracers

SIMPLE SUMMARY: HER3 is a known driver for oncogenesis and therapy resistance in solid cancers. PET imaging could be a useful tool to non-invasively detect and monitor HER3 expression and aid in the selection of patients for HER3-targeted therapy. PET tracers based on therapeutic antibodies have thus far shown limited success in reliably imaging HER3-expressing tumors in clinical trials. Smaller-sized tracers specifically designed for imaging might be needed for higher contrast imaging and sufficient sensitivity. Our group has previously studied the use of radiolabeled affibody molecules for imaging of HER3 expression. In the present study, we compared four different types of potential PET tracers for imaging of HER3 expression in a preclinical model. We demonstrated that the affibody-based tracer, [(68)Ga]Ga-Z(HER3), could provide overall superior imaging contrast to antibody- and antibody-fragment-based tracers shortly after injection. Our results indicate that HER3-targeting affibody molecules are promising agents for PET imaging of HER3 expression. ABSTRACT: 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 (89)Zr-labeled therapeutic antibody seribantumab, a seribantumab-derived F(ab)(2)-fragment labeled with (89)Zr and (68)Ga, and the (68)Ga-labeled affibody molecule [(68)Ga]Ga-Z(HER3). 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. [(68)Ga]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 [(89)Zr]Zr-DFO-seribantumab and [(89)Zr]Zr-DFO-seribantumab-F(ab’)(2) was achieved at 96 h and 48 h pi, respectively. Despite lower tumor uptake, [(68)Ga]Ga-Z(HER3) 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.