Increase in negative charge of (68)Ga/chelator complex reduces unspecific hepatic uptake but does not improve imaging properties of HER3-targeting affibody molecules

Upregulation of the human epidermal growth factor receptor type 3 (HER3) is a common mechanism to bypass HER-targeted cancer therapy. Affibody-based molecular imaging has the potential for detecting and monitoring HER3 expression during treatment. In this study, we compared the imaging properties of newly generated (68)Ga-labeled anti-HER3 affibody molecules (HE)(3)-Z(HER3)-DOTA and (HE)(3)-Z(HER3)-DOTAGA with previously reported [(68)Ga]Ga-(HE)(3)-Z(HER3)-NODAGA. We hypothesized that increasing the negative charge of the gallium-68/chelator complex would reduce hepatic uptake, which could lead to improved contrast of anti-HER3 affibody-based PET-imaging of HER3 expression. (HE)(3)-Z(HER3)-X (X = DOTA, DOTAGA) were produced and labeled with gallium-68. Binding of the new conjugates was specific in HER3 expressing BxPC-3 and DU145 human cancer cells. Biodistribution and in vivo specificity was studied in BxPC-3 xenograft bearing Balb/c nu/nu mice 3 h pi. DOTA- and DOTAGA-containing conjugates had significantly higher concentration in blood than [(68)Ga]Ga-(HE)(3)-Z(HER3)-NODAGA. Presence of the negatively charged (68)Ga-DOTAGA complex reduced the unspecific hepatic uptake, but did not improve overall biodistribution of the conjugate. [(68)Ga]Ga-(HE)(3)-Z(HER3)-DOTAGA and [(68)Ga]Ga-(HE)(3)-Z(HER3)-NODAGA had similar tumor-to-liver ratios, but [(68)Ga]Ga-(HE)(3)-Z(HER3)-NODAGA had the highest tumor uptake and tumor-to-blood ratio among the tested conjugates. In conclusion, [(68)Ga]Ga-(HE)(3)-Z(HER3)-NODAGA remains the favorable variant for PET imaging of HER3 expression.