Optimization of HER3 expression imaging using affibody molecules: Influence of chelator for labeling with indium-111

Abstract

Radionuclide molecular imaging of human epidermal growth factor receptor 3 (HER3) expression using affibody molecules could be used for patient stratification for HER3-targeted cancer therapeutics. We hypothesized that the properties of HER3-targeting affibody molecules might be improved through modification of the radiometal-chelator complex. Macrocyclic chelators NOTA (1,4,7-triazacyclononane-N,N′,N′′-triacetic acid), NODAGA (1-(1,3-carboxypropyl)-4,7-carboxymethyl-1,4,7-triazacyclononane), DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), and DOTAGA (1,4,7,10-tetraazacyclododececane,1-(glutaric acid)−4,7,10-triacetic acid) were conjugated to the C-terminus of anti-HER3 affibody molecule Z08698 and conjugates were labeled with indium-111. All conjugates bound specifically and with picomolar affinity to HER3 in vitro. In mice bearing HER3-expressing xenografts, no significant difference in tumor uptake between the conjugates was observed. Presence of the negatively charged 111In-DOTAGA-complex resulted in the lowest hepatic uptake and the highest tumor-to-liver ratio. In conclusion, the choice of chelator influences the biodistribution of indium-111 labeled anti-HER3 affibody molecules. Hepatic uptake of anti-HER3 affibody molecules could be reduced by the increase of negative charge of the radiometal-chelator complex on the C-terminus without significantly influencing the tumor uptake.

Benefit of Later-Time-Point PET Imaging of HER3 Expression Using Optimized Radiocobalt-Labeled Affibody Molecules

“…We and others have previously investigated the potential of radiolabeled affibody molecules as HER3-targeted imaging agents for PET and SPECT [15,16,[20][21][22]27]. It has been shown that an increase in the negative charge of the radionuclide-chelator complex can improve tumor-to-liver contrast for affibody molecules [26,36]. In addition, retention of activity in tumors is better than in normal tissues for several types of imaging probes.…”

“…Thus, imaging at later time points enables further increase of imaging contrast, and hence sensitivity, even for imaging probes with rapid in vivo kinetics and tumor targeting, such as bombesin analogues [38], somatostatin analogues [39], single chain variable fragments (scFv) [40], and affibody molecules [41]. Later-time-point imaging has been shown to improve the imaging quality of HER3-targeting affibody molecules [20,26,27]. The use of PET for imaging provides better sensitivity, spatial resolution and accuracy of quantification, and therefore a positron-emitting label is desirable.…”

“…It was hypothesized that an increased negative charge can reduce unspecific hepatic uptake and, therefore, enhance the tumorto-liver contrast [27]. This hypothesis was further supported by comparing the effect of differently charged radiometal-chelator complexes on the biodistribution of ZHER3 labeled with 68 Ga and 111 In and HER2-targeting affibody molecules [26,31,36].…”

“…Compared to HER2, the level of HER3 overexpression in cancer cells is low [25] and uptake in normal tissue complicates image interpretation. However, it was previously observed that uptake of HER3 affibody molecules in normal organs decreases over time, while tumor-associated activity cleared more slowly [26,27]. Therefore, HER3 imaging contrast could potentially benefit from delayed imaging (up to 24 h pi), which was previously observed for indium-111-labeled anti-HER3 affibody molecules [26] and for radiolabeled anti-HER1 (EGFR) affibody molecules [28][29][30].…”

“…However, it was previously observed that uptake of HER3 affibody molecules in normal organs decreases over time, while tumor-associated activity cleared more slowly [26,27]. Therefore, HER3 imaging contrast could potentially benefit from delayed imaging (up to 24 h pi), which was previously observed for indium-111-labeled anti-HER3 affibody molecules [26] and for radiolabeled anti-HER1 (EGFR) affibody molecules [28][29][30]. For PET imaging of HER3 expression, 18 F-and 68 Ga-labeled Z HER3 affibody molecules were able to visualize tumor uptake already 1 and 3 h pi in rodents.…”