Influence of Macrocyclic Chelators on the Targeting Properties of 68Ga-Labeled Synthetic Affibody Molecules : Comparison with 111In-Labeled Counterparts

Affibody molecules are a class of small (7 kDa) non-immunoglobulin scaffold-based affinity proteins, which have demonstrated substantial potential as probes for radionuclide molecular imaging. The use of positron emission tomography (PET) would further increase the resolution and quantification accuracy of Affibody-based imaging. The rapid in vivo kinetics of Affibody molecules permit the use of the generator-produced radionuclide 68Ga (T1/2 = 67.6 min). Earlier studies have demonstrated that the chemical nature of chelators has a substantial influence on the biodistribution properties of Affibody molecules. To determine an optimal labeling approach, the macrocyclic chelators 1,4,7,10-tetraazacylododecane-1,4,7,10-t​etraaceticacid (DOTA), 1,4,7-triazacyclononane-N,N,N-triacetic acid (NOTA) and 1-(1,3-carboxypropyl)-1,4,7- triazacyclononane-4,7-diacetic acid (NODAGA) were conjugated to the N-terminus of the synthetic Affibody molecule ZHER2:S1 targeting HER2. Affibody molecules were labeled with 68Ga, and their binding specificity and cellular processing were evaluated. The biodistribution of 68Ga-DOTA-ZHER2:S1, 68Ga-NOTA-ZHER2:S1 and 68Ga-NODAGA-ZHER2:S1, as well as that of their 111In-labeled counterparts, was evaluated in BALB/C nu/nu mice bearing HER2-expressing SKOV3 xenografts. The tumor uptake for 68Ga-DOTA-ZHER2:S1 (17.9±0.7%IA/g) was significantly higher than for both 68Ga-NODAGA-ZHER2:S1 (16.13±0.67%IA/g) and 68Ga-NOTA-ZHER2:S1 (13±3%IA/g) at 2 h after injection. 68Ga-NODAGA-ZHER2:S1 had the highest tumor-to-blood ratio (60±10) in comparison with both 68Ga-DOTA-ZHER2:S1 (28±4) and 68Ga-NOTA-ZHER2:S1 (42±11). The tumor-to-liver ratio was also higher for 68Ga-NODAGA-ZHER2:S1 (7±2) than the DOTA and NOTA conjugates (5.5±0.6 vs.3.3±0.6). The influence of chelator on the biodistribution and targeting properties was less pronounced for 68Ga than for 111In. The results of this study demonstrate that macrocyclic chelators conjugated to the N-terminus have a substantial influence on the biodistribution of HER2-targeting Affibody molecules labeled with 68Ga.This can be utilized to enhance the imaging contrast of PET imaging using Affibody molecules and improve the sensitivity of molecular imaging. The study demonstrated an appreciable difference of chelator influence for 68Ga and 111In