188Re-ZHER2:V2, a Promising Affibody-Based Targeting Agent Against HER2-Expressing Tumors: Preclinical Assessment

Affibody molecules are small (7 kDa) nonimmunoglobulin scaffold proteins with favorable tumor-targeting properties. Studies concerning the influence of chelators on biodistribution of Tc-99m-labeled Affibody molecules demonstrated that the variant with a C-terminal glycyl-glycyl-glycyl-cysteine peptide-based chelator (designated Z(HER2:V2)) has the best biodistribution profile in vivo and the lowest renal retention of radioactivity. The aim of this study was to evaluate Re-188-Z(HER2:v2) as a potential candidate for radionuclide therapy of human epidermal growth factor receptor type 2 (HER2)-expressing tumors. Methods: Z(HER2:V2) was labeled with Re-188 using a gluconate-containing kit. Targeting of HER2-overexpressing SKOV-3 ovarian carcinoma xenografts in nude mice was studied for a dosimetry assessment. Results: Binding of Re-188-Z(HER2:V2) to living SKOV-3 cells was demonstrated to be specific, with an affinity of 6.4 +/- 0.4 pM. The biodistribution study showed a rapid blood clearance (1.4 +/- 0.1 percentage injected activity per gram [%ID/g] at 1 h after injection). The tumor uptake was 14 +/- 2, 12 +/- 2, 5 +/- 2, and 1.8 +/- 0.5 %IA/g at 1, 4, 24, and 48 h after injection, respectively. The in vivo targeting of HER2-expressing xenografts was specific. Already at 4 h after injection, tumor uptake exceeded kidney uptake (2.1 +/- 0.2 %IA/g). Scintillation-camera imaging showed that tumor xenografts were the only sites with prominent accumulation of radioactivity at 4 h after injection. Based on the biokinetics, a dosimetry evaluation for humans suggests that Re-188-Z(HER2:v2) would provide an absorbed dose to tumor of 79 Gy without exceeding absorbed doses of 23 Gy to kidneys and 2 Gy to bone marrow. This indicates that future human radiotherapy studies may be feasible. Conclusion: (188)ReZ(HER2:v2) can deliver high absorbed doses to tumors without exceeding kidney and bone marrow toxicity limits.