Feasibility of affibody-based bioorthogonal chemistry-mediated radionuclide pretargeting

Affibody molecules constitute a new class of probes for radionuclide tumor targeting. The small size of Affibody molecules is favorable for rapid localization in tumors and clearance from circulation. However, high renal reabsorption of Affibody molecules prevents the use of residualizing radiometals, including several promising low-energy β- and α-emitters, for radionuclide therapy. We tested a hypothesis that Affibody-based pretargeting mediated by a bioorthogonal interaction between trans-cyclooctene (TCO) and tetrazine would provide higher accumulation of radiometals in tumor xenografts than in the kidneys. Methods: TCO was conjugated to the anti-human epidermal growth factor receptor 2 (HER2) Affibody molecule Z₂₃₉₅. DOTA-tetrazine was labeled with ¹¹¹In and ¹⁷⁷Lu. In vitro pretargeting was studied in HER2- expressing SKOV-3 and BT474 cell lines. In vivo studies were performed on BALB/C nu/nu mice bearing SKOV-3 xenografts. Results: ¹²⁵I-Z₂₃₉₅-TCO bound specifically to HER2-expressing cells in vitro with an affinity of 45 ± 16 pM. ¹¹¹In-tetrazine bound specifically and selectively to Z₂₃₉₅-TCO pretreated cells. In vivo studies demonstrated HER2- specific ¹²⁵I-Z₂₃₉₅-TCO accumulation in xenografts. TCO-mediated ¹¹¹In-tetrazine localization was shown in tumors, when the radiolabeled tracer was injected 4 h after an injection of Z₂₃₉₅-TCO. At 1 h after injection, the tumor uptake of ¹¹¹In-tetrazine and¹⁷⁷Lu-tetrazine was approximately 2-fold higher than the renal uptake. Pretargeting provided more than a 56-fold reduction of renal uptake of ¹¹¹In in comparison with direct targeting. Conclusion: The feasibility of Affibody-based bioorthogonal chemistry-mediated pretargeting was demonstrated. The use of pretargeting provides a substantial reduction of radiometal accumulation in kidneys, creating preconditions for palliative radionuclide therapy.