Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose211At-Labeled Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with ²¹¹At, because of their rapid accumulation in tumor and clearance from normal tissues. Here, we evaluate the therapeutic potential of this strategy with 5F7 and VHH_1028—2 sdAbs that bind with high affinity to domain IV of human epidermal growth factor receptor type 2 (HER2). Methods: The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using N-succinimidyl-3-²¹¹At-astato-5-guanidinomethyl benzoate (iso-²¹¹At-SAGMB). The cytotoxicity of iso-²¹¹At-SAGMB-5F7 and iso-²¹¹At-SAGMB-VHH_2001 were compared on HER2-expressing BT474 breast carcinoma cells. Three experiments in mice with subcutaneous BT474 xenografts were performed to evaluate the therapeutic effectiveness of single doses of iso-²¹¹At-SAGMB-5F7 (0.7–3.0 MBq), iso-²¹¹At-SAGMB-VHH_1028 (1.0–3.0 MBq), and iso-²¹¹At-SAGMB-VHH_1028 and iso-²¹¹At-SAGMB-VHH_2001 (∼1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso-²¹¹At-SAGMB-5F7 (D₀ = 1.313 kBq/mL) whereas iso-²¹¹At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with ²¹¹At-labeled HER2-specific 5F7 and VHH_1028 but not with HER2-irrelevant VHH_2001. At the 3.0-MBq dose, complete tumor regression was seen in 3 of 4 mice treated with iso-²¹¹At-SAGMB-5F7 and 8 of 11 mice treated with iso-²¹¹At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Conclusion: Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the iso-²¹¹At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.