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

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with At, 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). The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using -succinimidyl-3-At-astato-5-guanidinomethyl benzoate (-At-SAGMB). The cytotoxicity of At-SAGMB-5F7 and At-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 At-SAGMB-5F7 (0.7-3.0 MBq), At-SAGMB-VHH_1028 (1.0-3.0 MBq), and At-SAGMB-VHH_1028 and At-SAGMB-VHH_2001 (∼1.0 MBq). Clonogenic survival of BT474 cells was reduced after exposure to At-SAGMB-5F7 (D = 1.313 kBq/mL) whereas At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with At-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 At-SAGMB-5F7 and 8 of 11 mice treated with At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.