Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose ²¹¹At-Labeled Anti-HER2 Single Domain Antibody Fragment

Abstract

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy (TAT), 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 - two 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 1) iso-[²¹¹At]SAGMB-5F7 (0.7-3.0 MBq), 2) iso-[²¹¹At]SAGMB-VHH_1028 (1.0-3.0 MBq), and 3) 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) while 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 TAT of HER2-expressing cancers.