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

Single-domain antibody fragments (sdAbs) are attractive for targeted a-particle therapy, particularly with At-211, 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_10282 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-At-211-astato-5-guanidinomethyl benzoate (iso-At-211-SAGMB). The cytotoxicity of iso-At-211-SAGMB-5F7 and iso(-211)At-SAGMBVHH_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-At-211-SAGMB-5F7 (0.7-3.0 MBq), iso-(211)AtSAGMB-VHH_1028 (1.0-3.0 MBq), and iso-At-211-SAGMB-VHH_1028 and iso-At-211-SAGMB-VHH_2001 (similar to 1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso-(211)AtSAGMB-5F7 (D-0= 1.313 kBq/mL) whereas iso-At-211-SAGMB-VHH_ 2001 was ineffective. Dose-dependent tumor growth inhibition was observed with At-211-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-At-211-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-At-211-SAGMB residualizing prosthetic agent is a promising strategy for targeted alpha-particle therapy of HER2-expressing cancers.

Automated summary

This study evaluates the therapeutic potential of single-domain antibody fragments (sdAbs) combined with alpha-particle therapy for HER2-expressing cancers. The results demonstrate that HER2-specific sdAbs, such as 5F7 and VHH_1028, show better efficacy in tumor growth inhibition compared to HER2-irrelevant VHH_2001, suggesting that combining high-affinity HER2-targeted sdAbs with a residualizing prosthetic agent is a promising strategy for targeted alpha-particle therapy.