Site-Specific Radiohalogenation of a HER2-Targeted Single-Domain Antibody Fragment Using a Novel Residualizing Prosthetic Agent

Abstract

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter ²¹¹At. Herein, we have evaluated <i>iso</i>-[²¹¹At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. <i>Iso</i>-[²¹¹At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail. Both radiohalogenated PODS-5F7GGC conjugates were synthesized in good radiochemical yields and retained high binding affinity on HER2-positive BT474 breast carcinoma cells. <i>Iso</i>-[²¹¹At]AGMB-PODS-5F7GGC was considerably more stable <i>in vitro</i> than its maleimide analogue in the presence of cysteine and human serum albumin (HSA) and exhibited excellent tumor uptake and high <i>in vivo</i> stability. Superior tumor-to-kidney activity ratios were seen for both radiohalogenated PODS-5F7GGC conjugates compared with [¹⁷⁷Lu]Lu-DOTA-PODS-5F7GGC. These results suggest that <i>iso</i>-[²¹¹At]AGMB-PODS-5F7GGC warrants further evaluation for the treatment of HER2-expressing malignancies.