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

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 iso-[211At]­AGMB-PODS, a prosthetic agent that combines a functionality
for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone
(PODS) moiety for site-specific sdAb conjugation. Iso-[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. Iso-[211At]­AGMB-PODS-5F7GGC was considerably more stable in vitro than its maleimide analogue in the presence of
cysteine and human serum albumin (HSA) and exhibited excellent tumor
uptake and high in vivo 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 iso-[211At]­AGMB-PODS-5F7GGC
warrants further evaluation for the treatment of HER2-expressing malignancies.