Synthetic affibody molecules: A novel class of affinity ligands for molecular imaging of HER2-expressing malignant tumors

The Affibody molecule Z_{HER2:342-pep2}, site-specifically and homogeneously conjugated with a 1,4,7,10-tetra-azacylododecane-N,N′, N″,N‴-tetraacetic acid (DOTA) chelator, was produced in a single chemical process by peptide synthesis. DOTA-_{ZHER2:342-pep2} folds spontaneously and binds HER2 with 65 pmol/L affinity. Efficient radiolabeling with >95% incorporation of ¹¹¹In was achieved within 30 min at low (room temperature) and high temperatures (up to 90°C). Tumor uptake of ¹¹¹In-DOTA-Z_{HER2:342-pep2} was specific for HER2-positive xenografts. A high tumor uptake of 23% injected activity per gram tissue, a tumor-to-blood ratio of >7.5, and high-contrast gamma camera images were obtained already 1 h after injection. Pretreatment with Herceptin did not interfere with tumor targeting, whereas degradation of HER2 using the heat shock protein 90 inhibitor 17-allylaminogeldanamycin before administration of ¹¹¹In-DOTA-Z_{HER2:342-pep2} obliterated the tumor image. The present results show that radiolabeled synthetic DOTA-Z_{HER2:342-pep2} has the potential to become a clinically useful radiopharmaceutical for in vivo molecular imaging of HER2-expressing carcinomas.