L-Tyrosine Confers Residualizing Properties to a D-Amino Acid-Rich Residualizing Peptide for Radioiodination of Internalizing Antibodies

Purpose: The aims of the study were to develop and evaluate a novel residualizing peptide for labeling internalizing antibodies with 124I to support clinical development using immuno-positron emission tomography (PET). Methods: The anti-epidermal growth factor receptor antibody ch806 was radiolabeled directly or indirectly with isotopes and various residualizing peptides. Azido-derivatized radiolabeled peptides were conjugated to dibenzylcyclooctyne-derivatized ch806 antibody via click chemistry. The radiochemical purities, antigen-expressing U87MG.de2-7 human glioblastoma cell-binding properties, and targeting of xenografts at 72 hours post injection of all radioconjugates were compared. Biodistribution of 124I-PEG4-tptddYddtpt-ch806 and immuno-PET imaging were evaluated in tumor-bearing mice. Results: Biodistribution studies using xenografts at 72 hours post injection showed that 131I-PEG4-tptddYddtpt-ch806 tumor uptake was similar to 111In-CHX-A″-DTPA-ch806. 125I-PEG4-tptddyddtpt-ch806 showed a lower tumor uptake value but higher than directly labeled 125I-ch806. 124I-PEG4-tptddYddtpt-ch806 was produced at 23% labeling efficiency, 98% radiochemical purity, 25.9 MBq/mg specific activity, and 64% cell binding in the presence of antigen excess. Tumor uptake for 124I-PEG4-tptddYddtpt-ch806 was similar to 111In-CHX-A″-DTPA-ch806. High-resolution immuno-PET/magnetic resonance imaging of tumors showed good correlation with biodistribution data. Conclusions: The mixed d/l-enantiomeric peptide, dThr-dPro-dThr-dAsp-dAsp-Tyr-dAsp-dAsp-dThr-dPro-dThr, is suitable for radiolabeling antibodies with radiohalogens such as 124I for high-resolution immuno-PET imaging of tumors and for evaluation in early-phase clinical trials.