Copper-64-Labeled Antibody Fragments for Immuno-PET/Radioimmunotherapy with Low Renal Radioactivity Levels and Amplified Tumor-Kidney Ratios

[Image: see text] Copper-64 ((64)Cu)-labeled antibody fragments such as Fab are useful for molecular imaging (immuno-PET) and radioimmunotherapy. However, these fragments cause high and persistent localization of radioactivity in the kidneys after injection. To solve this problem, this study assessed the applicability of a molecular design to (64)Cu, which reduces renal radioactivity levels by liberating a urinary excretory radiometabolite from antibody fragments at the renal brush border membrane (BBM). Since 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) forms a stable complex with Cu, NOTA-conjugated Met-Val-Lys-maleimide (NOTA-MVK-Mal), which is a radio-gallium labeling agent for antibody fragments, was evaluated for applicability to (64)Cu. The MVK linkage was recognized by the BBM enzymes to liberate [(64)Cu]Cu-NOTA-Met although the recognition of the MVK sequence for the [(64)Cu]Cu-NOTA-MVK derivative was reduced compared with that of its [(67)Ga]Ga-counterpart, probably due to the difference in the charge of the metal-NOTA complexes. When injected into mice, [(64)Cu]Cu-NOTA-MVK-Fab resulted in similar renal radioactivity levels to the (67)Ga-labeled counterpart. In addition, [(64)Cu]Cu-NOTA-MVK-Fab resulted in lower renal radioactivity levels than those from (64)Cu-labeled Fab using a conventional method, without a reduction in the tumor radioactivity levels. These findings indicate that our approach to reducing renal radioactivity levels by liberating a radiolabeled compound from antibody fragments at the renal BBM for urinary excretion is applicable to (64)Cu-labeled antibody fragments and useful for immuno-PET and radioimmunotherapy.