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

Abstract

Copper-64 (⁶⁴Cu)-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 ⁶⁴Cu, 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 ⁶⁴Cu. The MVK linkage was recognized by the BBM enzymes to liberate [⁶⁴Cu]Cu-NOTA-Met although the recognition of the MVK sequence for the [⁶⁴Cu]Cu-NOTA-MVK derivative was reduced compared with that of its [⁶⁷Ga]Ga-counterpart, probably due to the difference in the charge of the metal-NOTA complexes. When injected into mice, [⁶⁴Cu]Cu-NOTA-MVK-Fab resulted in similar renal radioactivity levels to the ⁶⁷Ga-labeled counterpart. In addition, [⁶⁴Cu]Cu-NOTA-MVK-Fab resulted in lower renal radioactivity levels than those from ⁶⁴Cu-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 ⁶⁴Cu-labeled antibody fragments and useful for immuno-PET and radioimmunotherapy.