Abstract
1080
Objectives: Radiolabeled low molecular weight peptides exhibit high and persistent localization of radioactivity in the kidney after injection, which has hindered tumor visualization and limited therapeutic effectiveness. To solve this problem, we have recently developed radiogallium labeling agent (NOTA-MVK) with a linkage cleaved by enzymes present on the brush border membrane (BBM) lining the lumen of the renal tubule and demonstrated that 67Ga-labeled antibody fragment using this radiolabeling agent could reduce renal radioactivity levels. In this study, 64Cu-labeled antibody fragment was prepared using this radiolabeling agent and the renal radioactivity levels of the 64Cu-labeled antibody fragment were evaluated.
Methods: 64Cu-labeled S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (SCN-Bn-NOTA) was conjugated with an antibody Fab fragment through a Met-Val-Lys linkage (64Cu-NOTA-MVK-Fab) considering that a Met-Val sequence is a substrate of enzymes on the renal BBM and 64Cu-NOTA-Met is excreted from the kidney into the urine. The enzymatic recognition of the linkage was evaluated with a low-molecular-weight 64Cu-NOTA-Met-Val-Lys derivative. Biodistribution of radioactivity after injection of 64Cu-NOTA-MVK-Fab into mice was compared with a conventional thiourea linkage (64Cu-NOTA-Fab) and 67Ga-NOTA-MVK-Fab.
Results: The MVK linkage was recognized by enzymes on renal BBM to liberate 64Cu-NOTA-Met. When injected into mice, both 64Cu-labeled Fabs and 67Ga-NOTA-MVK-Fab exhibited similar blood clearance rates. Significant differences were observed in the kidney where 64Cu-NOTA-MVK-Fab registered lower renal radioactivity levels from early postinjection time (p < 0.05) compared with 64Cu-NOTA-Fab and similar renal radioactivity levels to 67Ga-NOTA-MVK-Fab. The tumor radioactivity levels of 64Cu-NOTA-MVK-Fab was similar to those of 64Cu-NOTA-Fab.
Conclusions: These findings indicated that our proposal of liberating a radiolabeled compound to urinary excretion from antibody fragments at the renal BBM to reduce the renal radioactivity levels was applicable to 64Cu-labeled antibody fragments. Since 64Cu-labeled antibody fragments and 67Ga-labeled antibody fragments showed similar biodistribution profiles, radiolabeling agent for low molecular weight peptides, NOTA-MVK, would be useful for radiotheranostics using 67/68Ga and 64/67Cu.