Preferential Cleavage of a Tripeptide Linkage by Enzymes on Renal Brush Border Membrane To Reduce Renal Radioactivity Levels of Radiolabeled Antibody Fragments.

The obstructive renal radioactivity after injection of antibody fragments/constructs labeled with metallic radionuclides would be improved by liberating a radiometal chelate of urinary excretion from the antibody molecules by enzymes on the renal brush border membrane (BBM). A tripeptide GFK sequence was newly evaluated as an enzyme-cleavable linkage and conjugated to a 99m Tc chelate of an isonicotinic acid derivative of 2-picolylglycine (99m Tc-IPG). 99m Tc-IPG-glycine was liberated from 99m Tc-IPG-GFK by the enzymes, while 99m Tc-IPG-GK (where the tripeptide GFK was substituted with a dipeptide GK) did not. When injected into mice, 99m Tc-IPG-GFK-conjugated Fab exhibited lower renal radioactivity levels than directly radioiodinated Fab shortly after injection without reducing the tumor radioactivity levels, due to a release and excretion of 99m Tc-IPG-glycine by enzymes present on the renal BBM. These findings would provide insights to develop antibody fragments/constructs labeled with metallic radionuclides of the clinical relevance for improved renal radioactivity levels.