Renal Handling of ^99mTc-Labeled Antibody Fab Fragments with a Linkage Cleavable by Enzymes on Brush Border Membrane

The high and persistent renal radioactivity levels after injection of radiolabeled low-molecular-weight polypeptides constitute a significant problem for their diagnostic and therapeutic applications, especially when they are labeled with metallic radionuclides. To improve the renal radioactivity levels of technetium-99m (^99mTc)-labeled Fab fragments, a mercaptoacetyltriglycine (MAG₃)-based new bifunctional chelating agent with a cleavable glycyl-phenylalanyl-lysine (GFK) linkage, MAG₃-GFK-suc-TFP, was designed, synthesized, and evaluated. ^99mTc-labeled Fab was obtained by reacting MAG₃-GFK-Fab conjugate with ^99mTc-glucarate. The GFK linkage remained stable in plasma but was cleaved by enzymes on the renal brush border membrane. The comparative biodistribution studies with indium-111 (¹¹¹In)-labeled Fab using SCN-CHX-A″-DTPA showed that while both radiolabeled Fabs exhibited similar elimination rates from the blood, [^99mTc]Tc-MAG₃-GFK-Fab registered much lower renal radioactivity levels from 30 min post-injection onward due to the release and subsequent urinary excretion of [^99mTc]Tc-MAG₃-Gly. However, [^99mTc]Tc-MAG₃-GFK-Fab showed an increase in the intestinal radioactivity levels with the time that was not observed with ¹¹¹In-labeled Fab. The analysis of the intestinal contents suggested the redistribution of [^99mTc]Tc-MAG₃-Gly to the intestine. The retrospective comparison of [^99mTc]Tc-MAG₃-GFK-Fab with the radiolabeled Fabs so far prepared under the identical concept suggested that some portion of [^99mTc]Tc-MAG₃-Gly was generated after the coated vesicle formation and they were excreted into the blood, and subsequently redistributed in the intestine via hepatobiliary excretion. In conclusion, MAG₃-GFK-suc-TFP provided ^99mTc-labeled Fabs that exhibit low renal radioactivity shortly after injection by the post-labeling procedure. The present study indicated that, contrary to our earlier proposal, the generation of the radiometabolites would proceed not only during the internalization process of the parental antibody fragments but also after coated vesicle formation. This study also showed that the intracellular behaviors of radiometabolites played crucial roles in the elimination rates and the routes of the radioactivity from the kidney.