Renal Uptake of 99mTc-Dimercaptosuccinic Acid Is Dependent on Normal Proximal Tubule Receptor–Mediated Endocytosis

Abstract

(99m)Tc-labeled dimercaptosuccinic acid ((99m)Tc-DMSA) accumulates in the kidney cortex and is widely used for imaging of the renal parenchyma. Despite its extensive clinical use, the mechanism for renal targeting of the tracer is unresolved. Megalin and cubilin are cooperating receptors essential to the proximal tubule endocytic uptake of proteins from the glomerular ultrafiltrate. We have used megalin/cubilin-deficient mice produced by gene knockout to determine whether receptor-mediated endocytosis is responsible for the renal uptake of (99m)Tc-DMSA. Control or megalin/cubilin-deficient mice were injected intravenously with 0.5 MBq of (99m)Tc-DMSA or (99m)Tc-mercaptoacetyltriglycine (MAG3). Whole-body scintigrams and the activity in plasma, urine, and the kidneys were examined 6 h after injection. The size and identity of (99m)Tc-DMSA-bound proteins in urine were analyzed by fractionation by centrifugation and separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by autoradiography and mass spectrometry. No renal accumulation of (99m)Tc-DMSA was identified in scintigrams of megalin/cubilin-deficient mice. The renal accumulated activity of the tracer was reduced to 11.4% (± 2.5%, n = 7) of the normal uptake in control mice, correlating with a reduction in renal megalin/cubilin expression in knockout mice to about 10% of normal. The reduced renal uptake in megalin/cubilin-deficient mice was accompanied by an increase in the urinary excretion of (99m)Tc-DMSA. Size separation of the urine by ultracentrifugation and sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that in megalin/cubilin-deficient mice an increased amount of (99m)Tc-DMSA was excreted in an approximately 27-kDa form, which by mass spectrometry was identified as the plasma protein α1-microglobulin, an established megalin/cubilin ligand. (99m)Tc-DMSA is filtered bound to α1-microglobulin and accumulates in the kidneys by megalin/cubilin-mediated endocytosis of the (99m)Tc-DMSA protein complex. Renal accumulation of (99m)Tc-DMSA is thus critically dependent on megalin/cubilin receptor function and therefore is a marker of proximal tubule endocytic activity.