Analysis of renal handling of radiopharmaceuticals.

Renal excretion is the preferable elimination pathway of radiopharmaceuticals and/or their metabolites from the body. The mechanisms of renal excretion involve glomerular filtration, tubular secretion, tubular reabsorption, and kidney metabolism. Radiopharmaceuticals of a molecular weight of up to 60 kDa are ultrafiltrated in the glomeruli at a rate dependent on their protein binding (the rate of glomerular filtration is a product of glomerular filtration rate and free fraction of the agent in plasma). Exclusive excretion by this route is typical for many chelates such us 99mTc-DTPA (diethylenetriaminepentaacetic acid). Some other radiopharmaceuticals are excreted in the renal tubuli into the tubular fluid by carrier-mediated and active processes involving different transport systems. Examples are ortho-iodohippurate and 99mTc-MAG-3 (mercaptoacetyltriglycine). Tubular reabsorption involves either passive diffusion of lipid-soluble radiopharmaceuticals from the glomerular filtrate back into blood or active, carrier-mediated process. Kidney metabolism and consequent renal uptake of the catabolites is of an outstanding importance for possible therapeutic applications of radiolabeled antibody fragments and peptides. These agents are partially reabsorbed from the ultrafiltrate by the cells of the renal proximal tubules by means of pinocytosis and subsequently degraded in the lysosomes. It limits their therapeutic use due to the potential radiation nephrotoxicity. For the analysis of elimination mechanisms of radiopharmaceuticals in the kidney, different approaches at various experimental levels (the kidney perfusion technique, isolated, functionally intact renal tubules, and isolated membranes) could be successfully employed.