Control of radioactivity pharmacokinetics of 99mTc-HYNIC-labeled polypeptides derivatized with ternary ligand complexes

An enhancement of the target/nontarget ratio of radioactivity levels enables reliable diagnosis and therapy using polypeptide radiopharmaceuticals in nuclear medicine. In the present study, we investigated the effects of the physicochemical properties of radiometabolites on the radioactivity pharmacokinetics after administration of Tc-99m-labeled polypeptides using 6-hydrazinopyridine-3-carboxylic acid (HYNIC). Four ternary ligands (L) [3-benzoylpyridine (BP), 3-acetylpyridine (AP), 3-nicotinic acid (NIC), pyridine (PY)] with different lipophilicity were selected as coligands for the preparation of Tc-99m-HYNIC-polypeptides. Each of the ternary ligands tested provided Tc-99m-HYNIC-labeled galactosyl-neoalbumin (NGA) and Fab fragments of high stability with high radiochemical purity. Moreover, after administration of each Tc-99m-HYNIC-labeled NGA into normal mice, the respective ternary ligand [Tc-99m](HYNIC-lysine)(tricine)(L) complexes were generated as final radiometabolites in the hepatic lysosome. The partition coefficients of [Tc-99m](HYNIC-lysine)(tricine)(BP), [99mTc](HYNIC-lysine)(tricine)(AP), [99mTc](FrYNIC-lysine)(tricine)(NIC), and [Tc-99m(HYNIC-lysine)(tricine)(PY) were determined to be -2.21, -2.37, -2.93, and -2.73, respectively. Elimination rates of these radiometabolites from the lysosome were enhanced in the order of increasing lipophilicity of the radiometabolites. After injection of the four Tc-99m-HYNIC-labeled Fab fragments into normal mice, blood clearances of radioactivity were similar while radioactivity elimination rates from the kidney were enhanced in the order of increasing lipophilicity of the radiometabolites. The present study indicated that the lipophilicity of the radiometabolites constitutes one important factor affecting their elimination rates from the tissues. Thus, as ternary ligands facilitate alteration of the physicochemical properties of radiometabolites, the use of ternary ligand complexes might be applicable for controlling the pharmacokinetics of Tc-99m-labeled polypeptides.