Synthesis and evaluation of a monoreactive DOTA derivative for indium-111-based residualizing label to estimate...

The purpose of this study was to develop an indium-111 (111In)-based residualizing label for estimating the pharmacokinetics of proteins. 1,4,7,10-Tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA), which produced a highly stable and hydrophilic 111In chelate, was selected as the chelating site, and the monoreactive DOTA derivative with a tetrafluorophenyl group as the protein binding site (mDOTA) was designed to avoid cross-linkings of proteins. mDOTA was synthesized with an overall yield of 11%. The stability in murine plasma, the radioactivity retention in the catabolic sites of proteins and the radiochemical yields of 111In-labelled proteins via mDOTA were investigated using human serum albumin (HSA), galactosyl-neoglycoalbumin (NGA) and cytochrome c (cyt c) as model proteins. 111In-labelled HSA via mDOTA was highly stable for 5 days after incubation in murine plasma. Long retention of radioactivity in the catabolic sites was observed after injection of 111In-DOTA-NGA in mice, due to the slow elimination of the radiometabolite from the lysosome. At a chelator concentration of 42.2 microM, 111In-DOTA-cyt c was produced with over 91% radiochemical yield. On the other hand, 111In-DOTA-lysine and 111In-DOTA were obtained with high radiochemical yields at lower chelator concentrations. These findings indicated that mDOTA would be an appropriate 111In-labelling agent for estimating protein pharmacokinetics. These findings also suggested that the introduction of a protein binding site at a position distal from the unmodified DOTA structure would be preferable to preparing 111In-DOTA-labelled proteins with higher specific activity.