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

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 μM, 111In‐DOTA‐cytc 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.