In-vivo evaluation of indium-111-diethylenetriaminepentaacetic acid-labelling for determining the sites and rates of protein catabolism in mice

Pharmacokinetic analyses of protein pharmaceuticals are of prime importance for their clinical application. Because many proteins have pharmacological activity at low concentrations, radiolabelling of proteins is widely used to identify the sites and determine the rates of protein catabolism in-vivo due to the high sensitivity of detection of radioactivity. Recently, a met allic radionuclide, In-111, has been used to trace the pharmacokinetics of proteins of interest after conjugation of the proteins with diethylenetriam inepentaacetic acid (DTPA). In this study, galactosyl-neoglycoalbumin (NGA) was reacted with the cyclic dianhydride of DTPA and labelled with In-111 t o estimate the validity of this radiolabelling procedure for pharmacokineti c analyses, For comparison, we also evaluated direct radioiodination, becau se directly-radioiodinated proteins are widely used to assess the pharmacok inetics of proteins of interest. The hepatic radioactivity profile after intravenous injection of [I-131]NGA or [In-111]DTPA-NGA into mice was analysed pharmacokinetically, and the fi rst-order rate constant representing the elimination of the respective radi ometabolite from hepatic parenchymal cells was determined. The results indicated that direct radioiodination is inappropriate for purs uing the pharmacokinetics of the proteins, because of rapid elimination of the radioactivity from the sites of protein catabolism. These findings also implied that the [In-111]DTPA label could be used to identify the cataboli c sites and determine the rates of catabolism of proteins with relatively s hort biological half-lives, although characterization of radiolabelled spec ies at the sites of accumulation would be required for accurate determinati on of the catabolic sites of proteins.