Disposition of radioactivity after injection of liver-targeted proteins labeled with In-111 or I-125. Effect of labeling on distribution and excretion of radioactivity in rats

The effect of radiolabeling liver-specific proteins on the in vivo disposit ion of radioactivity was investigated. The suitability of In-111 and I-125 as radiolabels for protein disposition studies in vivo was examined. Galact osylated and cationized bovine serum albumin were labeled with either I-125 by the chloramine-T method or In-111, using 1-(4-isothiocyanatobenzyl)ethy lenediaminetetraacetic acid (SCN-BZ-EDTA) or diethylenetriaminepentaacetic acid (DTPA) as bifunctional chelating agents (BCAs) and administered intrav enously to rats. I-125 radioactivity disappeared rapidly from the liver wit h subsequent excretion in the urine and bile, mainly in the TCA soluble fra ction. In-111-associated radioactivity, on the other hand, remained in the hepatic tissue in considerably higher amounts during the experiment and was excreted in the bile and urine to a lower extent when compared with I-125. When the effect of BCA on excretion of In-111 radioactivity was compared, no significant differences were observed in the urinary clearances. However , biliary excretion was significantly higher for In-111-SCN-BZ-EDTA-bound r adioactivity. In conclusion, when compared with I-125, In-111 labeling seem s to more accurately characterize the in vivo distribution of liver-targete d proteins after their iv administration in rats and allows a more accurate pharmacokinetic evaluation to be performed.