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

The effect of radiolabeling liver-specific proteins on the in vivo disposition of radioactivity was investigated. The suitability of 111In and 125I as radiolabels for protein disposition studies in vivo was examined. Galactosylated and cationized bovine serum albumin were labeled with either 125I by the chloramine-T method or 111In, using 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-BZ-EDTA) or diethylenetriaminepentaacetic acid (DTPA) as bifunctional chelating agents (BCAs) and administered intravenously to rats. 125I radioactivity disappeared rapidly from the liver with subsequent excretion in the urine and bile, mainly in the TCA soluble fraction. 111In-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 125I. When the effect of BCA on excretion of 111In radioactivity was compared, no significant differences were observed in the urinary clearances. However, biliary excretion was significantly higher for 111In-SCN-BZ-EDTA-bound radioactivity. In conclusion, when compared with 125I, 111In labeling seems to more accurately characterize the in vivo distribution of liver-targeted proteins after their iv administration in rats and allows a more accurate pharmacokinetic evaluation to be performed.