Radiolabeled products in rat liver and serum after administration of antibody—amide—DTPA—indium-111

Anti-human serum albumin antibody (Ab) was used as a model antibody. Ab was conjugated with DTPA using cyclic DTPA dianhydride reaction and radiolabeled with ¹¹¹In. The labeled Ab was purified by affinity chromatography. Size exclusion HPLC of this product showed 62% of ¹¹¹In bound to monomeric Ab and 38% of the activity bound to antibody oligomers with molecular weights ranging from 300,000 to 450,000. The labeled antibody preparation was injected into the tail vein of rats. The radioactive substances in serum and the supernatant from liver homogenates were analyzed for molecular weight and immunoreactivity. Size exclusion HPLC of the serum samples indicated that the monomeric and dimeric Abs disappeared from the serum at a similar rate over a 48 h period. In addition, a new radioactive substance with an estimated molecular weight of 35,000 appeared in the serum. The immunoreactive fraction of the circulating ¹¹¹In substances decreased slowly, somewhat proportional to the appearance of the metabolite. On the other hand, the immunoreactivity of the ¹¹¹In substances in the supernatant from the liver homogenate decreased rapidly and no appreciable immunoreactivity was observed after 48 h. The labeled antibody was catabolized very rapidly in the liver and the major activity in the supernatant was associated with a small molecular weight metabolite which had a HPLC retention time identical to that of DTPA-¹¹¹In. The second metabolite had an estimated molecular weight of 35,000. No radioactivity was associated with transferrin.