The identification of in vivo metabolites of antibodies, proteins, and polypeptides radiolabeled with metal radionuclides is important for understanding the mechanisms involved in the uptake and retention of

The in vivo fate of various “In-labeled polypeptides has been the subject of many investigations. Intracellular metabolism has been StUdied through the use of “In-labeled glycoproteins that are concentrated in the lysosome by receptor-mediated endocytosis. These studies have Indicated that the main lysosomal metabolite is “In-chelate-e-lysine, both in vitro and in vivo (Y. Arano et aL, J. NucI. Med., 35: 890—898, 1994; F. N. Franano et aL, Nucl. Med. Biol., 21: 1023—1034, 1994). Since the vast majority of radiolabeled antibodies do not localize within the target tissue, an understanding of the metabolism of “In-labeledantibodies in nontar get tissues Is important for the rational design of future radiolabeled antibodies. We investigated the in vivo metabolism of “11n.DTPA3-conjugated antibody in female Sprague-Dawley rats using the anticolorectal card noma monoclonal antibody (MAb) 1A3 and MAb 1A3-F(ab')2.Livers and kidneys were harvested from rats injected with either intact MAb or MAb fragments and analyzed by gel filtration chromatography. Thirty-five % ofthe radioactivity from Ҡ̃In-DTPA-1A3 MAb present in the liver was in the form of a low molecular weight species at 1 through 5 days. In contrast, @In-DTPA-lA3-F(ab')2 was >98% degraded to a low molecular weight species in the kidney after 1 day. In each case, the low molecular weight metabolites were collected and further analyzed by sifica gel thin-layer chromatography, reversed phase high-performance liquid chromatography, and ion-exchange chromatography and compared to “In-DTPAand Ҡ̃In-DTPA-e-lysine standards. In each system, the ma jor metabolite co-eluted with “In-DTPA-€-lysine, similar to the results obtained with WIn.labeled glycoproteins that are delivered to lysosomes by receptor-mediated endocytosis. A minor metabolite that was more highly charged than “In-DTPA was also observed. Analysis of urine and feces demonstrated that the main excretory product of both 1111n-labeled intact 1A3 and 1A3-F(ab')2 was †̃@In-DTPA-e-lysine. Based on this data, we propose that @In-DTPA-antibodies are degraded within lysosomes of nontarget organs such as the liver and kidneys.