The in vivo fate of various 111In-labeled polypeptides has been the subject of many investigations. Intracellular metabolism has been studied through the use of 111In-labeled glycoproteins that are concentrated in the lysosome by receptor-mediated endocytosis. These studies have indicated that the main lysosomal metabolite is 111In-chelate-ε-lysine, both in vitro and in vivo (Y. Arano et al., J. Nucl. 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 111In-labeled antibodies in nontarget tissues is important for the rational design of future radiolabeled antibodies.

We investigated the in vivo metabolism of 111In-DTPA3-conjugated antibody in female Sprague-Dawley rats using the anticolorectal carcinoma 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 % of the radioactivity from 111In-DTPA-1A3 MAb present in the liver was in the form of a low molecular weight species at 1 through 5 days. In contrast, 111In-DTPA-1A3-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 silica gel thin-layer chromatography, reversed phase high-performance liquid chromatography, and ion-exchange chromatography and compared to 111In-DTPA and 111In-DTPA-ε-lysine standards. In each system, the major metabolite co-eluted with 111In-DTPA-ε-lysine, similar to the results obtained with 111In-labeled glycoproteins that are delivered to lysosomes by receptor-mediated endocytosis. A minor metabolite that was more highly charged than 111In-DTPA was also observed. Analysis of urine and feces demonstrated that the main excretory product of both 111In-labeled intact 1A3 and 1A3-F(ab′)2 was 111In-DTPA-ε-lysine. Based on this data, we propose that 111In-DTPA-antibodies are degraded within lysosomes of nontarget organs such as the liver and kidneys.

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Presented at the “Fifth Conference on Radioimmunodetection and Radioimmunotherapy of Cancer,” October 6–8, 1994, Princeton, NJ. This work was funded by Department of Energy Grant DE-FG02-87ER-60512 (to M. J. W.) and the NIH (Grant CA-42925) with the National Cancer Institute (to M. J. W.).

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