In vivo and in vitro characterizations of three Tc-99m-labeled monoclonal antibody G250 preparations

In previous clinical studies, excellent visualization of tumor lesions has been observed with (131)l-labeled monoclonal antibody (mAb) G250 in patient s with renal cell carcinoma (RCC). In several cases,(131)l-cG250 immunoscin tigraphy disclosed tumor lesions that were not visualized by radiography or CT. To improve image quality, we aimed to develop a Tc-99m-labeled mAb G25 0 preparation for radioimmunodetection of RCC. We studied in vitro stabilit y, biodistribution and imaging potential of three Tc-99m-labeled G250 prepa rations in nude mice with subcutaneous RCC xenografts.(125)l-G250 and the n onspecific mAb (131)l- MN14 were used as control antibodies. Methods: The m Ab G250 was labeled with Tc-99m according to three methods using: (a) S-hyd razinonicotinamide (HYNIC), (b) S-benzoylmercapto-acetyltriglycine (MAG3) a nd (c) a direct labeling method (Schwarz method). The stability of all prep arations was tested in serum at 37 degrees C during 48 h. In addition, diet hylenetriamine pentaacetic acid, cysteine and glutathione challenge assays were performed. Results: All preparations showed good stability in serum du ring the 48-h incubation period. Tc-99m-G250 (Schwarz) showed release of th e radiolabel at a 100-fold or higher molar excess of cysteine and at a 10,0 00-fold or higher molar excess of glutathione. Tc-99m-MAG3-G250 showed rele ase of the radiolabel at a 10,000-fold molar excess of cysteine. (TC)-T-99m -HYNIC-G250 Was stable under all conditions. Tumors were clearly visualized with all preparations. Tc-99m-G250 (Schwarz) showed significantly lower bl ood levels (3.8 %ID/g) compared with all other preparations (11.2, 13.4 and 13.4 %ID/g for (TC)-T-99m-HYNIC-G250, (99m)TcMAG3-G250 and (125)l-G250, re spectively, 48 h postinjection). At 48-h postinjection, mean tumor uptake w as very high with all mAb G250 preparations: 92.4 ((TC)-T-99m-HYNIC-G250), 125.9 ((99m)TCMAG3-G250), 29.4 (Tc-99m-G250 Schwarz) and 75.4 (125)l-G250) %ID/g. Mean tumor uptake of the nonspecific (131)l-MN14,Ab was 6.6 %ID/g. C onclusion: In this study, Tc-99m-HYNIC-G250 showed excellent in vitro stabi lity and tumor targeting. Moreover, this preparation could be labeled with high efficiency (>95% at room temperature within 15 min. Therefore, Tc-99m- HYNIC-G250 seems to be an ideal candidate for radioimmunodetection of RCC.