In vivo and in vitro characterizations of three 99mTc-labeled monoclonal antibody G250 preparations.

UNLABELLED

In previous clinical studies, excellent visualization of tumor lesions has been observed with 131I-labeled monoclonal antibody (mAb) G250 in patients with renal cell carcinoma (RCC). In several cases, 131I-cG250 immunoscintigraphy disclosed tumor lesions that were not visualized by radiography or CT. To improve image quality, we aimed to develop a 99mTc-labeled mAb G250 preparation for radioimmunodetection of RCC. We studied in vitro stability, biodistribution and imaging potential of three 99mTc-labeled G250 preparations in nude mice with subcutaneous RCC xenografts.125I-G250 and the nonspecific mAb 131I-MN14 were used as control antibodies.

METHODS

The mAb G250 was labeled with 99mTc according to three methods using: (a) S-hydrazinonicotinamide (HYNIC), (b) S-benzoylmercaptoacetyltriglycine (MAG3) and (c) a direct labeling method (Schwarz method). The stability of all preparations was tested in serum at 37 degrees C during 48 h. In addition, diethylenetriamine pentaacetic acid, cysteine and glutathione challenge assays were performed.

RESULTS

All preparations showed good stability in serum during the 48-h incubation period. 99mTc-G250 (Schwarz) showed release of the radiolabel at a 100-fold or higher molar excess of cysteine and at a 10,000-fold or higher molar excess of glutathione. 99mTc-MAG3-G250 showed release of the radiolabel at a 10,000-fold molar excess of cysteine. 99mTc-HYNIC-G250 was stable under all conditions. Tumors were clearly visualized with all preparations. 99mTc-G250 (Schwarz) showed significantly lower blood levels (3.8 %ID/g) compared with all other preparations (11.2, 13.4 and 13.4 %ID/g for 99mTc-HYNIC-G250, 99mTc-MAG3-G250 and 125I-G250, respectively, 48 h postinjection). At 48-h postinjection, mean tumor uptake was very high with all mAb G250 preparations: 92.4 (99mTc-HYNIC-G250), 125.9 (99mTc-MAG3-G250), 29.4 (99mTc-G250 Schwarz) and 75.4 (125I-G250) %ID/g. Mean tumor uptake of the nonspecific 131I-MN14 mAb was 6.6 %ID/g.

CONCLUSION

In this study, 99mTc-HYNIC-G250 showed excellent in vitro stability and tumor targeting. Moreover, this preparation could be labeled with high efficiency (>95%) at room temperature within 15 min. Therefore, 99mTc-HYNIC-G250 seems to be an ideal candidate for radioimmunodetection of RCC.