Epitope blocking: Positive and negative effects on the biodistribution of ¹²⁵I-Labeled anti-tac disulfide-stabilized Fv fragment of two antibodies against different epitopes of the circulating antigen

Prior in vivo studies using the ¹²⁵I-labeled anti-Tac disulfide-stabilized variable region fragment (¹²⁵I-anti-Tac dsFv) of monoclonal antibody in the presence of the circulating soluble alpha subunit of the interleukin-2 receptor (sIL-2Rα) have shown formation of complexes which interfere with biodistribution. In this study we evaluated the effects of preinjecting HuTac and 7G7/B6, two immunoglobulin Gs (IgGs) that recognize different epitopes of sIL-2Rα, on the biodistribution of ¹²⁵I-anti-Tac dsFv in mice bearing SP2/Tac tumor xenografts, which produce sIL-2Rα, or on nude mice injected with 500 ng of sIL-2Rα. We also evaluated the biodistribution in mice of ¹²⁵I-labelcd sIL-2Rα injected alone or with HuTac and 7G7/B6. Injection of either HuTac or 7G7/B6 resulted in complexes with the sIL-2Rα in serum. Injection of HuTac before ¹²⁵I-anti-Tac dsFv, in SP2/Tac tumor-bearing mice, resulted in faster clearance of the dsFv from the blood (7.6% ID/g at 30 min), compared to 23.2% ID/g for the no-antibody control; preinjection of 7G7/B6 prolonged the retention of ¹²⁵I-anti-Tac dsFv to 35.3%ID/g, with more complexes in serum. In mice pre-injected with 7G7/B6 the concentration of ¹²⁵I-anti-Tac dsFv in tumor was lower (5.2 ± 0.3% ID/g) than in mice preinjected with HuTac (7.9 ± 1.2% ID/g) or in the control group (5.6 ± 0.7% ID/g). In conclusion, while both IgGs formed complexes with sIL-2Rα and prolonged its retention, preinjection of 7G7/B6 was detrimental, because the increased circulating sIL-2Rα still had the epitope recognized by the dsFv available for binding and neutralized the anti-Tac dsFv upon injection, whereas preinjection of HuTac blocked the epitope.