Epitope Blocking: Positive and Negative Effects on the Biodistribution of (125)I‐Labeled Anti‐Tac Disulfide‐stabilized Fv Fragment of Two Antibodies against Different Epitopes of the Circulating Antigen

Prior in vivo studies using the (125)I‐labeled anti‐Tac disulfide‐stabilized variable region fragment ((125)I‐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 (125)I‐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 (125)I‐labeled 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 (125)I‐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 (125)I‐anti‐Tac dsFv to 35.3%ID/g, with more complexes in serum. In mice pre‐injected with 7G7/B6 the concentration of (125)I‐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.