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-2R alpha) 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-2R alpha, on the biodistribution of 125I-anti-Tac dsFv in mice bearing SP2/Tac tumor xenografts, which produce sIL-2R alpha, or on nude mice injected with 500 ng of sIL-2R alpha. We also evaluated the biodistribution in mice of 125I-labeled sIL-2R alpha injected alone or with HuTac and 7G7/B6. Injection of either HuTac or 7G7/B6 resulted in complexes with the sIL-2R alpha 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-2R alpha and prolonged its retention, preinjection of 7G7/B6 was detrimental, because the increased circulating sIL-2R alpha 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.
Kobayashi
H
H
Department of Nuclear Medicine, Warren G. Magnuson Clinical Center, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892-1180, USA.
Sun
B F
BF
Han
E S
ES
Kim
M K
MK
Le
N
N
Wang
Q C
QC
Nelson
D L
DL
Pastan
I
I
Waldmann
T A
TA
Paik
C H
CH
Carrasquillo
J A
JA
D000818:Animals; D019138:Antibodies, Blocking / Q000378:metabolism; D000911:Antibodies, Monoclonal / Q000493:pharmacokinetics*; D004220:Disulfides; D005260:Female; D007128:Immunoglobulin Fragments / Q000378:metabolism*; D007074:Immunoglobulin G / Q000276:immunology; D007135:Immunoglobulin Variable Region / Q000378:metabolism*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D015375:Receptors, Interleukin-2 / Q000276:immunology* / Q000378:metabolism; D014018:Tissue Distribution; D014407:Tumor Cells, Cultured
D019138:Antibodies, Blocking; D000911:Antibodies, Monoclonal; D004220:Disulfides; D007128:Immunoglobulin Fragments; D007074:Immunoglobulin G; D007135:Immunoglobulin Variable Region; D007457:Iodine Radioisotopes; D015375:Receptors, Interleukin-2