Improved biodistribution of 125I-labeled anti-Tac disulfide-stabilized Fv fragment by blocking its binding to the alpha subunit of the interleukin 2 receptor in the circulation with preinjected humanized anti-Tac IgG.
Animal studies using radiolabeled anti-Tac disulfide-stabilized Fv (dsFv) monoclonal antibody have shown formation of complexes in serum with the soluble alpha subunit of the interleukin 2 receptor alpha (sIL-2R alpha). In this study, we improved the targeting of 125I-labeled anti-Tac dsFv to receptor-positive tumors in the presence of circulating receptor by preinjecting unlabeled humanized anti-Tac IgG antibody (HuTac IgG). We used mice bearing SP2/Tac tumor xenografts that express the IL-2R alpha. A positive correlation was seen between tumor size and the concentration of circulating receptor. Tumor-bearing mice were injected with 125I-labeled anti-Tac dsFv (400 ng), either alone or 15 min after injection of HuTac IgG. The 125I-labeled anti-Tac dsFv formed high molecular weight complexes with the sIL-2R alpha. The fraction of the dsFv present in the complexes increased as tumor size increased (greater sIL-2R alpha levels). The fractions of dsFv in the complexes were 9.9- to 11.6-fold higher when sIL-2R alpha was not blocked with preinjected HuTac IgG. The administration of a 12-fold molar excess of HuTac IgG over sIL-2R alpha resulted in >80% of the 125I activity present as the dsFv rather than in the complexes. Furthermore, the biodistribution of 125I-labeled anti-Tac dsFv was improved by blocking its binding to sIL-2R alpha by preinjecting HuTac IgG. Specifically, in the preinjected group, at 15 min postinjection, the 125I-labeled anti-Tac dsFv levels in tumor increased to 10.8% compared to 5.6% injected dose per gram in the non-preinjected group. In summary, our studies showed that preinjection of HuTac IgG can block the formation of complexes of circulating sIL-2R alpha and 125I-labeled anti-Tac dsFv. This blockade is associated with faster blood clearance, higher tumor uptake, and greater tumor:nontumor ratios of the radiolabeled antibody fragment.
Kobayashi
H
H
Department of Nuclear Medicine, National Cancer Institute, NIH, Bethesda, Maryland 20892-1180, USA.
Yoo
T M
TM
Drumm
D
D
Kim
M K
MK
Sun
B F
BF
Le
N
N
Webber
K O
KO
Pastan
I
I
Waldmann
T A
TA
Paik
C H
CH
Carrasquillo
J A
JA
D000818:Animals; D000911:Antibodies, Monoclonal / Q000493:pharmacokinetics; D004220:Disulfides / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D007128:Immunoglobulin Fragments / Q000378:metabolism*; D007074:Immunoglobulin G / Q000494:pharmacology*; D000922:Immunotoxins / Q000493:pharmacokinetics*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics*; D046911:Macromolecular Substances; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D009374:Neoplasms, Experimental / Q000378:metabolism / Q000648:ultrastructure; D015375:Receptors, Interleukin-2 / Q000097:blood / Q000276:immunology / Q000378:metabolism*; D014018:Tissue Distribution; D014183:Transplantation, Heterologous
D000911:Antibodies, Monoclonal; D004220:Disulfides; D007128:Immunoglobulin Fragments; D007074:Immunoglobulin G; D000922:Immunotoxins; D007457:Iodine Radioisotopes; D046911:Macromolecular Substances; D015375:Receptors, Interleukin-2