Relative therapeutic efficacy of (125)I- and (131)I-labeled monoclonal antibody A33 in a human colon cancer xenograft.
A33, a monoclonal antibody that targets colon carcinomas, was labeled with (125)I or (131)I and the relative therapeutic efficacy of the 2 radiolabeled species was compared in a human colon cancer xenograft system.


METHODS
Nude mice bearing human SW1222 colon carcinoma xenografts were administered escalating activities of (125)I-A33 (9.25-148 MBq) or (131)I-A33 (0.925-18.5 MBq), (125)I- and (131)I-labeled control antibodies, unlabeled antibody, or no antibody. The effects of treatment were assessed using the endpoints of tumor growth delay and cure.


RESULTS
Tumor growth delay increased with administered activity for all radiolabeled antibodies. Approximately 4.5 times more activity was required for (125)I-A33 to produce therapeutic effects that were equivalent to those of (131)I-A33. This ratio was approximately 7 for a nonspecific, noninternalizing isotype-matched, radiolabeled control antibody. Unlabeled A33 antibody had no effect on tumor growth. Approximately 10 times more activity of (125)I-A33 produced toxicity similar to that of (131)I-A33, and this ratio fell to approximately 6 for radiolabeled control antibody.


CONCLUSIONS
Treatment with (125)I-A33 resulted in a relative therapeutic gain of approximately 2 compared with (131)I-A33 in this experimental system.
Barendswaard
E C
EC
New York Branch, Ludwig Institute for Cancer Research, New York, New York, USA.
Humm
J L
JL
O'Donoghue
J A
JA
Sgouros
G
G
Finn
R D
RD
Scott
A M
AM
Larson
S M
SM
Welt
S
S
D000818:Animals; D000911:Antibodies, Monoclonal / Q000627:therapeutic use*; D003110:Colonic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes; D008562:Membrane Glycoproteins / Q000276:immunology*; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D016499:Radioimmunotherapy; D019275:Radiopharmaceuticals / Q000627:therapeutic use*; D014183:Transplantation, Heterologous
D000911:Antibodies, Monoclonal; C104258:GPA33 protein, human; C504395:Gpa33 protein, mouse; D007457:Iodine Radioisotopes; D008562:Membrane Glycoproteins; D019275:Radiopharmaceuticals