PT - JOURNAL ARTICLE AU - Barendswaard, Els C. AU - Humm, John L. AU - O’Donoghue, Joseph A. AU - Sgouros, George AU - Finn, Ronald D. AU - Scott, Andrew M. AU - Larson, Steven M. AU - Welt, Sydney TI - Relative Therapeutic Efficacy of <sup>125</sup>I- and <sup>131</sup>I-Labeled Monoclonal Antibody A33 in a Human Colon Cancer Xenograft DP - 2001 Aug 01 TA - Journal of Nuclear Medicine PG - 1251--1256 VI - 42 IP - 8 4099 - http://jnm.snmjournals.org/content/42/8/1251.short 4100 - http://jnm.snmjournals.org/content/42/8/1251.full SO - J Nucl Med2001 Aug 01; 42 AB - A33, a monoclonal antibody that targets colon carcinomas, was labeled with 125I or 131I 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 125I-A33 (9.25–148 MBq) or 131I-A33 (0.925–18.5 MBq), 125I- and 131I-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 125I-A33 to produce therapeutic effects that were equivalent to those of 131I-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 125I-A33 produced toxicity similar to that of 131I-A33, and this ratio fell to approximately 6 for radiolabeled control antibody. Conclusion: Treatment with 125I-A33 resulted in a relative therapeutic gain of approximately 2 compared with 131I-A33 in this experimental system.