Optimal quality I-131-monoclonal antibodies on high-dose labeling in a large reaction volume and temporarily coating the antibody with IODO-GEN

A novel, facile procedure for efficient coupling of high doses of I-131 to monoclonal antibodies (MAbs) was developed with minimal chemical and radiat ion damage. Methods: To diminish the radiation and chemical burden during l abeling, iodination was performed in a large reaction volume and by tempora rily coating the MAb with a minimal amount of IODO-GEN. The MAb was coated by injection of IODO-GEN (dissolved in acetonitrile [MeCN]) into the aqueou s MAb solution, and the coating was subsequently removed by addition of asc orbic acid. For chemoprotection before, during, and after PD-10 purificatio n of the I-131-MAbs, ascorbic acid and human serum albumin were used. The e ffects of autoradiolysis in the starting I-131 solution were countered by t reatment with NaOH and ascorbic acid. For this so-called IODO-GEN-coated MA b method, the sensitive chimeric MAb MOv18 (c-MOv18) and the more robust mu rine MAbs K928 and E48 were used. The high-dose I-131-labeled MAbs were cha racterized for radiochemical purity and MAb integrity by thin-layer chromat ography, high-performance liquid chromatography, and sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by phosphor imager quantificat ion. The high-dose I-131-labeled MAbs were also characterized for immunorea ctivity. The radiopharmacokinetics and biodistribution of I-131-c-MOv18 wer e analyzed in human tumor-bearing nude mice. For comparison, I-131-c-MOv18 batches were made using the conventional chloramine-T or IODO-GEN-coated vi al method. Results: Conventional high-dose labeling of 5 mg c-MOv18 with 4. 4 GBq I-131 resulted in a labeling yield of 60%, a radiochemical purity of 90%, an immunoreactive fraction of 25% (72% being the maximum in the assay used), and the presence of aggregation and degradation products. Using simi lar amounts of I-131 and MAb in the IODO-GEN-coated MAb method, 85%-89% ove rall radiochemical yield, at least 99.7% radiochemical purity, and full pre servation of MAb integrity and immunoreactivity were achieved. For this lab eling, 5 mg MAb were coated with 35 mug IODO-GEN during 3 min in a reaction volume of 6 mL. Also, biodistribution was optimal, and tumor accumulation was superior to that of coinjected I-125-c-MOv18 labeled according to the c onventional IODO-GEN-coated vial method. Conclusion: A new, facile, high-do se I-131-labeling method was developed for production of I-131-labeled MAbs with optimal quality for use in clinical radioimmunotherapy.