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¹³¹I-Rituximab: Relationship Between Immunoreactivity and Specific Activity

¹ Division of Nuclear Medicine, University Hospital of Lausanne, Lausanne, Switzerland
² Division of Nuclear Medicine, University Hospital of Geneva, Geneva, Switzerland
³ Center for Radiopharmaceutical Science, Paul-Scherrer-Institute, Villigen, Switzerland

As part of a search for optimal conditions for radioimmunotherapy of lymphoma, rituximab was labeled with 2 different specific activities of ¹³¹I and immunoreactivity was comparatively measured. Methods: Labeling was performed with chloramine T using as starting conditions 185 MBq of ¹³¹I per 1 mg and per 5 mg of antibody for labelings A and B, respectively. Six comparative labelings were performed over a period of 10 mo with similar efficacy and purified by anion-exchange chromatography. Immunoreactivity was determined immediately after labeling in parallel assays using different concentrations of fresh Raji and Daudi cells. Results were compared at maximal observed specific binding on 10⁷ cells and after extrapolation to infinite antigen excess. A statistical analysis was performed to predict the frequency of radiolabeled mono- and polyiodinated antibodies: First, a gaussian distribution predicted the number of iodine atoms per antibody in labelings A and B, respectively; then, the radiolabeling probability was developed according to the Newton binome. Results: Final radiochemical purity was >98.4% for all labelings. The final mean specific activities were 169.7 MBq/mg and 32.8 MBq/mg, corresponding to 0.87 and 0.17 iodine atoms per antibody in labelings A and B, respectively. Labeling B showed a significantly higher immunoreactivity than did labeling A, the mean relative increase in binding being 28% for both Raji cells and Daudi cells. The predictive statistical analysis indicated that 57.3% and 15.4% of radiolabeled antibodies in labelings A and B, respectively, were polyiodinated. Conclusion: The low specific activity of ¹³¹I-rituximab allowed preservation of a high immunoreactivity and correlated with the prediction of a low percentage of polyiodinated radiolabeled antibodies.

Key Words: rituximab • radioiodination • immunoreactivity • polyiodination

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