Radiolabeling of rituximab with Re-188 and Tc-99m using the tricarbonyl technology

Introduction: The most successful clinical studies of immunotherapy in patients with non-Hodgkin's lymphoma (NHL) use the antibody rituximab (RTX) targeting CD20(+) B-cell tumors. Rituximab radiolabeled with beta(-) emitters could potentiate the therapeutic efficacy of the antibody by virtue of the particle radiation. Here, we report on a direct radiolabeling approach of rituximab with the Tc-99m- and Re-188-tricarbonyl core (IsoLink technology). Methods: The native format of the antibody (RTXwt) as well as a reduced form (RTXred) was labeled with Tc-99m/Re-188(CO)(3). The partial reduction of the disulfide bonds to produce free sulfhydryl groups (-SH) was achieved with 2-mercaptoethanol. Radiolabeling efficiency, in vitro human plasma stability as well as transchelation toward cysteine and histidine was investigated. The immunoreactivity and binding affinity were determined on Ramos and/or Raji cells expressing CD20. Biodistribution was performed in mice bearing subcutaneous Ramos lymphoma xenografts. Results: The radiolabeling efficiency and kinetics of RTXred were superior to that of RTXwt (Tc-99m: 98% after 3 h for RTXred vs. 70% after 24 h for RTXwt). Tc-99m(CO)(3)-RTXred was used without purification for in vitro and in vivo studies whereas Re-188(CO)(3)-RTXred was purified to eliminate free Re-188-precursor. Both radioimmunoconjugates were stable in human plasma for 24 h at 37 degrees C. In contrast, displacement experiments with excess cysteine/histidine showed significant transchelation in the case of Tc-99m(CO)(3)-RTXred but not with pre-purified Re-188(CO)(3)-RTXred. Both conjugates revealed high binding affinity to the CD20 antigen (K-d=5-6 nM). Tumor uptake of Re-188(CO)(3)-RTXred was 2.5 %ID/g and 0.8 %ID/g for Tc-99m(CO)(3)-RTXred 48 h after injection. The values for other organs and tissues were similar for both compounds, for example the tumor-to-blood and tumor-to-liver ratios were 0.4 and 0.3 for Tc-99m(CO)(3)-RTXred and for Re-188(CO)(3)-RTXred 0.5 and 0.5 (24 h pi). Conclusion: Rituximab could be directly and stably labeled with the matched pair Tc-99m/Re-188 using the IsoLink technology under retention of the biological activity. Labeling kinetics and yields need further improvement for potential routine application in radioimmunodiagnosis and therapy. (C) 2011 Elsevier Inc. All rights reserved.