Radiolabeling of rituximab with 188Re and 99mTc using the tricarbonyl technology

• Published in: Nuclear medicine and biology 2011, Vol.38 (1), p.19-28
• Main Authors: Dias, Carla Roberta, Jeger, Simone, Osso, João Alberto, Müller, Cristina, De Pasquale, Christine, Hohn, Alexander, Waibel, Robert, Schibli, Roger
• Format: Article
• Language: English
• Subjects: 188Re; 99mTc; Antineoplastic agents; Biological and medical sciences; Chemotherapy; Contrast media. Radiopharmaceuticals; Medical sciences; Pharmacology. Drug treatments; Radioimmunoconjugate; Rituximab; Tricarbonyl core
• ISSN: 0969-8051; 1872-9614
• DOI: 10.1016/j.nucmedbio.2010.05.010

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 β − 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 99mTc- and 188Re-tricarbonyl core (IsoLink technology). The native format of the antibody (RTX wt) as well as a reduced form (RTX red) was labeled with 99mTc/ 188Re(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. The radiolabeling efficiency and kinetics of RTX red were superior to that of RTX wt ( 99mTc: 98% after 3 h for RTX red vs. 70% after 24 h for RTX wt). 99mTc(CO) 3-RTX red was used without purification for in vitro and in vivo studies whereas 188Re(CO) 3-RTX red was purified to eliminate free 188Re-precursor. Both radioimmunoconjugates were stable in human plasma for 24 h at 37°C. In contrast, displacement experiments with excess cysteine/histidine showed significant transchelation in the case of 99mTc(CO) 3-RTX red but not with pre-purified 188Re(CO) 3-RTX red. Both conjugates revealed high binding affinity to the CD20 antigen ( K d=5–6 nM). Tumor uptake of 188Re(CO) 3-RTX red was 2.5 %ID/g and 0.8 %ID/g for 99mTc(CO) 3-RTX red 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 99mTc(CO) 3-RTX red and for 188Re(CO) 3-RTX red 0.5 and 0.5 (24 h pi). Rituximab could be directly and stably labeled with the matched pair 99mTc/ 188Re 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.