Pretargeted cancer radioimmunotherapy and radioimaging using antibodies and antibody fragments
Over the last few decades, antibodies have become the mainstay of cancer diagnosis and therapeutics. In traditional radioimmunotherapy (RIT), tumor targeting antibodies are directly conjugated with radioisotopes and depending on the radionuclides's properties, the direct labeled antibodies can be used for diagnostic or therapeutic purposes. However, one of the major challenges of using radiolabeled antibodies for therapy is their long serum half-lives. It generally takes 5-7 days for antibodies to achieve maximum tumor binding. This slow blood clearance results in high normal tissue irradiation and a poor therapeutic index. This is exemplified by the fact that to date, only two radiolabeled antibodies have been approved by the FDA for radioimmunotherapy of cancer. … Thomsen-Friedenreich (TF) is a disaccharide (Galactose [beta]1-3 N-acetylgalactosamine) antigen, which is present on about [about]90% of carcinomas. The TF expression on the tumor cell is correlated with poor prognosis and tumor propagation. TF antigen is also involved in cell to cell adhesion and metastasis, making it a very good target for cancer imaging and therapy. Using phage display technology, TF binding scFv fragments were selected from the McCafferty antibody library. The selected scFv clones were characterized in vitro for their TF specificity and cell binding properties by ELISA and flow-cytometry assay. The selected TF specific clone (9C-scFv) was radiolabeled with [99m]Tc by directly conjugating [99m]Tc to the C-terminal 6x His-tag. The [99m]Tc-labeled 9C-scFv was injected in mice bearing MDA-MB-231 human breast cancer xenografts. The SPECT/CT images, acquired 4 hours post injection, revealed a moderate tumor uptake of radiolabeled scFvs with significant accumulation in the liver and kidneys. The phage display derived single-chain scFv fragments against the TF antigen demonstrated potential for development as an imaging agent but requires more work to achieve favorable pharmacokinetics.
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