| Abstract: |
Prolonged clearance kinetics have hampered the development of intact antibodies as imaging agents, despite their ability to effectively deliver radionuclides to tumor targets in vivo. Genetically engineered antibody fragments display rapid, high-level tumor uptake coupled with rapid clearance from the circulation in the athymic mouse/LS174T xenograft model. The anticarcinoembryonic antigen (CEA) T84.66 minibody (single-chain Fv fragment [scFv]-CH3 dimer, 80 kDa) and T84.66 diabody (non-covalent dimer of scFv, 55 kDa) exhibit pharmacokinetics favorable for radioimmunoimaging. The present work evaluated the minibody or diabody labeled with 124I, for imaging tumor-bearing mice using a high-resolution small-animal PET system. Methods: Labeling was conducted with 0.2-0.3 mg of protein and 65-98 MBq (1.7-2.6 mCi) of 124I using an iodination reagent. Radiolabeling efficiencies ranged from 33% to 88%, and immunoreactivity was 42% (diabody) or >90% (minibody). In vivo distribution was evaluated in athymic mice bearing paired LS174T human colon carcinoma (CEA-positive) and C6 rat glioma (CEA-negative) xenografts. Mice were injected via the tail vein with 1.9-3.1 MBq (53-85 μCi) of 124I-minibody or with 3.1 MBq (85 μCi) of 124I-diabody and imaged at 4 and 18 h by PET. Some mice were also imaged using 18F-FDG 2 d before imaging with 124I- minibody. Results: PET images using 124I-labeled minibody or diabody showed specific localization to the CEA-positive xenografts and relatively low activity elsewhere in the mice, particularly by 18 h. Target-to-background ratios for the LS174T tumors versus soft tissues using 124I-minibody were 3.05 at 4 h and 11.03 at 18 h. Similar values were obtained for the 124I-diabody (3.95 at 4 h and 10.93 at 18 h). These results were confirmed by direct counting of tissues after the final imaging. Marked reduction of normal tissue activity, especially in the abdominal region, resulted in high-contrast images at 18 h for the 124I-anti-CEA diabody. CEA-positive tumors as small as 11 mg (<3 mm in diameter) could be imaged, and 124I-anti-CEA minibodies, compared with 18F-FDG, demonstrated highly specific localization. Conclusion: 124I labeling of engineered antibody fragments provides a promising new class of tumor-specific probes for PET imaging of tumors and metastases. |
| Keywords: |
controlled study; unclassified drug; nonhuman; positron emission tomography; glioma; methodology; radiopharmaceuticals; quality control; mouse; animal; metabolism; animals; mice; animal tissue; image analysis; carcinoembryonic antigen; animal experiment; animal model; protein; in vivo study; cell line, tumor; immunoreactivity; histology; monoclonal antibody; colorectal neoplasms; soft tissue; imaging system; antibodies, monoclonal; correlation analysis; injection; antigen; antigens; diagnostic agent; radioactive iodine; isotope labeling; xenograft; nude mouse; tissue distribution; iodine radioisotopes; mice, nude; antigen specificity; colorectal tumor; tumor cell line; computer assisted emission tomography; radiopharmaceutical agent; scintiscanning; transplantation, heterologous; metabolic clearance rate; iodine 124; radioisotope; neoplasm transplantation; colon carcinoma; pet; molecular probe; tumor diagnosis; fragmentation reaction; prodrug; tomography, emission-computed; cancer transplantation; antibody engineering; prodrugs; antibody conjugate; immunoconjugates; 124i; radioimmunoassay; protein engineering; radioimmunoimaging; contrast radiography; humans; human; female; priority journal; article; engineered antibody fragments; neoembryonic antigen; anti cea scfv; anti-cea scfv
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