Evaluation of Tc-99m-immunoglobulins for imaging infection in the rat

Abstract

Three immunoglobulin molecules were evaluated as infection imaging agents in a rat model of S. aureas infection: 99mTc-infliximab, 99mTc-human immunoglobulin (HIG) and 99mTc-rat immunoglobulin (RIG). Infliximab is a chimeric monoclonal antibody specific for human tumour necrosis factor alpha (TNFα). 99mTc-HIG was chosen as an exogenous protein and 99mTc-RIG as an endogenous marker. Each immunoglobulin was treated with 2-mercaptoethanol and the reduced antibody was isolated by size exclusion chromatography. In combination with Sn II-methylenediphosphonic acid, cold kit formulations were prepared. Native and reduced infliximab were tested for rat TNFα binding ability in vitro. A focal intramuscular infection of S. aureus (1 × 108 colony forming units) was induced in the left thigh muscle of rats, that developed for 24 h. In separate experiments each tracer was administered by intravenous injection, then whole body scintigraphic imaging and biodistribution studies were performed at 1 and 4 h later. 99mTc-infliximab, 99mTc-HIG and 99mTc-RIG were prepared with ≥95% radiochemical purity from stable cold kits. Results from the organ assay gave infected (target) to non-infected (control) muscle ratios for 99mTc-infliximab as 5.7 ± 0.8, 7.1 ± 1.2, 99mTc-HIG gave 3.1 ± 1.1, 7.8 ± 1.2, and 99mTc-RIG 7.9 ± 0.3, 12.5 ± 1.5 at 1 and 4 h, respectively. Infliximab and SnII-infliximab did not bind to rat TNFα by the in vitro assay. Although lacking specific affinity for TNFα, 99mTc-infliximab accumulated at infectious sites in vivo. 99mTc-infliximab gave similar infection uptake ratios to 99mTc-HIG at 1 and 4 h, but these proteins were inferior in comparison to 99mTc-RIG, and is likely to be due to increased clearance associated with the foreign protein structure. Copyright © 2006 John Wiley & Sons, Ltd.