Evaluation of 99mTc-immunoglobulins for imaging infection in the rat

Three immunoglobulin molecules were evaluated as infection imaging agents in a rat model of S. aureas infection: Tc-99m-infliximab, Tc-99m-human immunoglobulin (HIG) and Tc-99m-rat immunoglobulin (RIG). Infliximab is a chimeric monoclonal antibody specific for human tumour necrosis factor alpha (TNF alpha). Tc-99m-HIG was chosen as an exogenous protein and Tc-99m-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 TNF alpha binding ability in vitro. A focal intramuscular infection of S. aureus (1 x 10(8) colony forming units) was induced in the left thigh muscle of rats, that developed for 24h. In separate experiments each tracer was administered by intravenous injection, then whole body scintigraphic imaging and biodistribution studies were performed at 1 and 4h later. Tc-99m-infliximab, Tc-99m-HIG and Tc-99m-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 Tc-99m-infliximab as 5.7 +/- 0.8, 7.1 +/- 1.2, Tc-99m-HIG gave 3.1 +/- 1.1, 7.8 +/- 1.2, and Tc-99m-RIG 7.9 +/- 0.3, 12.5 +/- 1.5 at 1 and 4 h, respectively. Infliximab and Sn-II-infliximab did not bind to rat TNF alpha by the in vitro assay. Although lacking specific affinity for TNF alpha, Tc-99m-infliximab accumulated at infectious sites in vivo. Tc-99m-infliximab gave similar infection uptake ratios to Tc-99m-HIG at 1 and 4h, but these proteins were inferior in comparison to Tc-99m-RIG, and is likely to be due to increased clearance associated with the foreign protein structure. Copyright (c) 2006 John Wiley & Sons, Ltd.