Technetium-99m labelled antimicrobial peptides discriminate between bacterial infections and sterile inflammations

The aim of this study was to select technetium-99m labelled peptides that c an discriminate between bacterial infections and sterile inflammations, For this purpose, we first assessed the binding of various Tc-99m-labelled nat ural or synthetic peptides, which are based on the sequence of the human an timicrobial peptide ubiquicidin (UBI) or human lactoferrin (hLF), to bacter ia and to leucocytes in vitro. In order to select peptides that preferentia lly bind to bacteria over host cells, radiolabelled peptides were injected into mice intraperitoneally infected with Klebsiella pneumoniae (K, pneumon iae) and the amount of radioactivity associated with the bacteria and with the leucocytes was quantitated. The next phase focussed on discrimination b etween bacterial infections and sterile inflammatory processes using Tc-99m -labelled peptides in mice intramuscularly infected with various bacteria ( e.g, multi-drug-resistant Staphylococcus aureus) and in animals that had be en injected with lipopolysaccharides (LPS) of bacterial origin to create a sterile inflammatory process. Also, we studied the distribution of Tc-99m-l abelled UBI 29-41 and UBI 18-35 in rabbits having an experimental thigh mus cle infection with K, pneumoniae and in rabbits injected with LPS. Based on the results of our in vitro and in vivo binding assays, two peptides, i.e. UBI 29-41 and UBI 18-35, were selected as possible candidates for infectio n imaging. The radiolabelled peptides can detect infections with both gram- positive and gram-negative bacteria in mice as early as 5-30 min after inje ction, with a target-to-nontarget (T/NT) ratio between 2 and 3; maximum T/N T ratios were seen within 1 h after injection. In rabbits, high T/NT ratios (>5) for Tc-99m-labelled UBI 29-41 were observed from 1 h after injection, No accumulation of the selected Tc-99m-labelled UBI-derived peptides was o bserved in thighs of mice and rabbits previously injected with LPS, Scintig raphic investigation into the biodistribution of Tc-99m-labelled UBI peptid es revealed that these peptides were rapidly removed fi om the circulation by renal excretion, Similar data were observed for Tc-99m-labelled defensin 1-3. Our data for Tc-99m-labelled hLF and related peptides indicate that t hese compounds are less favourable for infection detection. Taken together, Tc-99m-labelled UBI 18-35 and UBI 29-41 enable discrimination between bact erial infections and sterile inflammatory processes in both mice and rabbit s. Based on their characteristics, we consider these peptides the candidate s of preference for detection of bacterial infections in man.