Detection of sites of infection in mice using 99mTc-labeled PN2S-PEG conjugated to UBI and 99mTc-UBI: a comparative biodistribution study

The antimicrobial peptide ubiquicidin (UBI) directly labeled with technetium-99m (99mTc) has recently been shown to be specifically taken up at sites of infection; however, its chemical structure is not well defined. To address this problem, the aim of the present study was to label UBI using poly(ethyleneglycol)-N-(N-(3-diphenylphosphinopropionyl)glycyl) -S-tritylcysteine ligand (PEG-PN2S) in order to compare its ability to detect infection sites with that of 99mTc-UBI. Methods: The PN2S-PEG-UBI conjugate was prepared and labeled with 99mTc, and its radiochemical purity was subsequently assessed. The stability of the conjugate to cysteine challenge and dilution with both saline solution and phosphate buffer was determined and serum stability and protein binding were also assessed. In vivo studies were carried out in healthy mice to study the biodistribution of 99mTc-PN2S-PEG-UBI and its precursor 99mTc-PN2S-PEG and in infected mice to compare the uptakes of 99mTc-UBI and 99mTc-PN2S-PEG-UBI at the site of infection using scintigraphic imaging and ex vivo tissue counting. Results: 99mTc-PN2S-PEG-UBI was obtained with high radiochemical purity (98±1%) and high stability. The amphiphilic nature of the conjugate leads to a tendency to form micellar aggregates that explain the high protein binding values obtained. Biodistribution studies in mice showed low renal clearance followed by a predominant reticuloendothelial system clearance that limits its application in the abdominal area. Statistical analysis revealed no significant difference between 99mTc-UBI and 99mTc-PN2S-PEG-UBI uptake in infected mouse thigh, and the site of infection was clearly visualized using scintigraphic imaging. Conclusions: 99mTc-PN2S-PEG-UBI proved to be as effective as 99mTc-UBI in detecting sites of infection; however, the well-defined chemical structure of 99mTc-PN2S-PEG-UBI makes it a better candidate for clinical imaging of infection