Radiolabeled Cationic Peptides for Targeted Imaging of Infection

Molecular probes targeting bacteria provide opportunities to target bacterial infections in vivo for both imaging and therapy. In the current study, we report the development of positron emission tomography (PET) probes for imaging of live bacterial infection based on the small molecules HLys-DOTA, a polycationic peptide synthesized as the D-isomer (RYWVAWRNRG) conjugated to 1, 4, 7, 10-tetraazacyclododecane-N′,N″,N‴,N-tetraacetic acid (DOTA) and AB1-HLys-DOTA, which includes an unnatural amino acid AB1 that preferentially binds to bacteria membrane lipids with amine groups via formation of iminoboronates. HLys-DOTA and AB1-HLys-DOTA peptides were radiolabeled with (64)Cu and investigated as PET imaging agents to track bacterial infection in vitro and in intramuscularly infected (IM) mice models. Cell uptake studies at 37°C in Staphylococcus aureus (SA) show higher uptake of (64)Cu-AB1-HLys-DOTA; 98.47 ± 3.54% vs (64)Cu-HLys-DOTA; 39.12 ± 3.27% at 24 h. Standard uptake values (SUV) analysis of the PET images resulted in mean SUV of 0.70 ± 0.08, 0.49 ± 0.04, and 0.31 ± 0.01 for (64)Cu-AB1-HLys-DOTA and 0.17 ± 0.06, 0.16 ± 0.02, and 0.13 ± 0.01 for (64)Cu-HLys-DOTA at 1, 4, and 24 h post injection, respectively, in the infected muscles. Similarly, in the biodistribution studies, dose uptake in the infected muscles was 4 times higher in the targeted (64)Cu-AB1-HLys-DOTA group than in the (64)Cu-HLys-DOTA group and 2‐3 times higher than in the PBS control group at 1, 4, and 24 h post injection. (64)Cu-AB1-HLys-DOTA was able to distinguish between SA-infected muscle and Pseudomonas aeruginosa (PA) infected muscle with lower mean SUV of 0.28 ± 0.10 at 1 h post injection. This illustrates the utility of the AB1 covalently targeting group in synergy with the HLys peptide, which noncovalently binds to bacterial membranes. These results suggest that (64)Cu-labeled AB1-HLys-DOTA peptide could be used as an imaging probe for detection of bacterial infection in vivo with specificity for Gram-positive bacteria.