Radiotracers for in situ infection imaging: Experimental considerations for in vitro microbial uptake of gallium-68-labeled siderophores

In vitro screening of gallium-68(⁶⁸Ga)-siderophores in pathogens relevant to infections is valuable for determining species specificity, their effect on cell viability, and potential clinical applications. As the recognition and internalization of siderophores relies on the presence of receptor- and/or siderophore-binding proteins, the level of uptake can vary between species. Here, we report in vitro uptake validation in Escherichia coli with its native siderophore, enterobactin (ENT) ([⁶⁸Ga]Ga-ENT), considering different experimental factors. Compared with other reporting methods of uptake, ‘% Added dose/10⁹ CFU/mL (% AD/10⁹ CFU/mL),’ considering the total viable count, showed a better comparison among microbial species. Later, in vitro screening with [⁶⁸Ga]Ga-desferrioxamine B (DFO-B) showed high uptake by Staphylococcus aureus and S. epidermidis; moderate uptake by Pseudomonas aeruginosa; poor uptake by E. coli, Candida albicans, and Aspergillus fumigatus; and no uptake by Enterococcus faecalis and C. glabrata. Except for S. epidermidis, [⁶⁸Ga]Ga-DFO-B did not reduce the cell viability.