In vitro and in vivo comparison of siderophores labeled with 68Ga and 89Zr
Siderophores are high-affinity iron-chelating compounds, serving as carrier molecules transporting iron across the microbial cell membranes. Some 68Ga-siderophores seem favorable in imaging of infection where specificity and sensitivity are lacking. In this context, a recent publication  has compared the in vitro, in vivo and ex vivo behavior of selected siderophores labeled with 68Ga and 89Zr, considering that the latter might constitute a radioisotope-labeling alternative for imaging of infection.
Labeling with 68Ga and 89Zr
The selected siderophores were triacetylfusarinine C (TAFC), Desferriferichrome A (FCHA), ferrioxamine E (FOXE) and Desferrioxamine B (FOXB), which were labeled with 68Ga and 89Zr in HEPES or sodium acetate buffer systems, as required for each siderophore. In vitro analyses comprised stability tests, partition coefficient determination and definition of protein-binding affinities. In vivo (PET and CT imaging) and ex vivo assays (biodistribution analyses) were performed in Balb/c mice.
Differences were established in the labeling efficiency among the examined siderophores, as well as in the stability studies. High stability values were obtained for 68Ga-labeled TAFC and FOXE and for 89Zr-labeled TAFC, FOXE and FOXB (> 98% in human serum at all tested times). The highest protein-binding affinity for 68Ga-siderophores was approximately 11% for 68Ga-FOXB at 120 h, whereas after 24 h of incubation 89Zr-FOXE reached 16.6% and 89Zr-FCHA evinced 31.1% of protein binding.
Biodistribution and pharmacokinetics in mice were found to be similar for the 68Ga- and 89Zr-labeled siderophores TAFC, FCHA and FOXB, which were rapidly excreted by the renal system and had minimal retention in blood and other organs. 89Zr-FOXE showed significantly higher radioactivity accumulation in the intestines and liver, whereas 68Ga-FOXE had similar biodistribution to that of the other labeled siderophores, with marginally higher radioactivity values in the gastrointestinal tract. 89Zr-siderophores consistently displayed slightly higher bone uptake compared to their 68Ga-labeled counterparts, though still lower than 1% ID/g in all cases.
Overall, 89Zr- and 68Ga-labeled siderophores exhibited analogous in vitro and in vivo behavior. This way, 89Zr-siderophores are suggested as an alternative for longitudinal PET studies of fungal infections, and particularly employing TAFC for the development of novel bioconjugates.
*References* (Click on the PMID to see abstracts from PubMed/NCBI)
 Petrik M, Zhai C, Novy Z, Urbanek L, Haas H, Decristoforo C. In Vitro and In Vivo Comparison of Selected 68Ga and 89Zr Labelled Siderophores. Mol Imaging Biol. 2016 Jun; 18(3): 344-52. PMID: 26424719.