Biomimetic Analogues of the Desferrioxamine E Siderophore for PET Imaging of Invasive Aspergillosis: Targeting Properties and Species Specificity.

The pathogenic fungus Aspergillus fumigatus utilizes a cyclic ferrioxamine E (FOXE) siderophore to acquire iron from the host. Biomimetic FOXE analogues were labeled with gallium-68 for molecular imaging with PET. [68Ga]Ga(III)-FOXE analogues were internalized in A. fumigatus cells via Sit1. Uptake of [68Ga]Ga(III)-FOX 2-5, the most structurally alike analogue to FOXE, was high by both A. fumigatus and bacterial Staphylococcus aureus. However, altering the ring size provoked species-specific uptake between these two microbes: ring size shortening by one methylene unit (FOX 2-4) increased uptake by A. fumigatus compared to that by S. aureus, whereas lengthening the ring (FOX 2-6 and 3-5) had the opposite effect. These results were consistent both in vitro and in vivo, including PET imaging in infection models. Overall, this study provided valuable structural insights into the specificity of siderophore uptake and, for the first time, opened up ways for selective targeting and imaging of microbial pathogens by siderophore derivatization.

Preclinical characterisation of gallium-68 labeled ferrichrome siderophore stereoisomers for PET imaging applications.

BACKGROUND: Siderophores are small iron-binding molecules produced by microorganisms to facilitate iron acquisition from the environment. Radiolabelled siderophores offer a promising solution for infection imaging, as they can specifically target the pathophysiological mechanisms of pathogens. Gallium-68 can replace the iron in siderophores, enabling molecular imaging with positron emission tomography (PET). Stereospecific interactions play a crucial role in the recognition of receptors, transporters, and iron utilisation. Furthermore, these interactions have an impact on the host environment, affecting pharmacokinetics and biodistribution. This study examines the influence of siderophore stereoisomerism on imaging properties, with a focus on ferrirubin (FR) and ferrirhodin (FRH), two cis-trans isomeric siderophores of the ferrichrome type. RESULTS: Tested siderophores were labelled with gallium-68 with high radiochemical purity. The resulting complexes differed in their in vitro characteristics. [68Ga]Ga-FRH showed less hydrophilic properties and higher protein binding values than [68Ga]Ga-FR. The stability studies confirmed the high radiochemical stability of both [68Ga]Ga-siderophores in all examined media. Both siderophores were found to be taken up by S. aureus, K. pneumoniae and P. aeruginosa with similar efficacy. The biodistribution tested in normal mice showed rapid renal clearance with low blood pool retention and fast clearance from examined organs for [68Ga]Ga-FR, whereas [68Ga]Ga-FRH showed moderate retention in blood, resulting in slower pharmacokinetics. PET/CT imaging of mice injected with [68Ga]Ga-FR and [68Ga]Ga-FRH confirmed findings from ex vivo biodistribution studies. In a mouse model of S. aureus myositis, both radiolabeled siderophores showed radiotracer accumulation at the site of infection. CONCLUSIONS: The 68Ga-complexes of stereoisomers ferrirubin and ferrirhodin revealed different pharmacokinetic profiles. In vitro uptake was not affected by isomerism. Both compounds had uptake with the same bacterial culture with similar efficacy. PET/CT imaging showed that the [68Ga]Ga-complexes accumulate at the site of S. aureus infection, highlighting the potential of [68Ga]Ga-FR as a promising tool for infection imaging. In contrast, retention of the radioactivity in the blood was observed for [68Ga]Ga-FRH. In conclusion, the stereoisomerism of potential radiotracers should be considered, as even minor structural differences can influence their pharmacokinetics and, consequently, the results of PET imaging.

ETHANOL AS A MODIFIER OF RADIATION SENSITIVITY OF LIVING CELLS AGAINST UV-C RADIATION.

The protection of Escherichia coli bacteria and the yeast Saccharomyces cerevisiae against UV-C radiation by ethanol was studied. It was found that the fraction of surviving cells increases with increasing ethanol concentration. The specific protection depends on the dose rate, concentration range of ethanol, and it is higher for yeast compared to the bacteria.

RADIOPROTECTIVE EFFECT OF HYDROXYL RADICAL SCAVENGERS ON PROKARYOTIC AND EUKARYOTIC CELLS UNDER VARIOUS GAMMA IRRADIATION CONDITIONS.

The influence of various hydroxyl radical scavengers such as methanol, ethanol and dimethyl sulfoxide on radiation sensitivity of prokaryotic cells (bacteria Escherichia coli) and eukaryotic cells (yeast Saccharomyces cerevisiae and V79 cells-Chinese hamster pulmonary fibroblasts) irradiated by 60Co gamma radiation was investigated. The dependence of radiation sensitivity on dose rate in range from 1.8 to 100 Gy h-1 was evaluated. Survival of cells irradiated by increasing dose rates was followed using clonogenic assay. Specific protective effect was found to be a nonmonotonous function of dose rate with typical maximum at the dose rate range from 50 to 55 Gy h-1 in all studied cell types.

Effects of irradiation conditions on the radiation sensitivity of microorganisms in the presence of OH-radical scavengers.

PURPOSE: The purpose of the paper was to investigate the protective effect of some scavengers of OH radicals (hydroxyl radicals) on the radiation sensitivity of bacteria (in some cases also yeast) under normoxic (in air) or hypoxic (suboxic) conditions and to compare the obtained results with those published earlier for the yeast, all in a wide interval of irradiation conditions. Another aim was to investigate a possible impact of the reaction order of the reaction between the scavengers and the OH radicals on the protection of the cells. MATERIALS AND METHODS: In order to study the protective effect of OH scavengers we used various concentrations of methanol and potassium formate (in some cases also ethanol) in isotonic salt solutions. These solutions containing living bacteria (Escherichia coli) or yeast (Saccharomyces cerevisiae) were irradiated with 60Co radiation using various doses and dose rates. Irradiation was performed in air, in some cases under the hypoxic conditions. The number of surviving cells was determined prior to and after irradiation in suspension with and without scavengers. The surviving fractions after irradiation with and without scavenger were evaluated. RESULTS: The surviving fraction of bacterial cells increases linearly with increasing concentration of both scavengers. The fraction of surviving cells does not increase with increasing concentration of the scavengers under suboxic conditions. The protective effect Ϭ increases linearly with increasing scavenging efficiency and this dependence is much sharper under normoxic conditions than under suboxic ones. The specific protection k is much higher for the methanol than for the potassium formate. CONCLUSIONS: The basic characteristics of the impact of scavengers of OH radicals on radiation sensitivity of both bacteria and yeast are the same in a wide interval of doses and dose rates. The specific protection effect is much higher under the normoxic conditions. This protective effect is inversely proportional to the rate constant of the reaction between the scavenger and the OH radicals. It seems to be obvious that the presence of oxygen during irradiation is a necessary condition for the protective action of the scavengers which may be partially controlled by some transport processes and may be connected with the radiation sensitivity of the cells. On the other hand, the change of the reaction order of the reaction of the scavenger with the OH radicals has turned out to be unimportant.