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. 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. 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.

中文翻译：

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用于 PET 成像应用的镓 68 标记铁铬铁载体立体异构体的临床前表征

铁载体是微生物产生的小铁结合分子，有助于从环境中获取铁。放射性标记的铁载体为感染成像提供了一种有前景的解决方案，因为它们可以专门针对病原体的病理生理机制。Gallium-68 可以替代铁载体中的铁，从而实现正电子发射断层扫描 (PET) 的分子成像。立体特异性相互作用在受体、转运蛋白和铁利用的识别中发挥着至关重要的作用。此外，这些相互作用会对宿主环境产生影响，影响药代动力学和生物分布。本研究探讨了铁载体立体异构体对成像特性的影响，重点关注铁红素 (FR) 和铁红蛋白 (FRH)，这两种铁色素类型的顺反异构体铁载体。测试的铁载体用高放射化学纯度的镓68标记。所得复合物的体外特性有所不同。与[68Ga]Ga-FR相比，[68Ga]Ga-FRH表现出较低的亲水性和较高的蛋白质结合值。稳定性研究证实了所有检查介质中两种[68Ga]Ga-铁载体的高放射化学稳定性。发现这两种铁载体都被金黄色葡萄球菌、肺炎克雷伯菌和铜绿假单胞菌吸收，具有相似的功效。在正常小鼠中测试的生物分布显示，[68Ga]Ga-FR 具有快速的肾清除率和低血池滞留性，并且从检查的器官中快速清除[68Ga]Ga-FRH，而[68Ga]Ga-FRH 在血液中显示出中等的滞留性，导致药代动力学较慢。注射[68Ga]Ga-FR和[68Ga]Ga-FRH的小鼠的PET/CT成像证实了离体生物分布研究的结果。在金黄色葡萄球菌肌炎的小鼠模型中，两种放射性标记的铁载体均显示放射性示踪剂在感染部位积聚。铁红素和铁红素立体异构体的 68Ga 复合物显示出不同的药代动力学特征。体外摄取不受异构现象的影响。两种化合物均被相同的细菌培养物吸收，具有相似的功效。PET/CT 成像显示，[68Ga]Ga-复合物在金黄色葡萄球菌感染部位积聚，突出了[68Ga]Ga-FR 作为感染成像有前途的工具的潜力。相反，观察到[ 68 Ga]Ga-FRH 的放射性保留在血液中。总之，应考虑潜在放射性示踪剂的立体异构性，因为即使很小的结构差异也会影响其药代动力学，从而影响 PET 成像的结果。