Imaging is increasingly used to detect and monitor bacterial infection. Both anatomic (X-rays, computed tomography, ultrasound, and MRI) and nuclear medicine ([¹¹¹In]-WBC SPECT, [¹⁸F]FDG PET) techniques are used in clinical practice but lack specificity for the causative microorganisms themselves. To meet this challenge, many groups have developed imaging methods that target pathogen-specific metabolism, including PET tracers integrated into the bacterial cell wall. We have previously reported the d-amino acid derived PET radiotracers d-methyl-[¹¹C]-methionine, d-[3-¹¹C]-alanine, and d-[3-¹¹C]-alanine-d-alanine, which showed robust bacterial accumulation in vitro and in vivo. Given the clinical importance of radionuclide half-life, in the current study, we developed [¹⁸F]3,3,3-trifluoro-d-alanine (d-[¹⁸F]-CF₃-ala), a fluorine-18 labeled tracer. We tested the hypothesis that d-[¹⁸F]-CF₃-ala would be incorporated into bacterial peptidoglycan given its structural similarity to d-alanine itself. NMR analysis showed that the fluorine-19 parent amino acid d-[¹⁹F]-CF₃-ala was stable in human and mouse serum. d-[¹⁹F]-CF₃-ala was also a poor substrate for d-amino acid oxidase, the enzyme largely responsible for mammalian d-amino acid metabolism and a likely contributor to background signals using d-amino acid derived PET tracers. In addition, d-[¹⁹F]-CF₃-ala showed robust incorporation into Escherichia coli peptidoglycan, as detected by HPLC/mass spectrometry. Based on these promising results, we developed a radiosynthesis of d-[¹⁸F]-CF₃-ala via displacement of a bromo-precursor with [¹⁸F]fluoride followed by chiral stationary phase HPLC. Unexpectedly, the accumulation of d-[¹⁸F]-CF₃-ala by bacteria in vitro was highest for Gram-negative pathogens in particular E. coli. In a murine model of acute bacterial infection, d-[¹⁸F]-CF₃-ala could distinguish live from heat-killed E. coli, with low background signals. These results indicate the viability of [¹⁸F]-modified d-amino acids for infection imaging and indicate that improved specificity for bacterial metabolism can improve tracer performance.

中文翻译：

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用于细菌感染成像的肽聚糖靶向 [18F]3,3,3-三氟-d-丙氨酸示踪剂

成像越来越多地用于检测和监测细菌感染。解剖学（X 射线、计算机断层扫描、超声和 MRI）和核医学（[ ¹¹¹ In]-WBC SPECT、[ ¹⁸ F]FDG PET）技术均用于临床实践，但缺乏对致病微生物本身的特异性。为了应对这一挑战，许多研究小组开发了针对病原体特异性代谢的成像方法，包括整合到细菌细胞壁中的 PET 示踪剂。我们之前报道过d-氨基酸衍生的PET放射性示踪剂d-甲基-[ ¹¹ C]-蛋氨酸、d- [3- ¹¹ C]-丙氨酸和d- [3- ¹¹ C]-丙氨酸-d-丙氨酸，这表明细菌在体外和体内积累强劲。鉴于放射性核素半衰期的临床重要性，在本研究中，我们开发了[ ¹⁸ F]3,3,3-三氟-d-丙氨酸（d -[ ¹⁸ F]-CF ₃ -ala），一种氟 18标记的示踪剂。我们测试了以下假设：鉴于d -[ ¹⁸ F]-CF _{3 -ala 与}d -丙氨酸本身的结构相似，它将被纳入细菌肽聚糖中。 NMR分析表明，氟19母体氨基酸d- [ ¹⁹ F]-CF ₃ -ala在人和小鼠血清中稳定。d -[ ¹⁹ F]-CF ₃ -ala 也是d - 氨基酸氧化酶的不良底物，该酶主要负责哺乳动物d - 氨基酸代谢，并且可能是使用d - 氨基酸衍生的 PET 示踪剂的背景信号的贡献者。此外，通过HPLC/质谱检测，d- [ ¹⁹ F]-CF _{3 -ala 表现出与}大肠杆菌肽聚糖的牢固结合。基于这些有希望的结果，我们通过用[ ¹⁸ F]氟化物置换溴前体，然后进行手性固定相HPLC，开发了d- [ ^{18 F]-CF} _{3 -ala的放射合成方法。出乎意料的是，}体外细菌对d- [ ¹⁸ F]-CF ₃ -ala的积累对于革兰氏阴性病原体，特别是大肠杆菌是最高的。在急性细菌感染的小鼠模型中，d- [¹⁸ F]-CF ₃ -ala 可以区分活的和热灭活的大肠杆菌，背景信号低。这些结果表明[ ¹⁸ F]-修饰的d-氨基酸用于感染成像的可行性，并表明改善细菌代谢的特异性可以改善示踪剂性能。