PET imaging of viral infection

许多病毒通过利用宿主细胞提供的核苷酸进行复制来维持寄生生活方式。的 由SARS-CoV-2（SARS 2）引起的2019冠状病毒病（COVID-19）的表现和进展， 一种RNA病毒，伴随着病毒在受感染的宿主中利用宿主的机器进行扩增。的 SARS 2的快速扩增需要通过RNA合成的补救途径增加RNA合成，大多数 特别是在非分裂细胞中，这些细胞是SARS 2感染的目标。这提供了一个机会 如果可获得指示病毒RNA合成的成像示踪剂，则跟踪病毒感染的状态。 与宿主相似，病毒相关的RNA合成（病毒基因组RNA或病毒信使RNA）发生在 三个阶段：起始、延伸和终止。我们将重点关注病毒相关RNA的延伸 合成，这是重复添加核苷一磷酸（NMP）的3'端的增长 RNA链。由于SARS 2需要大量的宿主细胞尿苷，其基因组的1/3是尿苷，尿苷是 而且是亚基因组正链RNA合成/病毒功能所必需的。因此，放射性标记的 应使用尿苷通过PET成像跟踪嘧啶代谢，以增强RNA合成， SARS 2感染。然而，掺入病毒RNA的尿苷代谢的特异性补救途径 在SARS 2感染的组织中合成没有上调。然而，5-FU可以直接转化为尿苷 单磷酸（UMP）用于掺入RNA。因此，我们建议使用F-18标记的前体药物5- 氟尿嘧啶（5-[18 F]FU，与5-FU相同的分子，F-19被F-18同位素取代）代替。 尽管如此，5-[18F]FU的快速催化剂使PET图像的解释变得模糊，直到添加 eniluracil（5-乙炔基尿嘧啶），可防止5-[18 F]FU的催化作用，以获得有意义的5- [18F]FU摄取。因此，我们建议将5-[18F]FU重新用于SARS 2感染的PET成像， 在拟定的PET成像期间，将使用相同的恩尿嘧啶暂时保持5-[18 F]FU的完整性。 我们将使用现成的临床相关小鼠模型测试5-[18 F]FU的PET成像 肝炎病毒（MHV）感染。MHV是一种在结构上与SARS 2非常相似的鼠冠状病毒， 在小鼠和大鼠中广泛的疾病，包括肝炎、脑脊髓炎、肠炎和呼吸道疾病 疾病在人类SARS 2感染期间，许多相同的器官受到类似的影响， 急性肺损伤相关死亡率在公众认知中是COVID-19的象征。 5-[18F]FU PET成像的临床效用将补充基于PCR的COVID-19检测（黄金 标准）或相关肺部疾病的基于CT的评估（非特异性评估）。它也可以用来 促进新的抗病毒药物的开发（除了remdesivir），以优化治疗剂量和治疗 持续时间，以及将来的治疗监测。

Many viruses sustain a parasitic lifestyle by exploiting host cell supplies of nucleotides for replication. The manifestation and progression of Corona Virus Disease 2019 (COVID-19) caused by SARS-CoV-2 (SARS2), an RNA virus, is accompanied by the expansion of the virus in the infected host using host's machinery. The rapid SARS2 expansion requires augmented RNA synthesis via the salvage pathway of RNA synthesis, most especially in the non-dividing cells that are the targets of SARS2 infection. This provides an opportunity to trace the status of viral infection if an imaging tracer that is indicative of viral RNA synthesis is available. Similar to the host, viral related RNA synthesis (viral genomic RNA or viral messenger RNA) occurs in three stages: initiation, elongation and termination. We will focus on the elongation of viral related RNA synthesis, which is the repeated addition of a nucleoside monophosphate (NMP) to the 3’ end of the growing RNA chain. Since SARS2 requires massive amounts of host cell uridine, 1/3 of its genome is uridine, uridine is moreover required for sub-genome positive strand RNA synthesis/viral function. Accordingly, radiolabeled uridine shall be used to track pyrimidine metabolism by PET imaging for augmented RNA synthesis under SARS2 infection. However, the specific salvage pathway of uridine metabolism for incorporation into viral RNA synthesis was not up-regulated in SARS2 infected tissues. Yet, 5-FU can be converted directly into uridine monophosphate (UMP) for incorporated into RNA. We therefore propose to use F-18 labeled pro-drug 5- fluorouracil (5-[18F]FU, the same molecule as 5-FU with isotopic substitution of F-19 by F-18) instead. Still, the rapid catabolism of 5-[18F]FU obscures interpretation of the PET images until the addition of eniluracil (5-ethynyluracil) that prevents the catabolism of 5-[18F]FU to obtain meaningful PET images of 5- [18F]FU uptake. Consequently, we propose to re-purpose 5-[18F]FU for PET imaging of SARS2 infection, and will use the same eniluracil to transiently maintain the integrity of 5-[18F]FU during the proposed PET imaging. We will test PET imaging with 5-[18F]FU using the readily available and clinically relevant model of mouse hepatitis virus (MHV) infection. MHV, a murine coronavirus very similar to SARS2 in structure, can cause a wide range of diseases in the mice and rats, including hepatitis, encephalomyelitis, enteritis, and, respiratory diseases. Many of these same organs were similarly affected during human SARS2 infection even though the acute lung injury related mortality symbolized COVID-19 in the public perception. Clinical utility of 5-[18F]FU PET imaging would be complementing PCR-based testing of COVID-19 (gold standard) or CT-based assessment (non-specific evaluation) of related lung disease. It could also be used to facilitate new anti-viral drug development (beyond remdesivir) for optimizing therapeutic dose and treatment duration, and for treatment monitoring in the future.