PET imaging of viral infection

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.