Enhancing the Antiviral Efficacy of RNA-Dependent RNA Polymerase Inhibition by Combination with Modulators of Pyrimidine Metabolism

Qi Liu; Amita Gupta; Ayse Okesli-Armlovich; Wenjie Qiao; Curt R. Fischer; Mark Smith; Jan E. Carette; Michael C. Bassik; Chaitan Khosla

In mammalian cells, pyrimidine biosynthesis is a tightly regulated metabolic process. Two complementary pathways -de novo biosynthesis and pyrimidine salvage -are responsible for producing UTP and CTP for host as well as viral RNA synthesis (Figure 1 ). De novo pyrimidine biosynthesis is a resource-intensive process. In contrast, salvage occurs via phosphorylation of UMP and CMP derived from intracellular RNA degradation or via facilitated transport and phosphorylation of extracellular uridine, whose plasma concentration is tightly controlled in the low micromolar range (Traut, 1994) . Recently we discovered that GSK983, a broad-spectrum antiviral agent first reported in 2009 (Harvey et al., 2009) , is a potent inhibitor of dihydroorotate dehydrogenase (DHODH), a ratelimiting step in de novo pyrimidine biosynthesis (Deans et al., 2016) . In the course of those unbiased genome-wide studies, we also found that knockdown of uridine/cytidine kinase 2 (UCK2) and cytidine monophosphate kinase 1 (CMPK1) in the pyrimidine salvage pathway strongly sensitized cells to growth inhibition by GSK983 (Deans et al., 2016) . This finding was consistent with the observation that GSK983 often lacks antiviral efficacy in vivo despite high potency in vitro presumably due to salvage metabolism of circulating uridine by virus-infected cells (Traut, 1994; Wang et al., 2011) .