N7-Methyltransferase inhibitors of SARS-CoV-2: an unusual therapeutic target against COVID-19 – MetInCoV

The health emergency caused by the Covid-19 pandemic highlights the need to identify effective treatments against the virus (SARS-CoV-2). The first clinical trials have been testing repurposed drugs that show promising anti-SARS-CoV-2 effects in cultured cells. Although more than 2400 clinical trials are already under way, the actual number of tested compounds is still limited to approximately 20, alone or in combination. In addition, knowledge on their mode of action is currently insufficient. Importantly, efforts should also be made to test new compounds and strategies with a documented mode of action against SARS-CoV-2. The discovery of potent and specific antiviral inhibitors to fight this massive outbreak is an urgent research priority. The multidisciplinary approach as we propose in this project that combines molecular design, chemical synthesis and robust in vitro assays will accelerate the development of new antiviral molecules targeting an original enzymatic activity.

Enzymes involved in the capping pathway of viruses and more specifically RNA N7- or 2-O-methyltransferases (MTases) are now validated as potential targets for antiviral chemotherapy. Indeed, the N7- and 2-O-methylations of the viral RNA cap are key events for the viral infection as their inhibition might limit the synthesis of viral proteins and support virus elimination by stimulation of the immune response. Few viral MTase inhibitors have been developed so far, however SAM-mimetics acting as competitors of the MTase co-substrate merit attention. The rarity of specific inhibitors for viral MTases constitutes a stimulating challenge for new antiviral therapy but also for functional studies of these enzymes.

The proposal addresses the physio-pathogenesis of the COVID-19 with the development of novel nucleoside antivirals against the attractive but a less explored molecular target: SARS-CoV-2 N7-MTase nsp14. The new compounds to be synthesized have been designed and optimized from preliminary SAM-analogs derivatives displaying effective and specific inhibition in the submicromolar range against SARS-CoV nsp14. In addition, several compounds have shown a notable inhibitory activity (EC = 25 microM) in infected VeroE6 cells by SARS-CoV-2 and this, with low cytotoxicity (CC50 > 100 microM). These preliminary promising results prompt us to extend the series of the most potent derivatives. Molecular docking and previous conclusions are useful for designing new nucleoside SAM analogs. Enzymatic and cellular screening assays will be performed to validate the candidate molecules against SARS-CoV-2. Our approach is original, as the molecular target (nsp14) is an unusual target compared with the broadly targeted SARS-CoV-2 RNA polymerase (nsp12). In addition, our approach is based on strong and recent results obtained on SARS-CoV N7-MTase and thus the mode of action of the potential antiviral compounds is already assessed.