Positive-sense single-stranded RNA (+RNA) viruses have proven to be important pathogens that are able to threaten and deeply damage modern societies, as illustrated by the ongoing COVID-19 pandemic. Therefore, compounds active against most or many +RNA viruses are urgently needed. Here, we present PR673, a helquat-like compound that is able to inhibit the replication of SARS-CoV-2 and tick-borne encephalitis virus in cell culture. Using in vitro polymerase assays, we demonstrate that PR673 inhibits RNA synthesis by viral RNA-dependent RNA polymerases (RdRps). Our results illustrate that the development of broad-spectrum non-nucleoside inhibitors of RdRps is feasible.

Department of Experimental Biology Faculty of Science Masaryk University 62500 Brno Czech Republic

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic v v i Flemingovo nám 2 16610 Prague Czech Republic

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branišovská 1160 31 37005 Ceske Budejovice Czech Republic

Laboratory of Emerging Viral Diseases Veterinary Research Institute Hudcova 296 70 62100 Brno Czech Republic

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