Inhibitors of cellular RSK isoforms exhibit anti-SARS-CoV-2 activity, enhance efficacy of direct-acting antivirals, and suppress emergence of resistance.

Identifying novel antiviral treatment strategies to expand the existing repertoire of available small-molecule drugs is an important task in addressing both current and emerging viral diseases. Host-targeting antivirals (HTA) and direct-acting antivirals (DAA) represent two classes of such small-molecule drugs. While both classes of drugs are active in a stand-alone treatment, it may be a promising approach to increase the benefits of both by using them in combination which may lead to a synergistic amplification of the antiviral effects. In this study, we show that SARS-CoV-2 is sensitive to the inhibition of the cellular 90 kDa ribosomal S6 kinase (RSK) family, which are direct downstream effectors of the Raf/MEK/ERK signaling pathway. Specific RSK inhibitors (BI-D1870, BRD 7389) showed anti-SARS-CoV-2 properties in single and combination treatment. We could evaluate synergistic properties with the DAAs Remdesivir (RDV) and Nirmatrelvir (NTV). Serial passaging of δ-B.1.617.2 variant under permanent drug pressure did not alter the susceptibility to the RSK inhibitors, while attenuated responsiveness was found for the DAAs. Interestingly, this diminished drug sensitivity did not occur when BRD 7389 and NTV were combined. Furthermore, we demonstrated that combining RSK inhibitors with DAAs not only helps prevent the development of reduced drug susceptibility but may also synergistically compensate for diminished DAA sensitivity, a phenotype that was not found for combined DAA (NTV + RDV) treatments. These results are indicative of the potency and benefit of a combination treatment with DAAs and HTAs, which may likely be the basis for safe and long-lasting novel antiviral therapeutic approaches.