Insights into the complexity of SARS-CoV-2 M ^pro inhibition: Ebselen and its derivatives impair dimerisation of the enzyme

The SARS-CoV-2 Main Protease (M^pro), a key enzyme for viral replication, represents a highly attractive target for the development of broad-spectrum anti-coronavirus therapeutics. The organoselenium drug Ebselen has shown potent <i>in vitro</i> inhibition of M^pro as well as antiviral activity, granting clinical interest as a COVID-19 treatment option. Here we show that Ebselen and selected derivatives with human neutrophil elastase (HNE) inhibition and anti-radical activity are able to bind covalently to the viral enzyme with multiple stoichiometry, exhibiting inhibitory activity towards SARS-CoV-2 M^pro with potencies in the nanomolar range. Employing a mass spectrometry-based approach, we show that, upon binding to the target, Ebselen and its derivatives induce a dose-dependent shift in the dimer-monomer equilibrium, favouring the inactive monomeric state of the viral protease and possibly contributing to the observed <i>in vitro</i> inhibition.