Abstract
New Ebselen-like derivatives resulted to be very strong in vitro
inhibitors of SARS-CoV-2 main protease. We demonstrated that this activity mainly depends on the
electrophilicity of the selenium atom that is considerably higher in the N-substituted 1,2-
benzoselenazol-3(2H)-ones respect to the corresponding diselenides allowing it to be rapidly attached
by free thiols affording sulfur-selenium intermediates that are further subjected to thiol exchange
processes. This data paints a very complex scenario that requires us to consider Ebselen and Ebselen-like derivatives as potential electrophilic substrates for the several other free thiols present in the cell
beside the target free cysteine. The sulfur selenium intermediates are milder electrophiles that could
be theoretically implicated in both the detoxification process as well as in the final enzymatic
inhibition. We here demonstrated that the in vitro inhibition activity is not fully reproduced in the
prevention of viral replication in the cell-based assay. This indicates that the structure of the
substituents introduced in the Ebselen scaffold is a crucial factor to control the reactivity of the
selenated molecule in the network of thiol exchanges, as well as for molecular recognition of the
targeted enzymatic cysteine. For this reason, an in-depth investigation is strongly desirable to better
understand how to increase the activity and the selectivity of Ebselen derivatives overcoming the
issues of the apparent PAINS-like role of Ebselen. Furthermore, besides the antiviral activity, thee selected compounds also showed a different ability to reduce the virus-induced cytopathic effect, indicating that other mechanisms could be implicated. One may consider here the well-known cytoprotective antioxidant activity of Ebselen and its derivatives.