Mpro-Targeting Small Molecules and PROTACs with SARS-CoV-2 vs HCoV-OC43 Selectivity

Coronaviruses remain a major global health threat, highlighting the need for broad-spectrum antiviral agents effective against both circulating and emerging viral pathogens. The structural and functional conservation of the main protease (Mpro) across coronaviruses makes it an attractive target for broad-spectrum antiviral development. Herein, we report the design, MCR-based synthesis, and biological evaluation of a novel class of non-covalent, non-peptidic Mpro-targeting compounds and their corresponding PROTACs derivatives as antivirals candidates against coronaviruses infections. Starting from a fragment-based drug discovery approach, a series of small molecules was designed and synthesized one-pot via a Ugi four-component reaction (U-4CR). The compounds were evaluated for SARS-CoV-2 Mpro inhibition and antiviral activity in cellular models. The most active compound inhibited Mpro and showed antiviral activity against SARS‑CoV‑2 in the low‑micromolar range, with no cytotoxicity. Notably, a closely related analogue differing by only one methyl group displayed complementary antiviral activity against HCoV-OC43 while being inactive against SARS-CoV-2, although enzymatically inhibiting Mpro albeit with reduced potency. This divergent profile was retained in the corresponding PROTAC derivative, which inhibited HCoV‑OC43 replication with the same low micromolar EC₅₀ but showed no measurable activity against SARS‑CoV‑2. Overall, these results demonstrate antiviral activity against two distinct human coronaviruses within a closely related series of Mpro-targeting compounds, supporting these compounds as a promising starting point for the development of pan-coronavirus antivirals. Structure-activity relationship (SAR) studies are underway to optimize antiviral potency and broaden the spectrum of activity to other coronaviruses beyond the two studied so far.