Natural products as building blocks for bivalent inhibitors of SARS-CoV-2 Spike protein

SARS-CoV-2 is a virus responsible for COVID-19 pandemic, which caused more than 6.7 million deaths worldwide. Viral mutations could compromise the efficacy of vaccines, so developing drugs capable of suppressing viral replication to treat COVID-19 remains an open therapeutic challenge. In SARS-CoV-2 morphology, a crucial region is the receptor-binding domain (RBD), located in subunit S1 of Spike glycoproteins, since it is involved in the binding with Angiotensin-Converting Enzyme 2 (ACE2) receptor of human cells in the early internalization step. Through virtual screening of the RBD active recognition region, several natural products (NPs) capable to bind in two different portions of the RBD were identified. Triterpenoids were reported as pocket 1 binders, while bile acids derivatives should bind to pocket 5. In this work, bivalent compounds formed by a triterpenoid and a bile acid linked by a spacer were developed, with the aim of obtaining more potent and selective antiviral molecules capable of blocking the fusion between SARS-CoV-2 and host cell. Computational studies have guided the synthesis, providing indications on the size and flexibility of the compounds. Azide-alkyne cycloadditions and amide couplings were used as synthetic strategies to connect the selected triterpenoids (betulinic acid and glycyrrhetinic acid) and bile acid (obeticolic acid). Neutralization assays using pseudotyping lentiviral particles with specific SARS-CoV-2 Spike proteins were performed to evaluate the antiviral activity of these bivalent compounds, demonstrating a higher ability to reduce SARS-CoV-2 infection than single natural products.