N-(4-Hydroxyphenyl)retinamide suppresses SARS-CoV-2 spike protein-mediated cell-cell fusion and viral infection in vitro 

<jats:title>Abstract</jats:title> <jats:p>The coronavirus disease (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), persists worldwide with limited therapeutic options. Since membrane fusion between SARS-CoV-2 and host cells is essential for the early step of the infection, the membrane compositions, including sphingolipids, in host cells are considered to affect the viral infection. However, the role of sphingolipids in the life cycle of SARS-CoV-2 remains unclear. Here, we assessed several inhibitors of sphingolipid metabolism enzymes against SARS-CoV-2 spike protein-mediated cell-cell fusion and viral infection <jats:italic>in vitro</jats:italic>. Among the compounds tested, only <jats:italic>N</jats:italic>-(4-hydroxyphenyl)retinamide (4-HPR, also known as fenretinide), an inhibitor of dihydroceramide Δ4-desaturase 1 (DES1) and well known for having antitumour activity, suppressed cell-cell fusion (50% effective concentration [EC<jats:sub>50</jats:sub>] = 4.1 µM) and viral infection ([EC<jats:sub>50</jats:sub>] = 4.4 µM), wherein the EC<jats:sub>50</jats:sub> values are below its plasma concentration in previous clinical trials on tumours. DES1 catalyses the introduction of a double bond in dihydroceramide, and the inhibition efficiencies observed were consistent with an increased ratio of saturated sphinganine-based lipids to total sphingolipids and the decreased cellular membrane fluidity. These findings, together with the accumulated clinical data regarding the safety of 4-HPR, make it a likely candidate drug to treat COVID-19.</jats:p>