Uridine cytidine kinases govern molnupiravir bioactivation and anti-SARS-CoV-2 activity.

Molnupiravir is a nucleoside analogue antiviral drug against RNA viruses, including its clinical indication SARS-CoV-2. Whilst its mechanism-of-action is well defined, host factors that regulate its therapeutic responses have not been thoroughly deciphered and characterized. Here we show that uridine cytidine kinases (UCKs), key enzymes in pyrimidine salvage, effectively phosphorylate and thereby bioactivate N4-hydroxycytidine (NHC) - the active compound of molnupiravir, thus dictating its anti-SARS-CoV-2 efficacy and furthermore selectivity. In vitro, both isoforms of UCKs (UCK1 and UCK2) effectively phosphorylated NHC, where the structural basis of the catalysis was further deciphered via the first complete substrate bound co-crystal structure of UCK, i.e., UCK1-NHC-AMPPNP. In SARS-CoV-2-infected cells, UCK2 knockdown via siRNA hampered the intracellular accumulation of the tri-phosphorylated antiviral metabolite of NHC, resulting in a 10-fold reduction of the antiviral efficacy, and surprisingly, 2-fold reduction of its selectivity, which were critically recapitulated in a dose-dependent manner using a pan-UCK inhibitor. Altogether, this work underscores UCKs as pivotal players in upholding molnupiravir efficacy and therapeutic window, and furthermore as pharmacologically tractable targets for tailoring the drug response.