A Systematic Study on the Optimal Nucleotide Analogue Concentration and Rate Limiting Nucleotide of the SARS-CoV-2 RNA-Dependent RNA Polymerase.

The current COVID-19 pandemic has highlighted the necessity of more efficient antiviral compounds. The antiviral efficacy of adenosine-based analogs, the main repurposed drugs for SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) inhibition, is mainly assessed through in vitro or cell-free polymerization assays, under arbitrary conditions that do not reflect the physiological environment. We show that SARS-CoV-2 RdRp inhibition efficiency of remdesivir and cordycepin, two common adenosine analogs, is influenced by endogenous adenosine level, and that the current clinically approved concentrations for COVID-19 treatment are suboptimal for effective RdRp inhibition. Furthermore, we identified GTP as the rate-limiting nucleotide of SARS-CoV-2 replication. Our results demonstrate that nucleotide sensitivity of the RdRp complex and competition of nucleoside analog drugs against endogenous concentrations of nucleotides are crucial elements to be considered when designing new SARS-CoV-2 antiviral compounds.

Discovery of Taroxaz-104: The first potent antidote of SARS-CoV-2 VOC-202012/01 strain.

Polyhydroxyphenols and nitrogenous heterocyclics are two of the most powerful active species of molecules in pharmaceutical chemistry, as each of them is renowned for its various bioactivities for humans. One of their outstanding actions is the antiviral activities, which clearly appear if the principal functional entities of both classes meet into one compound. The recent COVID-19 pandemic pushed us to computationally sift and assess our small library of synthetic 2-(3,4,5-trihydroxyphenyl)-1,3,4-oxadiazoles against the main coronaviral protein/enzymatic targets. Surprisingly, few ligands exhibited interesting low binding energies (strong inhibitory affinities) with some SARS-CoV-2 proteins, mainly the pivotal enzyme RNA-dependent RNA polymerase (nCoV-RdRp). One of these compounds was Taroxaz-104 (5,5'-{5,5'-[(1R,2R)-1,2-dihydroxyethane-1,2-diyl]bis(1,3,4-oxadiazole-5,2-diyl)}dibenzene-1,2,3-triol), which presented lower binding free energies of about -10.60 and -9.10 kcal/mol (as compared to the reference agent, GS-443902, which presented about -9.20 and -7.90 kcal/mol) with nCoV-RdRp-RNA and nCoV-RdRp alone, respectively. Extensive molecular modeling examination disclosed the potent Taroxaz-104 inhibition of one of the possible active/allosteric sites of nCoV-RdRp, since Taroxaz-104 molecule interacts with at least seven main amino acids of the presumed pocket/cavity of this nCoV-RdRp active site. The effective repurposing of Taroxaz-104 molecule was attained after the satisfactorily interesting results of the anti-COVID-19 bioassay were secured, since these data demonstrated that Taroxaz-104 showed very efficient anti-COVID-19 actions (anti-SARS-CoV-2 EC50 = 0.42 µM) with specific promising efficacy against the new SARS-CoV-2 strains. Additional research studies for the progress of Taroxaz-104 and other related polyphenolic 2,5-disubstituted-1,3,4-oxadiazole analogs as successful anti-SARS-CoV-2 medications, via, e.g., preclinical/clinical trials, are pressingly required.