Carbovir

SARS-CoV-2, a member of the coronavirus family, is responsible for the current COVID-19 worldwide pandemic. We previously demonstrated that five nucleotide analogues inhibit the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp), including the active triphosphate forms of Sofosbuvir, Alovudine, Zidovudine, Tenofovir alafenamide and Emtricitabine. We report here the evaluation of a library of nucleoside triphosphate analogues with a variety of structural and chemical features as inhibitors of the RdRps of SARS-CoV and SARS-CoV-2. These features include modifications on the sugar (2' or 3' modifications, carbocyclic, acyclic, or dideoxynucleotides) or on the base. The goal is to identify nucleotide analogues that not only terminate RNA synthesis catalyzed by these coronavirus RdRps, but also have the potential to resist the viruses' exonuclease activity. We examined these nucleotide analogues for their ability to be incorporated by the RdRps in the polymerase reaction and to prevent further incorporation. While all 11 molecules tested displayed incorporation, 6 exhibited immediate termination of the polymerase reaction (triphosphates of Carbovir, Ganciclovir, Stavudine and Entecavir; 3'-OMe-UTP and Biotin-16-dUTP), 2 showed delayed termination (Cidofovir diphosphate and 2'-OMe-UTP), and 3 did not terminate the polymerase reaction (2'-F-dUTP, 2'-NH₂-dUTP and Desthiobiotin-16-UTP). The coronaviruses possess an exonuclease that apparently requires a 2'-OH at the 3'-terminus of the growing RNA strand for proofreading. In this study, all nucleoside triphosphate analogues evaluated form Watson-Crick-like base pairs. The nucleotide analogues demonstrating termination either lack a 2'-OH, have a blocked 2'-OH, or show delayed termination. Thus, these nucleotide analogues are of interest for further investigation to evaluate whether they can evade the viral exonuclease activity. Prodrugs of five of these nucleotide analogues (Cidofovir, Abacavir, Valganciclovir/Ganciclovir, Stavudine and Entecavir) are FDA-approved medications for treatment of other viral infections, and their safety profiles are well established. After demonstrating potency in inhibiting viral replication in cell culture, candidate molecules can be rapidly evaluated as potential therapies for COVID-19.

(-)-γ-Lactam ((-)-2-azabicyclo[2.2.1]hept-5-en-3-one) has attracted increasing attention as the chiral intermediate of carbocyclic nucleosides most of which serve as pharmaceutical agents such as anti-HIV/HBV drugs abacavir and carbovir. So far, developing in vitro (+)-γ-lactamase-mediated biotransformation has been one of the most efficient approaches for the production of (-)-γ-lactam. In this study, the catalytic activity of the (+)-γ-lactamase from Sulfolobus solfataricus P2 was engineered by semi-rational design. Molecular docking and molecular dynamics simulation were carried out to target the key positions relevant to catalytic activity. Nine amino acid residues were selected for site saturation mutagenesis. To expedite the screening process, a sensitive colorimetric high-throughput screening method was established based on the Rimini test which was originally applied to distinguish primary amines from secondary amines. The screening process resulted in the achievement of several efficient mutants: V203N, V203Q, I336H, I336R, and Y388H. Synergy effects led to four final mutants (V203N/I336R, V203N/Y388H, I336R/Y388H, and V203N/I336R/Y388H) with enhanced enzyme activity after the combination of positive single mutants. The best mutant V203N/Y388H/I336R displayed a 21-fold higher enzyme efficiency (k_cat/K_M) compared to the wild-type enzyme. The result demonstrated that the biotransformation using the triple mutant as the catalyst reached > 49% conversion and > 99% enantiomeric excess at 80 °C after 2 h, which made it a good catalyst candidate to produce (-)-γ-lactam. The possible mechanism responsible for the improvement in the catalytic activity was explicated by analyzing the protein-ligand binding modes and interaction between the protein and the ligand.