Every year, dengue virus affects hundreds of millions of individuals worldwide. To date, there is no specific medication to treat dengue virus infections. Nucleobases, the base of a nucleoside without ribose, are understudied as potential treatments for viral infections. Antiviral nucleobases are converted in infected cells to their corresponding nucleoside triphosphate active form. Importantly, the conversion of nucleobases to their active nucleotide form and their antiviral effect can be enhanced when combined with de novo nucleotide biosynthesis inhibitors. In this work, we evaluated seven purine and pyrimidine nucleobases alone or combined with six purine or pyrimidine de novo nucleotide biosynthesis inhibitors, including novel prodrugs. Our study revealed that while a strong potentiation of purine nucleobases by purine de novo nucleotide biosynthesis inhibitors was observed, the pyrimidine nucleobases were not potentiated by pyrimidine de novo nucleotide biosynthesis inhibitors, possibly highlighting a significant difference between the modulation of purine versus pyrimidine de novo pathways and their impact on nucleobase potentiation. Most significant antiviral effects and potentiation were observed for Favipiravir, T-1105, and ribavirin nucleobases combined with purine nucleotide de novo synthesis inhibitors. These results are significant because drug combinations may solve the limited efficacy observed for some antiviral nucleobase drugs such as Favipiravir.

Broad-spectrum antiviral therapies hold promise as a first-line defense against emerging viruses by blunting illness severity and spread until vaccines and virus-specific antivirals are developed. The nucleobase favipiravir, often discussed as a broad-spectrum inhibitor, was not effective in recent clinical trials involving patients infected with Ebola virus or SARS-CoV-2. A drawback of favipiravir use is its rapid clearance before conversion to its active nucleoside-5′-triphosphate form. In this work, we report a synergistic reduction of flavivirus (dengue, Zika), orthomyxovirus (influenza A), and coronavirus (HCoV-OC43 and SARS-CoV-2) replication when the nucleobases favipiravir or T-1105 were combined with the antimetabolite 6-methylmercaptopurine riboside (6MMPr). The 6MMPr/T-1105 combination increased the C-U and G-A mutation frequency compared to treatment with T-1105 or 6MMPr alone. A further analysis revealed that the 6MMPr/T-1105 co-treatment reduced cellular purine nucleotide triphosphate synthesis and increased conversion of the antiviral nucleobase to its nucleoside-5′-monophosphate, -diphosphate, and -triphosphate forms. The 6MMPr co-treatment specifically increased production of the active antiviral form of the nucleobases (but not corresponding nucleosides) while also reducing levels of competing cellular NTPs to produce the synergistic effect. This in-depth work establishes a foundation for development of small molecules as possible co-treatments with nucleobases like favipiravir in response to emerging RNA virus infections.

Background: p97 (also known as valosin-containing protein, VCP) is a member of the AAA+ ATPase family and is intimately associated with protein quality control and homeostasis regulation. Therefore, pharmaceutical inhibition of p97 has been actively pursued as an anticancer strategy. Recently, p97 has emerged as an important pro-viral host factor and p97 inhibitors are being evaluated as potential antiviral agents. Methods: We designed and synthesized novel p97 inhibitors based on the rearrangement of the central fused ring of our previously reported p97 inhibitors. These compounds were tested for inhibition of p97, cytotoxicity, and antiviral activity against SARS-CoV-2. Molecular docking was also performed on selected inhibitors to shed light on their binding modes. Results: Among these new p97 inhibitors, two compounds possess enhanced anti-p97 activity over their parent compounds. More significantly, these two inhibitors exhibit strong antiviral activity against SARS-CoV-2 at doses with no significant cytotoxicity. Molecular docking reveals no major change of the binding mode relative to that of their parent compounds, further supporting our design strategy. Conclusions: These compounds are structurally novel p97 inhibitors that display low toxicity and possess promising antiviral activity against SARS-CoV-2 and potentially other viruses. Further structural exploration is therefore justified and improved analogs will serve as useful tools for studying p97 as a promising host antiviral target.

We report the short synthesis of novel C-nucleoside Remdesivir analogues, their cytotoxicity and an in vitro evaluation against SARS-CoV-2 (CoV2). The described compounds are nucleoside analogues bearing a nitrogen heterocycle as purine analogues. The hybrid structures described herein are designed to enhance the anti-CoV2 activity of Remdesivir. The compounds were evaluated for their cytotoxicity and their anti-CoV2 effect. We discuss the impact of combining both sugar and base modifications on the biological activities of these compounds, their lack of cytotoxicity and their antiviral efficacy.