Synthesis and Antiviral Evaluation of Carbocyclic Nucleoside Analogs of Nucleoside Reverse Transcriptase Translocation Inhibitor MK-8591 (4'-Ethynyl-2-fluoro-2'-deoxyadenosine)

Potentiality of Nucleoside as Antioxidant by Analysis on Oxidative Susceptibility, Drug Discovery, and Synthesis

Nucleosides are sensitive sites towards oxidations caused by endogenous and exogenous oxidative resources, and a large number of the produced DNA lesions behave as pathogenesis eventually. We herein analyze oxidative modes of nucleosides and structure- activity relationships of some clinical nucleoside drugs. Together with our previous findings on the inhibitory effects of nucleoside derivatives against DNA oxidation, all these results imply a possibility for nucleoside to be a new member in the family of antioxidants. Then, some novel synthetic routines of nucleoside analogs are collected to reveal the applicability in the construction of nucleoside antioxidants. Therefore, it is reasonable to envision that the nucleoside antioxidant will be a novel topic in the research of both nucleosides and antioxidants.

4'-Ethynyl-2'-Deoxycytidine (EdC) Preferentially Targets Lymphoma and Leukemia Subtypes by Inducing Replicative Stress

Anticancer nucleosides are effective against solid tumors and hematologic malignancies, but typically are prone to nucleoside metabolism resistance mechanisms. Using a nucleoside-specific multiplexed high-throughput screening approach, we discovered 4'-ethynyl-2'-deoxycytidine (EdC) as a third-generation anticancer nucleoside prodrug with preferential activity against diffuse large B-cell lymphoma (DLBCL) and acute lymphoblastic leukemia (ALL). EdC requires deoxycytidine kinase (DCK) phosphorylation for its activity and induces replication fork arrest and accumulation of cells in S-phase, indicating it acts as a chain terminator. A 2.1Å cocrystal structure of DCK bound to EdC and UDP reveals how the rigid 4'-alkyne of EdC fits within the active site of DCK. Remarkably, EdC was resistant to cytidine deamination and SAMHD1 metabolism mechanisms and exhibited higher potency against ALL compared with FDA-approved nelarabine. Finally, EdC was highly effective against DLBCL tumors and B-ALL in vivo. These data characterize EdC as a preclinical nucleoside prodrug candidate for DLBCL and ALL.

4'-Ethynyl-2'-deoxy-2'-β-fluoro-2-fluoroadenosine: A Highly Potent and Orally Available Clinical Candidate for the Treatment of HIV-1 Infection

2'-Deoxy-2'-β-fluoroadenosines bearing 4'-azido or 4'-ethynyl groups designed for the treatment of HIV-1 infection have been synthesized. All these compounds possess nanomolar anti-HIV-1 activity, with the 4'-ethynyl-2-fluoroadenosine analog 1c (CL-197) being the most potent compound with low cytotoxicity (EC50 = 0.9 nM, CC50 > 100 μM). It also shows potent inhibitory activities on drug resistant and clinical HIV-1 strains. Oral administration of 1c to Beagle dogs resulted in high levels of its bioactive form 1c-TP in peripheral blood mononuclear cells, the HIV-1 target cells, where the resulting triphosphate exhibited a long-term intracellular retention and could prevent HIV-1 infection for an extended time. 1c displayed low in vivo toxicity and favorable pharmacokinetics profiles in Sprague-Dawley rats. The preclinical data support further development of 1c as a highly potent and orally bioavailable clinical candidate to treat HIV-1 infection. Currently, CL-197 is in clinical trials in China (registration number: CXHL2200529).

Synthesis of 4'-Substituted Carbocyclic Uracil Derivatives and Their Monophosphate Prodrugs as Potential Antiviral Agents

Over the past decades, both 4'-modified nucleoside and carbocyclic nucleoside analogs have been under the spotlight as several compounds from either family showed anti-HIV, HCV, RSV or SARS-CoV-2 activity. Herein, we designed compounds combining these two features and report the synthesis of a series of novel 4'-substituted carbocyclic uracil derivatives along with their corresponding monophosphate prodrugs. These compounds were successfully prepared in 19 to 22 steps from the commercially available (-)-Vince lactam and were evaluated against a panel of RNA viruses including SARS-CoV-2, influenza A/B viruses and norovirus.

4'-Modified Nucleosides for Antiviral Drug Discovery: Achievements and Perspectives

Modified nucleosides show therapeutic promise for antiviral therapies. However, issues including the emergence of drug resistance, toxicity, and coinfections have posed new challenges for nucleoside-based antiviral drug discovery, particularly in the era of the coronavirus disease 2019 (COVID-19) pandemic. Chemical manipulation could impact the antiviral potency, safety, and drug resistance of nucleosides. Generally, modified nucleosides are difficult to recognize by intracellular important enzymes as substrates and thus exhibit low toxicity. 4'-Modified nucleosides represent an important subclass of modified nucleosides for antiviral therapies. To prevent the occurrence of drug resistance, 4'-modified nucleosides should have 3'-OH, which should also be chemically unreactive for proviral DNA biosynthesis. The absence of 3'-OH may explain the occurrence of drug resistance for censavudine. The introduction of 4'-substituents improves enzymatic and acidic stability and makes the nucleosides more lipophilic, thus improving cell permeability and bioavailability. Steric hindrance between the 4'-substituent and 3'-OH changes the furanose conformation to the 3'-endo type, in which the oxygen lone pair on the furanose ring could not form an oxocarbonium ion for glycolysis. Currently, seven 4'-modified nucleoside drug candidates such as azvudine (also known as FNC), islatravir, censavudine, balapiravir, lumicitabine, AL-335, and 4-azidothymidine have progressed into clinical stages for treating viral infections. Of note, FNC was officially approved by NMPA in July 2021 for use in adult patients with high HIV-1 virus loads (nos. H20210035 and H20210036), providing an alternative therapeutic for patients with HIV-1. The long-term cellular retention of FNC suggests its potential as a long-lasting pre-exposure prophylaxis (PrEP) agent for preventing HIV-1 infection. Mechanistically, FNC not only inhibited HIV-1 reverse transcription and replication but also restored A3G expression in peripheral blood CD4⁺ T cells in HIV-1 patients receiving FNC. The 4'-azido group in azvudine stabilizes the 3'-C-endo (north) conformation by steric effects and the formation of an intramolecular hydrogen bond with the 3'-OH group, thus decreasing the nucleophilicity of 3'-OH. The north conformation may also enhance the phosphorylation efficiency of FNC by cellular kinases. Encouragingly, FNC, islatravir, and balapiravir show promise for the treatment of coronaviruses, of which FNC has advanced to phase 3 clinical trials in different countries to treat patients with COVID-19 (clinical trial numbers: NCT04668235 and NCT04425772). FNC cured the COVID-19 disease in almost all patients and showed better therapeutic efficacy than remdesivir. In this Account, we provide an overview of 4'-modified nucleoside analogs in clinical stages for antiviral therapies, highlighting the drug discovery strategies, structure-activity relationship studies, and preclinical/clinical studies and also give our perspectives on nucleoside-based antiviral drug discovery.

Diastereoselective Synthesis of Arabino- and Ribo-like Nucleoside Analogues Bearing a Stereogenic C3' All-Carbon Quaternary Center

The synthesis of novel nucleoside analogues bearing a C3' all-carbon quaternary center and a C2'-hydroxy substituent is described. The all-carbon stereogenic center was generated through an intramolecular 7-endo attack of a silyl-tethered allyl moiety on a tertiary radical using photoredox catalysis. Subsequent allylic oxidation and diastereoselective hydride reductions provided the hydroxy substituent at C2', which then controls the stereoselective introduction of pyrimidine nucleobases on the corresponding furanose scaffold. Density functional theory (DFT) calculations provided insights into the origin of the high syn diastereoselectivity resulting from the radical cyclization. This original methodology grants access to a wide range of 1',2'-cis and 1',2'-trans arabino- and ribo-like analogues bearing an all-carbon quaternary center at C3'. These molecules are currently being tested for their antiviral and anticancer properties.

Meeting report: 32nd International Conference on Antiviral Research

The 32nd International Conference on Antiviral Research (ICAR), sponsored by the International Society for Antiviral Research (ISAR), was held in Baltimore, Maryland, USA, on May 12-15, 2019. This report gives an overview of the conference on behalf of the Society. It provides a general review of the meeting and awardees, summarizing the presentations, and their main conclusions from the perspective of researchers active in many different areas of antiviral research and development. As in past years, ICAR promoted and showcased the most recent progress in antiviral research, and continued to foster collaborations and interactions in drug discovery and development. The 33rd ICAR will be held in Seattle, Washington, USA, March 30th-April 3rd, 2020.