Norovirus Antivirals - An Update

Introduction

Noroviruses are among the foremost causes of viral gastroenteritis globally, affecting all age groups. Infections caused by noroviruses lead to increased hospitalization and fatal consequences, especially in children, older adults, and immunocompromised individuals. There arises an immediate need for the development of specific and potent antivirals that are effective in reducing and managing norovirus infections such as gastroenteritis and other associated complications.

What Is the Need for a Recent Update on Norovirus Antiviral Therapeutics?

Norovirus is highly contagious and is one of the major causes of viral gastroenteritis worldwide, particularly severe pediatric gastroenteritis. People of all age groups get infected with norovirus. Human noroviruses (HuNoVs) spread through contaminated food and water and get transmitted from person to person.

Noroviruses cause significant outbreaks in closed settings such as hospitals, healthcare facilities, cruise ships, and restaurants leading to business closures and frequent hospitalizations. Below are some reasons for an urgent need for norovirus antiviral drugs.

• Currently, no licensed specific antiviral drugs or vaccines are available for noroviruses.

• Usually, norovirus‐associated infections are self‐limiting and get resolved in two to four days in healthy individuals. But they cause severe complications in high-risk populations.

• High‐risk groups such as neonates, children (below five years of age), elderly individuals, immunocompromised patients, and people who undergo transplantations or have chronic infections would greatly benefit from norovirus antivirals.

• People carrying norovirus infections shed millions of infectious norovirus particles, whereas only a few are enough to infect others. Appropriate norovirus antivirals would reduce the viral load and prevent transmission.

• Prophylactic use of norovirus antivirals in medical staff and people working in healthcare facilities and military settings would potentially reduce outbreaks and contain frequent epidemics.

• Noroviruses persist for extended periods in infected persons, even post-treatment (norovirus carriers). The use of antivirals would eliminate the carrier state of noroviruses.

• Moreover, numerous scientific reports on norovirus antivirals have emerged in the past three years, which warrants a comprehensive updated review of norovirus antivirals for their practical and authentic use.

• To date, no specific norovirus antiviral agent or vaccine has been approved exclusively for clinical use targeting only noroviruses.

How Are Norovirus Antivirals Classified?

Despite the growing demands, the development of norovirus antiviral therapeutics is considerably hindered due to the lack of specific cell culture systems. The rational development of any particular antiviral depends on efficient cell culture substrates to study antiviral activity and its effects on viral replication. Recent breakthroughs in developing norovirus antivirals include generating cell culture lines such as B cells, replicon systems, and intestinal cells to study the molecular details of norovirus proteins and their replication for assessing the efficacy of antiviral therapeutics of noroviruses.

Norovirus is an RNA virus, and so far, seven genogroups (G I to G VII) have been identified. The prototype (first identified) Norwalk virus and murine norovirus (MNV - mouse norovirus) are used as test virus models to evaluate the potency of antivirals in in-vitro clinical studies. The recent use of norovirus antivirals is primarily classified as follows:

• Antivirals target viral replication and proteins (viral protease inhibitors, polymerase inhibitors).

• Channelizing the antiviral effects of interferon.

• Antiviral agents targeting host cell factors (deubiquitinase inhibitors).

• Novel antivirals are inducing lethal mutagenesis of noroviruses.

• Recently recognized broad‐spectrum antivirals that possess anti-norovirus activity.

• Host-modulating compounds such as immunomodulators (Interferons).

What Are the Current Available Norovirus Antivirals?

In recent years, there has been significant progress in developing norovirus antivirals. These antiviral agents are tested in various methods, such as viral enzyme inhibition assays, MNV cell culture systems, and clinical trials. Some of the currently available norovirus antivirals for clinical use are as follows:

• Interferon (IFN) - IFNs (type I and type II) exhibit potent antiviral responses against noroviruses by inducing innate immunity in the host and reducing norovirus infections.

• Type III IFN and IFN-λ are used to control persistent norovirus infections by enhancing adaptive immune responses, suggesting the efficacy of interferons as the anti-noroviral potential for treating chronic norovirus infections in immunocompromised individuals.

• Viral Protease Inhibitors - norovirus proteases (NS6pro) is essential for viral replication and provide an optimal antiviral target.

• Rupintrivir is an effective viral protease inhibitor that blocks the NS6pro activity, thereby reducing norovirus multiplication and shedding.

• Polymerase Inhibitors - viral RNA– dependent RNA polymerases (RdRps) play an essential role in increasing viral load and are found to be suitable antiviral target sites. The norovirus RdRp inhibitors are divided into two groups: nucleoside analogs and non-nucleoside analogs.

• Nucleoside Analogs - target the active sites in norovirus polymerase enzymes and inhibit replication of noroviruses. 2′-C-methylcytidine (2CMC) is the most widely used nucleoside analog in treating norovirus infections.

• 2′-C-methylcytidine (2CMC) is used therapeutically to reduce diarrhea and prevent the shedding and transmission of noroviruses.

• It is also used in prophylactic treatment to ensure complete protection against norovirus infections.

• The FDA approval of 2CMC is still pending for treating viral infections because of its toxic side effects. The derivatives of 2CMC are under current clinical trials to improve their potential therapeutic use.

• Nucleoside analogs are 2′-C-MeC, 2′-F-2′-C-MeC, Lamivudine, Ribavirin, Favipiravir, and beta-D-N(4)-hydroxy cytidine [NHC] have been proven to control norovirus infections effectively.

• Non-nucleoside Inhibitors - express the narrow-spectrum antiviral activity and block viral polymerases by binding allosterically (competitive inhibition). Suramin and its derivatives, Filibuvir, Lomibuvir, Setrobuvir, and Tegobuvir, are some of the recently used non-nucleoside inhibitors that exhibit anti-norovirus activity.

• Host‐Targeted Norovirus Antivirals - these antivirals target the proteins (host cell factors) and cause their degradation resulting in the cellular deactivation of noroviruses. They reduce norovirus replication.

• Deubiquitinase (DUB) inhibitors like WP1130 and its derivatives demonstrate enhanced broad-spectrum antiviral activity without causing cellular toxicity.

• Antivirals With Known Targets - antiviral agents containing components like ZINC04041115 and human milk oligosaccharides (HMOs) block the target sites and prevent the attachment and entry of norovirus into the host cell.

• Antiviral Compounds With Unknown Targets - these compounds have antiviral activity and are effective against noroviruses, but their mechanism of inhibition remains unidentified.

• Nitazoxanide (NTZ) which has broad‐spectrum antimicrobial activity, also shows anti-norovirus activity and has completed clinical trials for therapeutic use in norovirus infections. It considerably decreases the symptom duration in norovirus-infected individuals.

What Are the Recent Developments in Norovirus Antiviral Therapies?

Recent clinical studies encourage the progressive development of new approaches and strategies to enhance the efficacy of norovirus antiviral therapeutics. Some of the recent developments in norovirus antivirals are as follows -

• Passive Immunotherapy Using Monoclonal Antibodies (mAbs) - also called nanobodies (Nbs), is generated by recombinant techniques using human antibodies and non-human (primates) antibodies.

  • These therapeutic nanobodies prevent norovirus cell attachment and entry and are particularly effective in immunocompromised patients.

  • The antiviral effects of nanobodies against norovirus need to be widely explored for further progression toward efficient clinical application.

• Lethal Mutagenesis - certain viruses undergo excessive mutations (genetic changes) at the time of replication, where the accumulation of these mutations (errors) leads to virus extinction, known as lethal Mutagenesis.

  • Inducing lethal Mutagenesis through therapeutic agents is now considered a novel antiviral strategy in controlling norovirus infections.

  • In-vitro studies suggest that Ribavirin and Favipiravir are currently identified as potent mutagenic-inducing agents in noroviruses.

  • Further studies are required in this field to validate their clinical use.

  • Certain phytochemicals (plant extracts) like curcumin and nutritional supplements (Vitamin A, flavonoids) also exhibit anti-norovirus effects but are not considered therapeutic norovirus antivirals because the active antiviral compound in these substances has not yet been identified (require further investigations).

Conclusion

Norovirus infections are an emerging global public health concern. Intensive research and studies are being conducted to develop safe and effective norovirus antivirals. The majority of norovirus antivirals are still in the preclinical developmental phases. A detailed update of the recent norovirus antivirals is essential to enhance the ongoing antiviral studies and provide a progressive platform for developing effective norovirus antivirals.