Influenza viruses constitute a major health threat and economic burden globally, frequently exacerbated by preexisting or rapidly emerging resistance to antiviral therapeutics. To address the unmet need of improved influenza therapy, we have created EIDD-2801, an isopropylester prodrug of the ribonucleoside analog N4-hydroxycytidine (NHC, EIDD-1931) that has shown broad anti-influenza virus activity in cultured cells and mice. Pharmacokinetic profiling demonstrated that EIDD-2801 was orally bioavailable in ferrets and nonhuman primates. Therapeutic oral dosing of influenza virus–infected ferrets reduced group pandemic 1 and group 2 seasonal influenza A shed virus load by multiple orders of magnitude and alleviated fever, airway epithelium histopathology, and inflammation, whereas postexposure prophylactic dosing was sterilizing. Deep sequencing highlighted lethal viral mutagenesis as the underlying mechanism of activity and revealed a prohibitive barrier to the development of viral resistance. Inhibitory concentrations were low nanomolar against influenza A and B viruses in disease-relevant well-differentiated human air-liquid interface airway epithelia. Correlating antiviral efficacy and cytotoxicity thresholds with pharmacokinetic profiles in human airway epithelium models revealed a therapeutic window >1713 and established dosing parameters required for efficacious human therapy. These data recommend EIDD-2801 as a clinical candidate with high potential for monotherapy of seasonal and pandemic influenza virus infections. Our results inform EIDD-2801 clinical trial design and drug exposure targets.

Morbidity and mortality resulting from influenza-like disease are a threat, especially for older adults. To improve case management, next-generation broad-spectrum antiviral therapeutics that are efficacious against major drivers of influenza-like disease, including influenza viruses and respiratory syncytial virus (RSV), are urgently needed. Using a dual-pathogen high-throughput screening protocol for influenza A virus (IAV) and RSV inhibitors, we have identified -hydroxycytidine (NHC) as a potent inhibitor of RSV, influenza B viruses, and IAVs of human, avian, and swine origins. Biochemical polymerase assays and viral RNA sequencing revealed that the ribonucleotide analog is incorporated into nascent viral RNAs in place of cytidine, increasing the frequency of viral mutagenesis. Viral passaging in cell culture in the presence of an inhibitor did not induce robust resistance. Pharmacokinetic profiling demonstrated dose-dependent oral bioavailability of 36 to 56%, sustained levels of the active 5'-triphosphate anabolite in primary human airway cells and mouse lung tissue, and good tolerability after extended dosing at 800 mg/kg of body weight/day. The compound was orally efficacious against RSV and both seasonal and highly pathogenic avian IAVs in mouse models, reducing lung virus loads and alleviating disease biomarkers. Oral dosing reduced IAV burdens in a guinea pig transmission model and suppressed virus spread to uninfected contact animals through direct transmission. Based on its broad-spectrum efficacy and pharmacokinetic properties, NHC is a promising candidate for future clinical development as a treatment option for influenza-like diseases.

Preparing antiviral defenses Antiviral drugs are an important tool in the battle against COVID-19. Both remdesivir and molnupiravir, which target the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) polymerase, were first developed against other RNA viruses. This highlights the importance of broad-spectrum antivirals that can be rapidly deployed against related emerging pathogens. Sourimant et al . used respiratory syncytial virus (RSV) as a primary indication in identifying further drugs that target the polymerase enzyme of RNA viruses. The authors explored derivatives of molnupiravir and identified 4′ fluorouridine (EIDD-2749) as an inhibitor of the polymerase of RSV and SARS-CoV-2. This drug can be delivered orally and was effective against RSV in mice and SARS-CoV-2 in ferrets. —VV

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SARS-CoV-2 variants of concern (VOC) have triggered infection waves. Oral antivirals such as molnupiravir promise to improve disease management, but efficacy against VOC delta was questioned and potency against omicron is unknown. This study evaluates molnupiravir against VOC in human airway epithelium organoids, ferrets, and a lethal Roborovski dwarf hamster model of severe COVID-19-like lung injury. VOC were equally inhibited by molnupiravir in cells and organoids. Treatment reduced shedding in ferrets and prevented transmission. Pathogenicity in dwarf hamsters was VOC-dependent and highest for delta, gamma, and omicron. All molnupiravir-treated dwarf hamsters survived, showing reduction in lung virus load from one (delta) to four (gamma) orders of magnitude. Treatment effect size varied in individual dwarf hamsters infected with omicron and was significant in males, but not females. The dwarf hamster model recapitulates mixed efficacy of molnupiravir in human trials and alerts that benefit must be reassessed in vivo as VOC evolve.