Molnupiravir maintains antiviral activity against SARS-CoV-2 variants and exhibits a high barrier to the development of resistance

Efficacy of late-onset antiviral treatment in immune-compromised hosts with persistent SARS-CoV-2 infection

The immunocompromised are at high risk of prolonged SARS-CoV-2 infection and progression to severe COVID-19. However, efficacy of late-onset direct-acting antiviral (DAA) therapy with therapeutics in clinical use and experimental drugs to mitigate persistent viral replication is unclear. In this study, we employed an immunocompromised mouse model, which supports prolonged replication of SARS-CoV-2 to explore late-onset treatment options. Tandem immuno-depletion of CD4 + and CD8 + T cells in C57BL/6 mice followed by infection with SARS-CoV-2 variant of concern (VOC) beta B.1.351 resulted in prolonged infection with virus replication for five weeks after inoculation. Early-onset treatment with nirmatrelvir/ritonavir (paxlovid) or molnupiravir was only moderately efficacious, whereas the experimental therapeutic 4'-fluorourdine (4'-FlU, EIDD-2749) significantly reduced virus load in upper and lower respiratory compartments four days post infection (dpi). All antivirals significantly lowered virus burden in a 7-day treatment regimen initiated 14 dpi, but paxlovid-treated animals experienced rebound virus replication in the upper respiratory tract seven days after treatment end. Viral RNA was detectable 28 dpi in paxlovid-treated animals, albeit not in the molnupiravir or 4'-FlU groups, when treatment was initiated 14 dpi and continued for 14 days. Low-level virus replication continued 35 dpi in animals receiving vehicle but had ceased in all treatment groups. These data indicate that late-onset DAA therapy significantly shortens the duration of persistent virus replication in an immunocompromised host, which may have implications for clinical use of antiviral therapeutics to alleviate the risk of progression to severe disease in highly vulnerable patients.

Importance

Four years after the onset of the global COVID-19 pandemic, the immunocompromised are at greatest risk of developing life-threatening severe disease. However, specific treatment plans for this most vulnerable patient group have not yet been developed. Employing a CD4 + and CD8 + T cell-depleted immunocompromised mouse model of SARS-CoV-2 infection, we explored therapeutic options of persistent infections with standard-of-care paxlovid, molnupiravir, and the experimental therapeutic 4'-FlU. Late-onset treatment initiated 14 days after infection was efficacious, but only 4'-FlU was rapidly sterilizing. No treatment-experienced viral variants with reduced susceptibility to the drugs emerged, albeit virus replication rebounded in animals of the paxlovid group after treatment end. This study supports the use of direct-acting antivirals for late-onset management of persistent SARS-CoV-2 infection in immunocompromised hosts. However, treatment courses likely require to be extended for maximal therapeutic benefit, calling for appropriately powered clinical trials to meet the specific needs of this patient group.

Why Certain Repurposed Drugs Are Unlikely to Be Effective Antivirals to Treat SARS-CoV-2 Infections

Most repurposed drugs have proved ineffective for treating COVID-19. We evaluated median effective and toxic concentrations (EC50, CC50) of 49 drugs, mostly from previous clinical trials, in Vero cells. Ratios of reported unbound peak plasma concentrations, (Cmax)/EC50, were used to predict the potential in vivo efficacy. The 20 drugs with the highest ratios were retested in human Calu-3 and Caco-2 cells, and their CC50 was determined in an expanded panel of cell lines. Many of the 20 drugs with the highest ratios were inactive in human Calu-3 and Caco-2 cells. Antivirals effective in controlled clinical trials had unbound Cmax/EC50 ≥ 6.8 in Calu-3 or Caco-2 cells. EC50 of nucleoside analogs were cell dependent. This approach and earlier availability of more relevant cultures could have reduced the number of unwarranted clinical trials.

SARS-CoV-2 resistance to monoclonal antibodies and small-molecule drugs

Over four years have passed since the beginning of the COVID-19 pandemic. The scientific response has been rapid and effective, with many therapeutic monoclonal antibodies and small molecules developed for clinical use. However, given the ability for viruses to become resistant to antivirals, it is perhaps no surprise that the field has identified resistance to nearly all of these compounds. Here, we provide a comprehensive review of the resistance profile for each of these therapeutics. We hope that this resource provides an atlas for mutations to be aware of for each agent, particularly as a springboard for considerations for the next generation of antivirals. Finally, we discuss the outlook and thoughts for moving forward in how we continue to manage this, and the next, pandemic.

Respiratory virus coinfections during the COVID-19 pandemic: epidemiologic analysis and clinical outcomes from the Phase 2/3 molnupiravir trial (MOVe-OUT)

This exploratory post hoc analysis assessed the incidence of respiratory viral coinfections and their impact on clinical outcomes in non-hospitalized adults with mild-to-moderate coronavirus disease-2019 (COVID-19) treated with molnupiravir versus placebo for 5 days in the Phase 2/3 MOVe-OUT trial (NCT04575597), which took place in October 2020 to January 2021 (Phase 2, n = 302) and May 2021 to October 2021 (Phase 3, n = 1,433). Among 1,735 total randomized participants, 1,674 had a baseline respiratory pathogen panel (NxTAG Respiratory Pathogen Panel for the Luminex MAGPIX instrument) performed and 69 (4.1%) were coinfected with at least one additional respiratory viral pathogen. Human rhinovirus/enterovirus (39/69, 56.5%) was the most common coinfection detected at baseline. In the modified intention-to-treat population, two participants with coinfecting respiratory RNA viruses were hospitalized and received respiratory interventions through Day 29, and none died; one participant in the molnupiravir group was coinfected with human rhinovirus/enterovirus, and one participant in the placebo group was coinfected with human metapneumovirus. Hospitalization or death occurred in 6.2% and 9.0% of non-coinfected participants in the molnupiravir versus placebo group, respectively, and over 90% did not require respiratory interventions. Most coinfecting respiratory RNA viruses detected at baseline were not detected at the end of therapy in both the molnupiravir and placebo groups. In summary, participants coinfected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and another respiratory RNA virus were not more likely to be hospitalized or die, or require respiratory interventions, compared to participants who were not coinfected with another respiratory RNA virus at baseline in both groups.

IMPORTANCE

Respiratory viral coinfections are known to occur with coronavirus disease-2019 (COVID-19). In a cohort of non-hospitalized adults with mild-to-moderate COVID-19 treated with molnupiravir versus placebo in the MOVe-OUT trial during October 2020 to October 2021, 4.1% of participants had a documented viral coinfection; human rhinovirus/enterovirus was the most common pathogen detected with the NxTAG Respiratory Pathogen Panel assay. Participants who had a coinfection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and another respiratory RNA virus were not more likely to have worse clinical outcomes compared to those participants without a viral coinfection, and many coinfecting respiratory RNA viruses were no longer detected at the end of the 5-day treatment period in both groups.

Molnupiravir: Mechanism of action, clinical, and translational science

Molnupiravir is an oral prodrug of the broadly active, antiviral ribonucleoside analog N-hydroxycytidine (NHC). The primary circulating metabolite NHC is taken up into cells and phosphorylated to NHC-triphosphate (NHC-TP). NHC-TP serves as a competitive substrate for viral RNA-dependent RNA polymerase (RdRp), which results in an accumulation of errors in the viral genome, rendering virus replication incompetent. Molnupiravir has demonstrated activity against SARS-CoV-2 both clinically and preclinically and has a high barrier to development of viral resistance. Little to no molnupiravir is observed in plasma due to rapid hydrolysis to NHC. Maximum concentrations of NHC are reached at 1.5 h following administration in a fasted state. The effective half-life of NHC is 3.3 h, reflecting minimal accumulation in the plasma following twice-daily (Q12H) dosing. The terminal half-life of NHC is 20.6 h. NHC-TP exhibits a flatter profile with a lower peak-to-trough ratio compared with NHC, which supports Q12H dosing. Renal and hepatic pathways are not major routes of elimination, as NHC is primarily cleared by metabolism to uridine and cytidine, which then mix with the endogenous nucleotide pools. In a phase III study of nonhospitalized patients with COVID-19 (MOVe-OUT), 5 days of treatment with 800 mg molnupiravir Q12H significantly reduced the incidence of hospitalization or death compared with placebo. Patients treated with molnupiravir also had a greater reduction in SARS-CoV-2 viral load and improved clinical outcomes, compared with those receiving placebo. The clinical effectiveness of molnupiravir has been further demonstrated in several real-world evidence studies. Molnupiravir is currently authorized or approved in more than 25 countries.

Post COVID-19 Reflections and Questions: How Prepared Are We for the Next Pandemic?

While the end of the COVID-19 pandemic was announced earlier in 2023 by WHO, the currently dominating COVID-19 virus variants, such as the omicron sub-lineages XBB [...].