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NIH animal study combines Paxlovid with Lagevrio against SARS-CoV-2 Delta

In a recent study posted to the bioRxiv* preprint server, researchers at the U.S National Institutes of Health demonstrated the efficacy of molnupiravir and nirmatrelvir, two oral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antivirals in the rhesus macaque model, the closest proxy to humans.

Study: Combined Molnupiravir and Nirmatrelvir Treatment Improves the Inhibitory Effect on SARS-CoV-2 in Rhesus Macaques. Image Credit: NIAID


Nirmatrelvir (PF-07321332) and molnupiravir (MK-4482) have each received the Food and Drug Administration (FDA) approval for use as monotherapy in high-risk coronavirus disease 2019 (COVID-19) patients.

The former is a 3C-like protease (3CLpro) inhibitor that interferes with the cleavage of the SARS-CoV-2 polyprotein to cease replication. Currently,  nirmatrelvir is used in combination with low-dose ritonavir, an inhibitor of cytochrome P450-3A4, which increases its serum half-life to increase its potency. Molnupiravir (MK-4482), on the other hand, is a nucleoside analog traded under the name  Lagevrio™. It induces a catastrophic rate of mutation that reduces the infectivity of SARS-CoV-2 progenies.

Although pre-clinical data from studies using rodent and ferret models is available, clinical studies have not evaluated these two drugs in combination, which as per some previous studies, improves their individual inhibitory effects.

About the study

In the present study, researchers assessed the efficacy of MK-4482 and PF-07321332 as monotherapy and in combination against infection with the SARS-CoV-2 Delta variant of concern (VOC) in the rhesus macaque model. Each group had five animals who received seven treatments administered by oral gavage every 12 hours, 12 hours post-infection (hpi) with 2×106 median tissue culture infectious dose (TCID50) of the Delta VOC. Animals either received a vehicle or 130mg/kg MK-4482 or 20mg/kg PF-073211332 with 6.5mg/kg ritonavir or a combination of the same amount of all three compounds.

The researchers euthanized animals four days post-infection (dpi) for tissue harvesting and analysis. Next, they used a scoring sheet to score animals for signs of disease, even before inoculation (zero dpi). They assigned a score of zero to 15  daily for appearance, skin, fur, head, nose, eyes, mouth, respiration, feces and urine, food consumption, and locomotor activity. Finally, the researchers performed the area under the curve (AUC) analysis for an aggregate measure of significant differences between the three study groups.

Study findings

Animals in the vehicle-treated group had the highest scores peaking at two dpi, while those receiving the combination treatment scored the lowest scores.

The AUC analysis showed a significant difference between the vehicle-treated group and the group receiving the combination therapy. For instance, the results of subgenomic E(sgE)-based reverse transcription-polymerase chain reaction (RT-PCR), which indicated active SARS-CoV-2 replication, showed lower infectious viral titers at one dpi in all treated groups than the vehicle-treated group. Also, sgE viral RNA loads in the nasal swabs remained significantly lower in the combination therapy group for the entire study.

The AUC analysis of the bronchoalveolar lavages (BALs) also revealed a significant difference between the vehicle-treated and combination-treated groups. Infectious viral titers in the BAL samples were lower in all treatment groups at one and two dpi compared to the vehicle-treated group. Also, the effect of combination therapy was significantly lower than the vehicle-treated controls at one dpi.

Furthermore, animals receiving PF-07321332 and the combination therapy had lower levels of sgE ribonucleic acid (RNA) viral loads in the lungs. However, at four dpi, infectious viral titers in the lungs diminished across all the treatment groups. Combination therapy also reduced pneumonia, with only one of five animals displaying minimal lesions in two lung lobes.

While 800mg oral molnupiravir was prescribed twice daily within five days of symptom onset, 300mg oral nirmatrelvir was prescribed twice daily with 100mg ritonavir. In addition, the researchers evaluated plasma levels of each pharmaceutically active compound at each clinically relevant examination point before dosing and in lung homogenates at necropsy. These evaluations confirmed the drug presence at the desired level and ensured the absence of any unanticipated drug interaction between the two treatments. Notably, the combined treatment regimen did not negatively affect the levels of each active drug.

The study findings also suggested that the combination therapy of molnupiravir and nirmatrelvir may counteract the rebound effect by promoting a shorter five-day treatment regime. Further, it might reduce the possibility of viral escape, as seen for other mutation-prone viruses, such as the hepatitis C virus.


Overall, the current study highlighted the need for continuous improvement in the dosing regimens of existing COVID-19 drugs, including molnupiravir and nirmatrelvir. Amid the continued emergence and selection of SARS-CoV-2 variants (13 identified so far), all monoclonal antibody therapies and current SARS-CoV-2 vaccines are failing because the viral spike (S) protein-based, which continues to mutate as SARS-CoV-2 evolves. However, MK-4482 and PF-07321332 work independently from the SARS-CoV-2 S protein and target SARS-CoV-2 polymerase and protease, respectively.

In addition, the mode of action of both MK-4482 and PF-07321332 is discrete; thus, combined therapy could provide a potential benefit over monotherapy. Moreover, the rhesus macaque model well-tolerated the combined administration of MK-4482 and PF-07321332 drugs with no adverse reactions, as indicated by clinical observation, blood chemistry, and hematology analyses.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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