In a recent study published on bioRxiv* pre-print server, Belgian researchers assessed the effectiveness of antiviral drugs against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a combined severe immunodeficient (SCID) mouse model.
Study: A SCID mouse model to assess the efficacy of antivirals against SARS-CoV-2 infection. Image Credit: Kateryna Kon/Shutterstock
Due to the inability of ancestral strains of SARS-CoV-2 to bind to mouse angiotensin-converting enzyme 2 (ACE2) receptor, mouse-adapted (MA) viruses or mice genetically modified (GM) have been used to conduct SARS-CoV-2 – related preclinical studies.
However, some of the evolved SARS-CoV-2 variants of concern (COVs), such as COV beta (B.1.351), bind to the murine ACE2 receptor and replicate in some species of wild-type (wt) mice, including including C57BL/6 and BALB/c mice. COV B.1.351 carries mutations in the spike (S) protein, primarily N501Y, facilitating efficient binding of murine receptors in wt mice. Similarly, a study demonstrated that the K417N mutation was prevalent in a SARS-CoV-2 MA variant.
Together, these results indicated that the K417N and E484K mutations in the beta variant receptor binding domain (RBD) facilitate efficient binding to murine ACE2 receptors than SARS-CoV-2 alpha variants.
About the study
In the present study, the researchers used the SCID mouse infection model to study the live efficacy of antiviral drugs against SARS-CoV-2, molnupiravir and nirmatrelvir. Using a small animal model allowed researchers to test small amounts of antiviral drugs in convenient housing conditions and avoid using SARS-CoV-2 MA strains and GM mice.
The team intranasally infected nine mice of each wt species with 105 tissue culture median infectious doses (TCID50) of the Beta B.1.351 variant. They euthanized all mice on day 3 post infection (pi) to harvest their lungs and quantify infectious viral titers.
In addition, researchers explored the replication kinetics of COV beta (B.1.351) in SCID mice. To this end, they infected seven- to nine-week-old male SCID mice with 105 TCID50 VOC B.1.351. Between three and seven days pi, the team euthanized 10 test animals and removed their lungs to quantify infectious viral titers.
Finally, to assess the potential antiviral efficacy of molnupiravir and nirmatrelvir against the beta variant (B.1.351), male SCID mice were treated twice daily with oral doses of the drug or vehicle. Additionally, they treated test animals with 200 mg/kg molnupiravir and 300 mg/kg nirmatrelvir for three days from the day of infection. The team euthanized antiviral-treated mice at day 3 pi for lung collection.
Replication of beta (B.1.351) SARS-CoV-2 in different mice. Infectious viral titers in the lungs of male SCID, male BALB/c and male C57BL/6 mice infected with 105 TCID50 of SARS-CoV-2 beta variants at 3 days post-infection (pi) are expressed as logten TCID50 per mg of lung tissue. Individual data and median values are presented. Data were analyzed with the Mann™ Whitney U test. *P<0.05, **P<0.01). Data are from two independent experiments with n=9 per group.
Male SCID mice infected with Beta B.1.351 exhibited high infectious viral titers and aberrant pathology in the lungs at day 3 pi. The authors noted significantly higher infectious viral titers in the lungs of infected SCID mice than in infected BALB/c and C57BL/6 mice.
From day 4 pi, titers of infectious virus in the lungs began to decline. After day 3 µl, test animals also began to gain weight normally. However, when monitored until day 14 pi, the animals tested showed no signs of weight loss or morbidity. However, histological examinations showed slight peri-bronchial inflammation. Additionally, there was significant perivascular inflammation and intra-alveolar hemorrhage.
Replication kinetics of SARS-CoV-2 beta (B.1.351) in male SCID mice. (A) Infectious viral loads in the lungs of 10-infected male SCID mice.5 TCID50 beta variants of SARS-CoV-2 at different days post-infection (pi) are expressed as logten TCID50 per mg of lung tissue. Individual data and median values are presented. Data were analyzed with the Mann™ Whitney U test. *** P = 0.0003, **** P < 0.0001 (B) Weight change at different days pi in percent, normalized to body weight at time of infection. Bars represent means ± SD. All data are from 2 independent experiments with 10 animals per group. (C) Representative H&E image of the lung of a SCID mouse infected with the beta variant at day 3 pi showing limited peri-bronchial inflammation (blue arrows), extensive peri-vascular inflammation (red arrows), and intra-alveolar hemorrhage (green arrow). Scale bar = 100 µM
Infected mice treated with SARS-CoV-2 antiviral drugs, molnupiravir or nirmatrelvir, showed a substantial drop in infectious virus titers in their lungs. The groups treated with molnupiravir and nirmatrelvir showed a decrease of 1.9 and 3.8 log10 TCID50/mg tissue, respectively, in lung virus titers. Additionally, it significantly improved the lung pathology of test animals. Notably, four and eight of 14 animals in the molnupiravir and nirmatrelvir-treated groups, respectively, had no infectious viral titer. As expected, these animals did not lose weight due to the medications.
The results of the present study demonstrate the benefits of using SCID mice/B.1.351 variant infection models to assess the potential activity of SARS-CoV-2 antivirals in preclinical studies.
The SCID mouse infection model was not suitable for evaluating vaccines and therapeutic antibodies against SARS-CoV-2. However, they have been shown to be beneficial for live efficacy studies of drug molecules inhibiting SARS-CoV-2 replication.
Molnupiravir and nirmatrelvir have previously been shown to inhibit the replication of the B.1.351 variant in Syrian hamsters. Likewise, they demonstrated high efficacy in the SCID mouse model. Notably, both drugs significantly reduced viral loads in the lungs of infected mice with similar potency as they had previously in the Syrian hamster model.
These results are encouraging, especially since molnupiravir, marketed as Lagevrio, is used as an oral medication in several countries for the treatment of coronavirus disease 2019 (COVID-19). Nirmatrelvir and ritonavir tablets, marketed as Paxlovid, are also approved for oral use by the US Food and Drug Administration (FDA) against SARS-CoV-2 and other coronavirus.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.
- A SCID mouse model to assess the efficacy of antivirals against SARS-CoV-2 infection, Rana Abdelnabi, Caroline S. Foo, Suzanne Kaptein, Robbert Boudewijns, Laura Vangeel, Steven De Jonghe, Dirk Jochmans, Birgit Weynand , Johan Neyts, bioRxiv pre-print 2022, DOI: https://doi.org/10.1101/2022.05.13.491916, https://www.biorxiv.org/content/10.1101/2022.05.13.491916v1
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