As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapidly spread to almost every country around the world, healthcare professionals desperately searched for readily available drugs that could potentially cure infected individuals, most notably of which include RdRp inhibitors.
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What is RdRp?
The widespread of many viruses is largely attributed to their rapidly mutating ribonucleic acid (RNA) genomes that allow the infection to persist despite challenges posed by host cells.
Within the RNA genome of viruses exists RNA dependent RNA polymerase (RdRp), a highly versatile enzyme that assists in RNA synthesis by catalyzing the RNA-template dependent formation of phosphodiester bonds.
The lack of exonuclease activity during the RNA synthesis process causes RdRps to have a high rate of copying error, which is estimated to be around 10-4. Although mutations are generally considered unfavorable to host cells, the progeny viral population of RNA viruses benefit from increased mutation rates.
As these viruses mutate under extreme pressures caused by the host organism’s defense physiology, the variants that can withstand these environmental factors survive.
As a result, more lethal and resistant virus progeny can continue to reproduce without interference from organisms' immune cells.
An overview of RdRp inhibitors
The clinical significance of preventing the activity of RdRp has led to the development of several inhibitors that have been used to treat a wide range of RNA viruses, including Ebola, the human immunodeficiency virus (HIV), and the Zika virus.
Although RdRp inhibitors can achieve these antiviral effects through different mechanisms, one notable way in which these drugs halt the RNA viral genome replication process is by competing with adenosine triphosphate.
Once metabolized, these RdRp inhibitors compete with the viral ATP molecules for incorporation into the nascent RNA strand. Once the RdRp drug replaces ATP in the new strand, the RNA synthesis process is terminated, thereby preventing further replication of the virus from occurring.
Although many viruses, particularly coronaviruses, are equipped with a powerful proofreading process that can normally discard unwanted nucleoside analogs, certain RdRp inhibitors have been found to maintain their antiviral activity by overcoming this proofreading process.
RdRp and SARS-CoV-2
Human coronaviruses like the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are made up of structural proteins, of which include the spike (S) protein that surrounds the outer surface of the virus particle and gives it its crown-like appearance, as well as the nucleocapsid (N), matrix (M) and envelope (E).
In addition to these structural proteins, human coronaviruses are also made up of various non-structural proteins like the proteases nsp3 and nsp5, as well as RdRp.
Like many other pharmaceutical drugs that have been used to combat RNA viruses, RdRp inhibitors have also been investigated and demonstrated unique success in eliminating SARS-CoV-2.
Attacking SARS-CoV-2 with RdRp inhibitors
Several different in vitro and in vivo studies have investigated the efficacy of different RdRp inhibitors for treating human coronaviruses including SARS-CoV-2. Typically, in vitro studies will provide information on the half-maximal effective concentration (EC50), which is a value that represents the drug concentration that was responsible for half of the maximum response to occur, which, in this case, would be antiviral activity.
Similarly, half-maximal inhibitor concentration (IC50) will inform researchers of the drug concentration that results in half of the drug's peak inhibiting effect against certain viral functions.
When both the EC50 and IC50 values are low, high drug potency can be determined. Recent work has shown that RdRp inhibitors used against SARS-CoV-2 in Vero E5 cells resulted in EC50 values ranging between 0.77 µM up to 109.5 µM.
Once in vitro data has been confirmed and low EC50 and IC50 values are found, animal studies are used to investigate further how these drugs can combat the virus and protect against its associated pathologies.
Typically, in vivo studies on coronaviruses have been conducted in transgenic mouse species and rhesus macaques. In one recent study, the RdRp inhibitor was used to treat SARS-CoV-2 in the rhesus macaque model and successfully prevented the subjects from experiencing any of the classical signs associated with this respiratory disease.
Furthermore, test subjects receiving the RdRp inhibitor had reduced pulmonary infiltrates on their radiographs and lower virus titers in collected bronchoalveolar lavage samples as early as 12 hours following the first treatment.
Several different RdRp inhibitors have shown promising results for their use in treating the novel SARS-CoV-2. While further work must still be conducted to fully understand the mechanisms responsible for reducing the antiviral activity of SARS-CoV-2, their potential in curing infected individuals and even putting an end to this pandemic is extremely valuable.
References and Further Reading
- Venkataraman, S., Prasad, B. V. L. S., & Selvarajan, R. (2018). RNA Dependent RNA Polymerases: Insights from Structure, Function, and Evolution. Viruses 10(2). doi:10.3390/v10020076.
- Elfiky, A. A. (2020). Ribavirin, Remdesivir, Sofosbuvir, Galidesivir, and Tenofovir against SARS-CoV-2 RNA dependent RNA polymerase (RdRp): A molecular docking study. Life Sciences 253. doi:10.1016/j.lfs.2020.117592.
- Venkateshan, M., Muthu, M., Suresh, J., & Kumar, R. R. (2020). Azafluorene derivatives as inhibitors of SARS-CoV-2 RdRp: Synthesis, physicochemical, quantum chemical, modeling and molecular docking analysis. Journal of Molecular Structure 1220. doi:10.1016/j.molstruc.2020.128741.
- Amirian, E. S., & Levy, J. K. (2020). Current knowledge about the antivirals remdesivir (GS-5734) and GS-441524 as therapeutic options for coronaviruses. One Health 9. doi:10.1016/j.onehlt.2020.100128.
- Williamson, B. N., Feldmann, F., Schwarz, B., Meade-White, K., et al. (2020). Clinical benefit of remdesivir in rhesus macaques infected with SARS-CoV-2. Nature. doi:10.1038/s41586-020-2423-5.