A new study from Gilead Sciences published in the preprint journal bioRxiv in April 2020 adds support for remdesivir, an antiviral that showed promise early in the COVID-19 pandemic.
What is Remdesivir?
Remdesivir is an antiviral first developed by Gilead in 2014 as a treatment for Ebola and Marburg viruses. However, scientists explored its possibilities as a broad-spectrum drug when it proved effective against SARS-CoV and MERS-CoV, coronaviruses which were responsible for earlier outbreaks.
The function of any antiviral drug is affected by the presence of gene variants at the target gene locus, the distribution of the drug in the host, and the expression of molecules on host tissues and cells that affect the drug's stability and metabolism.
How Does Remdesivir Work?
Remdesivir is a nucleoside analog. It was first designed to fit an earlier model of the SARS-CoV-1 virus. It was designed to work with the viral RNA polymerase complex of enzymes, which comprises non-structural proteins nsp12, nsp7, and nsp8. The model showed that the prototype drug would be rapidly taken up and incorporated into the growing RNA strand, ending the replication process, thereby after the addition of three more nucleotides. The drug thus hijacks the viral RNA and prevents viral replication.
The drug was refined following the current outbreak, based on the most current cryo-electron microscopic structure of the RNA polymerase of the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It reveals a particular domain of nsp12, which was unclear in the older viral structure.
Novel Coronavirus SARS-CoV-2 Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
Remdesivir Safe, Effective in Cell Cultures
In the current study, researchers studied the effect of remdesivir on human lung cells and primary human airway epithelial cultures.
In cell culture, remdesivir and its parent nucleoside GS-221524 prevented the virus from replicating. In two types of cells, replication was inhibited in a dose-dependent manner.
In primary human airway epithelial cultures, the drug demonstrated potent antiviral activity, with very low effective concentration (EC50) values of 0.0010 and 0.009 128 μM. This is the concentration at which 50% of viruses die following exposure to the drug.
Such cultures reflect the complexity and function of the human airway to a significant extent. The reduction in infectious viral particle count increased 100 times from the lowest to the highest tested concentration.
A previous study had shown that remdesivir did not kill cells at or below a concentration of 10 μM, thus proving that its antiviral effect on the human airway epithelial cells was virus-specific. Thus, it also has a high selectivity index of over 1,000, ensuring a wide margin of safety.
The study also showed that the effectiveness of the drug on different types of cells depended on the cell's ability to metabolize it to its active form. It had a more significant antiviral effect on cells that had higher metabolizing capacity, such as the human airway epithelium.
Remdesivir Active in Infected Mice
Remdesivir is active against the SARS-CoV-2 RNA polymerase enzyme in vivo, as shown by a chimeric viral model encoding this enzyme. After injecting this viral strain into an adapted mouse breed, the mice were then treated with remdesivir.
The mice were followed up by clinical examination and daily whole-body plethysmography (WBP), which is a substitute for lung obstruction.
Both treated and control mice lost weight at the same rate after infection. However, treatment with remdesivir reduced lung hemorrhaging during the period of infection. These mice also preserved more lung function, while the viral load went down markedly with treatment.
Thus, the researchers say that "therapeutically administered RDV can reduce virus replication and improve pulmonary function in an ongoing infection" in mice infected with an engineered virus expressing the remdesivir target, the RNA polymerase of SARS-CoV-2.
The Importance of Remdesivir
Remdesivir is a broad-spectrum antiviral capable of activity against many coronaviruses with different genetic make-ups, as well as unrelated viruses like the Ebola virus.
The current study shows that the drug can suppress the replication of SARS-CoV-2 in the human airway epithelial cell line, which is a potential reflection of its physiological efficacy. The presence of the triphosphate metabolite is responsible for its activity against the virus and is higher in airway epithelium compared to laboratory cultures of lung cells.
The drug matches the host expression of relevant enzymes that activate the drug. The paper says, "This is the first rigorous demonstration of potent inhibition of SARS-CoV-2 in continuous and primary human lung cultures and first study suggesting efficacy of RDV against SARS-CoV-2in mice."
Explaining Differences in Results
The paper also offers explanations for the variability of differences in EC50 in experiments carried out by various researchers. The results are probably affected by intrinsic differences in the virus isolate and experimental conditions.
The type of cell in which the culture is carried out is also an important effect modifier since the most commonly employed Vero E6 cell line shows twofold reduced drug potency for remdesivir compared to the prodrug. This is due to the lower uptake or metabolism of the active drug in this cell line, unlike in the human airway epithelium culture.
Thirdly, the rt-PCR test used to estimate the potency may detect not only the intact virus but even viral nuclear components, giving a falsely high viral count. This could lead to underestimation of the reduction in viral load with this method.
Prior use of remdesivir in preclinical as well as phase I and phase II trials in the Ebola outbreaks have proved its safety and tolerability, putting it in position for immediate, compassionate use in very ill COVID-19 patients. However, the final verdict will come from ongoing phase III randomized controlled trials, which will monitor the safety, effectiveness, and optimal dosing strategy of remdesivir in patients at different stages of COVID-19.
The researchers also caution against the emergence of drug resistance, which is likely if single antivirals are used on a large scale in the pandemic. In general, they say, coronaviruses are slow to develop resistance to remdesivir, which requires two separate mutations in the gene encoding the RNA polymerase. These resistant strains are also less robust in replication, besides being more vulnerable to the toxic effects of another potent nucleoside analog viral inhibitor called β-D-N4-272 hydroxycytidine (NHC).
The researchers advise: "Therapies combining direct-acting antivirals (DAAs) such as RDV and NHC, along with other DAAs such as antibodies and protease inhibitors that target different stages of the viral replication cycle, could be considered for counteracting resistance if it emerges in patients treated with antiviral monotherapy."
They say that the concomitant use of anti-inflammatory drugs may also help to buy time for the antiviral drug to suppress viral replication and improve the outcomes. With the continued spread of several human coronavirus pathogens in bats, new outbreaks are likely. This makes it urgent to find and develop broad-spectrum therapies against the coronaviruses speedily.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.