As the COVID-19 pandemic continues to spread despite vaccination roll-outs, albeit at a slower pace, the fact that universal vaccination would not be reached for another year or two has prompted continuous efforts to develop an effective antiviral therapy.
According to a recent study published on the bioRxiv* preprint server, varenicline tartrate, a nicotinic acetylcholine receptor (nACHR) agonist, exhibits antiviral action against the virus.
The SARS-CoV-2 virus penetrates host cells via the angiotensin-converting enzyme-2 (ACE2), and the host transmembrane serine protease 2 (TMPRSS2) enzyme is also involved in priming the virus spike protein. Because the nasal mucosa produces both of these proteins on the epithelium, it may be the most susceptible area of the airway to infection.
Due to early research indicating that a low incidence of smoking is associated with lower COVID-19 hospitalization rates, some scientists hypothesized that the nicotinic cholinergic system is involved in the virus spread.
According to binding simulations, the virus spike protein is believed to be able to bind to nAChR, and one study identified a potential region within the receptor binding domain (RBD) of the SARS-CoV-2 spike glycoprotein that acts similarly to nAChR-binding snake venom.
These interactions may account for the immunologic pathology observed in COVID-19, including disease flares or acute myasthenia-like signs in patients with the autoimmune disease following infection with SARS-CoV-2 or following vaccination with mRNA vaccines against this virus.
In this case, the researchers investigated the possible role of the selective small-molecule nAChR agonist varenicline tartrate. This substance mimics nACh activity at the α7 receptor and is a partial agonist at the α3β4, α3α5β4, α4β2, and α6β2 receptors.
The study found that at concentrations ranging from 0.3 M to 0.5 M – the half-maximal inhibitory concentration – varenicline decreased virus titers in two separate cell cultures to half the predicted level (IC50). When tested against the alpha version, the IC50 was significantly lower, at 0.13 M, whereas it was 4M for the beta VOC.
The cells exhibited no toxicity in either of these circumstances.
A second phase of the trial examined the drug’s efficacy in rhesus macaques infected with SARS-CoV-2. This animal model consistently demonstrated mild to moderate illness following virus infection, with elevated viral loads in the respiratory system and several patches of lung inflammation.
Varenicline prevented SARS-CoV-2 infection in monkeys without affecting breathing rate, respiratory distress, or fecal consistency. Bodyweight and temperature changes were minimal.
When varenicline OC-01 nasal spray was used, viral loads were significantly reduced as determined by reverse transcriptase-polymerase chain reaction (RT PCR) analysis of genomic and subgenomic (sg) ribonucleic acid (RNA). These should be present in abundance while the virus is actively reproducing.
The amounts of genomic and subgenomic RNA were reduced approximately 100-fold and 200-fold, respectively, in comparison to control cells. sgRNA levels had decreased to undetectable levels four days after exposure.
The researchers have presented the results of a pioneering study showing the antiviral activity of a nAChR agonist against SARS-CoV-2. The growth of the virus in cell cultures, following treatment with varenicline over a range of dosages and using wild-type, alpha and beta variants, was severely suppressed, while leaving the cell viability intact.
Using a rhesus monkey model, in vivo investigations revealed that varenicline given as a nasal spray at an estimated dose of 1 mM successfully inhibited virus infection and replication in the nasal cavity within 24 hours of administration.
As varenicline is an aqueous nasal spray that can be applied topically, it can achieve high local concentrations in the nasal mucosa. Furthermore, this avoids any negative effects associated with systemic dosing. Furthermore, it combats the infection at the point of entry. Finally, it has the same efficiency against alpha and beta versions as the wild-type virus.
It confirms earlier in silico studies that showed varenicline’s ability to bind directly to the SARS-CoV-2 RBD, at the hinge site, with great affinity. This is hypothesised to prevent the spike protein from changing into the ‘up’ conformation, which is required for binding to the ACE2 receptor or the nAChR and subsequent infection of the host cell.
The spike binds to the nAChR directly as well, although not via the RBD, but rather at the Y674-R685 area. In fact, following binding to the 42 and 7 nAChR subtypes, which have significant binding affinity for this drug, this portion of the protein may adopt various conformations.
This second action implies that varenicline will continue to be effective against new VOCs since its affinity for the Y674-R685 region of the spike or for nAChRs is independent of RBD conformation.
Finally, the nicotinic cholinergic system is hypothesised to protect against severe COVID-19, which is caused by the development of a hyperactive immune-inflammatory response, resulting in a cytokine storm and multi-organ dysfunction.
“Given the in vitro and in vivo effectiveness seen in the studies, varenicline nasal spray warrants further investigation as an antiviral agent for pre-exposure/post-exposure prophylaxis, and/or prevention of transmission of SARS-CoV-2 wild-type and variants,” write the researchers.
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