A recent study published in the American Journal of Physiology-Lung Cellular and Molecular Physiology investigated the effects of thiol drugs on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Coronavirus disease 2019 (COVID-19), caused by SARS-CoV-2, is a multidimensional disease characterized by pneumonia progressing to respiratory failure and death. The spike (S) protein of the virus invades host cells by binding to angiotensin-converting enzyme 2 (ACE2), the human cell receptor.
Natural thiol/disulfide rearrangements could result in conformational changes promoting viral entry. However, removing the disulfide bridge by mutating cysteines could disrupt viral binding to host receptors and thus prevent infection. Evidence indicates that disulfide bonds are structurally and functionally crucial for SARS-CoV-2 S protein.
Previously, the current study’s researchers revealed that mucin disulfide crosslinks could be targeted using drugs that contain functional thiol groups, which led to speculation that thiol drugs could cleave cystines in the S protein’s receptor-binding domain (RBD) to disrupt ACE2 binding.
The study and findings
In the present study, researchers evaluated the antiviral effects of thiol drugs on SARS-CoV-2 using ACE2 binding and cell entry assays. The RBD of ancestral SARS-CoV-2 isolate (Wuhan-1) was exposed to eight thiol drugs in a plate-based binding assay to test the cleavage of cystines in the S protein.
A commercial binding assay was optimized wherein the ancestral RBD was coupled to the plates functionalized with an amine-reactive maleic anhydride. Binding was measured after one hour of incubation with the recombinant ACE2. They noted that all tested drugs inhibited RBD-ACE2 binding in a dose-dependent manner.
Next, they tested the inhibition of SARS-CoV-2 infection of HEK293T cells using vesicular stomatitis virus (VSV)-derived pseudoviruses (PVs) coated with SARS-CoV-2 S protein. PVs were pretreated with thiol drugs for two hours, and a diluted mix was used to infect cells. As a control, cells were treated with drugs for two hours, followed by drug removal and infection with untreated PVs.
Pretreatment of PVs with thiol drugs exhibited significant dose-dependent inhibition of viral entry. Potent inhibitory effects were noted with 2-mercaptoethane sulfonate sodium salt (Mesna), cysteamine, bucillamine, and WR-1065 (the active metabolite of the amifostine prodrug). The effectiveness of WR-1065 and cysteamine was higher than tiopronin and N-acetyl cysteine (NAC).
In the control experiment, pretreatment of cells with drugs resulted in minor inconsistent effects on the infectivity of untreated PVs. This confirmed that thiol drugs predominantly act on viral S protein and not off-targets (host cell proteins). Further experiments involved infection of Vero E6 cells with authentic Wuhan-1 strain and the most potent drugs. They found that WR-1065 and cysteamine were highly effective in reducing cytopathic effects (CPE). Notably, infection inhibition was minimal when cells were treated with diluted drugs and infected with the virus.
Subsequently, the VSV-PV infection assay was repeated with PVs coated separately with S proteins of SARS-CoV-2 variants (Delta, Kappa, and Omicron) using the most potent thiol drugs. Additionally, methyl 6-thio-6-deoxy-α-D-galactopyranoside (TDG), a novel thiol saccharide molecule, was tested. Consistently, they observed that WR-1065 and cysteamine potently inhibited PV entry. Pretreatment of cells with drugs had no significant effect.
The half-maximal inhibitory concentration (IC50) of thiol drugs was higher for Delta variant inhibition than that for Omicron or Kappa variant. IC50 values of Mesna and bucillamine for the Omicron variant were low, whereas cysteamine, TDG, and WR-065 exhibited similar inhibition of Kappa and Omicron PVs. Again, WR-1065 and cysteamine were the most potent drugs with IC50 values in the low millimolar range.
In additional tests, the authentic Delta variant was used to infect Vero E6 cells, and they noted lower IC50 values of drugs for authentic Delta virus than Delta-PV. Interestingly, the team observed an inverse relation between drug potency and the acid dissociation constant (pKa). As such, they compared the efficacy of cysteine derivatives such as NAC (pKa = 9.5) and cysteine methyl ester [CME] (pKa = 7) in a Kappa-PV assay. They found that IC50 of CME was 25-fold lower than that of NAC.
Lastly, the research team assessed the action of cysteamine on SARS-CoV-2 infection in Syrian hamsters. The drug was intraperitoneally (i.p.) administered at a 100 mg/kg dose. The first dose was injected two hours before infection with Wuhan-1 strain, and subsequent doses were administered twice daily for five days. They noted that different measures of lung inflammation were lower in cysteamine-treated hamsters. In contrast, viral load was not significantly lower in the drug-treated animals compared to controls.
The authors demonstrated the inhibitory effects of thiol drugs in vitro and found cysteamine as the most potent drug. However, cysteamine did not show an antiviral effect in vivo, though it exerted anti-inflammatory effects. The researchers posit that the cysteamine concentration upon i.p. delivery might have been insufficient for antiviral activity but sufficient for anti-inflammation effect.
The present work demonstrated that thiol drugs, particularly cysteamine, could decrease lung inflammation and injury due to SARS-CoV-2 infection. Future studies should investigate whether aerosol delivery of these drugs to the airways could achieve in vivo antiviral activity.