Hopeaphenol inhibits entry of SARS-CoV-2 variants of concern into cells

New antivirals are being sought worldwide to combat the ever-growing threat of coronavirus 2019 (COVID-19) disease, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has already claimed more than three million human lives.

A new study, published as a preprint on the bioRxiv server*, reports promising results with the natural product hopeaphenol, which successfully prevented the virus from entering host cells.

Several vaccines have been deployed in many countries, but it is now clear that the mammoth task of global immunization will not be completed within the current year.

In the meantime, new variants of the new coronavirus are constantly emerging around the world. They are often more infectious or virulent than the parent strain. In addition, they appear to have acquired immune escape capabilities, resisting neutralization by antibodies produced by previous strains or by vaccines.

Study: Natural stilbenoid (-)-hopeaphenol inhibits cell entry of USA-WA1/2020, B.1.1.7 and B.1.351 variants of SARS-CoV-2. Image credit: Imilian / Shutterstock
Study: Natural stilbenoid (-)-hopeaphenol inhibits cellular entry of USA-WA1/2020, B.1.1.7 and B.1.351 variants of SARS-CoV-2. Image credit: Imilian / Shutterstock
Stilbenoid activity
Many plants, including Hopea, Vitis, Shorea, and Anisoptera, are sources of stilbenoids such as (-)-hopeaphenol, vatalbinoside A, vaticanol B and their stereoisomers. Previous work has shown that, in vitro, these substances inhibit cell proliferation, Gram-negative bacteria, fungi and the herpes simplex virus, and have anti-inflammatory properties.

In studies conducted on mice, hoperephenol also normalized plasma triglyceride levels after administration of olive oil, as well as glucose levels in mice fed with sugars. In addition, it prevented liver damage from lethal lipopolysaccharides (LPS) that serve as Gram-negative endotoxins.

In these studies, stilbenoids were found to be tolerable and safe in vivo, even at high concentrations. These results indicate the importance of extending these short-term in vivo studies to understand the efficacy of stilbenoids against SARS-CoV-2.

What were the conclusions?
There is a need to develop antivirals that are effective against the virus. The current effort began with a library of purified derivatives of natural products.

The analysis yielded three compounds from the stilbenoid group. The compound (-)-hopeaphenol is considered representative of the three.

The researchers found that hopeaphenol prevented the viral epi receptor binding domain (RBD), expressed on pseudoviruses, from binding to its host cell receptor, angiotensin converting enzyme 2 (ACE2). It also prevents virus replication in vitro without affecting host cell viability.

Hoperaphenol remained effective against two of the SARS-CoV-2 variants of concern (VOC), the British and South African, despite the presence of mutations that allow them to escape immune neutralization and be more infectious than the Wuhan or D614G strain.  

Thus, these stilbenoid analogues, particularly hopeaphenol, offer hope for the development of a broad-spectrum SARS-CoV-2 antiviral, inhibiting entry into host cells. They could be used alone or in combination with antivirals already used against other viral targets.

A more recent molecular docking analysis showed the potential of stilbenoids to prevent RBD-ACE2 binding. The compound kobophenol A, also a stilbenoid, was also shown to prevent this interaction at a low inhibitory concentration (50% of inhibited binding at 1.8 μM.

Kobophenol A also inhibited SARS-CoV-2 replication, with 50% inhibition observed at 71.6 μM. The current results with hopeaphenol confirm these earlier findings.

However, the researchers failed to observe similar antiviral activity with resveratrol, even at 100 μM, despite earlier reports of 50% efficacy in preventing infection of cultured cells by the virus at one-tenth that concentration. One likely reason for this discrepancy may be that previous work used supernatant polymerase chain reaction to verify cell infection, whereas the current work relies on observing cytopathic effects (CPE) caused by the virus in infected cell cultures.

What are the findings?
The three compounds studied here showed selective activity against RBD-ACE2 binding compared to another ligand-receptor pair with unrelated physiological activity. However, they need to be studied in more detail to rule out cytotoxicity.

Further research may lead to the detection or synthesis of analogues with improved safety and therapeutic margins.

Stilbenoids are also easily degraded by oxidation, light, and pH changes. This vulnerability may explain why vaticanol B was three times less effective than the other two compounds (hopeaphenol and vatalbinoside A) in preventing RBD-ACE2 binding in vivo, both in pseudovirus inhibition studies and in PBS assays. However, it was most potent in vitro.

However, the scientists did not observe significant inhibitory activity against the main protease of SARS-CoV-2, as had been the case recently in a virtual screening study. This indicates that the antiviral activity observed here is probably not due to Mpro inhibition.

This hypothesis is supported by the finding of higher activity against the South African variant of the spike, compared to the Wuhan and UK variants.

However, it may be possible to identify stilbenoid derivatives that are active against both the spike entry protein and the Mpro enzyme, thereby striking the virus twice. This would not only improve efficacy, but also reduce the risk of developing resistance.