Glycyrrhiza glabra L. Extracts and Other Therapeutics against SARS-CoV-2 in Central Eurasia: Available but Overlooked

Abstract

In Central Eurasia, the availability of drugs that are inhibitors of the SARS-CoV-2 virus and have proven clinical efficacy is still limited. The aim of this study was to evaluate the activity of drugs that were available in Kazakhstan during the acute phase of the epidemic against SARS-CoV-2. Antiviral activity is reported for Favipiravir, Tilorone, and Cridanimod, which are registered drugs used for the treatment of respiratory viral infections in Kazakhstan. A licorice (Glycyrrhiza glabra) extract was also incorporated into this study because it offered an opportunity to develop plant-derived antivirals. The Favipiravir drug, which had been advertised in local markets as an anti-COVID cure, showed no activity against SARS-CoV-2 in cell cultures. On the contrary, Cridanimod showed impressive high activity (median inhibitory concentration 66 μg/mL) against SARS-CoV-2, justifying further studies of Cridanimod in clinical trials. Tilorone, despite being in the same pharmacological group as Cridanimod, stimulated SARS-CoV-2 replication in cultures. The licorice extract inhibited SARS-CoV-2 replication in cultures, with a high median effective concentration of 16.86 mg/mL. Conclusions: The synthetic, low-molecular-weight compound Cridanimod suppresses SARS-CoV-2 replication at notably low concentrations, and this drug is not toxic to cells at therapeutic concentrations. In contrast to its role as an inducer of interferons, Cridanimod is active in cells that have a genetic defect in interferon production, suggesting a different mechanism of action. Cridanimod is an attractive drug for inclusion in clinical trials against SARS-CoV-2 and, presumably, other coronaviruses. The extract from licorice shows low activity against SARS-CoV-2. At the same time, high doses of 2 g/kg of this plant extract show little or no acute toxicity in animal studies; for this reason, licorice products can still be considered for further development as a safe, orally administered adjunctive therapy.

Keywords: COVID-19; Cridanimod; Favipiravir; Glycyrrhiza glabra; Glycyrrhizin; SARS-CoV-2 virus; Tilorone; antivirals; influenza virus; licorice; phytochemicals.

Figure 1

Diagram of the process of obtaining a dry extract from licorice roots.

Figure 2

HPLC chromatograms of the Glycyrrhizin standard (upper panel) and obtained licorice extract (lower panel).

Figure 3

Four substances, Favipiravir, CMA (Cridanimod), Tilorone, and a G. glabra extract, were used to measure cytotoxicity in Vero E6 cells and their potential to inhibit SARS-CoV-2 replication in cell cultures. (Left panels): As described in the Materials and Methods, 50% inhibitory concentrations (IC₅₀) were measured in 96-well-plate cultures. Tilorone is distinguished from other substances by its much higher cytotoxicity. (Right panels): As described in the Materials and Methods, 50% effective concentrations (EC₅₀) were measured using the virus yield reduction assay with cell cultures grown in P100 dishes. No virus inhibition was found for Tilorone at non-cytotoxic concentrations. On the contrary, for the distantly related compound CMA, EC₅₀ = 66 μg/mL, which is the lowest value, indicating that it has the highest activity among the tested substances.

Figure 4

Antiviral activities of Favipiravir and G. glabra extract against type A influenza virus in the VYRA test.