Broadly active antiviral therapeutics are of great interest to medicine, as drugs with too high of a specificity rely on quick and accurate pathogen identification and may fail to target genetic variants or newly emerging viruses. Due to the sheer number of different viruses capable of causing respiratory disease and the speed at which symptoms can develop, readily available and broadly effective therapeutics would be highly desirable for both prophylaxis and treatment of respiratory infections. However, for most respiratory viruses, no specific antiviral therapy is available. Effective broad-spectrum antivirals would reduce the severity of illness, reduce transmission and prevent secondary infections, thereby lessening the general burden and morbidity of these viruses. Given their penchant for zoonotic transmission, antiviral treatments against highly pathogenic coronaviruses are of particular interest and the current SARS-CoV-2 outbreak further illustrates the need for accessible, fast-acting anti-coronavirals.
Herbal preparations of Echinacea have traditionally been used to prevent and treat symptoms of colds and flu and are still widely used (9, 12). Echinaforce, an Echinacea purpurea extract, has been shown to broadly inhibit the infectivity of influenza A and B, RSV, parainfluenza virus, and herpes simplex virus in-vitro and to interfere with cytokine production induced upon viral infection (19-21). Results from the current study complement these previous findings by demonstrating a direct antiviral activity of Echinaforce both against common cold coronavirus 229E (HCoV-229E) and highly pathogenic coronaviruses (SARS-CoV and MERS-CoV). We observed a dose dependent inactivation of HCoV-229E upon direct exposure to the extract and 50% reduction of HCoV-229E infectivity (IC50) was achieved at 3.2 µg/ml. As previously seen for RSV, limited intracellular effect was observed for HCoV-229E, as virus replication was not affected by the addition of Echinaforce prior to infection. This observation, along with the fact that treatment of cell cultures with the extract post infection has only a limited effect at the highest concentration (50µg/ml), suggests that the observed antiviral effects against coronaviruses are primarily restricted to the extracellular phases, i.e prior to viral entry into the cell and/or during progeny virus release. Furthermore, this antiviral activity is not strain-specific since the related coronaviruses SARS-CoV and MERS-CoV were inactivated in a comparable manner. Interestingly, even unrelated enveloped RNA viruses such as yellow fever virus were sensitive to Echinaforce treatment indicating a broad antiviral activity against enveloped viruses.
Mechanism of action of different Echinacea extracts are currently unclear, however, for most viruses, Echinaforce seems to exert its antiviral effect upon direct contact, leading to a permanent inactivation of the virus particles. In the current study, inhibition of HCoV-229E infectivity after direct exposure could not be reverted by washing. This observed effect is likely due to a stable alteration of viral components, presumably, the viral membrane, or membrane proteins. Although specific inhibition has been suggested for Influenza (19), the heterogeneity of the envelope proteins and cell receptors used by all the different viruses susceptible to Echinacea treatment strongly argues against a specific mechanism of action. Rather, the broad antiviral activity of Echinacea on various membranous RNA viruses points to a more general inhibitory effect. Non-enveloped rhinoviruses are sensitive to Echinaforce at high concentrations while adenoviruses and mouse parvovirus are not (20). Interestingly, Echinacea does not inhibit vaccinia virus, a large, enveloped DNA virus. So far, it is the only enveloped virus found to be resistant to treatment with Echinaforce.
We investigated whether a protective effect in the upper-respiratory tract could be reproduced in-vitro, in re-constituted three-dimensional nasal epithelium, i.e air-liquid interface (ALI) cell cultures, where the apical side is exposed to air resembling the human airways in-vivo. This cell culture system recapitulates many of the characteristics of the human respiratory tract, including ciliary beating and mucus production (27, 28). Regular intake of Echinaforce was simulated by overlaying cells with a thin layer of the extract and this treatment was sufficient to either prevent or reduce infection with HCoV-229E in respiratory epithelium. Almost complete protection was observed in respiratory epithelium treated with 50 µg/ml. At a lower concentration (10 µg/ml), the protection was less efficient but detectable. These results are in agreement with observations made in clinical studies investigating the effect of Echinaforce on the incidence of respiratory tract infections in 755 volunteers. In this randomized, double blind, placebo controlled, clinical study the numbers of cold episodes were significantly lower in the volunteers receiving Echinaforce. While the placebo group had 188 cold episodes, with a collective duration of 850 days, the Echinaforce-treated group had 149 with a duration of 672 days. Throughout the whole study period, 54 viral infections, of which 21 were caused by coronaviruses (9: 229E, 11: HKU1, 1: OC43) were detected in the treated group and 74, of which 33 were coronaviruses (15: 229E, 17: HKU1, 1: OC43) in the placebo group. The same study found that the infection rates of membranous respiratory viruses (including HCoV-229E, NL-63 and OC-43) could be reduced in adults by approximately 50% (p=0.0114) during a 4-month prophylactic treatment with Echinaforce (15). Furthermore, very similar results were recently obtained in a pediatric study where similar reduction in infection rates was observed in 203 children, aged 4-12 years (p=0.0218) after Echinaforce treatment (Ogal M, unpublished data).
These studies indicate a clinically relevant protection against coronaviruses with prophylactic Echinaforce treatment at tolerable and safe dosages. Furthermore, we have also observed partial protection at lower concentrations. In vivo, this might be due to insufficient dosage. A better protection may be achieved by ingesting higher doses of the extract or a more directed distribution of Echinaforce in the airways, e.g. by aerosol delivery. Furthermore, isolation and concentration of the active compounds in Echinacea products could result in smaller daily doses and increased activity.
As previously mentioned, in addition to direct inactivation of viral particles, Echinacea also inhibits cytokine secretions during virus infection. Excessive production of interleukin-6 (IL-6) or IL-8 have been associated with symptomatic development of viral infections and such responses, i.e. a cytokine storm, are likely responsible for many of cold-associated symptoms such as runny nose, coughing, sneezing et cetera (29). During certain viral infections (e.g. influenza), the heightened immune response may actually contribute to the destruction of respiratory epithelium and may even be the dominant reason for symptoms in absence of virus-mediated cytopathicity (30, 31). In these cases, the inhibition of virus-induced cytokine production by Echinaforce may be beneficial by limiting the damage of the respiratory epithelium provoked by the immune system (13). For many other viruses, including coronaviruses, no direct cell destruction is observed during infection. This is in accordance with the fact that coronaviruses, in general, do not elicit a pronounced cytokine response upon infection (32). Despite severe symptoms and pulmonary pathology, the highly pathogenic MERS-CoV does not elicit an overwhelming cytokine response in primary respiratory epithelial cells in the early course of infection. However, later on, a marked induction of the pro-inflammatory cytokines/chemokines IL-1β, IL-8 and IL-6 was observed (33). Even if the anti-inflammatory action of Echinaforce is less relevant for coronaviruses, treatment with 50µg/ml Echinaforce inactivated both MERS-CoV and SARS-CoV particles to similar levels as observed for HCoV-229E.