Can essential oils act as antivirals in HRSV infections?

Introduction: Human Respiratory Syncytial Virus (hRSV) is of medical importance because it causes acute respiratory infections mainly in children up to five years old. The infection affects the airways, being more severe when it reaches the lower respiratory tract, causing bronchiolitis, pneumonia, and bronchopneumonia. With the estimation that one out of every 50 deaths in children younger than 5 years is attributable to RSV, the impact of this virus on global child health is very high. In addition, RSV infection in early life is associated with long-term respiratory dysfunction and it is estimated that of the 101,400 global deaths secondary to RSV in 2019, 97% occurred in low- and middle-income countries. According to PAHO/WHO (Pan American Health Organization/World Health Organization) in Brazil hRSV remains at high levels, with an increase in the number of weekly cases in children, mainly under 2 years old, from April 2023. So far, treatment is based on the use of Ribavirin and the monoclonal antibody Palivizumab, which are high-cost alternatives that can generate serious side effects. Therefore, hRSV infection needs safe and more efficient treatment alternatives. Products of natural origin, such as essential oils, extracted from plant organs, such as leaves, barks, roots and flowers, have shown antiviral potential against SARS-CoV, Influenzas and other viruses. Thus, essential oils may be effective in combating hRSV infection. Objective: The aim of this work was to investigate the virucidal potential and protective effect of compound 1 essential oil on hRSV-infected HEp-2 cell cultures. Methods: HEp-2 cells were adhered in 96-well plates. Upon reaching 80% confluence, different concentrations of the essential oil (0.64%, 0.31%, 0.16%, 0.07%, 0.04%, 0.02% and 0.01%), by 48 hours were tested for determination of CC50 by MTT. Verification of virucidal activity was performed by bathing the virus sample (MOI 0.1) for 30 minutes at concentrations below that determined by the CC50 of the compound 1 and 2. It was also evaluated in pretreatment plate assay and virucidal test at different MOI (0.01, 0.05, 0.1, and 0.5) for 3 hours. The viral aliquot was bathed for 1 hour with the same concentrations before the infection process and the plate was incubated in a CO2 incubator for 5 days. After this period, 10% formaldehyde was added overnight, and the cells stained with 1% crystal violet. Plaque forming units (PFU) were counted and compared untreated cells. Results: Characterization of the CC50 of compound 1, demonstrated 50% of cell viability is 0.02% compound 1 was used, compound 2 demonstrated cytotoxic effect on the cell at the same tested concentrations of compound 1. Compared to compound 2 which did not show efficacy in the viral inhibition effect, allowing the formation of syncytia. In the virucidal test, the same pattern observed by the MTT assay was observed. In the plaque assay, compound 1 showed promising effects. MOI 0.01 showed an inhibition of approximately 47.3%, preventing plaque formation, compared to the viral control, with a decrease in plaque forming units (PFU). High concentrations of essential oils were cytotoxic to the cells, high concentrations of MOIs were aggressive, making the compound unable to perform its treatment. Conclusion: This essential oil may have future therapeutic applications against the hRSV virus, because in the assay it was possible to observe positive results for viral inhibition.