The in-vitro antiviral activity of Carissa edulis, Tulbaghia acutiloba, and Tetradenia riparia on severe acute respiratory syndrome coronavirus-2

Abstract

Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative virus of the coronavirus disease of 2019 (COVID-19). Despite the availability of some antiviral drugs, the treatment options for COVID-19 remain limited, which highlights the need for alternative approaches. Traditional medicines represent an underexplored source of potential antiviral compounds. This study assessed the antiviral activity of three South African medicinal plants that are traditionally used to treat lower respiratory tract infections such as SARS-CoV-2. Methods: Different concentrations of ethanolic extracts of Carissa edulis, Tulbaghia acutiloba, and Tetradenia riparia were prepared and screened for their cytotoxicity on human non-small cell lung carcinoma cell line (H1299 cells) using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The extracts with non-cytotoxic effects were further investigated for their antiviral activity against SARS-CoV-2-infected H1299 cells using a time-of-addition assay. Real-time polymerase chain reaction quantified viral loads post-treatment and the anti-inflammatory activity was determined by measuring inflammatory cytokine levels via enzyme-linked immunosorbent assay (ELISA). The anti-oxidant capacity of the extracts was assessed using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Phytochemical screening of the plant extracts was performed using the coulometric assay. Results: Both pre-treatment and post-treatment of H1299 cells with C. edulis extract significantly reduced viral loads by two- to four-fold decreases as compared to untreated controls, which suggests potential antiviral activity. T. acutiloba and T. riparia also exhibited moderate antiviral suppression, especially when the extracts were pre-treated prior to infection at early time points (24 hours). The anti-inflammatory effects were assessed by measuring pro-inflammatory cytokine (interleukin [IL]-6, IL-8, and tumour necrosis factor-alpha [TNF-α]) levels. All three plants extracts demonstrated greater than two-fold decreases in IL-6 and TNF-α production over time compared to controls, which is indicative of potential anti-inflammatory activity. The anti-oxidant capacity was determined using the DPPH radical scavenging assay, with T. riparia exhibiting the strongest activity with an inhibitory concentration of 50% (IC50) of 144 μg/ml, followed by T. acutiloba (IC50 158 μg/ml) and C. edulis (IC50 234 μg/ml). Phytochemical analysis revealed the presence of flavonoids, saponins, tannins, and alkaloids, with T. riparia containing the highest phenolic content (12.57 g GAE/g) and alkaloid levels (7.86 mg BE/g). Discussion: The study investigated the antiviral and anti-inflammatory potential of three South African medicinal plants and revealed significant insight into traditional medicine’s role in contemporary therapeutic approaches. The findings from this research demonstrate C. edulis’ potent antiviral activity and positions it as a promising candidate for further research in antiviral therapy development. Similarly, T. acutiloba and T. riparia demonstrate moderate antiviral suppression, which is particularly notable when the extracts were applied as a pre-treatment before infection. The effectiveness observed at early time points (24 hours) suggests that the timing of treatment application plays a crucial role in maximising antiviral effects, which highlights the potential for these plants to contribute to early intervention strategies. The study also delved into the anti-inflammatory properties of the plant extracts by measuring the levels of pro-inflammatory cytokines such as IL-6, IL-8, and TNF-α. The results showed greater than two-fold decreases in IL-6 and TNF-α production over time in comparison to controls, which indicates significant anti-inflammatory activity. This outcome is particularly relevant in the context of SARS-CoV-2 infections, where the cytokine storm is a critical factor in disease severity, which suggests that these plant extracts could offer dual benefits in both antiviral and anti-inflammatory capacities. T. riparia displayed the highest anti-oxidant capacity, followed by T. acutiloba and C. edulis. This anti-oxidant activity is crucial as oxidative stress plays a significant role in the pathogenesis of viral infections, including those caused by SARS-CoV-2. The phytochemical analysis conducted as part of the study revealed the presence of flavonoids, saponins, tannins, and alkaloids, with T. riparia exhibiting the highest levels of phenolic content and alkaloid levels. These compounds are known for their therapeutic effects, including antiviral, anti-inflammatory, and anti-oxidant properties, which could explain the observed biological activities of the extracts. Conclusion: This study not only reaffirms the traditional use of C. edulis, T. acutiloba, and T. riparia in treating respiratory infections but also highlights their potential as sources of bioactive compounds for the development of novel antiviral and anti-inflammatory therapies. The significant reductions in viral load, coupled with anti-inflammatory and anti-oxidant effects, highlights the need for further investigation into these plants’ mechanisms of action and their potential integration into treatment protocols for viral infections, including COVID-19. Future research should aim to isolate and characterise the specific active compounds that are responsible for these effects, thereby paving the way for the development of new pharmacological agents derived from traditional medicinal plants.