Isolation and characterisation of antibacterial and anti-inflammatory compounds from Gnaphalium polycaulon

Abstract Ethno-pharmacological relevance Gnaphalium polycaulon commonly known as “cudweed” has been used throughout South America as an infusion to treat colds, bronchitis, fever or pneumonia. Aim of the study This study aimed to determine the antibacterial and anti-inflammatory activities of the aqueous extract of Gnaphalium polycaulon and identify the related compounds. Materials and methods A bio-guided isolation of the active compounds of Gnaphalium polycaulon was carried out, selecting the fractions depending on their antibacterial, anti-inflammatory and cytotoxic activities. The antibacterial effect was studied against Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus and Streptococcus pneumoniae; and the anti-inflammatory study was performed by measuring the inhibition of NF-κB in BEAS-2B and IMR-90 cell cultures. Results Three compounds were obtained and characterised by nuclear magnetic resonance and mass spectrometry. These compounds are 2-(4-(1-H-tetrazol-1-yl) phenyl)-2-aminopropanoic acid (1), N-phenyl-4-(3-phenyl-1,2,4-thiadiazol-5-yl) piperazine-1-carboxamide (2) and N-(4-ethoxyphenyl)-4-(2-methylimidazo-[1,2-α] pyridine-3-yl) thiazol-2-amine (3). All compounds showed antibacterial activity with MIC values of 44.80–44.85, 0.017–0.021 and 0.0077–0.0079 μM, respectively, in the Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus and Streptococcus pneumoniae strains, while the positive control, Ofloxacin, had a MIC value of 27.64–27.67 μM. This was corroborated through a zone inhibition assay, where compound 3 (11.36–11.67 mm) was much more active than the positive control (Ofloxacin, 23.41–24.12 mm), while compounds 2 (26.47–27.64 mm) and 1 (28.39–29.76 mm) displayed similar antibacterial potential to the positive control. Finally, all the compounds presented NF-κB inhibitory activity, compounds 3 (IC50 = 0.0071–0.0073 μM) and 2 (IC50 = 0.016–0.019 μM) being the most promising. Compound 1 (IC50 = 44.24–44.26 μM) had less anti-inflammatory potential, being also the closest to the values displayed by the positive control (Celastrol, IC50 = 7.41 μM). Conclusion In the present study, three compounds were isolated for the first time from the aqueous extract of Gnaphalium polycaulon. Their antibacterial and anti-inflammatory potential was tested and showcased.