Secondary metabolites from Lobaria pulmonaria (L.) Hoffm. target key metabolic enzymes: a novel strategy against multidrug-resistant tuberculosis

Abstract

Numerous cultures have traditionally utilised the foliose lichen Lobaria pulmonaria (L.) Hoffm. (“Oak Lung” or “Lungs of Oak” in English; family: Lobariaceae) as a Tuberculosis (Tb) treatment. The present study aimed to scientifically validate the folkloric use of L. pulmonaria in treating Tb by investigating its antimycobacterial profile against Mycobacterium tuberculosis H37Ra (M.tb) and six other MDR-Tb isolates. The preliminary results obtained from XRMA revealed the notable inhibitory activity of LP and Fraction (F)-3 against M.tb, displaying IC₅₀ values of 7.74 ± 0.27 and 6.26 ± 0.04 µg mL⁻¹, respectively; followed by F2 (IC₅₀ value: 38.82 ± 0.34 µg mL⁻¹) and F5 (IC₅₀ value: 46.69 ± 1.13 µg mL⁻¹). The purification process of these bioactive fractions resulted in the identification of four known secondary metabolites: fukinanolide A, pinastric acid, stictic acid, and scrobiculin. Furthermore, the MICs from REMA showed that LP, stictic acid, and fukinanolide A have greater efficacy in controlling the growth of all six tested MDR-Tb isolates, compared to rifampicin. Notably, LP exhibited superior antimycobacterial activity against all six tested MDR strains as compared to all isolated compounds and rifampicin, possibly due to the synergistic effect of its metabolites. Furthermore, the IC₅₀ values of LP, stictic acid, and fukinanolide A on THP-1 macrophages were considerably higher than MICs against the tested mycobacterial strains, suggesting that THP-1 remained unaffected at concentrations effective against M.tb and MDR-Tb isolates. The deliberated SI ratio values indicated that LP, stictic acid, and fukinanolide A were more active and less toxic to MDR-Tb strains than rifampicin. The molecular docking studies on 1EA1, 4V1F and 3VIU revealed that fukinanolide A and stictic acid bind effectively and selectively to 3VIU (β-ketoacyl reductase FabG4), thereby conferring their anti-TB potential. The outcomes provide a validation for the traditional use of L. pulmonaria in Tb treatment, with stictic acid and fukinanolide A identified as key biomarkers. Hence, L. pulmonaria presented as a promising source for the development of novel drugs targeting against MDR-Tb.