Antimycobacterial activity of scopoletin from ethanolic extract of Hymenodictyon floribundum (Hochst. & Steud.) B.L.Rob. Stem bark

The available tuberculosis (TB) therapies are taken for long time with complex regimens associated with adverse effects and drug-drug interactions. The emergence of multidrug resistant (MDR) and extensively drug resistant (XDR) strains of Mycobacterium tuberculosis and HIV co-infections necessitates the need for continued search for new effective anti-TB alternative leads with different mechanisms of action and safety profile. The aim of this study was to isolate and characterize antimycobacterial compounds from Hymenodictyon floribundum (Hochst. & Steud.) B.L.Rob. a plant used traditionally in the management of HIV/AIDS-related conditions. Antimycobacterial activity of the plant extracts and its constituents against Mycobacterium indicus pranii (MIP) and Mycobacterium madagascariense (MM) was tested using a twofold broth micro-dilution technique. Bioassay-guided isolation was used to isolate active compounds. Isolated compounds were also tested for activity against clinical isolates of Mycobacterium tuberculosis (Mtb). The crude plant extract had antimycobacterial activity and its ethyl acetate fraction was the most active with minimum inhibitory concentration (MIC) values of 97 µg/ml and 197 µg/ml against MIP and MM respectively. Three compounds were isolated from the ethyl acetate fraction; 7‑hydroxy-6-methoxycoumarin (1), 7,7′-dihydroxy-6,6′-dimethoxy-3,3′-biscoumarin (2) and 7,7′-dihydroxy-6,6′-dimethoxy-8,8′-biscoumarin (3). Compound 1 had growth inhibitory activity against MIP, MM and Mtb with MIC values of 0.2 M against MIP, 0.41 M against MM, 0.78 M against Mtb1 and 1.63 M against Mtb2. In conclusion, 7-Hydroxy-6- methoxycoumarin exhibited moderate antimycobaterial activity. The findings from this study support the use of H. floribundum by traditional health practitioners for treatment of HIV and AIDS-related conditions.