Anti-mycobacterial triterpenes from the Canadian medicinal plant Alnus incana

Ethnopharmacological relevanceAlnus incana, commonly known as the gray or speckled alder, is a medicinal plant used by some Canadian First Nations to treat symptoms associated with tuberculosis. The aim of this study was to assess the anti-mycobacterial activity of an Alnus incana bark extract and to identify the active constituents of the extract.Materials and MethodsMethanolic extracts of the bark of A. incana were subjected to bioassay guided fractionation using Mycobacterium tuberculosis (H37Ra). The active constituents were identified by NMR and MS.ResultsFour pentacyclic lupane triterpenes were isolated and were identified as betulin, betulinic acid, betulone and lupenone. Betulin displayed a MIC of 12.5 μg/mL and an IC50 of 2.4 μg/mL against M. tuberculosis (H37Ra). Betulinic acid and betulone showed lower anti-mycobacterial activities with IC50 values of 84 and 57 μg/mL respectively. Lupenone was inactive against M. tuberculosis (H37Ra).ConclusionsBetulin, betulinic acid and betulone were identified as the major anti-mycobacterial constituents in the bark of A. incana and the functionality at carbons 3 and 28 of the lupane skeleton would seem to be important in determining the anti-mycobacterial activity of the triterpenes. This work supports the ethnopharmacological use of A. incana by Canadian First Nations communities as a treatment for tuberculosis.

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