In vitro antituberculosis activity of diterpenoids from the Vietnamese medicinal plant Croton tonkinensis

Diterpenoids from the Vietnamese medicinal plant Croton tonkinensis are rich in ent-kaurane, kaurane and the grayanane class and are valuable intermediate plant metabolites with different bioactivities. In this study, we report the antituberculosis activity of these diterpenoids against both susceptible and resistant strains of M. tuberculosis for the first time. All of the ent-kaurane, kaurane and grayanane diterpenoids showed high to moderate activity against Mycobacterium. The highest antituberculosis activity was observed for ent-1β,7α,14β-triacetoxykaur-16-en-15-one (cpp604), with MIC values of 0.78, 1.56 and 3.12-12.5 µg/ml against H37Ra, H37Rv and all other resistant strains of Mycobacterium tuberculosis examined. In addition, other ent-kaurane-type diterpenoids also showed very high activities against mycobacterium, including cpp609 (1.56 µg/ml), cpp610 (1.56 µg/ml), cpp601 (3.12-6.25 µg/ml), cpp602 (3.12-6.25 µg/ml), cpp607 (3.12-6.25 µg/ml) and cpp608 (3.12-6.25 µg/ml). From the structure-activity relationship, functional groups R3 and R5 of the ent-kaurane skeleton were found to modulate the antimycobacterial activity.