Since the first discovery in 1961, more than 1300 ent-kaurane diterpenoids have been isolated and identified from different plant sources, mainly the genus Isodon. Chemically, they consist of a perhydrophenanthrene subunit and a cyclopentane ring. A large number of reports describe the anticancer potential and mechanism of action of ent-kaurane compounds in a series of cancer cell lines. Oridonin is one of the prime anticancer ent-kaurane diterpenoids that is currently in a phase-I clinical trial in China. In this review, we have extensively summarized the anticancer activities of ent-kaurane diterpenoids according to their plant sources, mechanistic pathways, and biological targets. Literature analysis found that anticancer effect of ent-kauranes are mainly mediated through regulation of apoptosis, cell cycle arrest, autophagy, and metastasis. Induction of apoptosis is associated with modulation of BCL-2, BAX, PARP, cytochrome c, and cleaved caspase-3, -8, and -9, while cell cycle arrest is controlled by cyclin D1, c-Myc, p21, p53, and CDK-2 and -4. The most common metastatic target proteins of ent-kauranes are MMP-2, MMP-9, VEGF, and VEGFR whereas LC-II and mTOR are key regulators to induce autophagy.
Three isopimarane diterpenes [fladins B (1), C (2), and D (3)] were isolated from the twigs and leaves of Chinese folk medicine, Isodon flavidus. The chemical structures were determined by the analysis of the comprehensive spectroscopic data, and the absolute configuration was confirmed by X-ray crystallographic analysis. The structures of 1–3 were formed from isopimaranes through the rearrangement of ring A by the bond break at C-3 and C-4 to form a new δ-lactone ring system between C-3 and C-9. This structure type represents the first discovery of a natural isopimarane diterpene with an unusual lactone moiety at C-9 and C-10. In the crystal of 1, molecules are linked to each other by intermolecular O-H···O bonds, forming chains along the b axis. Compounds 1–3 were evaluated for their bioactivities against different diseases. None of these compounds displayed cytotoxic activities against HCT116 and A549 cancer cell lines, antifungal activities against Trichophyton rubrum and T. mentagrophytes, or antiviral activities against HIV entry at 20 µg/mL (62.9–66.7) µM. Compounds 1 and 3 did not show antiviral activities against Ebola entry at 20 µg/mL either; only 2 was found to show an 81% inhibitory effect against Ebola entry activity at 20 µg/mL (66.7 µM). The bioactivity evidence suggested that this type of compound could be a valuable antiviral lead for further structure modification to improve the antiviral potential.
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