Cytotoxic ent-kaurane diterpenoids from Isodon eriocalyx var. laxiflora

Three new ent-kaurane diterpenoids laxiflorin E (1), laxiflorin H (6) and laxiflorin I (8) were isolated from the leaves of Isodon eriocalyx var. laxiflora, along with nine known diterpenoids, eriocalyxin A (2), laxiflorin C (3), laxiflorin D (4), laxiflorin A (5), maoecrystal S (7), maoecrystal Q (9), eriocalyxin B (10), maoecrystal C (11) and enmelol (12). The structures of the new compounds were determined by spectroscopic methods. Compounds 1-5 are 6,7-seco-ent-kaurane-7,20-olide diterpenoids, and 6-12 belong to 7,20-epoxy-ent-kauranoids. The spectral data of enmelol (12) are reported here for the first time. Ten of these compounds were tested for their cytotoxicity toward K562 and T24 human tumor cells. Compounds 1, 3 and 10 showed significant inhibitory effects on K562 cells with IC50 values 0.077, 0.569 and 0.373 mug/mL, and compounds 1 and 10 also demonstrated significant inhibitory activities toward T24 cells with IC50 values 0.709 and 0.087 mug/mL. Compounds 8 and 11 also displayed inhibitory effect on both two kinds of cells with IC50 value less than 6.5 mug/mL.