Geliboluols A-D: Kaurane-Type Diterpenoids from the Marine-Derived Rare Actinomycete Actinomadura geliboluensis.

Four new kaurane-type diterpenoids, geliboluols A-D (1-4), along with one known analog (5), were isolated from the culture broth of the marine-derived rare actinomycete Actinomadura geliboluensis. The structures of compounds 1-4 were determined by spectroscopic analysis (HR-ESIMS, 1D, and 2D NMR), the MPA method, and by comparing their optical rotation values with those in the literature. The new compounds were evaluated for their cytotoxicity against seven blood cancer cell lines by a CellTiter-Glo (CTG) assay and six solid cancer cell lines by a sulforhodamine B (SRB) assay. Among the new compounds, compound 4 exhibited moderate cytotoxic activity against some blood cancer cell lines, with GI50 values ranging from 2.59 to 19.64 µM, and against solid cancer cell lines with GI50 values ranging from 4.34 to 7.23 µM.