Cytotoxic Phenanthrenequinones and 9,10-Dihydrophenanthrenes from Calanthe arisanensis

Two new phenanthrenequinones, calanquinones B and C (2 and 3) and four new 9,10-dihydrophenanthrenes, calanhydroquinones A-C (4-6) and calanphenanthrene A (7), along with five known compounds (1 and 8-11), were isolated from an EtOAc-soluble extract of Calanthe arisanensis through bioassay-guided fractionation. Their structures were identified from spectroscopic data, and the compounds were tested for in vitro cytotoxic activity against human lung (A549), prostate (PC-3 and DU145), colon (HCT-8), breast (MCF-7), nasopharyngeal (KB), and vincristine-resistant nasopharyngeal (KBVIN) cancer cell lines. Compound 1 showed the highest potency (EC50 < 0.5 mu g/mL) against all seven cancer cell lines, with the greatest activity against breast cancer MCF-7 cells (EC50 < 0.02 mu g/mL). Generally, except for 7, compounds 2-11 also showed significant cytotoxic activity (EC50 < 4 mu g/mL) against some cell lines (especially PC-3 and MCF-7) in the panel.