Antibacterial and cytotoxic biflavonoids from the root bark of Ochna kirkii

The new isoflavonoid kirkinone A (1) and biflavonoid kirkinone B (2) along with six known compounds (3-8) were isolated from the methanolic extract of the root bark of Ochna kirkii. The compounds were identified by NMR spectroscopic and mass spectrometric analyses. Out of the eight isolated natural products, calodenin B (4) and lophirone A (6) showed significant antibacterial activity against the Gram-positive bacterium Bacillus subtilis with MIC values of 2.2 and 28 μM, and cytotoxicity against the MCF-7 human breast cancer cell line with EC₅₀ values of 219.3 and 19.2 μM, respectively. The methanolic crude extract of the root bark exhibited cytotoxicity at EC₅₀ 8.4 μg/mL. The isolated secondary metabolites and the crude extract were generally inactive against the Gram-negative Escherichia coli (MIC ≥400 μg/mL). Isolation of biflavonoids and related secondary metabolites from O. kirkii demonstrates their chemotaxonomic significance to the genus Ochna and to other members of the family Ochnaceae.