Discovery of undescribed anthracycline-derived polyketides with cytotoxicity from endophytic Streptomyces chartreusis M7

Endophytic actinomycetes exhibit considerable potential for the production of biologically active metabolites due to their coevolution with plant hosts. In this study, an endophytic Streptomyces chartreusis M7 was isolated from Houttuynia cordata Thunb. Bioactivity-guided investigation of the metabolites produced by this strain led to the identification of thirteen anthracycline-derived polyketides, including five unreported anthraquinones designated streptoquinones A-E (1-5) and two undescribed angular polyketides named chartins A and B (6-7) along with six knowns. Their structures were elucidated through comprehensive spectroscopic analysis and ECD calculations. Notably, chartins A (6) and B (7) feature angular tetracyclic and pentacyclic skeletons, respectively, which have undergone several oxidative rearrangements. Moreover, streptoquinone A (1) exhibited moderate cytotoxicity against A549 cells, with an IC₅₀ value of 4.8 μM.