| Summary: | Marine actinomycetes are prolific chemical sources of complex and novel natural products, providing an excellent chance for new drug discovery. The chemical investigation of the marine-derived <i>Streptomyces</i> sp. ITBB-ZKa6, from Zhaoshu island, Hainan, led to the discovery of two unique antimycin-type depsipeptides, zhaoshumycins A (<b>1</b>) and B (<b>2</b>), along with the isolation of the four known neoantimycins A (<b>3</b>), F (<b>4</b>), D (<b>5</b>), and E (<b>6</b>). The structures of the new compounds <b>1</b> and <b>2</b> were elucidated on the basis of the analysis of diverse spectroscopic data and biogenetic consideration. Zhaoshumycins A (<b>1</b>) and B (<b>2</b>) represent a new class of depsipeptides, featuring two neoantimycin monomers (only neoantimycin D or neoantimycins D and E) linked to a 1,4-disubstituted benzene ring via an imino group. Initial toxicity tests of <b>1</b>–<b>6</b> in MCF7 human breast cancer cells revealed that compounds <b>5</b> and <b>6</b> possess weak cytotoxic activity. Further structure–activity relationship analysis suggested the importance of the NH<sub>2</sub> group at C-34 in <b>5</b> and <b>6</b> for cytotoxicity in MCF7 cells.
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