Seven new octapeptides from the mangrove rhizosphere-derived <i>Streptomyces</i> sp. GXIMD 03507

Mangrove-derived actinomycetes are a rich reservoir of bioactive natural products, offering promising opportunities for new lead discovery. Seven new quinomycin-type octapeptides, designated quinomycin Q-U and triostin D-E, were isolated from the mangrove rhizosphere-derived <i>Streptomyces</i> sp. GXIMD 03507. The structures of compounds 1-7, including their absolute configurations were elucidated through a combination of chemical and spectroscopic analyses. The anti-proliferation activities of compounds 1-7 were evaluated against five tumor cell lines. None of the compounds (1-7) exhibited significant cytotoxic activity. These results suggested that the bicyclic framework bridged by ester linkages is crucial for biological activity, hydrolysis of one or both of these ester bonds dramatically decreases the cytotoxic potency.