Anti-MRSA actinomycins D1-D4 from the marine sponge-associated Streptomyces sp. LHW52447

Actinomycins D1−D4 (1–4), four new D-type actinomycin analogues, were isolated from the fermentation broth of a strain of marine sponge-associated Streptomyces sp. LHW52447, together with actinomycin D (5). The structures of 1−4 were determined by a combination analysis of HRMS and NMR spectroscopic methods, and their absolute configurations of amino acids were determined by Marfey's analysis. Actinomycins D1 (1) and D2 (2) are the first two naturally occurring actinomycins with incorporation an oxazole unit into the central phenoxazinone chromophore. Both 1 and 2 showed more potent antibacterial activities against three strains of pathogenic methicillin-resistant Staphylococcus aureus (MRSA) with MIC values of 0.125–0.25 μg/mL than those of 3−5 with MIC values of 0.5–1.0 μg/mL, which suggested that the anti-MRSA activity might be enhanced by the incorporation of an additional oxazole unit. In addition, the cytotoxicity evaluation against WI-38 human diploid lung fibroblasts revealed that the incorporation of oxazole unit could decrease the cytotoxicity of actinomycins on human normal cells.