New Cytotoxic Cyclic Peptide from the Marine Sponge-Associated sp. UR67.

A new cyclic hexapeptide, nocardiotide A (), together with three known compounds-tryptophan (), kynurenic acid (), and 4-amino-3-methoxy benzoic acid ()-were isolated and identified from the broth culture of sp. UR67 strain associated with the marine sponge sp. from the Red Sea. The structure elucidation of the isolated compounds were determined based on detailed spectroscopic data including ¹D and ²D nuclear magnetic resonance (NMR) experimental analyses in combination with high resolution electrospray ionization mass spectrometry (HR-ESI-MS), while the absolute stereochemistry of all amino acids components of nocardiotide A () was deduced using Marfey's method. Additionally, ten known metabolites were dereplicated using HR-ESI-MS analysis. Nocardiotide A () displayed significant cytotoxic effects towards the murine CT26 colon carcinoma, human HeLa cervix carcinoma, and human MM.1S multiple myeloma cell lines. The results obtained revealed sponge-associated as a substantial source of lead natural products with pronounced pharmacological activities.