Isolation, Structural Elucidation, and Biological Evaluation of Pyrrole-Based Alkaloids from Sea Anemone-Associated Streptomyces sp. S1502

Three new pyrrole alkaloids, streptopyrroles D-F (1-3), along with four known analogs (4-7) were isolated from Sea Anemone-Associated Streptomyces sp. S1502 via an OSMAC (One Strain Many Compounds)-based strategy. Their structures were elucidated through comprehensive spectroscopic analyses, including HRESIMS and 1D/2D NMR experiments (COSY, HSQC, and HMBC), and further confirmed by X-ray crystallography. Biological evaluation identified streptopyrrole (4) as an anti-MRSA (methicillin-resistant Staphylococcus aureus) agent, while 4 and 6 displayed broad-spectrum cytotoxicity and good selectivity against a panel of human cancer cell lines. Notably, 4 and 6 showed particularly potent activity against the lung cancer cell lines H1299, SW1573, and A549, with IC₅₀ values ranging from 5.43 to 16.24 μM. Further mechanistic investigation revealed that both compounds suppress the proliferation of lung cancer cells by inducing cell cycle arrest at the G₀/G₁ phase and impair metastatic potential by inhibiting migration and invasion. These findings not only expand the structural diversity of marine-derived pyrrole alkaloids but also reveal the anticancer mechanisms of 4 and 6, highlighting their promise as active candidates for further antitumor drug development, particularly in lung cancer.