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.