Discovery of Spirosnuolides A–D, Type I/III\nHybrid Polyketide Spiro-Macrolides for a Chemotherapeutic Lead against\nLung Cancer

Four new macrolides,\nspirosnuolides A–D (<b>1</b>–<b>4</b>, respectively),\nwere discovered from the termite nest-derived <i>Kitasatospora</i> sp. INHA29. Spirosnuolides A–D are\n18-membered macrolides sharing an embedded [6,6]-spiroketal functionality\ninside the macrocycle and are conjugated with structurally uncommon\nside chains featuring cyclopentenone, 1,4-benzoquinone, hydroxyfuroic\nacid, or butenolide moieties. Structure elucidation was achieved using\na combination of spectroscopic analyses, multiple chemical derivatizations\n(methylation, methanolysis, Luche reduction, and Mosher’s reaction),\nX-ray diffraction analysis, and computational ECD calculations. Interestingly,\ngenome sequencing analysis suggests that spirosnuolides were biosynthesized\nthrough a rare type I/III hybrid polyketide synthase. Importantly,\nspirosnuolide B displayed potent antiproliferative effects against\nvarious cancer cell lines at nanomolar concentrations, particularly\nagainst HCC827 cells, an EGFR mutant non-small-cell lung cancer (NSCLC)\ncell line, with a high safety index value. Based on <i>in vitro</i> studies, the antiproliferative mechanism of spirosnuolide B involved\nthe activation of AMPK signaling, leading to cell cycle arrest and\napoptosis in HCC827 cells. Its potent efficacy was also proven <i>in vivo</i> by the effective inhibition of tumor growth in mouse\nxenograft studies. Moreover, cotreatment with spirosnuolide B and\ngefitinib, synergistically enhanced the antiproliferative activity\nand apoptosis, suggesting a potential strategy to overcome gefitinib\nresistance in EGFR mutant NSCLC.