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

Four new macrolides, spirosnuolides A-D (<b>1</b>-<b>4</b>, respectively), were discovered from the termite nest-derived <i>Kitasatospora</i> sp. INHA29. Spirosnuolides A-D are 18-membered macrolides sharing an embedded [6,6]-spiroketal functionality inside the macrocycle and are conjugated with structurally uncommon side chains featuring cyclopentenone, 1,4-benzoquinone, hydroxyfuroic acid, or butenolide moieties. Structure elucidation was achieved using a combination of spectroscopic analyses, multiple chemical derivatizations (methylation, methanolysis, Luche reduction, and Mosher's reaction), X-ray diffraction analysis, and computational ECD calculations. Interestingly, genome sequencing analysis suggests that spirosnuolides were biosynthesized through a rare type I/III hybrid polyketide synthase. Importantly, spirosnuolide B displayed potent antiproliferative effects against various cancer cell lines at nanomolar concentrations, particularly against HCC827 cells, an EGFR mutant non-small-cell lung cancer (NSCLC) cell line, with a high safety index value. Based on <i>in vitro</i> studies, the antiproliferative mechanism of spirosnuolide B involved the activation of AMPK signaling, leading to cell cycle arrest and apoptosis in HCC827 cells. Its potent efficacy was also proven <i>in vivo</i> by the effective inhibition of tumor growth in mouse xenograft studies. Moreover, cotreatment with spirosnuolide B and gefitinib, synergistically enhanced the antiproliferative activity and apoptosis, suggesting a potential strategy to overcome gefitinib resistance in EGFR mutant NSCLC.