Molecular Networking Reveals Indolo-Sesquiterpene Hybrids from the Marine-Derived Fungus Aspergillus terreus N4-9

Tandem mass spectrometry (MS/MS)-based molecular networking has emerged as a powerful tool for rapid dereplication of known compounds and discovery of novel structural analogues within the same metabolite class. In this study, the chemical diversity of indolo-sesquiterpene hybrids from the mangrove rhizosphere soil-derived fungus, Aspergillus terreus N4-9, was investigated by using molecular networking strategies. The known indolo-sesquiterpene hybrid terreuside B (1) along with three new analogues, terreusides C-E (2-4), were targeted isolation from the fungal cultures. Additionally, three putative new congeners, terreusides F-H (8-10), were tentatively identified through systematic analysis of their characteristic MS/MS fragmentation patterns. Detailed fragmentation studies revealed two predominant cleavage pathways for these hybrids related to fracture of the methylene bridge connecting Rings A and B (Type I) and furan opening in Ring C (Type II). Compound 2 demonstrated significant growth inhibitory activity against human gastric cancer SGC-7901 cells with an IC₅₀ value of 6.25 μM.