BACKGROUND: Marine organisms are established to be a wealthy source of bioactive compounds with diverse chemical structures and bioactivities. Acanthostrongylophora ingens is known to be rich with pyrimidine b-carboline and manzamine-type alkaloids. The goal of the present work is to isolate and identify new alkaloids from A. ingens as well as to assess the cytotoxic potential of these metabolites towards various cancer cell lines. METHODS: The crude MeOH extract of the sponge was separated by vacuum liquid chromatography (VLC), using n-hexane, EtOAc, and MeOH. The EtOAc fraction was chromatographed on VLC, SiO2, sephadex LH-20, and RP(18) columns, affording four metabolites. Their structures were identified using infrared, ultraviolet, high-resolution mass spectrometry, and nuclear magnetic resonance spectroscopic techniques, as well as comparison with the published data. RESULTS: A new 1,2,3,4-tetrahydro-beta-carboline (THbetaCs) alkaloid, ingenine F (4) and three known compounds: Annomontine (1), acanthomine A (2), and 1-oxo-1,2,3,4-THbetaCs (3) were isolated and identified. Ingenine F (4) exhibited cytotoxic activity toward hormone-dependent breast carcinoma (MCF7), colon carcinoma (HCT116), and lung carcinoma (A549) cell lines with IC50 values of 2.82, 1.00, and 2.37 muM, respectively, compared to doxorubicin (IC50 0.012, 0.036, and 0.102 muM, respectively). CONCLUSION: It is the first report for the isolation of THbetaCs alkaloids from A. ingens. The THbetaCs alkaloid with N-methylbutyramide unit as found in ingenine F is very rarely encountered in nature. Ingenine F may provide new promising candidates for potential cytotoxic agent. SUMMARY: Ingenine F, a new 1,2,3,4-THbetaCs derivative (4) and three known alkaloids (1-3) were isolated from A. ingens. Their structures were verified by various spectroscopic analyses. Compound 4 had potent cytotoxic effect toward MCF7, HCT116, and A549 cancer cell lines. Abbreviations used: 1D: One-dimensional; 2D: Two-dimensional; CC: Column chromatography; COSY: Correlations spectroscopy; DMSO: Dimethyl sulfoxide; HMBC: Heteronuclear multiple bond correlation experiment; HRESIMS: High resolution electrospray ionization mass spectrometry; HSQC: Heteronuclear single quantum correlation; IR: Infrared; LCQ: Liquid chromatography quadrupole; LTQ: Linear trap quadropole; NMR: Nuclear magnetic resonance; RP: Reversed phase; SiO2: Silica gel; TLC: Thin-layer chromatography; UV: Ultraviolet; VLC: Vacuum liquid chromatography.