Three new spirocyclic terpenoids from Euphorbia amygdaloides exhibit cytotoxicity against cancerous cell lines through early and late apoptosis

Bioassay-guided fractionation led to the isolation of three new spirocyclic terpenoid compounds from Euphorbia amygdaloides L., named Zagrosin I–III. Their structures were identified by 1D and 2D NMR (¹H NMR, ¹³C NMR, DEPT 135, HMBC, and HSQC-TOCSY) and LC-MS-MS spectrometry. The cytotoxicity of the isolated spirocyclic terpenoids (Zagrosin I-III) was assessed against human breast cancer (MCF-7), human fibrosarcoma (HT1080), and normal human foreskin fibroblast cells with MTT assays (24, 48, and 72 h treatments). The FITC-Annexin V apoptosis flow cytometry assays and cell cycle analysis were performed for Zagrosin I–III.

These isolated compounds were identified as: (9)-8a-((benzoyloxy)methyl)-2-methoxy-4,9-dimethyltetrahydro-4H,5H-2,4a-methanobenzo[d] [1,3] dioxine-4-carboxylate (Zagrosin I), ((9)-4-hydroxy-2-methoxy-4,9-dimethyltetrahydro-4H,8aH-2,4a-methanobenzo[d] [1,3] dioxin-8a-yl) methyl benzoate (Zagrosin II), and (9)-2-methoxy-4,9-dimethyl-8a-(phenoxy methyl) tetrahydro-4H,5H-2,4a-methanobenzo[d][1,3]dioxin-4-yl 4-methylpentanoate (Zagrosin III).

The IC₅₀ of Zagrosin I on 48-h-treated MCF-7 was calculated as 1.5 μg/mL. Zagrosin II and III exhibited cytotoxicity on 48-h-treated MCF-7 with IC₅₀s of 14.04 and 12.50 μg/mL, respectively. The IC₅₀ of Zagrosin I on human fibrosarcoma (HT1080) was 115.5 μg/mL, Zagrosin III, 16.81 μg/mL (48 h treatment), and Zagrosin II, 142.7 μg/mL (72 h treatment). Zagrosin I-III exhibited significant cytotoxicity against the MCF-7 cell line and human fibrosarcoma (HT1080), with the mechanism of early and late apoptosis affecting cells mostly in G0/G1 fallowed by S and G2 phases. MCF-7 had a higher rate of phosphatidyl serine exposure on the cell membrane than two other studied cells. The cytotoxicity on normal human foreskin fibroblasts was low. Zagrosin I-III can be considered an effective chemical backbone for anticancer drug development.

Abbreviations: 2D NMR: Two-Dimensional Nuclear Magnetic Resonance Spectroscopy; API: Atmospheric Pressure Ionization; DEPT: Distortionless Enhancement by Polarization Transfer; ELISA: Enzyme-Linked Immunosorbent assay; ERK: Extracellular signal-regulated kinase; ESI: Electrospray ionization; FBS: Fetal Bovine Serum; FITC: Fluorescein isothiocyante; fr: fraction; HMBC: Heteronuclear Multiple Bond Correlation; HPLC: High-Performance Liquid Chromatography; HSQC: Heteronuclear Single Quantum Coherence; HT1080: Human fibrosarcoma cell line; IC₅₀: Half-maximal inhibitory concentration; MCF-7: Human breast cancer cell line; MHz: Megahertz; MTT: 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide; NMR: Nuclear Magnetic Resonance; PBS: Phosphate Buffered Saline; PI: Propidium Iodide; ppm: Part Per Million; R_f: Retention Factor; TLC: Thin-layer chromatography; TOCSY: Total Correlation Spectroscopy; VLC: Vacuum Liquid Chromatography.