Improving Cytotoxicity of Saporin with Saponin SO1406 Isolated from the Roots of Saponaria Officinalis

Background/Objectives: Saponins have recently emerged as promising natural products that enhance toxin-based anticancer therapeutics by improving cytosol uptake. This study aimed to identify structurally defined novel natural saponins and evaluate their ability to enhance anticancer cytotoxicity. Methods: The roots of Saponaria officinalis L. were extracted with aqueous ethanol and purified by silica gel column chromatography and reverse-phase high-performance liquid chromatography (RP HPLC). The structures of new saponins were elucidated by NMR spectroscopy and mass spectrometry. Biological activity was assessed in vitro using multiple cancer cell lines. Results: Two pairs of structurally defined pure saponins were obtained: SO1406 and SO1448, and SO1684 and SO1726. Structural elucidation revealed that SO1684 and SO1726 share the core structure 3-O-β-D-Gal-(1→2)-[β-D-Xyl-(1→3)]-β-D-GlcA-gypsogenin-28-O-β-D-Qui-(1→4)-[β-D-Xyl-(1→3)-β-D-Xyl-(1→4)]-α-L-Rha-(1→2)-β-D-Fuc, with SO1684 acetylated at Qui O-4 and SO1726 bearing additional acetylation at Qui O-3. Deacetylation of either SO1684 or SO1726 afforded a known saponin SA1641 isolated from Saponinum album (Merck). Similarly, SO1406 and SO1448 were identified as 3-O-β-D-Gal-(1→2)-[β-D-Xyl-(1→3)]-β-D-GlcA-gypsogenin-28-O-β-D-Xyl-(1→4)-α-L-Rha-(1→2)-β-D-Fuc derivatives, each acetylated at Fuc O-4, with SO1448 containing an additional acetyl group at Fuc O-3. Among the isolated compounds, SO1684 is a known saponin and SO1406 exhibited the most pronounced biological activity, significantly enhancing the cytotoxicity of the ribosome-inactivating protein saporin (SAP) in the MDA-MB231 (triple-negative breast cancer) cell line. Conclusions: SO1406 demonstrates strong cytotoxicity-enhancing activity, highlighting the significant potential of structurally defined natural saponins to advance intracellular delivery and improve the therapeutic performance of protein-based anticancer agents.