Analysis of Triterpenoid Saponins Reveals Insights into Structural Features Associated with Potent Protein Drug Enhancement Effects
Xue‐Wei Cao, Fujun Wang, Oi‐Wah Liew, Ye-Zhou Lu, Jian Zhao
Abstract
Plant-based\nsaponins are amphipathic glycosides composed of a hydrophobic\naglycone backbone covalently bound to one or more hydrophilic sugar\nmoieties. Recently, the endosomal escape activity of triterpenoid\nsaponins has been investigated as a potentially powerful tool for\nimproved cytosolic penetration of protein drugs internalized by endocytic\nuptake, thereby greatly enhancing their pharmacological effects. However,\nonly a few saponins have been studied, and the paucity in understanding\nthe structure–activity relationship of saponins imposes significant\nlimitations on their applications. To address this knowledge gap,\n12 triterpenoid saponins with diverse structural side chains were\nscreened for their utility as endosomolytic agents. These compounds\nwere used in combination with a toxin (MAP30-HBP) comprising a type\nI ribosome-inactivating protein fused to a cell-penetrating peptide.\nSuitability of saponins as endosomolytic agents was assessed on the\nbasis of cytotoxicity, endosomal escape promotion, and synergistic\neffects on toxins. Five saponins showed strong endosomal escape activity,\nenhancing MAP30-HBP cytotoxicity by more than 10<sup>6</sup> to 10<sup>9</sup> folds. These saponins also enhanced the apoptotic effect\nof MAP30-HBP in a pH-dependent manner. Additionally, growth inhibition\nof MAP30-HBP-treated SMMC-7721 cells was greater than that of similarly\ntreated HeLa cells, suggesting that saponin-mediated endosomolytic\neffect is likely to be cell-specific. Furthermore, the structural\nfeatures and hydrophobicity of the sugar side chains were analyzed\nto draw correlations with endosomal escape activity and derive predictive\nrules, thus providing new insights into structure–activity\nrelationships of saponins. This study revealed new saponins that can\npotentially be exploited as efficient cytosolic delivery reagents\nfor improved therapeutic drug effects.