Effect of pH-Responsive Ligands on mRNA Knockdown in EGFR-Targeting Ligand-Conjugated siRNAs

Ligand-conjugated small interfering RNAs (siRNAs) have emerged as a powerful approach to developing nucleic acid-based medicines. To achieve efficient mRNA knockdown, it is important to select targeting receptors with high expression and ligands that exhibit rapid internalization. However, the key characteristics of ligand-receptor sets involved in the postinternalization process remain largely unclear. In this study, we investigated the effect of ligand-receptor binding dissociation under low pH conditions, known as a postendocytic environment. Specifically, we chemically synthesized several modified epidermal growth factor (EGF) ligands that showed a variety of binding activities to the EGF receptor (EGFR) at low pH. Among these modified ligands, the siRNA conjugate with chemically synthesized EGF H10Y/H16Y, which is a less pH-responsive variant, exhibited reduced internalization and mRNA knockdown activity at high concentrations in EGFR-expressing cells. Additionally, we explored the use of antibody-related molecules (anti-EGFR IgG and Fab) as targeting moieties for siRNA conjugates. The anti-EGFR Fab-siRNA, which showed dissociation of EGF under low pH conditions, demonstrated stronger internalization and mRNA knockdown activity compared to the anti-EGFR IgG-siRNA, which strongly binds EGF at low pH. These data emphasize the importance of intracellular ligand-receptor dissociation and provide insights for future advancements in the field.