Drug delivery system based on the novel acidic self-assembling peptide hydrogels: Insights into N-terminal...

Self-assembling peptide hydrogels show considerable promise as drug delivery platforms. However, their application in the gastrointestinal tract is hindered by pepsin-induced degradation and inadequate pH-responsiveness. Herein, this study systematically investigates the regulatory role of N-terminal appended residues in modulating the solubility, self-assembly, and physicochemical properties of (RADA)₄ peptides. The X(RADA)₄ variants (X = G/S/A/M/Q/D) formed nanofibrous hydrogels at pH 1.2, characterized by a typical β-sheet secondary structure. Notably, their fiber diameters and thermal stability varied significantly depending on the specific N-terminal residue, with fiber diameters ranging from 5.0 to 10.6 nm and melting temperature (T_m) ranging from 42.1 to 71.5 °C respectively. In contrast, the X(RADA)₄ variants with X = C or H lacked β-sheet structures and failed to assemble into hydrogels. Among all variants, G(RADA)₄ was identified as the optimal candidate, exhibiting high-yield production (25.9 mg/L, 70.6 % recovery rate) and exceptional acid and pepsin-resistance at pH 1.2 over 14 days, rendering them suitable as oral carriers for sulfasalazine and insulin in simulated gastrointestinal environments. At gastric pH 2.0, less than 5 % of sulfasalazine was released within the initial 2 h, whereas intestinal pH 7.2 triggered 70 % release over 24 h. Insulin exhibited analogous pH-dependent release with less than 10 % release at pH 2.0 and 75 % release at pH 7.2. Transmission electron microscopy and cryo-scanning electron microscopy analysis revealed that pH-mediated disassembly of nanofibers and loss of porous architecture, transitioning from entangled fibers at pH 2.0 to aggregated short fibers at pH 7.2, accounted for the pH-responsive release behavior of the encapsulated drugs. In conclusion, the acid-stable G(RADA)₄ hydrogel system minimized gastric drug loss and enabled targeted intestinal delivery, highlighting its significant potential in gastrointestinal drug delivery.