Influence of Formulation Composition and Rheological Properties on the Stability of PnPP19 Peptide in Thermoresponsive Hydrogels

Purpose Peptide-based drugs present high specificity and therapeutic potential but remain limited by poor physicochemical stability, particularly in aqueous and semi-solid systems. Thermoresponsive poloxamer hydrogels have emerged as promising platforms for the formulation of peptide-based therapeutics. This study investigates the influence of polymer concentration, buffer composition, and co-solvents on the stability of a synthetic bioactive peptide (PnPP19) incorporated into Poloxamer 407 (P407) hydrogels. Methods Formulations were characterized in terms of sol–gel transition temperature (Tsol–gel), rheological behavior, and peptide stability under different storage conditions (5, 25, and 40 °C), including photostability. Results Increasing P407 reduced the Tsol–gel, while propylene glycol increased it. Phosphate buffer (pH 5.8) promoted a greater reduction in Tsol–gel compared to acetate buffer (pH 5.0). Formulations with higher P407 content (16% w/w) and phosphate buffer exhibited non-Newtonian Herschel–Bulkley behavior. This formulation showed improved peptide stability during storage, with the optimized hydrogel (16 F-PEHG) maintaining higher HPLC-quantified peptide content compared to a low-viscosity formulation (10 S-PEHG) (19.39% vs. 25.71% loss at 25 °C after 9 months). Similar trends were observed at 40 °C, while at 5 °C the structured formulation significantly improved stability. Kinetic analysis indicated that formulation composition influenced the degradation behavior of PnPP19, with differences observed in degradation rate constants and indicative activation energy estimates. In contrast, photodegradation was not significantly affected by formulation composition or rheological behavior. Conclusion These findings suggest that formulation composition and rheological behavior significantly influence the long-term stability of PnPP19 and support the rational development of thermoresponsive peptide formulations.