Burst-Free Sustained Release of Proteins from Thermal Gelling Polymer Solutions

Objectives: Thermo-gelling hydrophilic polymers like PLGA-PEG-PLGA are known as injectable sustained-release depots for biologics, but they face challenges due to the occurrence of severe burst release. This study aimed to develop a strategy to avoid the initial burst release by pre-encapsulating proteins in polysaccharide microparticles through an aqueous-aqueous emulsion mechanism, thereby enhancing therapeutic retention and linear release kinetics. Methods: Five model proteins (G-CSF, GM-CSF, IGF-1, FVIII, BSA) were encapsulated in dextran microparticles, using an organic solvent-free aqueous-aqueous emulsion method. These particles were dispersed in a 23% (w/w) PLGA-PEG-PLGA solution and injected into a 37 °C release buffer to form a gel depot. The in vitro release profiles were quantified using ELISA and MicroBCA assays over 9-42 days. The bioactivity of the proteins was validated using cell proliferation assays (NFS-60, TF-1, MCF-7) and chromogenic kits. The in vivo pharmacokinetics of the FVIII-loaded formulations were evaluated in Sprague-Dawley rats (n = 5/group) over 28 days. Results: Protein-loaded dextran particles retained their structural integrity within the hydrogel and exhibited minimal burst release (≤5% within 30 min vs. >25% for free proteins). Sustained near-linear release profiles were observed for all the proteins, with complete release by day 9 (G-CSF, GM-CSF, BSA) or day 42 (FVIII). Rats administered with the thermal gel with FVIII-dextran particles showed a significantly lower peak plasma concentration (C_max: 88.25 ± 30.21 vs. 132.63 ± 66.67 ng/mL) and prolonged therapeutic coverage (>18 days vs. 15 days) compared to those administered with the thermal gel with the FVIII solution. The bioactivity of the released proteins remained at ≥90% of the native forms. Conclusions: Pre-encapsulation in dextran microparticles effectively mitigates burst release from thermosensitive hydrogels, while preserving protein functionality.