Subcutaneous Delivery of Albumin: Impact of Thermosensitive Hydrogels

Albumin demonstrates remarkable promises as a versatile carrier for therapeutic and diagnostic agents. However, noninvasive delivery of albumin-based therapeutics has been largely unexplored. In this study, injectable thermosensitive hydrogels were evaluated as sustained delivery systems for Cy5.5-labeled bovine serum albumin (BSA-Cy5.5). These hydrogels were prepared using aqueous solutions of Poloxamer 407 (P407) or poly(lactide-co-glycolide)-block-poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PLGA-PEG-PLGA), which could undergo temperature-triggered phase transition and spontaneously solidify into hydrogels near body temperature, serving as in situ depot for tunable cargo release. In vitro, these hydrogels were found to release BSA-Cy5.5 in a sustained manner with the release half-life of BSA-Cy5.5 from P407 and PLGA-PEG-PLGA hydrogels at 16 h and 105 h, respectively. Without affecting the bioavailability, subcutaneous administration of BSA-Cy5.5-laden P407 hydrogel resulted in delayed BSA-Cy5.5 absorption, which reached the maximum plasma level (T_max) at 24 h, whereas the T_max for subcutaneously administered free BSA-Cy5.5 solution was 8 h. Unexpectedly, subcutaneously injected BSA-Cy5.5-laden PLGA-PEG-PLGA hydrogel did not yield sustained BSA-Cy5.5 plasma level, the bioavailability of which was significantly lower than that of P407 hydrogel (p < 0.05). The near-infrared imaging of BSA-Cy5.5-treated mice revealed that a notable portion of BSA-Cy5.5 remained trapped within the subcutaneous tissues after 6 days following the subcutaneous administration of free solution or hydrogels, suggesting the discontinuation of BSA-Cy5.5 absorption irrespective of the formulations. These results suggest the opportunities of developing injectable thermoresponsive hydrogel formulations for subcutaneous delivery of albumin-based therapeutics.