In vitro and in vivo study of thymosin alpha1 biodegradable in situ forming poly(lactide-co-glycolide) implants

The purpose of this study was to develop poly(lactide-co-glycolide) (PLGA) based in situ forming implants (ISFI) for controlled release of thymosin alpha 1 (Tα1). The ISFI was prepared by dissolving PLGA in N-methyl-2-pyrrolidone (NMP) or mixtures of NMP and triacetin. Tα1 microparticles, prepared by spray-freeze drying method with chitosan or bovine serum albumin as a protectant, were suspended in PLGA solutions. The effects of Tα1 pre-encapsulation, PLGA molecular weight, PLGA concentration and organic solvents composition on the in vivo Tα1 release were evaluated by subcutaneously injecting Tα1-loaded ISFI into Sprague–Dawley Rats. The pharmacological efficacy of Tα1-loaded ISFI was examined using immunosuppressive BALB/c mice induced by cyclophosphamide. The ISFI composed of Tα1 pre-encapsulated with chitosan, higher molecule-weight PLGA at higher concentration and more triacetin showed a lower initial release and a longer sustained release period. The optimal prescription of our study showed a low initial release of 29.3% (24 h), followed by a slow and continuous drug release up to 28 d in vivo. An in vitro release device was designed to mimic the in vivo release of Tα1, and good correlation was observed between the in vitro and in vivo releases, with the linear correlation coefficient of 0.9899. Tα1-loaded ISFI showed low cytotoxicity as tested by CCK-8 assay. Tα1-loaded ISFI significantly increased the thymic index and spleen index of immunosuppressive mice. These results suggest that the ISFI is a suitable system for controlled release of Tα1.