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Thermosensitive polymeric delivery system (PLA-PEG-PLA) loaded with chitosan-zinc-insulin complex was designed for continuous in vivo insulin delivery at basal level for prolonged period after a single subcutaneous injection. Chitosan-zinc-insulin complex was optimized to restrict the diffusion of insulin from the delivery system by forming large complexes and thereby reducing the initial burst release. The in vivo absorption and bioactivity of insulin released from the delivery systems were studied in streptozotocin-induced diabetic rat model. The amount of insulin released in vivo was quantified using Enzyme Linked Immunosorbent Assay (ELISA), and its bioactivity was determined by its ability to reduce the blood glucose levels in diabetic rats. An indirect ELISA was performed to determine the immunogenic potential of insulin released from the formulations. Furthermore, the in vitro and in vivo biocompatibility of the delivery system was studied using an MTT assay, and by studying the histology of skin samples, respectively. Chitosan-zinc-insulin complex significantly (P<0.05) reduced the initial burst release of insulin from the polymeric delivery system in comparison to zinc-insulin or insulin alone. The delivery system released insulin for ~3 months in biologically active form with corresponding reduction in blood glucose levels in diabetic rats. The insulin released from the delivery systems did not provoke any immune response. The delivery systems demonstrated excellent biocompatibility both in vitro and in vivo and were non-toxic. The results indicate that the chitosan-zinc-insulin complex incorporated in the thermosensitive polymeric delivery system can be used as an alternative to the conventional daily basal insulin therapy.

The purpose of this study was to investigate the effects of size and type of incorporated model molecules on the polymer degradation and release profile from thermosensitive triblock copolymer based controlled delivery systems. In vitro release of the incorporated molecules demonstrated slow release for risperidone (molecular weight (M w ) = 410.48 Da; partition coefficient (K o/w ) = 3.49), while bovine serum albumin (BSA) (M w = "66,400 Da; K o/w = 0.007) and insulin (M w = 5808 Da; K o/w = 0.02) showed initial burst release followed by controlled release. The proton NMR, Gel Permeation Chromatography, and Cryo-SEM studies suggest that the size and partition coefficient of incorporated molecules influence the pore size, polymer degradation, and their release. In spite of using a similar polymer delivery system the polymer degradation rate and drug release notably differ for these model molecules. Therefore, size and oil-water partition coefficient are important factors for designing the controlled release formulation of therapeutics from triblock copolymer based delivery systems.Polymers 2015, 7 1511