In vitro release of insulin and biocompatibility of in situ forming gel systems

The purpose of this study was to develop single-dose insulin delivery system based on in situ forming gel to provide basal insulin level for a prolonged period. The in situ forming gel formulation was prepared by dissolving poly(d,l-lactic acid) (PLA) in hydrophobic (benzyl benzoate) and hydrophilic (benzyl alcohol) solvent mixtures. In vitro release was carried out in phosphate buffered saline (PBS) (pH 7.4) and the amount of released insulin was quantified by MicroBCA assay. In vivo biocompatibility study of in situ forming gel system was based on the histological evaluation of the tissue samples retrieved from injection sites at different time points. The tissue reaction was evaluated over 12 weeks. Throughout this period, all formulations showed normal inflammatory and foreign body reactions characterized by the presence of macrophages, fibroblasts and foreign body giant cells. Neither necrosis nor tissue damage could be identified. At the end of 12 weeks, no distinct histological differences were observed in comparison to the control tissue samples. The comparable results between blank and insulin-loaded in situ forming gel system indicated that the insulin itself did not induce additional inflammatory reactions. The results suggested that in situ forming gel system was biocompatible.