Influence of the co-encapsulation of different non-ionic surfactants on the properties of PLGA insulin-loaded microspheres

The aim of this work was to produce insulin-loaded microspheres allowing th e preservation of peptide stability during both particle processing and ins ulin release. Our strategy was to combine the concepts of using surfactants to improve insulin stability while optimising overall microsphere characte ristics such as size, morphology, peptide loading and release. Bovine insul in was encapsulated within poly(lactide-co-glycolide) (PLGA 50:50, Resomer RG504H) microspheres by the multiple emulsion-solvent evaporation technique . Microspheres were prepared by adding to the primary emulsion three non-io nic surfactants, poloxamer 188, polysorbate 20 and sorbitan monooleate 80, at different concentrations (1.5 and 3.0% w/v). The presence of surfactants was found to decrease the mean diameter and to affect the morphology of th e microspheres. Insulin encapsulation efficiency was reduced in the presenc e of surfactants and especially for sorbitan monooleate 80, in a concentrat ion-dependent mode. The influence of the surfactants on the interactions be tween insulin and PLGA together with the primary emulsion stability were fo und to be the major determinants of insulin encapsulation. The release of i nsulin from microspheres was biphasic, showing an initial burst effect foll owed by a near zero-order release for all the batches prepared. The initial burst was related to the presence of insulin molecules located onto or nea r to the microsphere surface. In the presence of surfactants, a faster insu lin release with respect to microspheres encapsulating insulin alone was ob served. Insulin stability within microspheres after processing, storage and release was evaluated by reversed phase- and size-exclusion-HPLC. The anal ysis of microsphere content after processing and 6 months of storage showed that insulin did not undergo any chemical modification within microspheres . On the contrary, during the period of sustained release insulin was trans formed in a high-molecular weight product, the amount of which was related to the surfactant used. In conclusion, polysorbate 20 at 3% w/v concentrati on was the most effective in giving regular shaped particles with both good insulin loading and slow release, and limiting insulin modification within microspheres. (C) 2000 Elsevier Science B.V. All rights reserved.