Stereocomplexes based on poly(lactic acid) and insulin: formulation and release studies

Diastereoisomeric complexes of insulin with D-poly(lactic acid) (D-PLA) or stereocomplexes of D- and L-PLA entrapping insulin were discovered. The complexes were spontaneously formed when insulin and D-PLA were mixed together in acetonitrile solution. Complexes of insulin-D-PLA formed a microparticulate precipitate after a few hours in the solution. The porous 1-3 microm precipitate, which contained both insulin and D-PLA, was insoluble in solvents that dissolve isotactic PLA, and had an additional transition temperature at 169 degrees C. When suspending these particles in buffer solution of pH 7.4, 37 degrees C, insulin was constantly released for a few weeks. L-PLA or D,L-PLA did not form a precipitate with insulin, which indicates stereospecificity to the complex formation. Microparticulates were also obtained when L-PLA was added to the D-PLA-insulin solution. In this case two types of complexes, D-PLA-insulin and D-PLA-L-PLA complexes, were formed. These macromolecular stereocomplexes may form the onset of the development of a new generation of controlled release systems for peptides and proteins, by molecular complexation with enantiomeric polymers.