Preparation and evaluation of insulin-loaded poly(DL-lactide) microspheres using an experimental design

Twenty formulations of bovine insulin in poly(DL-lactide) (DL-PLA) microspheres were elaborated to study the influence of polymer molecular weight, insulin content and polyvinylalcohol (PVA) concentration on microsphere characteristics and in vitro release profile. Ten of them released nearly all the incorporated insulin in 24 h. Of the rest, four were porous, presenting a burst effect over 40% and releasing 70% in 48 h. The other six formulations were non-porous; four released the insulin in 1 week and the other two only released 28% in 15 days. None of these 10 formulations were able to release the total amount of incorporated insulin, probably due to the adsorption of insulin by the polymer. From the results obtained with the experimental design, one more lot was prepared and tested in vitro and in vivo. The in vitro release profile was much slower than the in vivo one; 16% and 28% were released in vitro on the first day and in the following 10 days, respectively, while the corresponding figures for in vivo release were 60% and 90%.