Biodegradable Triblock Copolymer Microspheres Based on Thermosensitive Sol-Gel Transition

Purpose. The purpose of this study is to design microspheres for sustained protein delivery using thermosensitive, biodegradable triblock copolymer, poly (D,L-lactide-co-glycolide)-b-poly (ethylene glycol)-b-poly (D,L-lactide-co- glycolide) (PLGA-PEG-PLGA) without using organic solvent. Methods. Microspheres of the triblock copolymer PLGA-PEG-PLGA were prepared in an aqueous-based method without using methylene chloride (Msp A). This method used the sol-gel transition property of the polymer. The size and morphology of the microspheres were examined by optical microscopy and scanning electron microscopy (SEM). Zinc crystalline recombinant human insulin was incorporated in Msp A as well as in the microspheres of the same polymer prepared by the conventional water-in-oil-in-water (w/o/w) double emulsion method using methylene chloride (Msp B). Insulin release from both microspheres was carried out using high-performance liquid chromatography (HPLC) as well as circular dichroism (CD) spectroscopy of released insulin. FITC-insulin-loaded Msp A and Msp B were observed under confocal microscopy. Both microspheres were injected subcutaneously to SD rats with diabetes induced by streptozotocin. Blood glucose and plasma insulin levels were monitored. Results. Although the insulin release from Msp B exhibited initial burst and incomplete release. Msp A showed significant reduction of initial burst and continuous release over 3 weeks (>85%). CD spectra of released insulin showed that insulin from Msp A preserved its secondary structural integrity, whereas that from Msp B indicated changes in conformation. Confocal microscopy of FITC-insulin-loaded microspheres (both A and B) showed that the observed release profile may be attributed to homogeneous distribution of FITC-insulin within Msp A but inhomogeneiety in Msp B. Both microspheres were injected s.c. to diabetic rats. Whereas Msp B caused a burst effect (hypoglycemia) followed by quick change in blood glucose and insulin level. Msp A exhibited relatively sustained release of insulin and blood glucose level for at least 10 days. Conclusions. The PLGA-PEG-PLGA microspheres (Msp A) demonstrated continuous release of insulin in vitro and in vivo without serious burst effect and incomplete release, as shown by Msp B.