Micelles of polylactide–poly(ethylene glycol)–polylactide (LA n –EG m –LA n ) triblock copolymers as insulin delivery system: spectroscopic studies
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A series of triblock copolymers are synthesized by the ring-opening polymerization of l-lactide in the presence of PEG of various molecular weights. Firstly, the (lactic acid) n –(ethylene glycol) m –(lactic acid) n (LA n –EG m –LA n ) copolymers are characterized by 1H NMR and GPC studies. Micelles, prepared in aqueous media by the direct dissolution method, are characterized by TEM and DLS. In vitro release behavior of bovine insulin from LA37–EG136–LA37 micelles is evaluated at 37 °C. According to results, nearly 90 wt% of insulin is released over 6 days by diffusion in a biphasic pattern. Secondly, a comparative study of the interactions occurring between insulin (both bovine and human) and the LA37–EG136–LA37 triblock copolymer is carried out using different spectroscopic techniques. UV–visible spectroscopy shows that the copolymer binds spontaneously to both proteins with a similar stoichiometry. However, a higher affinity was observed for bovine insulin. Far-UV CD spectroscopy reveals that the secondary structure of both proteins changes in the presence of the copolymer, although the extent of changes in bovine insulin is larger. Acrylamide quenching experiments display reduced accessibility of tyrosines at high copolymer concentrations. In these conditions, the major decrease in the α-helical content of the proteins causes tyrosines to be exposed to the nonpolar environment of the micelles and thus shielded from the aqueous solvent. Results demonstrate that upon interaction with the copolymer, the microenvironment around tyrosines is rearranged, the extent of which depends on the copolymer concentration. Altogether, drug–copolymer interactions need to be considered when attempting to use polymeric formulations as delivery carriers of peptide drugs.
KeywordsInsulin Circular dichroism (CD) Fluorescence spectroscopy Polylactide (PLA) Triblock copolymer
Hepatitis B surface antigen
(Lactic acid) n –(ethylene glycol) m –(lactic acid) n
Gel permeation chromatography
The authors thank the Iran National Science Foundation (INSF) and Research Council of University of Tehran for their financial support.
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