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Open AccessArticle

Addition of Zinc Improves the Physical Stability of Insulin in the Primary Emulsification Step of the Poly(lactide-co-glycolide) Microsphere Preparation Process

1
Department of Pharmaceutical Sciences, College of Pharmacy, Nursing, and Allied Sciences, North Dakota State University, Dept# 2665, Sudro Hall, PO Box 6050, Fargo, ND 58108-6050, USA
2
Isis Pharmaceuticals, Inc., 2855 Gazelle Court, Carlsbad, CA 92010, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Cyrille Boyer
Polymers 2015, 7(5), 836-850; https://doi.org/10.3390/polym7050836
Received: 14 November 2014 / Revised: 21 April 2015 / Accepted: 21 April 2015 / Published: 28 April 2015
(This article belongs to the Special Issue Polymers and Polymeric Nanoparticles for Therapy and Imaging)
In this study, the effect of zinc on insulin stability during the primary emulsification step of poly(lactide-co-glycolide) microspheres preparation by the water-in-oil-in-water (w/o/w) double emulsion solvent evaporation technique was evaluated. Insulin was emulsified at homogenization speeds of 5000 and 10,000 rpm. Insulin was extracted from the primary w/o emulsion by a method previously reported from our laboratory and analyzed by comprehensive analytical techniques. The differential scanning calorimetry thermograms of insulin with zinc showed a single peak around 83 °C with calorimetric enthalpy values similar to native insulin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of extracted insulin showed a single intense band around 6 kDa, demonstrating the preservation of primary structure. High performance liquid chromatography (HPLC) analysis revealed that no degradation products were formed during the homogenization process. Insulin aggregates residing at the w/o interfaces were found to be of non-covalent nature. In addition, observation of a single characteristic peak for insulin at m/z 5808 in the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum confirmed the absence of insulin degradation products and covalent dimers. Presence of zinc preserved the secondary structure of insulin as indicated by circular dichroism. In conclusion, these results show that with the addition of zinc, insulin stability can be improved during the primary emulsification step. View Full-Text
Keywords: insulin; stability; poly(lactide-co-glycolide); microspheres; double emulsion solvent evaporation insulin; stability; poly(lactide-co-glycolide); microspheres; double emulsion solvent evaporation
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MDPI and ACS Style

Manoharan, C.; Singh, J. Addition of Zinc Improves the Physical Stability of Insulin in the Primary Emulsification Step of the Poly(lactide-co-glycolide) Microsphere Preparation Process. Polymers 2015, 7, 836-850.

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