Enhanced stability of horseradish peroxidase encapsulated in acetalated dextran microparticles stored outside cold chain conditions

Abstract

Micro- and nanoparticles have been shown to improve the efficacy of safer protein-based (subunit) vaccines. Here, we evaluate a method of improving the vaccine stability outside cold chain conditions by encapsulation of a model enzyme, horseradish peroxidase (HRP), in an acid-sensitive, tunable biodegradable polymer, acetalated dextran (Ac-DEX). Vaccines that are stable outside the cold chain would be desirable for use in developing nations. Ac-DEX particles encapsulating HRP were prepared using two different methods, probe sonication and homogenization. These particles were stored under different storage conditions (-20 °C, 4 °C, 25 °C or 45 °C) for a period of 3 months. On different days, the particles were characterized for various physical and chemical measurements. At all conditions, Ac-DEX particles remained spherical in nature, as compared to PLGA particles that fused together starting at day 3 at 45 °C. Furthermore, our results indicated that encapsulation of HRP in Ac-DEX reduces its storage temperature dependence and enhances its stability outside cold chain conditions. Homogenized particles performed better than probe sonicated particles and retained 70% of the enzyme's initial activity as compared to free HRP that retained only 40% of the initial activity after 3 months of storage at 25 °C or 45 °C. Additionally, HRP activity was more stable when encapsulated in Ac-DEX, and the variance in enzyme activity between the different storage temperatures was not observed for either particle preparation. This suggests that storage at a constant temperature is not required with vaccines encapsulated in Ac-DEX particles. Overall, our results suggest that an Ac-DEX based micro-/nanoparticles system has wide applications as vaccines and drug delivery carriers, including those in developing nations.