Synthesis and Characterization of an Acetalated Dextran Polymer and Microparticles with Ethanol As a Degradation Product

Dextran is a polysaccharide biopolymer composed of linked glucose molecules.  One of its several FDA-approved uses is as an antithrombotic agent, and it is highly biocompatible and biodegradable.  Using dextran as the molecular foundation of our polymeric technology, our group has initiated and continues to expand a family of biobased polymers derived from dextran that have numerous potential drug delivery applications, including protein-based vaccine delivery, immunosuppressant delivery to phagocytic cells, and chemotherapeutic delivery to pulmonary cancer cells.  Currently, poly(lactic-co-glycolic acid) (PLGA) is commonly used as a polymeric delivery vehicle.  However, PLGA is pH-insensitive, its degradation profile is not widely tunable, and its degradation products create an acidic microenvironment.  Thus, our group has created and continues to improve upon the biobased material acetalated dextran (Ac-DEX), a biopolymer synthesized by appending cyclic and acyclic methoxy acetal groups onto the hydroxyl groups of dextran.  Ac-DEX possesses a tunable, acid-sensitive degradation profile and could be used in numerous in vivo applications where acidic environments are present (e.g. the delivery of vaccines via the endocytic pathway of immune cells).  Additionally, Ac-DEX has three pH-neutral degradation products: acetone, dextran, and methanol.  A drawback of Ac-DEX is the presence of its methanol degradation product.  It is known that methanol is metabolized to formic acid, causing metabolic acidosis, which can lead to blindness or death.  Although using lower doses of Ac-DEX microparticles would likely not pose a problem in vivo, other potential uses of Ac-DEX would require higher amounts of the biopolymer, such as multiple microparticle doses or microfibers in tissue engineering applications.  Therefore, in this study our group has developed another biobased material, ethoxy-derivatized acetalated dextran (Ace-DEX).  Ace-DEX has nearly the same structure as Ac-DEX; one structural difference is the presence of an extra methyl group on the acyclic acetal groups, creating acyclic ethoxy acetals.  This additional methyl group eliminates the methanol byproduct seen with Ac-DEX degradation and replaces it with an ethanol byproduct.  Ethanol is more desirable, because it is much more biologically tolerable.  The presence of the ethanol has been confirmed using ¹H-NMR.  Several material properties of Ac-DEX have been previously characterized: the effects of reaction time on cyclic and acyclic acetal coverage, the degradation profiles in extracellular (pH 7.4) and phagosomal (pH 5.0) conditions, and the cell viability of macrophages treated with Ac-DEX microparticles.  Thus, in this study we have examined these same characteristics of Ace-DEX.  As compared to Ac-DEX, using reaction times spanning from 5 minutes to 8 hours, Ace-DEX possesses a slightly lower degree of substitution of cyclic acetals and a slightly higher degree of substitution of acyclic acetals.  This difference in the degrees of substitution indicates that the reaction rate of Ace-DEX is slower than that of Ac-DEX, because cyclic acetals replace acyclic acetals over time.  It is hypothesized that the slower reaction rate is caused by a minor steric hindrance effect from the 2-ethoxypropene reactant (as opposed to the 2-methoxypropene reactant used in Ac-DEX synthesis).  It is shown that the degradation profiles of Ace-DEX in acidic (pH 5.0) and extracellular (pH 7.4) conditions are easily tunable via simply altering the reaction time; extending the reaction time creates more cyclic acetals and causes a slower degradation to occur.  As compared to Ac-DEX, Ace-DEX is shown to have less of a burst and a more sustained degradation (indicated by a longer half-life, the time at which 50% of the material has degraded).  Thus, Ace-DEX could be used for a more sustained delivery of a therapeutic.  Finally, the cell viabilities of macrophages treated with Ace-DEX microparticles are shown to be statistically similar to cells treated with Ac-DEX or PLGA microparticles at concentrations ranging from 0 to 1 mg/mL.  It is very promising that the biobased Ace-DEX polymer shows very similar cell viabilities to that of FDA-approved PLGA.  Overall, Ace-DEX represents the continuing development and growth of our family of biobased, acid-sensitive polymers that have tunable degradation profiles.  Ace-DEX is very attractive for applications requiring higher amounts of polymer, because the methanol degradation product of Ac-DEX has been replaced with an ethanol byproduct.  In the future, we intend to explore the use of Ace-DEX in studies requiring multiple microparticle doses or in tissue engineering applications that require microfibrous scaffolds.