(596l) Encapsulation of Adjuvant in Acetalated Dextran Microparticles for Production of Subunit Vaccines Using an Electrospray Method

Vaccines are arguably the most effective tool for prevention of disability and disease.  Subunit or protein based vaccines are among the safest types of vaccines, due to the fact that they contain elements of the pathogen but not the entire pathogen.  However, they also tend to have low immunostimulatory capabilities.  One solution has been to add an immunostimulatory molecule as an adjuvant, which can enhance the immune response to the subunit protein antigen.  For example, agonists of toll like receptors (TLRs) have the capacity to promote an immune response.  In the case of TLR-7, the agonist must be delivered intracellularly, which can be achieved by encapsulating the drug in a delivery vehicle. 

Imiquimod, a TLR-7 agonist, has previously been encapsulated in acid-sensitive acetalated dextran (Ac-DEX) microparticles, using a single emulsion method.  The encapsulated drug has demonstrated improved immunostimulatory capabilities over that of free drug.  Resiquimod is similar to imiquimod, but has greater potency for promoting cytokine production.  However, the emulsion method yields low encapsulation efficiency for resiquimod, since it is partially soluble in water.  Furthermore, it is a batch process, making high scale production difficult.  This work investigates the use of electrohydrodynamic spraying (electrospray) to encapsulate resiquimod in Ac-DEX microparticles. 

A single capillary electrospray apparatus was used to generate an aerosol from a solution of resiquimod and Ac-DEX in a solvent.  Six straight chain alcohols were tested as the solvent, and the resulting particle size and morphology were analyzed by scanning electron microscopy (SEM).  The Peclet number (Pe) was calculated and used to explain the observed collapsed particle morphology.  Ethanol was chosen as an ideal solvent, and solutions of Ac-DEX and resiquimod in ethanol were electrosprayed to generate approximately 2 μm Ac-DEX particles containing resiquimod with an encapsulation efficiency of 85%.  For comparison, resiquimod loaded Ac-DEX particles were prepared using a single emulsion method, with a resulting encapsulation efficiency of 6%.   To improve the particles’ ability to disperse in an aqueous medium, Tween 80 was blended with the Ac-DEX at ~10% w/w of total polymer and particles containing resiquimod were formed via electrospray with encapsulation efficiencies between 40% and 60%.  In vitro release profiles of resiquimod from Ac-DEX/Tween 80 particles exhibited the acid-sensitive nature of Ac-DEX, with 100% drug release after 8 h at pH 5 (phagosomal pH)  and after 48 h at pH 7.4 (physiological pH).  RAW macrophages that were treated with resiquimod loaded Ac-DEX/Tween 80 particles displayed a significantly greater immune response than macrophages treated with the same dose of free drug.  Overall, we present a scalable method that incorporates the blending of Tween 80 to produce Ac-DEX microparticles for vaccine applications.  This is an elegant method for the encapsulation of adjuvant into an acid sensitive delivery vehicle.  This work opens future opportunities for application of resiquimod for the production of effective subunit vaccines.