Encapsulation of the immune potentiators MPL and RC529 in PLG microparticles enhances their potency.

Purpose: Monophosphoryl lipid A (MPL) and the synthetic LPS mimetic RC529, encapsulated in poly(lactide-co-glycolide) (PLG) microparticles, were evaluated as immune potentiators in the presence of either HIV-1 gp120 protein or antigen from Neisseria meningitidis serotype B (Men B). The immunogenicity of these formulations was evaluated in mice and compared to CpG containing oligonucleotide. This work was done as part of an ongoing effort to enhance the potency of vaccine candidates against HIV and Men B.

Methods: Microparticles were made by a solvent evaporation method. Blank microparticles as well as microparticles with encapsulated MPL or RC529 were made using the PLG polymer RG503 and the ionic surfactant Dioctylsulfosuccinate by the water-in-oil-in-water emulsion technique. Antigens from HIV-1 and Men B were adsorbed on the surface of these anionic microparticles and the final formulations characterized for protein loading, release, and integrity. The formulations were then tested in mice for their ability to elicit antibodies and bactericidal activity in comparison with CpG containing oligonucleotide.

Results: We have found that adding soluble immune potentiators to Men B antigen formulated on PLG microparticles significantly enhanced the immune response to a level comparable to that obtained using CpG. In a separate study, we found that encapsulating MPL or RC529 in PLG microparticles further enhanced the response in comparison to soluble CpG, which is our control group. Similarly, adding soluble immune potentiators to gp120 antigen formulated on PLG microparticles resulted in a significant enhancement of the immune response. Moreover, delivering MPL or RC529 encapsulated in PLG microparticles with gp120 adsorbed on PLG microparticles, resulted in even further enhancement of serum titers over those obtained with soluble immune potentiators. These titers were comparable to or greater than those obtained with soluble CpG, the control group. This effect was observed for both antigens regardless of whether or not the immune potentiator and the antigen were used with the same or with separate particles. In conclusion, the advantages of encapsulating MPL and RC529 lie not only in the enhanced immune response they elicit, but also in the convenience of handling these relatively insoluble compounds, and flexibility in vaccine design. The fact that MPL and RC529 are readily soluble in methylene chloride used for the manufacturing of PLG microparticles makes it easy to avoid solubility issues. Moreover, formulating antigen and immune potentiator with the same particle offers an attractive approach to vaccine delivery.