Advances in vaccines against neglected tropical diseases: enhancing physical stability of a recombinant hookworm vaccine through biophysical and formulation studies.

A bivalent recombinant vaccine for human hookworm disease is under development. One of the lead candidate antigens in the vaccine is a glutathione S-transferase cloned from the hookworm Necator americanus (Na-GST-1) which is expressed in the yeast Pichia pastoris. Based on preliminary studies demonstrating that the recombinant protein was not stable in an acetate buffer at pH 6, we undertook an extensive stability analysis of the molecule. To improve and optimize stability we complemented traditional methods employed for macromolecule and vaccine stabilization with biophysical techniques that were incorporated into a systematic process based on an eigenvector approach. Large data sets, obtained from a variety of experimental methods were used to establish a color map ("empirical phase diagram") of the physical stability of the vaccine antigen over a wide range of temperature and pH. The resulting map defined "apparent phase boundaries" that were used to develop high throughput screening assays. These assays were then employed to identify excipients that stabilized the antigen against physical degradation that could otherwise result in losses of physicochemical integrity, immunogenicity, and potency of the vaccine. Based on these evaluations, the recombinant Na-GST-1 antigen was reformulated and ultimately produced under Good Manufacturing Practices and with an acceptable stability profile.

Biophysical characterization and conformational stability of Ebola and Marburg virus-like particles.

The filoviruses, Ebola virus and Marburg virus, cause severe hemorrhagic fever with up to 90% human mortality. Virus-like particles of EBOV (eVLPs) and MARV (mVLPs) are attractive vaccine candidates. For the development of stable vaccines, the conformational stability of these two enveloped VLPs produced in insect cells was characterized by various spectroscopic techniques over a wide pH and temperature range. Temperature-induced aggregation of the VLPs at various pH values was monitored by light scattering. Temperature/pH empirical phase diagrams (EPDs) of the two VLPs were constructed to summarize the large volume of data generated. The EPDs show that both VLPs lose their conformational integrity above about 50°C-60°C, depending on solution pH. The VLPs were maximally thermal stable in solution at pH 7-8, with a significant reduction in stability at pH 5 and 6. They were much less stable in solution at pH 3-4 due to increased susceptibility of the VLPs to aggregation. The characterization data and conformational stability profiles from these studies provide a basis for selection of optimized solution conditions for further vaccine formulation and long-term stability studies of eVLPs and mVLPs.

Charge-charge interactions in the denatured state influence the folding kinetics of ribonuclease Sa.

Gaining a better understanding of the denatured state ensemble of proteins is important for understanding protein stability and the mechanism of protein folding. We studied the folding kinetics of ribonuclease Sa (RNase Sa) and a charge-reversal variant (D17R). The refolding kinetics are similar, but the unfolding rate constant is 10-fold greater for the variant. This suggests that charge-charge interactions in the denatured state and the transition state ensembles are more favorable in the variant than in RNase Sa, and shows that charge-charge interactions can influence the kinetics and mechanism of protein folding.