Stability of aluminium-containing adjuvants to autoclaving

Key Process Parameters Study for the Fill Finish of Vaccines Containing Aluminum Hydroxide Adjuvant

Vaccine manufacturing is one of the most challenging and complex processes in pharmaceutical industry, and the process control strategy is critical for the safety, effectiveness, and consistency of a vaccine. The efficacy of aluminum salt adjuvant on vaccines strongly depends on its physicochemical properties, such as size, structure, surface charge, etc. However, stresses during the vaccine manufacturing may affect the stability of adjuvant. In this study, the impacts of cold/thermal stress, autoclaving, pumping, mixing, and filling shear stress on the physicochemical properties of aluminum hydroxide (AH) adjuvant were evaluated as part of the manufacturing process development. The results showed that the autoclaving process would slightly influence the structure and properties of the investigated AH adjuvant, but thermal incubation at 2-8 °C, 25 °C and 40 °C for 4 weeks did not. However, -20 °C freezing AH adjuvant led to the adjuvant agglomeration and rapid sedimentation. For the high shear stress study with mixing at 500 rpm in a 1-L mixing bag and pumping at 220 rpm for up to 24 h, the average particle dimension of the bulk AH adjuvant decreased, along with decreasing protein adsorption ratio. The studies indicate that various stresses during manufacturing process could affect the structure and physicochemical properties of AH adjuvant, which calls for more attention on the control of adjuvant process parameters during manufacturing.

Autoclave-Induced Changes in the Physicochemical Properties and Antigen Adsorption of Aluminum Adjuvants

Aluminum hydroxide adjuvants are widely used in human vaccines, such as diphtheria, tetanus, hepatitis A and hepatitis B vaccines. The adsorption of antigens on aluminum hydroxide adjuvants determines the immune boosting effect of vaccines, but it is not clear how changes in physicochemical properties resulting from the production and formulation processes affect the adsorption of aluminum hydroxide adjuvants with antigens. In this study, the commercial aluminum hydroxide adjuvant Alhydrogel® was pretreated by commonly used processes such as autoclaving and calcination, and the changes of aluminum hydroxide adjuvant in physicochemical properties during the treatment were then comprehensively characterized. The adsorption of ovalbumin (OVA) with treated Alhydrogel®, was also investigated, it was found that the decrease in specific surface area caused by the autoclaving process reduced the adsorptive capacity of the antigen, and the adsorptive strength of antigen was decreased only when the surface hydroxyl groups and chemically bound water of adjuvant were reduced by calcination. These findings help to optimize the production and formulation process of adjuvants for the rational regulation of antigen adsorption in vaccines.

Use of a quartz crystal microbalance platform to study protein adsorption on aluminum hydroxide vaccine adjuvants: Focus on phosphate-hydroxide ligand exchanges

Aluminum hydroxide (AH) salts are widely used as vaccine adjuvants and controlling antigen-AH interactions is a key challenge in vaccine formulation. In a previous work, we have developed a quartz crystal microbalance (QCM) platform, based on stable AH-coated sensors, to explore the mechanisms of model antigen adsorption. The QCM study of bovine serum albumin (BSA) adsorption at different pH and ionic strength (I) values showed that protein adsorption on AH adjuvant at physiological pH cannot be explained mainly by electrostatic interactions, in contrast with previous reports. Here, we exploit further the developed QCM platform to investigate the role of phosphate-hydroxyl ligand exchanges in the adsorption mechanism of BSA, human serum albumin (HSA) and ovalbumin (OVA) on two commercial AH adjuvants. BSA adsorption decreased on immobilized AH particles previously treated with KH₂PO₄, highlighting the role of exchangeable sites on AH particles in the adsorption process. BSA and OVA were dephosphorylated by treatment with an acid phosphatase to decrease their phosphate content by about 80% and 25%, respectively. Compared to native BSA, adsorption of dephosphorylated BSA decreased significantly on one AH adjuvant at pH 7. Adsorption of dephosphorylated OVA was comparable to the one of native OVA. Further QCM assays showed that phospho-amino acids (PO₄-serine and PO₄-threonine) displaced previously adsorbed BSA and OVA from AH particles in conditions that were depending on the protein and the AH. Taken together, these observations suggest that phosphate-hydroxyl ligand exchange is an important adsorption mechanism of proteins on AH. These results moreover confirm that the developed AH-coated QCM sensors offer a new platform for the study of antigen adsorption, to the benefit of vaccine formulation.

An intrinsic fluorescence method for the determination of protein concentration in vaccines containing aluminum salt adjuvants

Determination of protein concentration in vaccines containing aluminum salt adjuvant typically necessitates desorption of the protein prior to analysis. Here we describe a method based on the intrinsic fluorescence of tyrosine and tryptophan that requires no desorption of proteins. Adjuvanted formulations of three model Bordetella pertussis antigens were excited at 280 nm and their emission spectra collected from 290 to 400 nm. Emission spectra of protein antigens in the presence of aluminum salt adjuvants were able to be detected, the effects of adjuvants on the spectra were analyzed, and linear regressions were calculated. The fluorescence method proved to be very sensitive with a limit of quantification between 0.4 and 4.4 µg/mL and limit of linearity between 100 and 200 µg/mL, across the formulations tested. The fluorescence method was found to be influenced by adjuvant presence, type of adjuvant, adjuvant concentration, buffer and pH conditions. The method also demonstrated ability to monitor the percent adsorption of antigens to the adjuvants. Furthermore, intrinsic fluorescence showed good correlation with micro-Kjeldahl elemental assay in quantifying protein concentration. Being a non-invasive, quick and sensitive method, intrinsic fluorescence has the potential to be utilized as a high throughput tool for vaccine development and conceivably implemented in-line, using in-line fluorimeters, to monitor antigen concentration during formulation processing.

Immobilization of Aluminum Hydroxide Particles on Quartz Crystal Microbalance Sensors to Elucidate Antigen-Adjuvant Interaction Mechanisms in Vaccines

Aluminum hydroxide (AH) salts are the most widely used adjuvants in vaccine formulation. They trigger immunogenicity from antigenic subunits that would otherwise suffer from a lack of efficiency. Previous studies focusing on antigen-AH interaction mechanisms, performed with model proteins, suggested that electrostatic interactions and phosphate-hydroxyl ligand exchanges drive protein adsorption on AH. We however recently evidenced that NaCl, used in vaccine formulation, provokes AH particle aggregation. This must be taken into account to interpret data related to protein adsorption on AH. Here, we report on the successful development and use of a stable AH-coated surface to explore the mechanisms of protein adsorption by means of ultrasensitive surface analysis tools. Bovine serum albumin (BSA) adsorption was studied at different pHs and ionic strengths (I) using quartz crystal microbalance. The results show that protein adsorption on the AH adjuvant cannot be explained solely by electrostatic interactions and ligand exchanges. Hence, a higher adsorption was observed at pH 3 compared to pH 7, although AH and BSA respectively undergo repulsive and attractive electrostatic interactions at these pH values. Almost no effect of I on adsorption was moreover noted at pH 7. These new developments and observations not only suggest that other mechanisms govern protein adsorption on AH but also offer a new platform for the study of antigen adsorption in the context of vaccine formulation. Immobilizing particles on QCM sensors also enriches the range of applications for which QCM can be exploited, especially in colloid science.

NaCl strongly modifies the physicochemical properties of aluminum hydroxide vaccine adjuvants

The immunostimulation capacity of most vaccines is enhanced through antigen adsorption on aluminum hydroxide (AH) adjuvants. Varying the adsorption conditions, i.e. pH and ionic strength (I), changes the antigen adsorbed amount and therefore the ability of the vaccine to stimulate the immune system. Vaccine formulations are thus resulting from an empirical screening of the adsorption conditions. This work aims at studying the physicochemical effects of adjusting the ionic strength of commercial AH adjuvant particles suspensions with sodium chloride (NaCl). X-ray photoelectron spectroscopy data show that AH particles surface chemical composition is neither altered by I adjustment with NaCl nor by deposition on gold surfaces. The latter result provides the opportunity to use AH-coated gold surfaces as a platform for advanced surface analysis of adjuvant particles, e.g. by atomic force microscopy (AFM). The morphology of adjuvant particles recovered from native and NaCl-treated AH suspensions, as studied by scanning electron microscopy and AFM, reveals that AH particles aggregation state is significantly altered by NaCl addition. This is further confirmed by nitrogen adsorption experiments: I adjustment to 150mM with NaCl strongly promotes AH particles aggregation leading to a strong decrease of the developed specific surface area. This work thus evidences the effect of NaCl on AH adjuvant structure, which may lead to alteration of formulated vaccines and to misinterpretation of data related to antigen adsorption on adjuvant particles.

Development of safe, effective and immunogenic vaccine candidate for diarrheagenic Escherichia coli main pathotypes in a mouse model

Background

Enteric and diarrheal diseases are important causes of childhood death in the developing world. These diseases are responsible for more than 750 thousand deaths in children under 5 years old worldwide, ranking second cause of death, after lower respiratory diseases, in this age group. Among the major causative agents of diarrhea is Escherichia coli. There are several vaccine trials for diarrheagenic E. coli. However, diarrheagenic E. coli has seven pathotypes and vaccines are directed for one or two of the five main pathotypes-causing diarrhea. Currently, there are no combined vaccines available in the market for all five diarrheagenic E. coli pathotypes. Therefore, we aimed to develop a low-cost vaccine candidate combining the five main diarrheagenic E. coli to offer wide-spectrum protection. We formulated a formalin-killed whole-cell mixture of enteroaggregative, enteropathogenic, enteroinvasive, enterohemorrhagic, and enterotoxigenic E. coli pathotypes as a combined vaccine candidate.

Results

We immunized Balb/C mice subcutaneously with 10⁹ CFU of combined vaccine candidate and found a significant increase in survival rate post challenge compared to unimmunized controls (100 % survival). Next we aimed to determine the immunological response of mice to the combined vaccine candidate compared to each pathotype immunization. To do so, we immunized mice groups with combined vaccine candidate and monitored biomarkers levels over 6 weeks as well as measured responses post challenge with relevant living E. coli. We found significant increase in specific systemic antibodies (IgG), interferon gamma (IFNγ) and interleukin 6 (IL-6) levels elicited by combined vaccine candidate especially in the first 2 weeks after mice immunization compared to controls (p < 0.05). We also evaluated alum and cholera toxin B subunit (CTB) as potential adjuvant systems for our candidate vaccine. We found that CTB-adjuvanted combined vaccine candidate showed significantly higher IgG and IFNγ levels than alum.

Conclusions

Overall, our combined vaccine candidate offered protection against the five main diarrheagenic E. coli pathotypes in a single vaccine using mouse model. To the best of our knowledge, this is the first combined vaccine against the five main diarrheagenic E. coli pathotypes that is cost-effective with promise for further testing in humans.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-016-1891-z) contains supplementary material, which is available to authorized users.

Shear effects on aluminum phosphate adjuvant particle properties in vaccine drug products

Adjuvant-containing drug products can be exposed to high levels of interfacial shear during manufacture. This may affect the integrity of the adjuvant, alter its interaction with the drug substance or change the physical characteristics of the drug product. In this study, a solid-liquid interfacial shear device was used to investigate the shear response of aluminum phosphate adjuvant alone and two adjuvant containing vaccine drug products (DP1 and DP2). The relationship between the shear sensitivity of each and its resuspension properties was determined. Changes in the particle dimensions of the bulk adjuvant were minimal at shear strain rates of 10,900 s(-1) . However, at 25,500 s(-1) , the median particle diameter was reduced from 6.2 to 3.5 μm and was marked by the presence of sub-micron fines. A formulation without drug substance and DP2 produced similar shear responses but with less impact on particle diameter. The behavior of DP1 was less predictable. Sheared DP1 was characterized by prolonged sedimentation because of the presence of fine particulates and required in excess of 300 rotations to resuspend after extended storage. The study confirms that the solid-liquid interfacial shear device may be applied to understand product shear sensitivity associated with vaccine manufacturing.

Freeze-thaw stress of Alhydrogel ® alone is sufficient to reduce the immunogenicity of a recombinant hepatitis B vaccine containing native antigen

Preventing losses in vaccine potency due to accidental freezing has recently become a topic of interest for improving vaccines. All vaccines with aluminum-containing adjuvants are susceptible to such potency losses. Recent studies have described excipients that protect the antigen from freeze-induced inactivation, prevent adjuvant agglomeration and retain potency. Although these strategies have demonstrated success, they do not provide a mechanistic understanding of freeze-thaw (FT) induced potency losses. In the current study, we investigated how adjuvant frozen in the absence of antigen affects vaccine immunogenicity and whether preventing damage to the freeze-sensitive recombinant hepatitis B surface antigen (rHBsAg) was sufficient for maintaining vaccine potency. The final vaccine formulation or Alhydrogel(®) alone was subjected to three FT-cycles. The vaccines were characterized for antigen adsorption, rHBsAg tertiary structure, particle size and charge, adjuvant elemental content and in-vivo potency. Particle agglomeration of either vaccine particles or adjuvant was observed following FT-stress. In vivo studies demonstrated no statistical differences in IgG responses between vaccines with FT-stressed adjuvant and no adjuvant. Adsorption of rHBsAg was achieved; regardless of adjuvant treatment, suggesting that the similar responses were not due to soluble antigen in the frozen adjuvant-containing formulations. All vaccines with adjuvant, including the non-frozen controls, yielded similar, blue-shifted fluorescence emission spectra. Immune response differences could not be traced to differences in the tertiary structure of the antigen in the formulations. Zeta potential measurements and elemental content analyses suggest that FT-stress resulted in a significant chemical alteration of the adjuvant surface. This data provides evidence that protecting a freeze-labile antigen from subzero exposure is insufficient to maintain vaccine potency. Future studies should focus on adjuvant protection. To our knowledge, this is the first study to systematically investigate how FT-stress to adjuvant alone affects immunogenicity. It provides definitive evidence that this damage is sufficient to reduce vaccine potency.

Gamma irradiation of active self-healing PLGA microspheres for efficient aqueous encapsulation of vaccine antigens

PURPOSE

To investigate the effect of γ-irradiation of poly(lactic-co-glycolic acid) (PLGA)/Al(OH)₃/0 or 5 wt% diethyl phthalate (DEP) microspheres for active self-healing encapsulation of vaccine antigens.

METHODS

Microspheres were irradiated with ⁶⁰Co at 2.5 and 1.8 MRad and 0.37 and 0.20 MRad/h. Encapsulation of tetanus toxoid (TT) was achieved by mixing Al(OH)₃-PLGA microspheres with TT solution at 10-38°C. Electron paramagnetic resonance (EPR) spectroscopy was used to examine free radical formation. Glass transition temperature (T(g)) and molecular weight of PLGA was measured by differential scanning calorimetry and gel permeation chromatography, respectively. Loading and release of TT were examined by modified Bradford, amino acid analysis, and ELISA assays.

RESULTS

EPR spectroscopy results indicated absence of free radicals in PLGA microspheres after γ-irradiation. Antigen-sorbing capacity, encapsulation efficiency, and T(g) of the polymer were also not adversely affected. When DEP-loaded microspheres were irradiated at 0.2 MRad/h, some PLGA pores healed during irradiation and PLGA healing during encapsulation was suppressed. The molecular weight of PLGA was slightly reduced when DEP-loaded microspheres were irradiated at the same dose rate. At the 0.37 MRad/h dose rate, these trends were not observed and the full immunoreactivity of TT was preserved during encapsulation and 1-month release. Gamma irradiation slightly increased TT initial burst release. The small increase in total irradiation dose from 1.8 to 2.5 MRad had insignificant effect on the polymer and microspheres properties analyzed.

CONCLUSIONS

Gamma irradiation is a plausible approach to provide a terminally sterilized, self-healing encapsulation PLGA excipient for vaccine delivery.

Aluminum adjuvants: preparation, application, dosage, and formulation with antigen

Important new knowledge about the effect of aluminum adjuvants on the immune response in terms of their impact on cytokine profiles, uptake by antigen-presenting cells (APC), and surface marker expression has been published in recent years. However, although the knowledge about these adjuvants is thus more comprehensive now than ever before, the user is often still confined to a more empirical approach when confronted with practical issues when it comes to the handling and use of these adjuvants. In this chapter we have given focus to the user's perspective, discussing practicalities like dosage, temperature stability, relevant monographs, and preparation with antigen.

Inhibition of aggregation of aluminum hydroxide adjuvant during freezing and drying

Aluminum-salt adjuvants are widely used to increase immunogenicity of recombinant protein vaccines. However, when vaccines formulated with these adjuvants are frozen or lyophilized, losses of efficacy are often reported. This loss of potency is usually attributed to the aggregation of adjuvant particles during processing. In this study, we examine the aggregation behavior of Alhydrogel, a commercial aluminum hydroxide adjuvant, during freeze-thawing and freeze-drying. By cooling Alhydrogel formulations at faster rates or by the addition of sufficient amounts of a glass forming excipient such as trehalose, aggregation of Alhydrogel, can be prevented or minimized. We propose that freeze-concentration of buffer salts induces modifications in adjuvant surface chemistry and crystallinity, which in turn favor aggregation. These modifications, and the resulting aggregation of Alhydrogel particles can be minimized through choice of buffer ions, or kinetically inhibited by rapidly forming a glassy state during freezing.

Relationship between physical and chemical properties of aluminum-containing adjuvants and immunopotentiation

Aluminum-containing adjuvants are an important component of many vaccines because they safely potentiate the immune response. The structure and properties of aluminum hydroxide adjuvant, aluminum phosphate adjuvant and alum-precipitated adjuvants are presented in this review. The major antigen adsorption mechanisms, electrostatic attraction and ligand exchange, are related to the adjuvant structure. The manner by which aluminum-containing adjuvants potentiate the immune response is related to the structure, properties of the adjuvant and adsorption mechanism. Immunopotentiation occurs through the following sequential steps: inflammation and recruitment of antigen-presenting cells, retention of antigen at the injection site, uptake of antigen, dendritic cell maturation, T-cell activation and T-cell differentiation.

Particle size determination in aluminum hydroxide suspensions using near-infrared transmittance spectroscopy

A new method for particle size determination in polystyrene and aluminum hydroxide suspensions using near-infrared transmittance spectroscopy is described. Mono-dispersed polystyrene particle size standards were used to establish the calibration model. The particle sizes used in the study are similar to the wavelength range of 700-1300 nm, where light scattering is wavelength dependent. The wavelength dependency of near-infrared (NIR) absorbance is found to be linear with the particle size when the analysis is based on the same spectrum starting point (the same absorbance at 700 nm). Partial least squares regression (PLSR) is applied to model this linear relationship. Compared to laser diffraction (LD) the NIR method has similar accuracy and precision in the measurement of particles with a uniform size. For a sample containing multiple sizes of particles, the mean size measured by the NIR method is shown to be weighted by the particle mass. The application of the model to aluminum hydroxide suspension shows that the NIR method is suitable for the detection of particle size changes during the production process and storage. The advantages of the NIR method are that no knowledge of the refractive index and the concentration of a sample are necessary and that the method is fast and easy to operate.

Concentration determination of a recombinant vaccine antigen adsorbed onto an alum adjuvant by chemiluminescent nitrogen detection

PURPOSE

A chemiluminescent nitrogen detector (CLND) has been evaluated for determining the concentration of an aluminum-adsorbed recombinant vaccine antigen.

METHODS

Quantification of the antigen was based upon several nitrogen-containing compounds used to calibrate the CLND. All calibrants (6.75-400 microg/ml) generated linear standard curves, with slopes being directly proportional to the % nitrogen. The limit of quantification (LOQ) was determined to be 6.75 microg/ml based on the performance of the antigen standard curve, and the limit of detection (LOD) was defined by setting the CLND minimum peak area to 40,000 U. The CLND was capable of analyzing antigen-adjuvant suspensions (adsorbed + unbound antigen) without any sample pretreatment. To measure unbound antigen, the suspension was centrifuged and an aliquot of supernatant removed for analysis; the difference between these two measurements was the amount of adsorbed antigen.

RESULTS

The adjuvant exhibited no significant matrix effect. Samples were analyzed in triplicate with observed relative standard deviation values ranging from 0.065% to 10.0%. The most accurate concentrations of the antigen were recovered relative to the antigen itself and to glycine as standards.

CONCLUSION

This methodology provides a direct measurement of the concentration of a vaccine antigen adsorbed onto an aluminum adjuvant.

Spectroscopic characterization of antibodies adsorbed to aluminium adjuvants: correlation with antibody vaccine immunogenicity

MMA383 is an anti-idiotypic antibody designed as an immunogenic surrogate for the cancer specific Lewis Y antigen. Lewis Y is expressed in 70-90% of tumours of epithelial origin with limited expression in normal tissue. Five different MMA383 vaccines were prepared by mixing a MMA383 antibody solution with an Alhydrogel aluminium hydroxide adjuvant and tested on the biological activity in a rat model. The immunogenicity increased when: (i) the adjuvant was sterilized at 121 degrees C compared to no sterilization, (ii) the adjuvant was suspended in a phosphate buffer compared to water and (iii) the MMA383 solution was at a pH of 7.2. The immunogenicity of a ready-to-use MMA383 aluminium hydroxide suspension was the lowest. The in vivo data show that small differences in vaccine formulations before injection can generate significant changes in immunogenicity. Prior to mixing with the adjuvant, the physical and chemical characteristics of MMA383 antibodies were the same in all vaccines. Fluorescence and light scattering methods were developed to characterize antibodies in the presence of the adjuvant. Compared to the least active vaccines, the two most biologically active vaccines showed an increase in the antibody Trp fluorescence intensity, anisotropy, fluorescence lifetime, 90 degrees light-scatter, sedimentation velocity and rotational correlation time. Analysis of the 90 degrees light-scatter sedimentation kinetics indicates that stronger immune responses of vaccines can be related to the stronger binding of the antibodies to the adjuvants and the formation of more compact and condensed particles. Taken together, these results show a correlation between the in vitro fluorescence and light-scatter data and the in vivo immune response of the five MMA383 vaccines. The spectroscopic techniques described offer a new in vitro approach for the prediction of immune responses of different vaccine formulations.

Using Rate of Acid Neutralization to Characterize Aluminum Phosphate Adjuvant

Five aluminum phosphate adjuvants having P/Al molar ratios ranging from 0.74 to 0.26 were prepared. The adjuvants were characterized by both protein adsorptive capacity and rate of acid neutralization at pH 2.25, 25 degrees C. The protein adsorptive capacity was not a useful parameter to compare the initial properties of the adjuvants, as differences in surface charge of the adjuvants required the use of different proteins. In contrast, the rate of acid neutralization allowed a comparison of the freshly precipitated adjuvants and revealed that the rate of acid neutralization was directly related to the P/Al molar ratio. The protein adsorptive capacity decreased slightly during 39 weeks of aging at room temperature. The changes in the rate of acid neutralization were much greater and indicated that a P/Al molar ratio of at least 0.5 was required to minimize the aging of the adjuvants. Thus, the rate of acid neutralization was found to be the most sensitive parameter to characterize aluminum phosphate adjuvants.

Effect of thermal treatment during the preparation of aluminum hydroxide adjuvant on the protein adsorption capacity during aging

Six aluminum hydroxide adjuvants, poorly crystalline aluminum oxyhydroxide (AlOOH) were prepared using different thermal treatments of amorphous aluminum hydroxide (Al(OH)3) in an effort to increase the protein adsorption capacity. All of the adjuvants initially exhibited a higher protein adsorption capacity. However, the protein adsorption capacity decreased during aging at room temperature. X-ray and differential centrifugal sedimentation analysis revealed that complete dehydration of amorphous aluminum hydroxide to aluminum oxyhydroxide is required to produce a stable adjuvant. Any residual amorphous aluminum hydroxide will spontaneously transform to crystalline aluminum hydroxide during aging at room temperature. Since crystalline aluminum hydroxide has a small surface area, the protein adsorption capacity of adjuvants containing amorphous aluminum hydroxide decreased by 30-40% when stored for 6 months at room temperature.

Measuring the surface area of aluminum hydroxide adjuvant

The traditional method of determining surface area, nitrogen gas sorption, requires complete drying of the sample prior to analysis. This technique is not suitable for aluminum hydroxide adjuvant because it is composed of submicron, fibrous particles that agglomerate irreversibly upon complete removal of water. In this study, the surface area of a commercial aluminum hydroxide adjuvant was determined by a gravimetric/FTIR method that measures the water adsorption capacity. This technique does not require complete drying of the adjuvant. Five replicate determinations gave a mean surface area of 514 m(2)/g and a 95% confidence interval of 36 m(2)/g for a commercial aluminum hydroxide adjuvant. The X-ray diffraction pattern and the Scherrer equation were used to calculate the dimensions of the primary crystallites. The average calculated dimensions were 4.5 x 2.2 x 10 nm. Based on these dimensions, the mean calculated surface area of the commercial aluminum hydroxide adjuvant was 509 m(2)/g, and the 95% confidential interval was 30 m(2)/g. The close agreement between the two surface area values indicates that either method may be used to determine the surface area of aluminum hydroxide adjuvant. The high surface area, which was determined by two methods, is an important property of aluminum hydroxide adjuvants, and is the basis for the intrinsically high protein adsorption capacity.

Surface phosphophilicity of aluminum-containing adjuvants probed by their efficiency for catalyzing the P--O bond cleavage with chromogenic and fluorogenic substrates

Aluminum-containing adjuvants are widely used in a variety of vaccine products, such as recombinant proteins, virus-like particles, conjugated polysaccharides, and recently DNA vaccines. Aluminum-containing adjuvants are also known to have a high affinity to inorganic phosphate and its mono- or diesters. Since phosphate groups are present in many antigens as well as the natural physiological environment, a better understanding of the interactions between phosphate and phospho-containing species could help in the design of improved vaccines. This report describes a convenient and novel continuous procedure to measure the avidity denoted by the new term "phosphophilicity" of phosphate and phosphate esters to the surface of aluminum-containing adjuvants. The assay measures the rate of hydrolysis of a fluorogenic substrate-6,8-difluoro-4-methylumbelliferyl phosphate (DiFMUP)-with a microplate reader. This method was based on the fundamental bioorganic phenomenon that when a tight binding event occurs, the effective concentration of nucleophile(s) will be significantly increased in the proximity of the P atom for a nucleophilic reaction (i.e., the cleavage of the P&bond;O bond) to take place. A very good leaving group (pK(a) of DiFMU approximately 4.7) in the phosphate monoester substrate makes the assay highly sensitive. Top reading of the nascent fluorescence makes the assay very convenient with no need to separate the particulate adjuvants from the reaction mixtures. The results from this assay are consistent with catalysis of the chromogenic phosphate mono- or diesters.

Stability of aluminium-containing adjuvants during aging at room temperature

Aluminium phosphate adjuvant and aluminium hydroxide adjuvant became more ordered during aging at room temperature. The increased degree of order was accompanied by a decrease in protein adsorption capacity.