Vaccine Adjuvant Filtration

Vaccine manufacturers are increasingly using adjuvants to boost the immune response to an antigen. Examples are the pandemic H1N1 vaccines now available in Europe. Adjuvants are substances added to vaccine formulations to enhance, accelerate and/or prolong the immune response to a vaccine. Adjuvants are a very heterogeneous group of compounds, including single compounds with intrinsic immuno-stimulating properties (e.g. MPL or QS21) and antigen carriers such as liposomes, microparticles or aluminum salts, and often are a combination of both.

The properties of these adjuvants or adjuvanted vaccines, in combination with specific process operating conditions, may create some challenges for sterilizing filtration. The particulate character of some adjuvants can make these products difficult to filter and cause premature plugging of the filter membranes. In addition, their low surface tension may reduce a filter’s bacterial retention capacity and may lead to a failing bacterial challenge test during process validation.

Scientists at Pall Life Sciences have been studying the sterilizing filtration challenges presented by influent adjuvants and have developed strategies and recommendations for membrane filter selection and process conditions that may enhance filter capacity and avoid costly rework by increasing the chance of passing the bacterial retention challenge validation.

Filterability of Vaccine Adjuvants

The particulate size of some adjuvants (e.g. aluminum salts, liposomes and emulsions) may be too large to pass through the pores of a sterilizing grade filter membrane, or may be at levels that cause premature plugging of the filter membrane, reducing filter capacity. The flux decays seen with such adjuvants or adjuvanted vaccines may be a function of the filter membrane structure, its’ effective pore size distribution and total porosity. Filter capacity can be maximized by selecting the appropriate filter for the adjuvanted product. Pre-filtration can be a useful procedure for improving throughput by removing larger emulsion droplets or liposomes that otherwise plug the sterilizing membrane. Pall Scientific and Laboratory Services groups help customers conducting feasibility (filterability) trials, selecting appropriate filter-media grades and reviewing process parameters such as pressure, temperature and flux which can all have a large impact on the filter throughput and capacity.

Bacterial Retention Validation

Sterilizing grade membrane filters rated at 0.2 micron are typically effective in removing bacteria with a high degree of assurance. During process validation, this is confirmed by a high titer bacterial challenge using a small model bacterium, Brevundimonas diminuta, in the process fluid or an appropriate surrogate. On rare occasions, "worst case" conditions may be such that the membrane’s normally high efficiency of bacterial removal is reduced to a level where some bacterial penetration can occur. A review of field and laboratory bacterial retention validation data for a variety of fluids and challenge conditions suggests that low surface tension fluids, such as many adjuvants and adjuvanted vaccines, present a higher risk of bacterial penetration under the high challenge conditions applied during sterilizing filtration validation 1. Among the characterized solutions examined, liposome suspensions represent the highest risk followed by lipid emulsions and finally surfactant solutions. B. diminuta bacteria loading above 1 x 108 CFU/cm2 (> 10 times the minimum required challenge density) increases the chance of a penetrative event, as does a loading rate of greater than 1 x 105 CFU/min. For liposome, lipid and surfactant solutions, bacterial challenges using constant flow for the process may present a higher risk than using constant pressure and we would hence recommend conducting validation studies and operating processes of low surface tension adjuvants and adjuvanted vaccines at constant pressure 1.

For further investigation, our global validation services team coordinated B. diminuta challenge studies on several integral Pall and competitor filters under process conditions recognized to facilitate microbial breakthrough, using a liposome carrier challenge fluid 2. The data illustrates the influence this type of challenge fluid may have on the microbial removal performance of the tested filters. As a conclusion, Pall’s latest generation of sterilizing grade filters, the Fluorodyne® EX range, are recommended to deliver the highest levels of safety 2.

Pall Scientific and Laboratory Services

For the successful sterilizing filtration of adjuvants and adjuvanted vaccines, we recommend filter users consider the risk of reduced bacterial retention efficiency very early in the process, i.e. when designing the formulation, the filtration operating conditions and sequence, and when drafting the sterilizing filtration validation bacterial challenge test protocol. Several membrane candidates for bacterial retention should be tested along with filterability studies, to arrive at the candidate offering both complete bacterial retention under "worst case" validation conditions and providing the highest filtration capacity for superior process economy. Due to the impact of operating conditions on bacterial retention, it is important to maintain process consistency during scale-up and to design the process with scalability in mind. With this pro-active approach, Pall Life Sciences is pleased to work with you to qualify the most appropriate filter to ensure sterilization of your vaccine product. Inquire today

Links and References

1Onraedt, A.; Folmsbee, M.; Kumar, A.; Martin, J. (2010). Sterilizing filtration of adjuvanted vaccines: ensuring successful filter qualification. Supplement to BioPharm International October 2010.

2 Bacterial Penetration of 0.2 μm Sterilizing-Grade Filters with a Cholesterol Liposome Carrier: A Comparison of Data.

3 Folmsbee, M.; Moussourakis, M. (2012). Sterilizing filtration of liposome and related lipid containing solutions: enhancing successful filter qualification. PDA Journal of Pharmaceutical Science and Technology, 66, 161-167.
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