Adjuvants are becoming more common in vaccine and other drug formulations to increase therapeutic response. Some of these
substances, however, are close enough in size to bacteria that they are unable to pass through sterilizing-grade filters.
Others have low surface tension that can reduce a filter's bacterial retention. As a result, adjuvants can cause premature
plugging of filter membranes and reduce filter capacity. Pharmaceutical Technology spoke to several industry experts to gain insight on resolving these technical challenges.
Forum participants include Holger Bromm, director of marketing and product management filtration technologies at Sartorius
Stedim Biotech; Jerold Martin, senior vice-president of Global Scientific Affairs at Pall Life Sciences; Peter Koklitis, a
technical filtration specialist at 3M Purification in the United Kingdom; and Jim Powell, business development manager of
Asahi Kasei Bioprocess.
IMAGE: COURTESY SARTORIUS STEDIM
PharmTech: Novel adjuvants are often based on emulsions or liposomes, which are suspensions of small particles made up of surfactant
or lipid particles. Because these formulations have a relatively high viscosity and because the typical particle size of the
micelles or liposomes is close to the size of the smallest bacteria to retain, they result in a difficult separation process.
In addition, these fluid streams often contain high particle loads which can cause premature plugging of sterilizing-grade
filters. How can pharmaceutical or filter manufacturers reduce such filter plugging or pore blockage?
Bromm (Sartorius Stedim): One possibility for filter manufactures to deal with these challenges is to develop sterilizing-grade filters that specifically
address these needs. According to our experience at Sartorius Stedim, highly asymmetric membranes, such as polyethersulfone
(PES) membranes provide higher flow rate and capacity for such type of formulations compared with symmetric membranes. According
to practical experiences, the use of a heterogeneous double-layer membrane construction provides total throughput advantages
compared with single layer membrane filters. The prefilter (i.e., upstream layer) protects the final membrane (i.e., downstream
layer) from premature plugging. Of high importance is to find the optimal graduation between two membranes. Studies with
model solutions and test results with actual formulations in field tests have demonstrated that the combination of a finer
prefilter membrane with the final 0.2 Ám membrane achieves better results compared with combinations with a coarser prefilter
membrane for adjuvants applications.
Pharmaceutical manufacturers should carry out filtration studies to compare the performance of different membrane materials
and construction principals of filters to find out the optimal solution for their specific formulation. Furthermore, the use
of prefilters should be considered in such studies to protect final sterilizing-grade filters effectively and to reduce costs
and filtration time. These studies can be used to determine the optimal parameters for the filtration process, such as differential
pressure or temperature. Increasing the temperature can enhance filterability depending on the stability of the solution at
higher temperatures. The same filter-selection process may be applied for other protein therapeutics or vaccines.
Martin (Pall): Pharmaceutical manufacturers can reduce filter plugging by optimizing formulation and process conditions for desired filter
life, along with selection of appropriate filters with suitable capacity. Filter manufacturers can provide technical support
for this process by conducting feasibility (filterability) trials, selecting appropriate filter-media grades, sizing of filter
cartridges or capsules, as well as ultimately applying that knowledge to the development of new filters capable of providing
Process parameters such as pressure, temperature, and flux (i.e., flow per unit area) can have a large impact on filter throughput
and capacity. For example, with complex plugging biological fluids, performing the filtration in a constant flow mode, increasing
pressure differential to maintain flux rather than operating under a constant pressure mode can often have a positive impact
on filtration throughput (capacity). Process temperature can also have an impact but is product-dependent and needs feasibility
(filterability) tests to determine whether an improvement can be achieved through modification. Optimizing these performance
variables is an acceptable (and recommended) technique to reduce the risk of premature blockage for vaccines or protein therapeutics.
Koklitis (3M): The plugging of membrane filter systems by adjuvants is particularly undesirable when the process step has been validated
to provide sterility assurance. The risk of filter plugging can be reduced by careful control of the filtration operating
conditions, such as inlet pressure and optimum flux. The lifetime of the sterilizing-grade filter membrane will be greatly
determined by the particle load in the process feedstream and the capacity may be extended with a prefiltration stage. A prefilter
rated at 0.45 Ám will remove larger emulsion micelles or liposomes which might ordinarily plug a sterilizing 0.2 Ám membrane.
Another option is to consider a 0.2 Ám-rated bioburden reduction membrane as a prefilter. This can be of the same material
as the final sterilizing membrane to simplify validation and may be effective for removing larger particle sizes from the
process stream as a result of its pore size distribution. The prefiltration system selected should be sized appropriately
to meet the demands of the process stream to minimize the expense associated with the final sterilizing membrane stage. When
emulsions are used, the pharmaceutical manufacturer could investigate an adjuvant formulation with a sufficiently small particle
size to make it filter-sterilizable.
Some studies with oil-in-water emulsions have shown that increasing the pressure drop across the membrane can increase filter
capacity. The coating of bacteria on the membrane with emulsion has been considered to contribute to bacterial penetration.
In such instances, higher bacterial retention may be achieved by increasing the temperature if cold conditions are currently
used. However, the reasons for adopting cold filtration (e.g., to maintain protein stability) may present an obstacle to implementing
Powell (Asahi): This is rather hard to answer because the blocking can occur due to a wide range of issues related to product use and conditions
such as pH, conductivity, protein concentration, viscosity, temperature, membrane incompatibility with what is in the adjuvant,
and so forth. The best solution would be to better characterize the adjuvant, the product, and the combination to find the
most stable and best filter condition possible, where material is not precipitating, too viscous, too high a concentration,
and/or at the early stage or "edge" of aggregation and the filter type where the adjuvant's oil, if present, does not bind
to or change/damage the membrane itself.
There are really two choices: the brute force method, where one throws more membrane at the problem, or the better method,
which would be to choose the right adjuvant for the job and choose conditions that fit into a high stability window of operation
for the API. Another more sophisticated solution to these kinds of clogging problems is to use a cascade of filters that
end in the final desired porosity. The upstream filter(s) can act as prefilters to increase final filter capacity.