Figure 1: Apparatus used to recirculate the antimicrobial preservative-containing solution through the test filter. (IMAGE
IS COURTESY OF MERCK & CO.)
Sterility assurance is critically important for liquid parenteral products. For pharmaceuticals that are not heat-labile,
terminal sterilization may be performed. However, in most cases, sterilizing-grade filters must be used. Following their use
in manufacturing, these filters are tested to ensure that the membrane remained integral. A method referred to as the bubble-point
integrity test is commonly performed. This test may be completed manually or with an automated bubble-point integrity tester.
More than 85% of companies use the bubble point as the test for filter integrity when manufacturing sterile products (1).
In addition, the US Food and Drug Administration guideline states that the bubble-point test is mandatory to ensure that filter
leaks or perforations did not occur during the filtration process (2).
Table I: Preservatives used in filter bubble-point studies and typical amounts found in pharmaceutical products. (IMAGE IS
COURTESY OF MERCK & CO.)
Before using a sterilizing-grade filter to manufacture a parenteral formulation, a bubble-point value is determined with the
filter and the product in the laboratory. A procedure for performing and determining the product-specific bubble point has
been described in the literature (3). In this procedure, the product is first pumped briefly through tubing connected to a
filter. Once the filter has been wetted with the product, it is removed, and the bubble point is determined, typically using
an automated bubble-point integrity machine. Three replicates are performed, and the average product-specific bubble point
is established. The bubble-point value is then transferred to a batch document that is used during the manufacturing process.
If the filter fails the minimum product-specific bubble point following the manufacture of a product, a series of tests may
be performed on the filter to determine whether a true leak is present or whether a false failure occurred (3).
Table II: Tubing types evaluated in filter bubble-point studies. (IMAGE IS COURTESY OF MERCK & CO.)
The authors demonstrated that in some cases the failure of the product-specific bubble point may not result from a leak or
loss in membrane integrity, but from factors such as interaction between the product and materials used during the formulation
and filling process (4). The selection of tubing was shown to be critically important when filling parenteral liquid products
that contain antimicrobial preservatives such as phenol, m-cresol, and benzyl alcohol (4). In these studies, the interaction of these preservatives with the tubing resulted in the
release of polydimethyl siloxane (silicone oil), which lowered the product-specific bubble point (4). The lowering of the
bubble point resulted from a reduction of the liquid surface tension of the wetted membrane pores and was caused by the adsorption
of polydimethyl siloxane to the filter membrane. This article will review these findings and discuss factors to consider when
defining a product-specific bubble point.
Materials and methods
Table III: Corrected product-wetted bubble-point (CPBP)–preservative solutions. (IMAGE IS COURTESY OF MERCK & CO.)
The equipment that was used to evaluate the effect on filters with various types of tubing and antimicrobial solutions is
depicted in Figure 1 (4). In this procedure, the solution was recirculated at flow rates of 160–200 mL/min for a minimum of
15 h. The bubble point was measured using an automatic integrity tester (Sartocheck 4, Sartorius, New York).
Table IV: Effect of preservative-containing solutions and platinum-cured silicone tubing on the bubble point. (IMAGE IS COURTESY
OF MERCK & CO.)
A series of tests was performed using various types of tubing and antimicrobial preservative-containing solutions. Table I
lists some of the antimicrobial preservatives that are used in parenteral products and the typical amounts found in these
products (5). The levels of antimicrobial preservatives ranged from 0.0715% to 1.9% (5). Solutions containing 0.25% of each
preservative in saline solution were used in previous filtration studies (4). Each of these preservative-containing solutions
was recirculated through several types of tubing (listed in Table II) with a sterilizing-grade, 0.22-Ám filter (Millipak 20,
Millipore, Bedford, MA).
Table V: Effect of preservative-containing solutions and C-Flex tubing on the bubble point. (IMAGE IS COURTESY OF MERCK &
The experiments were performed by first determining the corrected product-wetted bubble point (CPBP) for the preservative
solutions tested. Each filter was flushed with water, and the bubble point was determined. These tests were performed with
replicates according to the procedures outlined by the Parenteral Drug Association (3). The results of these tests are summarized
in Table III.
Table VI: Effect of preservative-containing solutions and PharMed tubing on the bubble point. (IMAGE IS COURTESY OF MERCK
Before the 15-h recirculation experiments were initiated, the filters were wetted with the preservative-containing solutions
(see Tables IV–VII), and the bubble point at time zero (i.e., preuse bubble point) was measured (4). The solution was then
recirculated for 15 h, then a postuse bubble-point test of the filter was performed (see Tables IV–VII). The results indicated
that the bubble point of the filter was affected the most when using platinum-cured silicone tubing (see Table IV). The smallest
effects on the bubble point occurred when using C-Flex, PharMed, and Biopharm tubing (Cole-Parmer, Vernon Hills, IL) (see