Container-seal tests

Of the different test methods described, the assessment of the container-closure is arguably the most important because it indicates whether the device is at risk from extraneous microbial contamination. Pharmaceutical containers constructed of materials such as plastic and glass must be qualified and meet USP <661> Containers and <671> Containers-Permeation standards. The user will therefore need to undertake additional tests that examine the physical seal of the closure in the vial, i.e., when the stopper is fully inserted and crimped, usually by of an aluminium band. The choice to conduct a physical test or a microbial ingress test for this purpose is a matter of debate. Some practitioners argue that the physical methods of measuring the system's integrity are preferred because they are more reproducible, faster, less expensive, more reliable, and quantitative. Others argue that, as the objective is to ensure that the product is safe from microbial contamination, a microbial test is the only true test. Some opt to undertake both physical and microbial tests.

A review of industry practices suggests that failures occur with container-closure seals for a variety of reasons (8). These failures include poor quality starting materials, an improper fit of the container-closure combination, the lack of sufficient inspection as part of batch release, insufficient process monitoring or process control, the use of unreliable manual or visual inspection techniques, the use of methods that produce subjective results, and the lack of proper process validation. The latter point is addressed through the tests described below.

Alternative and novel test methods to assess container-closure integrity include the use of hygroscopic powder and near-infrared (NIR) spectroscopy as a means of visualization. A second example is with airborne ultrasonic technology where a sound wave is directed towards the container-closure and visualized through the creation of a high-resolution image. An alternative to ultrasound is the use of a laser diode or the utilization of high- voltage technology. These new techniques have the advantage of being non-destructive and they allow for a larger proportion of the batch to be tested, which increases the level of confidence in the integrity of the seal. These techniques are also more accurate in allowing identification of small pinholes, micro cracks and seal imperfections that cannot visually be seen.

With microbiological testing, a sterility test of the end product or a microbial ingress test can be considered. The sterility test is unsuitable because the test will only detect viable microorganisms present at the time of the test and those that are capable of growth within the culture media used. The microbial ingress test involves direct microbial challenge and is, therefore, a more robust test. The objective is to detect microbial ingress based on 1) the probability that the challenge microorganisms can find a container-closure leak, 2) the ability of the microorganisms to traverse the leak, and 3) the capability of the microorganisms to grow in the internal container environment.

The microbial ingress test can be performed in different ways. One of the key criteria is the selection of the microorganisms. It is more common to use two different microorganisms of different sizes and with different methods of motility. For example, Brevundimonas diminuta, a very small bacterium, and Escherichia coli, a bacterium with a relatively powerful motility, are often used in combination (10). The complexity with the test relates to achieving a sufficiently high microbial population.

To conduct a microbial challenge test, vials are filled with a microbiological growth medium before stoppering and crimping, and are immersed in a 35 °C bath containing magnesium ion as well as 8 to 10 logs of viable bacterial cells for 24 hours. The test units are then incubated at 35 °C for 7 or 14 days. Microbial ingress is detected by turbidity and plating on blood agar.

The described tests, or a selection thereof, should ensure that the integrity is verified over the product's shelf-life, simulating the stresses the product will be subjected to, including sterilization, handling, and storage conditions. The tests, therefore, need to be made more rigorous in order to simulate "real life" events, for example by exposing test vials to stresses of temperature and pressure conditions, which the vials are subjected to when being transported for distribution and sales. The level of confidence is increased if three different batches are assessed. Another option is to assess vials as part of a stability trial program, which includes a time point at the end of the shelf-life.