Lyophilisation – the process in which a liquid drug formulation is frozen and placed under a vacuum such that the ice changes directly from solid to vapour without passing through a liquid phase – has many advantages for moisture-sensitive drug products. In addition to preserving the characteristics of a potentially unstable drug, the benefits of the lyophilisation process include enhanced product stability in a dry state, rapid and easy dissolution of a reconstituted product, and ease of processing.1
Lyophilisation is an ideal process for drug products that may have a limited shelf-life or sensitivities to external influences. Some drug formulations are unstable in aqueous solutions; the molecules can interact and degrade quickly in water, so moisture-sensitive drugs are often lyophilised not only to protect them from outside elements, but also to protect them from their own eventual decay.2
Maintaining the stability of the lyophilised drug product during its shelf-life can be difficult for moisture-sensitive drug products
However, maintaining the stability of the lyophilised drug product during its shelf-life can be difficult for moisture-sensitive drug products, particularly if the elastomeric component selected for the primary containment system allows moisture vapour to migrate into the drug. Proper selection of the lyophilisation component and the rubber formulation can help to prevent risks and costs associated with disintegration of the freeze-dried drug product, often described as ‘lyo cake’, resulting in loss of drug product and patient confidence.
Design options for closures
Glass has a very low moisture vapour transmission rate, so when selecting a primary container for a moisture-sensitive drug product, glass vials are often used. However, when selecting a stopper for the system, pharmaceutical manufacturers should be aware of the design and feature options, and consider the rubber formulation as well as its processing to ensure optimised storage and moisture protection for the drug product.
Igloo stoppers offer a more stable positioning in the freeze-drying phase due to their increased contact area with the glass vial orifice. They are also less flexible than split (or two-leg) designs. One drawback to this option is the asymmetric balance point. The stopper may shift out of the vertical axis during stoppering, which could lead to technical issues on the filling line, e.g. during camera inspection of the seated stopper for In Process-Control, or even a closure displacement and, in a worst case scenario, the stopper falling off the vial.
The split design offers a more flexible option during stopper insertion, and its symmetric design keeps it horizontal during freeze-drying. However, the twinning effect, where the stopper legs could intertwine during processing, may cause issues during the filling line process, e.g. in the closure feeder bowl and on the transportation rails until the closures reach the stopper insertion station.
Of greater concern to pharmaceutical manufacturers is that moisture can migrate through the rubber stopper itself during long-term storage of the freeze-dried drug product
Either option will work well for lyophilisation as both offer a gate where water vapour can be released from the vial headspace during the freeze-drying process. Of greater concern to pharmaceutical manufacturers is that moisture can migrate through the rubber stopper itself during long-term storage of the freeze-dried drug product, so selecting the right rubber formulation and processing steps for the lyophilisation closure are of utmost importance.
Rubber formulations that reduce moisture transmission in lyophilisation stoppers: To assure a clean and effective drug product, the lyophilisation process should begin with primary packaging component selection. High-quality elastomeric components used for aseptic filling will be washed and dried, then steam sterilised and dried again to help ensure cleanliness and reduced residual moisture content of the closures. The stoppers will then be used in the lyophilisation process to contain the freeze-dried drug product. The fully stoppered vials are released from the freeze drier, crimped with seals and stored.
It is impossible to keep moisture out of lyophilisation closure processing as there are two distinct points where moisture is driven into the stopper: initial pharmaceutical washing and the steam sterilisation process. Both are followed by a drying period that should be optimised before use with a moisture-sensitive drug product.
Moisture can reach a lyophilised drug product in a variety of ways. The simplest source of moisture ingress is a lack of seal integrity in the stopper-vial combination. If there is not a tight seal, moisture can travel easily into the vial and permeate into the freeze-dried drug product. Residual moisture driven into the closure, mostly during steam sterilisation and not removed due to insufficient drying conditions, may affect the drug product as the captured excess water vapour could be released from the rubber stopper into the vial headspace and permeate into the lyophilised drug product over time.
Finally, water vapour can migrate from the environment through the rubber stopper during long-term storage. Every rubber formulation has a characteristic rate for water vapour to migrate through this material over time which is called Moisture Vapour Transmission Rate (MVTR). This can be measured during development of a rubber formulation on a vulcanised rubber plate of specific thickness that is used as a permeation barrier in a humidity chamber. The MVTR is expressed in g/m2 x day on a 0.035 inch thick vulcanised test plate of a specific rubber formulation.
Figure 1: Residual moisture content in lyophilisation stoppers of different rubber formulation
The amount of moisture that could negatively affect the drug product varies based on the size of the freeze-dried drug product and its contents. As noted above, when moisture is driven into the elastomeric component during the washing and steam sterilisation processes, the rubber stopper reacts like a sponge. It can take in and release moisture. Each rubber formulation has its equilibrium moisture content under certain conditions, and the environment also influences the final moisture ingress into the vials. An environment with high humidity could affect the humidity in the headspace of the vial, which may influence the rate of moisture travelling time through the rubber closure during long-term storage.
West Pharmaceutical Services conducted a study to determine whether various drying periods affected moisture uptake within the rubber component. In doing so, it was discovered that the MVTR was more important to long-term storage than initial dryness. Residual moisture was studied on the content of a 100mg lactose lyo cake and lyophilisation stoppers for a three-year period. Different rubber formulations were compared and different drying periods for the closures after steam sterilisation were tested.
Method: Samples were prepared as follows. Stoppers were subjected to a washing and drying process. Steam sterilisation was conducted on all stoppers (60min, 121°C). Stoppers were dried at 105°C for 1, 4 and 8 hrs. Vials were filled with 2.0mL of 5% lactose in water solution. Stoppers were seated onto the vials. Lactose solution was lyophilised by a contract processing agent. Three samples were tested at time 0. Lyophilised product vials were stored for three years at 25°C/60% relative humidity (RH). Sample pulls were at: 0, 1, 3, 6, 12, 18, 24 and 36 months.
Seal integrity was tested on all the vials using Helium-Leak detection. Samples for helium leak testing were prepared by inserting a metal cannula through the stopper target area, flooding the vial with helium, and then sealing the punctured stoppers with epoxy. The sealed vials had to have an actual helium leak rate below 6 x 10-6 stdandard cc/sec in order to proceed with the study. Rubber stopper samples and corresponding lyo cakes were analysed for moisture content using a Karl Fischer coulometric titration method developed by West.
The results noted that moisture migrates from the environment into the stopper over time and that residual moisture in the stopper is dependent on its rubber formulation and applied drying time. In fact, the drying conditions had an effect on the residual moisture content in the stoppers. It was noted that a one-hour drying cycle is insufficient to remove the moisture driven into the stopper during the autoclave cycle. Drying for four hours returned the moisture content within the stopper back to the amount of moisture prior to steam sterilisation, but the best results were gained with an eight-hour drying period at 105°C.
Figure 2: Residual moisture content in lactose lyo cakes using lyophilisation stoppers of different rubber formulations (dried for 8hrs)
Figure 2 shows that the lyo cake also takes up moisture. Moisture uptake by the lyo cake correlates with the MVTR of the rubber formulation, not with the moisture uptake of the stopper after washing or steam sterilisation. Selecting the wrong rubber formulation may allow excessive moisture to reach the lyo cake, and the cake itself will collapse at some point during storage. This may result in costly recalls or the rejection of the drug product at its point of use.
As noted in Figure 3, even though stoppers could be very dry at Time 0, it is important to check how much moisture is taken up by the lyo cake over the long-term storage period. This moisture uptake is dependent on the stopper’s rubber formulation and its MVTR. The solid, horizontal blue line denotes for comparison the maximum moisture content after 36 months in the lyo cake in a vial stoppered with a bromobutyl type closure. As noted, although the bromobutyl stopper had more moisture at Time 0, it prevents moisture from reaching the lyo cake over time.
If the drying conditions for the stopper are not optimised, residual moisture can migrate into the lyophilised drug product over time
In contrast, the example of a very dry butyl type rubber formulation at Time 0 allowed water vapour to travel through the elastomer more easily and so the moisture content measured after 36 months in the lyo cake contained in these vial-stopper systems was higher than the one in the bromobutyl sample. Elastomeric rubber formulations that have reduced moisture vapour transmission rates are better suited to keeping the drug product relatively dry after lyophilisation.
Residual moisture in elastomeric stoppers can cause degradation of lyophilised drug product. If the drying conditions for the stopper are not optimised, residual moisture can migrate into the lyophilised drug product over time. Best practices for the elastomeric components include optimised drying time after the steam sterilisation process, and consideration of a rubber formulation with a reduced MVTR. After drying correctly, and before introduction to the filling line, packaging must also be considered. The use of a moisture barrier bag rather than a steam sterilisable bag will help to maintain the achieved and specified residual moisture in the stoppers after steam sterilisation and appropriate drying process. Moisture barrier bags will help to keep the stoppers dry prior to use.
Figure 3: Moisture uptake of lyo cake and stoppers (bromobutyl vs butyl rubber)
To select the best primary container system for a lyophilised drug product, pharmaceutical manufacturers must consider closures carefully. The design should be selected to ensure dimensional fit of the stopper to the vial, eliminating the possibility of a ‘mis-fit’ during the capping process, which would allow moisture an easy access through gaps between the vial and stopper. Rubber formulations with low MVTR should be selected to ensure optimised processing, and sufficient drying time should be allowed to help ensure that long-term storage will not affect the drug product.
There is no stopping the uptake of moisture over time, but with the right consideration given to the selection and preparation of the elastomeric components of a container closure system, lyophilised drug products will maintain stability with respect to moisture sensitivity and related efficacy over the long journey to their use by the patient.
References
1. US Department of Health and Human Services, (2014). Lyophilization of Parenteral (7/93) Guide to Inspection of Lyophilization of Parenterals. Retrieved from: http://www.fda.gov/ICECI/Inspections/InspectionGuides/ucm074909.htm
2. Mayberry, J. (2012). The New Scope of Pharmaceutical Lyophilization. Retrieved from: http://www.pharmpro.com/articles/2012/06/new-scope-pharmaceutical-lyophilization
The author wishes to acknowledge that this article is based on a study conducted by Amy Miller and Jennifer Riter of West Pharmaceutical Services, Inc., without whose significant efforts this article could not have been written.