February 2012
Freeze drying is carried out to enable long term storage. Water affects many products both chemically and physically, so removing the moisture stabilises the product so that its immunologic potency, viability and integrity are not compromised. Dried products are not completely inert, and the moisture content within a freeze dried material has a direct effect on the glass transition of the material which in turn impacts on long term stability and storage conditions. Therefore the final moisture content of a product must be controlled within established tolerances.
Determining Target Moisture Content To assume that the ideal moisture content for one product will apply to another could be costly, resulting in product losing activity or degrading too quickly. Stability studies should be carried out to ascertain the ideal moisture content. Variations are inevitable, and a range of tolerance should be defined that is safe for the product, that is achievable, and that is identifiable by the measurement type specified.
It’s important to note that drying becomes increasingly slow and expensive as the product dries due to impedance of the cake layer above. An ideal residual moisture content might be uneconomical or even unachievable, requiring formulation or process revision. These issues should be addressed early on in development.
Moisture content is an important metric, but it is not the whole story. Percentage moisture content refers to an average whereas in practice water distribution may vary across the product. Water may be present in a variety of “forms” – free, adsorbed, chemically bound, hydration shells (e.g. of proteins), water of crystallisation, not all of which may be directly linked to the activity or stability of the product in question.
Different types of measurement methods measure the water differently. For example thermogravimetric analysis measures mainly surface water and some loosely bound water of hydration, whereas Karl Fischer measures surface moisture and most types of bound water (so long as the whole product is soluble in KF reagent). Thermogravimetry may cause thermal decomposition of the sample and it is often used in conjunction with Mass Spectrometry for moisture studies. It is advisable to use several measurement techniques in combination, especially during development and trials.
Over time the location of the water in a dried sample may change. The extent to which this occurs will depend on many factors including the hydroscopicity of the excipients and environmental conditions. Moisture studies which can be carried out on the same samples over time, for example by Frequency Modulated Spectroscopy, can provide invaluable insight into these changes.
Moisture content can vary across each batch, from freeze dryer shelf to shelf and across each shelf. Variations are to be expected, testing and processing methods should be developed to take this into account. Wherever possible measures should be taken to reduce variation. Some examples include:
Container type: The container type and size should be such that heat transfer is facilitated as much as possible. Any changes to containers should be evaluated, trays may warp over time leading to uneven drying.
A freeze dryer shelf of vials mapped for final moisture content
Container treatment: Oven-drying stoppers before freeze drying can reduce the level of moisture ingress from the stopper during and after processing.
Processing: Products that have undergone collapse will dry poorly and unevenly, and cycles that run too close to the collapse temperature risk product failure that may not be visually obvious. The freezing step is vital for creation of ideal drying conditions, and thermal treatments such as annealing and controlled ice nucleation can result in more uniform ice crystal structure and therefore more even drying.
Formulation: Products must be formulated to provide favourable thermal characteristics. Different excipients display different dynamics of interaction with water and may exhibit complex drying behaviour.
Moisture content analysis is important during R&D, to design and optimise freeze drying cycles, and for validation and regulatory purposes. BTL offers moisture content analysis by Karl Fisher Titration and FMS. We can also offer formulation and cycle development services specific to freeze drying, and can test-run your product and process in our pilot-scale freeze dryers.
>>Applications of headspace moisture analysis (presentation)
>>Moisture Analysis service from BTL
>>FDA Guideline for the determination of residual moisture in dried biological products (pdf)
>> Controlled ice nucleation: case studies (pdf)
Tags: moisture, moisture analysis, moisture content, moisture determination, stability