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When freeze drying biological materials, the major concern is achieving product consistency both within a batch and between batches.
When freeze drying biological materials, the major concern is achieving product consistency both within a batch and between batches. Consistency is judged over a number of parameters, such as:
container headspace atmospheric content/vacuum
Heterogeneity of product in any of these factors will, in the customer’s mind, call into question the quality of the manufacturing process. Furthermore, when dealing with parenterals, assuring product sterility is critical.
Ideally, manufacturers need to achieve consistency while maximising process efficiency; they need to try to minimise run times and maximise profitability.
In early‑stage development, selecting and optimising a formulation that will preserve your product so as to maximise the shelf storage stability are two considerable processes. Other important issues include the product delivery format and the concentration of the biological material for many protein therapeutics, higher concentrations to deliver the necessary dosage in limited injection volume are becoming a priority.
Understand the product and control the process
Understanding your product is fundamental to being in control of the freeze‑drying process. During the last decade great emphasis has been placed on the use of analytical techniques that provide the critical glass transition (Tg) and eutectic properties of materials to be lyophilised. These can indicate what freezing temperature is necessary (as this is an expensive and time‑consuming step) and what product temperature is critical to the sublimation stage; from the latter studies, the maximum applicable shelf temperature can then be selected.
The current focus on PAT and building QbD into manufacturing processes has sparked a significant interest in developing effective PAT tools for lyophilisation. A number of different approaches are being developed, but, at present, I believe the leader is the monitoring of bulk vapour flow from chamber to condenser, as this allows manufacturers to estimate the sublimation time for the whole batch, not just individual vials in a fill. There are still some limitations with these methods, but the evidence produced in the literature so far is very encouraging.
It is vital to develop the optimal formulation early in a product’s lifecycle so that sub‑optimal choices do not let the product down in the later and much more expensive stages of development. The issues include selecting a formulation that does not infringe intellectual property rights while delivering maximal recovery of activity, and then establishing the stability of the product by accelerated environmental stress conditions. Traditional pilot studies are valuable and can be speeded up through the application of Design of Experiment strategies on small volumes to identify key stabilisers and destabilising conditions, while the availability of a developmental protein is still at a premium.
Methods of moisture determination
The destructive water content determination methods of colorimetric Karl Fischer and thermogravimetric analysis are still the industry standards and are likely to remain so for the immediate future. However, noninvasive monitoring of product moisture, currently by infra‑red methods, can be used in‑line or at‑line, and so gives a far more representative data for the batch as a whole. These methods, however, require calibration back to the destructive methods and, for those methods that examine the freeze‑dried cake, the infra‑red absorption by formulation components can be a problem and lack of penetration into the cake may mean that any heterogeneity in moisture content across the depth of the cake goes unidentified. The methods are container size/dimension specific so validation of each format is necessary. There has been interesting work on analysing headspace water vapour content where infrared laser light is passed through the headspace of the vial and the water vapour above the cake measured,1 but this also requires careful validation on a formulation and product presentation basis against conventional methods.
The importance of Tg
The Tg of the lyophilised product has been a popular parameter for study for many years. It is readily analysed by modern differential scanning calorimetry methods, though lyophilised materials are by nature hygroscopic and any assessment of Tg on such materials must control for the ingress of environmental water vapour.
Recent publications, however, indicate that it is certainly not the whole story; some formulations for biologicals of higher Tg may actually be less stabilising than those with lower Tg values. Very interesting work has been published on the molecular mobility and relaxation events that occur in the dried state of these largely amorphous products and these require other analytical techniques, such as solid state nuclear magnetic resonance and neutron backscattering, as well as infra‑red spectroscopy and vapour sorption techniques.
Clinicians will always want a product that is ready to use and so the reconstitution time for dried biological materials is a constant consideration. Biological materials are, however, often labile and so stabilisation by dehydration is attractive. Syringe formats offer advantages of convenience and are an upcoming trend; however, I think the lyophilised vial format will remain with us for many years to come, particularly for the most labile molecules. Freeze drying is a very expensive unit process and so alternative strategies, such as atmospheric drying and spray freeze drying, which reduce the processing costs, will gain in popularity, but will also bring practical problems of their own, which cannot be underestimated.
I. Cook, K. Ward and D. Duncan (Freeze Drying of Pharmaceuticals and Biologicals’ conference, CO, USA; August 2008).
Based on a contribution by Dr Paul Matejtschuk, Principal Scientist, Standardisation Science and LTN Business Fellow, at NIBSC, Health Protection Agency (UK).