Downstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 171

levels as high as 3 mol l 1 . Increased solute concentration alone can accelerate chemical reactions
damaging to the protein product. In addition, such concentration effectively brings individual
protein molecules into more intimate contact with each other, which can prompt protein-protein
interactions and, hence, aggregation.
As the temperature drops still lower, some of the solutes present may also crystallize, thus being
effectively removed from the solution. In some cases, individual buffer constituents can crystallize
out of solution at different temperatures. This will dramatically alter the pH values of the remain-
ing solution and, in this way, can lead to protein inactivation.
As the temperature is lowered further, the viscosity of the unfrozen solution increases dramati-
cally until molecular mobility effectively ceases. This unfrozen solution will contain the protein,
as well as some excipients, and (at most) 50 per cent water. As molecular mobility has effectively
stopped, chemical reactivity also all but ceases. The consistency of this 'solution' is that of glass,
and the temperature at which this is attained is called the glass transition temperature T g´ . For most
protein solutions, T g´ values reside between 40 C and 60 C. The primary aim of the initial
stages of the freeze-drying process is to decrease the product temperature below that of its T g´ value
and as quickly as possible in order to minimize the potential negative effects described above.
The next phase of the freeze-drying process entails the application of a vacuum to the system.
When the vacuum is established, the temperature is increased, usually to temperatures slightly in
excess of 0

C. This promotes sublimation of the crystalline water, leaving behind a powdered cake
of dried material. Once satisfactory drying has been achieved, the product container is sealed.
The drying chamber of industrial-scale freeze dryers usually opens into a cleanroom ( Figure 6.23).
This facilitates direct transfer of the product-containing vials into the chamber. Immediately prior to
fi lling, rubber stoppers are usually partially inserted into the mouth of each vial in such a way as not
to hinder the outward fl ow of water vapour during the freeze-drying process. The drying chamber
normally contains several rows of shelves, each of which can accommodate several thousand vials
( Figure 6.25). These shelves are wired to allow their electrical heating, cooling, and their upward or
downward movement. After the freeze-drying cycle is complete (which can take 3 days or more),
the shelves are then moved upwards. As each shelf moves up, the partially inserted rubber seals are
inserted fully into the vial mouth as they come in contact with the base plate of the shelf immediately
above them. After product recovery, the empty chamber is closed and is then heat-sterilized (using its
own chamber-heating mechanism). The freeze-drier is then ready to accept its next load.

6.9.5 Labelling and packing

After the product has been fi lled (and sealed) in its fi nal product container. QC personnel then remove
representative samples of the product and carry out tests to ensure conformance to fi nal product speci-
fi cation. The most important specifi cations will relate to product potency, sterility and fi nal volume
fi ll, as well as the absence of endotoxin or other potentially toxic substances. Detection and quantifi ca-
tion of excipients added will also be undertaken. Product analysis is considered in Chapter 7.
Only after QC personnel are satisfi ed that the product meets these specifi cations will it be labelled
and packed. These operations are highly automated. Labelling, in particular, deserves special atten-
tion. Mislabelling of product remains one of the most common reasons for product recall. This can
occur relatively easily, particularly if the facility manufactures several different products, or even a sin-
gle product at several different strengths. Information presented on a label should normally include: