The rate at which the bulk drug product is frozen is only one of the factors that affect protein recovery. Container dimensions
also play an important part in API recovery. The freeze distance is the space from the edge of a container to its center and
thus it is not always accurate to use scaled-up or down models to describe API stability in bulk-freeze containers. To receive
accurate data, studies must be conducted for each individual container used for a freeze-thaw process.
Thermodynamics of Freezing
The freezing of bulk drug product is a complicated process that can be divided into several segments. In the initial cooling
phase, the product is chilled to the freezing point of the liquid. The initial drop of the freeze curve eventually plateaus
off at this point. This plateau occurs because at this point, the liquid is in the middle of its liquid–solid phase transition
state. The liquid–solid phase transition state is completed when the liquid's latent heat of freezing is overcome. Next, the
frozen solution is cooled down from the freezing point to the set point of the system after the latent heat of freezing has
been overcome.
Another variable that must be considered is bulk-scale freeze concentration. Bulk-scale freeze concentration is a phenomenon
that can happen if a solution is frozen in a way that allows for solutes to disseminate from the slowly growing ice surfaces,
which can open a way for convection to occur in the unfrozen parts of the solution. This can be a major issue to deal with
because it may lead to variances in a drug product's stability and formulation concentration. The freeze concentration can
lead to changes in the protein and excipient concentrations in a solution, which means that certain parts of the solution
will freeze faster or slower than others. Another issue that can be a problem when considering freeze concentration is scale-up
because models on a small level may not be representative of what happens when container shape, size, and volume change.
The most important point that occurs on the freeze characteristic graph is called the last point to freeze (LPTF). LPTF is
defined as the longest freezing time experienced by a product in a vessel in which the product is in contact with the liquid
phase. The LPTF always occurs in the geometric center of the product when freezing because bulk product freezes from the outside
in and from the bottom up. Because the LPTF is representative of the worst-case scenario for protein stability, it can be
used to correlate freeze results between stainless steel vessels and smaller-scale systems. As a result of a disposable freeze
study at Genentech, freeze curves of disposable systems were shown to be very similar to curves found for stainless steel
containers, thus proving the comparability of disposable bulk freeze-thaw containers with their stainless steel counterparts.
Inherent Risks of Bulk Shipping
The risk in using disposable freezing and shipping systems in bulk storage and transfer is obvious. Many biotechnology companies
are reluctant to use disposables because they are novel and untried systems. Additionally, there are other issues regarding
the use of disposable bulk freezing and bulk storage systems that are not present with stainless steel applications. Some
of them include the following:
- leaching
- absorption of bulk product to the bag
- variation of the APIs because of storage in the bag (stability)
- light sensitivity
- integrity issues with manufacturing of the bag (e.g., connectors, ports, transport).
Testing for extractables or leachables is a requirement before implementing any disposable bioprocess container into a manufacturing
facility. Leaching can be described as the removal of soluble or insoluble materials from the container that are released
into a product solution during a process. Obviously, this can be an problem when dealing with bulk product and purified substances.
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