Best Practices for Formulation and Manufacturing of Biotech Drug Products - How to maintain product stability and prevent particulates. - BioPharm International
The large-scale storage systems discussed here attempt to control the rate of heat removal and thereby obtain a reproducible
process. In the case of bottles or carboys, the exact pretreatment and placement of the containers and load in the freezer
must be defined. Similarly, the thawing conditions and placement must be established. The active systems from Sartorius-Stedim
can be programmed to provide reproducible temperature profiles in the bulk. The actual profile is determined by the load,
but a range of loads (fill volumes) can be defined, qualified, and validated. During operation, after a small degree of supercooling,
ice formation is generally nucleated throughout the bulk, although the growth is faster at the edges than in the center. Depending
on the rate and nature of ice growth in relation to diffusion rate, solutes get trapped between growing ice crystals. Thus,
concentration gradients are generated and "frozen-in" in such systems. An example of concentration gradients observed in bottles
is shown in Figure 2(a). Such gradients can persevere if thawing is carried out without mixing, as shown in Figure 2(b). Our
in-house observations show that proteins and other excipients cryoconcentrate to the same extent. Cryoconcentration occurs
in the cryovessels and bags.21,22 Because a degree of cryoconcentration is unavoidable and the protein is most vulnerable when the solute concentration is
high but the mass has not been completely immobilized, it is best to freeze as rapidly as possible. Doing so minimizes the
time the protein spends in the partially frozen high concentration but still mobile transition region.
Once processed, these bulk containers must be stored for a period of time. The storage temperature is determined by the nature
of the formulation as well as practical and logistical considerations. Bottles and carboys can be placed in deep freezers
at any desired temperature that can be tolerated by the material of construction. The glass transition temperature of high
density polyethylene (HDPE) is –145 °C for the amorphous portion (brittle temperature quoted as –100 to –70 °C), making it
suitable for most applications.23 For polypropylene (PP), the glass transition temperature ranges from –15 to –10 °C requiring that such containers be handled
carefully in cold storage. Similar care is required for containers based on ethylene vinyl acetate (EVA), which has a transition
around –15 °C although the brittleness temperature for film-grade EVA is stated to be as low as –75 °C or below. Large stainless
steel cryovessels have an operational temperature limit around –60 °C, although custom-built warehouses are required if storage
below –20 °C is needed.
The process of thawing, while simple in principle, must be controlled properly to ensure that the wall temperatures at the
heat transfer surfaces do not exceed allowable limits for the product. To ensure that the thawed material does not overheat
while a remainder is still in the frozen state, the mass should be agitated during processing, thereby ensuring efficient
heat transfer as well as preventing hot spots. Finally, a comparable system to perform long-term stability studies is required
to support regulatory filings. In the case of a bottle or carboy storage, smaller units are used. Simple dimensional considerations
imply that these cannot be completely representative. For the cryovessels and bag freezing systems, small-scale models are
available to do process development. Their utility as stability models must be evaluated.
Anurag S. Rathore, PhD, is a consultant, Biotech CMC Issues, and a member of the faculty in the department of chemical engineering at the Indian Institute of Technology. Rathore is also a member of BioPharm International's Editorial Advisory Board.
Articles by Anurag S. Rathore, PhD
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