One of the fundamental problems during freezing is that it often creates heterogeneous ice-crystal structures among the vials
within a batch as well as among different batches. "It is a problem that has persisted and assumed to be inherent in freezing,"
says Robert Sever, Group Leader of research and development at Praxair (Chicago Technical Center, Burr Ridge, IL). The problem
is that the vials do not all freeze at the same time nor at their expected freezing point.
For an aqueous solution, the freezing point is approximately 0 °C. In a pharmaceutical environment, however, these containers
will cool well below their freezing point (subcooling), to perhaps 20° or 30° below 0 °C, before they will actually start
to form ice (nucleation). These first few molecules are critical for obtaining crystal growth. In traditional lyophilization,
nucleation temperatures range widely and are significantly below the thermodynamic freezing point. This variation is a result
of pharmaceutical cleanroom environments where there is a lack of particulate contamination that helps ice nuclei form. As
a result nucleation doesn't occur very easily and occurs only at very cold and highly variable temperatures. The freezing
process is therefore random because the exact nucleation temperature for an individual vial cannot be predicted.
The nucleation temperature dictates many of the structural characteristics of ice. "The main problem that comes from not having
control over this phenomenon, is that you have different ice structures in each vial, and some of these ice structures turn
out to be very hindering when it comes to subsequent processing," says Sever. Vials that nucleate at a very cold temperature
produce very dense ice structures, which impede subsequent sublimation in the drying steps and lead to very long drying times.
"You may have some vials in a batch that have nucleated at warmer temperatures and are able to dry more quickly, but you ultimately
have to run your freeze-drying cycle for your worst-case vials, which are those that have nucleated at very cold temperatures."
Dense ice structures also tend to negatively affect yield, depending on the type of structure that is created. Ice structures
with large surface areas have the potential to damage active ingredients. Sensitive therapeutics such as proteins can aggregate
or denature on these ice surfaces and may be damaged in subsequent processes.
After completion of the freezing step, it is possible to undergo an annealing stage to try to restructure the crystals. This
strategy is practiced in the industry. However, as Sever points out, annealing often requires a substantially long time to
complete. More important, annealing generally requires raising the product temperature above the critical collapse temperature
of the material so that there is enough mobility in the system to allow the ice to restructure. Some consider this approach
relatively risky for sensitive products. Annealing is an after-the-fact process, and it does not control the nucleation step.
A novel Praxair process technology uses inert gas pressure in a freeze dryer to induce nucleation during the freezing step.
By precisely controlling this pressure, vials can be made to nucleate at a selected temperature. Having control over nucleation
provides some control over stresses occurring during freezing and, in some cases, reduces aggregation events in proteins.
To date, Praxair has tested the method within 5m2 freeze-drying chambers and is expanding the process for 30 m2 units. It has used visual monitoring and various analytical methods to demonstrate the approach for a wide variety of model
Having greater control of this step may also lead to having the ability to tighten specifications, narrowing variability and
increasing level of control, which is encouraged by FDA's quality-by-design initiative. "The process can deliver a much shorter
drying cycle on the order of many hours or even days in some cases. However, the potential benefits of controlling nucleation
go beyond controlling your ice structure for the sake of reducing drying time, as the preservation of the active ingredients
is also strongly connected to the ice-crystal structure. This is a method for improving the product quality attributes and
providing uniformity within a batch and among batches. And with the growing demand in protein therapeutics and increasingly
sensitive biologicals, we are optimistic that there may be an important role for this technology."
1. A.A. Barresi et al., "In-line Control of the Lyophilization Process: A Gentle PAT Approach to Using Software Sensors,"
Internat. J. Refrigeration,