Ice Fog as a Means to Induce Uniform Ice Nucleation During Lyophilization (Peer Reviewed) - The authors describe a novel means to control ice nucleation using a sterile cryogenic ice fog that is appl

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Ice Fog as a Means to Induce Uniform Ice Nucleation During Lyophilization (Peer Reviewed)
The authors describe a novel means to control ice nucleation using a sterile cryogenic ice fog that is applicable to laboratory-, pilot-, and production-scale lyophilizers.


BioPharm International
Volume 25, Issue 1, pp. 33-38

From a regulatory standpoint, creation of the ice fog at production scale does not introduce anything fundamentally new to the system. The ice fog is produced inside the ejector using steam and sterile-filtered nitrogen gas, both of which are already used in lyophilizers today (e.g., steam for sterilization and nitrogen for inerting or backfilling). All components downstream of the sterile nitrogen gas filter and up to the output of the ejector that releases the ice fog into the lyophilizing chamber have been designed to be sterilized in place. Hence, all the surfaces the ice fog touches before being introduced into the lyophilizer are sterile. All surfaces within the lyophilizer itself, including the vials, are sterilized, and the ice fog does not touch anything that is nonsterile, even after being introduced into the freezing chamber. In summary, the introduction of a sterile ice fog is no different from the introduction of any inert gas, such as nitrogen, that is used today for backfilling vials. No additional sterility concerns should arise regarding the surfaces the ice fog touches inside the lyophilizer.

Introducing water in the form of ice crystals into a finished formulation may raise concerns initially. However preliminary tests have shown that ice-fog derived water is a small fraction of the total water already present in the formulation, and comparable with the prevalent chamber moisture content that formulations routinely encounter when loaded into lyophilizers.


Figure 5: Sequence of still frames from a 7-second video in increasing order of time from left to right. The first image shows the chamber before the introduction of the ice fog. The last image shows the chamber 7 seconds after the introduction of the ice fog.
Ice nucleation in all vials was further confirmed visually and through video recording. Figure 5 depicts a 7-s video as a sequence of still frames separated by 0.4 s in real time. It shows the lyophilizer being filmed from outside the plexiglass door during the introduction of the ice fog. The first image in the sequence shows the chamber before the introduction of the ice fog, and the last image shows it 7 s after the introduction of the ice fog. The images clearly show a dense ice fog distributed throughout the chamber within this time.


Figure 6: Sequence of still frames from a 4-second video in increasing order of time from left to right. The first image shows three consecutive vials placed in the center of the middle shelf in the lyophilizer before introduction of the ice fog. The last image shows the same vials 4 seconds after the introduction of the ice fog.
Ice nucleation inside the vials can be visualized in Figure 6, which shows a 4-s video as a sequence of still frames separated by 0.3 s in real time. It shows the close-up of three consecutive vials placed in the center of the middle shelf of the lyophilizer, where ice fog reach is expected to be the most challenging. The first image shows the close-up of one vial just as it begins to nucleate after the introduction of the ice fog. Within 4 s, vials adjacent to it also nucleate and at the end of 4 s, all three vials have completely nucleated. On a macro scale, this phenomenon is seen in all vials inside the chamber, and all vials nucleate within 4–10 s following the introduction of ice fog. This result is a significant improvement over the 20-min vial-to-vial nucleation variability seen in the absence of ice fog.

Scale-up considerations and potential regulatory concerns

The scalability of the technique has been verified by replicating it on a lab-scale (MINIFAST) and a pilot-scale (LYOMAX) lyophilizer. It is expected to be easily scalable to larger sizes. The water-vapor source for ice-fog generation can be chosen based on ease of use and infrastructure availability. For instance, in nonindustrial, nonaseptic laboratories, a humidified gas stream may be the preferred source, whereas on the aseptic production floor, steam would be the preferred fluid.


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