Deep Freeze: Innovations in Lyophilization

Although lyophilization as a practice of preservation has existed for many years, perfecting the science continues to be a highly sought after objective of parenteral drug manufacturers. The strong push for increasing the number of protein-based therapeutics and other injectable products through development has contributed to the need for leaner, more cost-efficient freeze-drying methods without compromising quality. Boosting quality involves identifying vulnerable critical process parameters. To address the needs for safety, reduced risk, and better control, industry has designed solutions for use in the three areas of the lyophilization process well known to be highly susceptible: stopper placement, lyophilizer loading and unloading, and freezing.

(ILLUSTRATION BY M. MCEVOY. IMAGES:BAXTER PHARMACEUTICAL SOLUTIONS LLC AND DON FARRAL/GETTY IMAGES)

PAT and QbD: drivers of innovation

The US Food and Drug Administration's efforts to cultivate better understanding and control into many sectors of the process development cycle, including lyophilization, are evident.

"A lot of the emphasis has been spent in the past couple of years on identifying the limits of the freeze-drying process," says Heinrich Meintrup, managing director of GEA Pharma Systems (Hurth, Germany). "The basis is having to identify the weak points. The efforts in lyophilization are going toward trying to get a closer understanding of what is going on in a big installation, across all the vials, and within individual vials. How can we eliminate variability of the results in individual vials and how do we improve the design of the equipment to reduce this variability? It comes from better capturing the process conditions and understanding."

Solutions for monitoring lyophilizer shelf temperature and chamber pressure over time has involved the implementation of radio-frequency transitioned data, process analytical technology (PAT)-inspired wireless sensors, manometric temperature measurement approaches, and "smart" vials (1). Other methods include barometric temperature measurement (BTM).

This approach measures the broad temperature barometrically without the need for placing a real sensor. The strategy is especially advantageous in fully automated systems in which it may be difficult to first place a real sensor in a vial and then downstream to remove this single vial.

Figure 1. (ALL FIGURES ARE COURTESY OF W.L. GORE & ASSOCIATES.)

"It is a method that is based on physics ... now integrated into the control system as part of the PAT initiative," says Meintrup. "If you look at what has been driven by PAT, so clearly it is about temperature distribution, having clear information about what stage the process is at any time inside of the freeze dryer. Because it is a big chamber closed under vacuum conditions, you cannot take a sample, there is nothing you can see from outside other than frozen vials." Measuring equipment added to freeze dryers provides a level of understanding of what is going on in the chamber as well as some justification of the particular stage of the freeze-drying process.

Shrinking the cleanroom

For contract facilities or clinical-trial material processes, innovative solutions must take place within a limited space. These facilities may not have the resources to set up and maintain large expensive cleanrooms and must rely on other means to control contamination at critical points. One of these process points is stopper placement. Traditionally, sterile split-stoppers are placed only part way down the vials, allowing vapor to escape during lyophilization. After lyophilization, under vacuum or atmospheric conditions, the stopper is pushed into its final position by the freeze-dryer shelves to seal the vials. However, industry researchers point out that the semi-open vials are at risk of being exposed to potential contaminants.

At the 2008 American Association of Pharmaceutical Scientists annual meeting, W.L. Gore and Associates of Elkton, MD, introduced a bicomponent material technology to seal vials off from external environments (Gore Lyoguard vial isolator). The system incorporates a microporous vent to protect the product inside the vial, which effectively shrinks the cleanroom to a one-vial size. The unit provides a level of filtration such that the product can be freeze-dried through the membrane.

The device isolates the material inside the vial and is applied directly after the filling step. However, instead of incorporating a lyophilization stopper, a standard stopper can be used. The only time the vial is exposed to the environment is the time between when it is filled and the when the device is applied, which can be the very next station on the fill line. "At that point, it is protected until you are in the freeze dryer," says Fred Giordano, application engineer at Gore. "When the shelves in the freeze dryer collapse, the stopper is in the final position and the material inside has never really been exposed to the environments, which depending on the freeze dryer you are using and the manufacturing facility, can give you a lot more flexibility around the plant layout and around your capabilities."

Currently, the device is available for 20-mm neck vials and for small production runs such as for research and development, preclinical, and early-phase development stages. The company is working on a 13-mm version as well.

Figure 2. (ALL FIGURES ARE COURTESY OF W.L. GORE & ASSOCIATES.)

Toward full automation

One of the most vulnerable stages in the efforts to reduce the risk of contamination is the loading and unloading of the freeze-dryer trays of the lyophilizer. Eliminating the human factor from production provides an extra level of safety in terms of sterility assurance levels, less risk of encountering problems during processing, and faster cycle times.

"Freeze drying is a critical batch process, and it is a very slow process, usually taking days, followed by subprocesses such as cleaning, sterilization, and filter testing. Carrying trays or frames with vials brings humans in fairly close contact with the open vials that are often filled with very sensitive and very expensive product. Because these vials are open, having semi-inserted stoppers, they are exposed in the cleanroom," says Meintrup. "The process involves handling sterile solutions that are filled into sterile vials which, in turn, have to be handled in a sterile environment and processed under sterile conditions. Any human interference is a risk in and a potential source of contamination. Automation takes this risk out, and the automatic system can be enclosed in an environment such as a RABS (restricted access barrier system) or an isolator."

The automatic loading and unloading of lyophilization systems, depending on the various types and executions of freeze dryers, helps reduce risk during this step. There are a few companies that design this type of equipment and the technology is not new to the industry. Nonetheless, a fully automated system, in line with automated vial-washing lines and filling lines that have no human interference, closes the gaps in the process between the filling line and the freeze drying step. "These transitions between various steps in the process are a matter of concern," says Meintrup. "We automate not only for performance but also to reduce the risk for the operators and for the product as well as to make the whole process safer, leaner, and more comprehensive."

Improvements to automated equipment include reducing its footprint, incorporating new refrigerants, and optimizing the refrigeration system. Reducing unproductive time through, for example, fast sterilization methods such as VHP (vaporized hydrogen peroxide) will increase the overall system productivity. "It's basically about improving the product quality and reducing the total cost of ownership that are the two main drivers in this area," says Meintrup. "That is the challenge in lyophilization."

Controlled freezing

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 formulations.

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."

Reference

1. A.A. Barresi et al., "In-line Control of the Lyophilization Process: A Gentle PAT Approach to Using Software Sensors," Internat. J. Refrigeration,www.sciencedirect.com (2008).