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Cynthia A. Challener is a contributing editor to Pharmaceutical Technology.
As an important method for improving the stability of parenterals, lyophilization is fairly well understood, but can still benefit from several advancements in the technology.
Many biological APIs are not stable in aqueous solutions, making preparation of shelf-stable sterile formulations difficult or impossible. Removal of the water at low temperature is a gentle method for overcoming this issue. To achieve effective lyophilization processes, careful selection of ingredients and knowledge of their properties and behavior at different temperatures is necessary. Parenteral manufacturers are benefiting from the use of process analytical technology (PAT) and increased automation. High hopes are also placed on controlled ice nucleation technology, with the first commercial product implementations expected soon.
An effective solution for improved stability
For parenterals, the preferred product presentation is a sterile, ready-to-use solution, largely because of ease of use. For many drugs, protein pharmaceuticals in particular, the active ingredient is not stable enough in an aqueous solution to make a sterile solution formulation feasible, according to Steven L. Nail, a senior research scientist at Baxter Biopharma Solutions. Robert Sever, business development manager for life sciences and laboratories at Praxair, also notes that freeze-drying is a relatively gentle method for obtaining the desired shelf life while maintaining the efficacy of the product, which can be more difficult with other options, such as spray-drying.
There are additional benefits as well. “Not only can shelf life be extended from weeks or months to years, but because the product has long-term stability, it can be produced in larger batch sizes, which provides economies of scale,” observes Isobel Cook, a principal scientist at Biopharma Technology. Freeze-dried products also do not require a carefully controlled environment during shipment, which is a cost savings, according to lyophilization consultant Larry Ulfik of Applewood Scientific.
“As importantly,” Ulfik says, “the final product is whole and complete, meaning it can be produced under sterile conditions with dosage accuracy, even when only miniscule amounts of API are required, such as for cytotoxic anticancer formulations and vaccines. The finished lyophilized vial is in its own sterile boundary and protected from environmental gases, moisture, and particulate contamination, and thus remains sterile over the entire expiry period and perhaps even beyond without using antibacterial-mold preservatives.” By freeze-drying parenterals, it is also possible to ensure that the API is in its preferred crystalline or amorphous form and helps speed up the time to market for complex and unstable products.
Proper formulation is critical
These benefits only accrue, of course, if the appropriate lyophilization conditions are used. Determining these parameters can be a challenge, though, given the various ingredients found in parenterals formulations. “In general, the composition of the formulation is critical in achieving the desired product quality attributes,” Nail states. “The formulation may include a buffer, a bulking agent, a stabilizer, and (perhaps) a surfactant.” Excipients can also provide stability to complex molecules by replacing the water in the matrix so that the molecular structure of the API is retained after reconstitution, according to Ulfik. For proteins, the stabilizer is generally a disaccharide, which protects against damage to the API, often due to unfolding caused by the stresses associated with freezing, with the drying process, or even reconstitution.
Excipients can also be added to adjust the critical temperature. “The lower the freezing temperature, the longer the cycle time; excipients are thus often added that can raise the critical temperature,” explains Cook. In fact, the idea is to dry the product at as high a temperature as possible without exceeding the critical, or collapse, temperature where the microstructure established during freezing has the potential to collapse due to viscous flow of solutes during primary drying when the ice is sublimed away. Shorter cycle times are important for controlling costs, which are a major consideration given freeze-drying is a very expensive process, according to Sever.
Knowledge of the formulation and its effect on the API during lyophilization can be obtained using appropriate analytical techniques, such as differential scanning calorimetry, freeze-drying microscopy, Karl Fischer analysis, X-ray diffraction, Raman spectroscopy, infrared spectroscopy, gas chromatography/liquid chromatography analysis, and final inspection via noninvasive methodologies, according to Ulfik.
Although freeze-drying technology tends to advance at a slow pace, some incremental advances in vial design have been beneficial, according to Cook. Examples include vials that can be stoppered and crimped in the freeze-dryer for improved seal integrity and enhanced glass systems for reduced breakage and cycle times and improved quality.
The use of PAT in recent years has had a significant impact. “Devices that monitor the global moisture levels of drying products to ensure that the process endpoint is reached consistently are an important example,” says Ulfik. Nail points to tunable diode laser absorption spectroscopy (TDLAS), a near-IR-based in-process flow meter that gives an instantaneous reading of the mass flow rate of water vapor during drying. “We use this instrumentation as a part of our quality-by-design approach to freeze-dry process development,” he notes. Nail, however, has found that TDLAS suffers at present from poor quantitative accuracy in large-scale equipment, but expects improvements as the understanding of vapor flow in the freeze-dryer increases.
The implementation of advanced sterile practices enabled by the burgeoning isolator and robotization industries has also allowed automated loading and unloading of freeze-dryers.”With this technology, the product, container, closure, and sealing cap are never exposed or manipulated by operators after sterilization at a cost that is competitive with and less risky than conventional methods in classified facilities,” Ulfik explains. He notes that the important benefit of this isolator technology is an overall improvement in product quality and worker safety.
The technology attracting the most attention, however, is controlled ice nucleation. Using current freezing methodology, ice nucleation occurs randomly and often at very low temperatures. According to Sever, the vial-to-vial nucleation variability leads to heterogeneity in ice structure, which can impact critical product and process attributes, leading to sub-optimal processes and challenges in scale-up. Very cold nucleation temperatures can significantly reduce the primary drying rate. “The reduction is due to the fact that vials that freeze at lower temperatures have smaller ice crystals, which leave smaller pores behind when they sublime, thus slowing the drying rate.” Praxair has developed a technology that makes it possible to ensure nucleation at not only a specific temperature for all vials, but generally at warmer temperatures than previously possible. “Such controlled nucleation has the potential to make both the freezing process and the drying process more consistent and efficient,” observes Nail.
Praxair has worked with freeze-dryer manufacturers to make the controlled nucleation technology readily available to the industry. SP Scientific provides laboratory scale freeze-dryers equipped with the new technology; both SP Scientific and GEA Lyophil can provide the technology on new or existing freeze-dryers at pilot and manufacturing scales. Sever notes that the technology has been demonstrated to be effective at manufacturing scales with no scale-up issues. Although no commercial process has yet adopted controlled nucleation, many are looking to implement the technology in the near future. Nail adds that there are several approaches to controlled nucleation that are currently under development that will benefit the industry.
Issues beyond ice nucleation
In addition to ice-nucleation issues, there are several other limitations of the current freeze-drying process that need to be addressed, including protein/nucleic acid stabilization in the solid state, heat transfer, vial-fogging, high-concentration formulations. Improvements are also desired in the reconstitution process, the scale-up process, and characterization and analysis tools. “Each drug formulation has its own unique characteristics, and therefore its own set of issues depending on those properties,” asserts Cook.
In general, though, Nail sees a real need for better screening techniques to quickly identify the most promising formulations, particularly for protein pharmaceuticals since, in general, solid state stability is very much affected by the composition of the formulation. Nail also believes that the inefficiency of current freeze-drying technology is a serious issue. “Freeze-dryers basically need to be redesigned to make heat transfer more efficient by taking more effective advantage of thermal radiation.” Ulfik notes that there have been improvements in shelf fabrication methodologies that have contributed to increased heat transfer uniformity.
He adds, though, that inexpensive workable, noninvasive methods for measuring product conditions in all areas of the freeze-drying shelf stack are still needed to ensure that the product process envelope can be maintained over each and every product vial in the entire freeze-dryer load. “The shelf position in the dryer can have an effect during processing. Knowing these influences will reduce risks to product efficacy and overall customer safety, and of course save tossing batches if the control system can maintain conditions based upon these individualized measurements,” Ulfik states.
Improvements in technologies for cleaning in place and sterilization are also needed. Ulfik believes that the use of sterilizing gas rather than steam at high pressure is promising and expects that sterilization gases that remain as ideal gases will ultimately replace the use of hydrogen peroxide vapor.
Solutions under development
Manufacturers and users of freeze-dryers are also investigating other improvements as well. “Users of lyophilizers want engineered solutions to the problems they face to maintain product process envelopes. That can include tempered jacketed chambers to reduce chamber-heat gain to edge-of-shelf vials, solutions for choked flow during processing, the use of non-hardwired temperature sensors during validation, and non-temperature-based methods for robustly determining process endpoints. Additional improvements include streamlining of the interiors and operation of freeze-drying systems.
With respect to formulating parenterals for freeze-drying, drug manufacturers are exploring different buffer types and concentrations to minimize pH effects on product integrity during freezing and reconstitution, according to Ulfik. The use of more complex excipients, such as amino acids and glass-forming polysaccharides, is also increasing.
Meanwhile, some pharmaceutical companies are entering into what has previously been thought to be forbidden territory and are exploring drying above the critical temperature of the frozen solid. “Drug manufacturers are looking for ways to reduce the cycle time and lower the cost of freeze-drying. Raising the temperature is one way to do so,” Sever explains. “What is most interesting is that it has been found in some cases that the stability and reconstitution properties of the freeze-dried formulations are acceptable despite the potential for collapse of the cake microstructure.”
New characterization technologies are also under development. Baxter is closely following optical coherence tomography, which offers potential improvement over current methodology for freeze-dry microscopy, according to Nail. Biopharma Technology, meanwhile, has investigated headspace moisture analysis. “Moisture analysis is very important in freeze-drying because moisture impacts product quality and storage stability. An approach based on headspace analysis is beneficial because it is both rapid and non-destructive and accurately indicates whether the product is within the specified moisture range, which can be critical for biologics,” Cook says.
Finally, Sever believes that one of the biggest challenges is educating practitioners of freeze-drying technology about recent scientific advances so that they can be put into commercial practice. There has been significant effort to develop effective process modeling tools and describe robust process design strategies that can help reduce the amount of trial and error needed to optimize lyophilization, but they are not yet widely being used in the industry today,” he says.