Department of Pharmaceutical Analysis, Faculty of Pharmaceutical Sciences
Following the current trends in drug development, the focus of pharmaceutical companies is shifting from small molecule compounds towards biological drug products such as therapeutic proteins and vaccines. However, the stability of biopharmaceuticals formulated as an aqueous drug solution is often restricted due to water driven degradation pathways. Freeze-drying is a low-temperature drying process commonly applied to extend the shelf life of these biopharmaceuticals. About half of the group of biopharmaceutical therapeutics approved by the regulatory authorities (> 300) consists of freeze-dried formulations, despite the long processing time and high costs associated with this production process.1,2
State-of-the-art pharmaceutical freeze-drying is a batch-wise process during which an entire batch of vials (i.e., unit doses) is simultaneously processed through a fixed sequence of consecutive steps. Sterile glass vials are aseptically filled with the aqueous drug formulation and automatically loaded onto the temperature-controlled shelves in the drying chamber of the batch freeze-dryer. The freezing step is then initiated by gradually cooling these shelves leading to crystallization of most of the water into ice, hence, concentrating the solutes in the drug formulation between the growing ice crystals. Some of these solutes crystallize at the eutectic temperature, while those that do not are transformed into a rigid glass when reaching the glass transition temperature. Upon complete solidification of the product, the drying chamber is evacuated to induce primary drying, while energy is supplied by increasing the shelf temperature to enhance ice sublimation (endothermic process). The shelf temperature is set so the temperature at the sublimation front stays below the collapse temperature during the entire primary drying stage, avoiding structural product collapse and ensuring an acceptable cake appearance.
During the secondary drying stage, remaining unfrozen water (i.e., water dissolved in the amorphous phase) is removed by diffusion and desorption. Once the desired residual moisture content of the dried end product is obtained, the glass vials are stoppered and capped under specified pressure conditions in a controlled environment.
At the end of the freeze-drying process, a solid and rigid dried cake with a prolonged shelf life is obtained. The main Critical Quality Attributes (CQAs) of freeze-dried products are (i) the stability and therapeutic activity of the biopharmaceutical, related to the conformational state (e.g., protein conformation); (ii) the residual moisture content; (iii) the cake appearance and (iv) the reconstitution time. For each freeze dried batch, these CQAs are evaluated for randomly selected vials via time-consuming off-line analytical techniques.
For several decades, pharmaceutical freeze-drying has been conducted via this unchanged batch-wise approach as described in the previous paragraphs. However, this traditional batch approach is inherently associated with several disadvantages:
- Batch freeze-drying is an inefficient, time and energy consuming process, with cycle times which can run up from one to even seven days for one batch of vials. The freeze-drying equipment, including the handling equipment for (un)loading the vials and the necessary buffer systems, require large space which must meet the Class 100 clean room standards, mandatory in the sterile production of biopharmaceuticals for parenteral administration. These strict requirements in terms of cleanliness and sterility strongly increase production costs and resource consumption.
- Industrial batches contain a large amount of vials (often more than 10,000) which is highly impractical for cycle development. The initial development of freeze-drying cycles occurs in lab-scale equipment, but eventually requires scale-up from lab-scale to pilot-scale and finally to industrial scale freeze-dryers for production. Differences in heat (and mass) transfer between the different equipment scales require re-optimization and re-validation of previously developed cycles. These additional development steps significantly increase the time to market when launching a new drug product, leading to a shorter time under patent protection and faster loss of market exclusivity.
- The freezing step is uncontrolled by nature, which has a significant impact on the consecutive drying steps. Freezing initially involves cooling of the vials until ice nucleation occurs, which is generally several degrees below 0°C (i.e., supercooling). Ice nucleation is a stochastic event, hence, it occurs at a different temperature for each vial in the batch, resulting in a difference in degree of supercooling: a high degree of supercooling results in a high number of small ice crystals, while a lower degree of supercooling leads to a lower number of large ice crystals. The size of the ice crystals in the frozen matrix relates to the final pore size in the dried layer formed during the sublimation process. As a larger pore size is associated with a lower dried product mass transfer resistance and vice versa, the impact upon the sublimation rate during the primary drying stage is enormous. Consequently, the uncontrolled freezing step is linked to the vial-to-vial variability in sublimation rate within one single batch and between batches.3
- As the heat transfer in the drying chamber is uneven, the energy transfer among vials positioned at different locations on the freeze-dryer shelves can highly deviate. For instance, vials situated at the edge of the shelves are exposed to additional radiant heat coming from the warmer surroundings (i.e., door and walls of the freeze dryer) compared to the vials situated in the middle of the shelves. This vial-to-vial variability in heat flux leads to higher product temperatures for the edge vials, related to higher drying rates and an increased risk for product collapse.4
- Both the uncontrolled freezing and the unequal energy transfer result in different process conditions for each individual vial in the batch, linked with uncontrolled vial-to-vial and batch to-batch end product variability. Product quality is only tested upon a small portion of vials, before releasing the entire batch. This uncontrolled vial-to-vial variability and manufacturing (quality) approach do not meet the most recent guidelines issued by the regulatory authorities regarding Quality-by-Design (QbD) and Process Analytical Technology (PAT) which state that quality should be guaranteed by building it into the product (i.e., in each released vial), instead of elaborate time consuming off -line testing.5-7
Two years ago, an innovative continuous and controlled freeze drying concept was developed overcoming all above described disadvantages and challenges related to standard batch freeze drying.8-11 Continuous manufacturing integrates all unit operations in a single production line with continuous feeding of raw materials and the concomitant removal of finished products. This manufacturing approach offers many advantages, like avoiding scale-up issues, reducing cycle times and production costs, smaller manufacturing installations and clean room facilities and improved product quality (uniformity). The continuous freeze-drying technology for unit doses is currently being commercialized by the Belgian company RheaVita (www.rheavita.com).
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At the start of the continuous freeze-drying process, sterile glass vials are automatically and aseptically filled with the aqueous drug formulation and transferred to the continuous freezing unit. Here, the vials are gripped at the bottom of their cylindrical wall and rotated along their longitudinal axis resulting in a thin homogeneous product layer spread over the entire inner vial wall (i.e., spin-freezing, see Figure 1). The flow of a cold, inert and sterile gas is jetted upon the spinning vial for cooling and freezing of the solution. Both the temperature and the flow of the gas can be adapted to obtain a specified cooling and freezing regime, varying from very fast to slow cooling. When at the end of the spin freezing step the product is completely solidified, a thin product layer with a uniform thickness spread over the entire inner vial surface is formed (i.e., large surface area and thin product layer). Optionally, annealing can be applied to obtain the desired solid state of the solutes or to alter the ice crystal size in the frozen matrix by transferring the vials to a separate chamber with a controlled temperature.
A suitable load-lock system is used for quick transfer of the spin frozen vials between the continuous freezing and the continuous drying compartment, while maintaining the specific conditions of pressure and temperature in each chamber, hence, guaranteeing the continuity of the process. In the drying chamber, an endless belt system facilitates the transport of the spin frozen vials positioned in front of individually controlled radiators which provide a uniform and adequate heat transfer to the entire vial surface to achieve an efficient and homogeneous drying behavior (Figure 2). Optionally, if secondary drying would require a pressure level different from primary drying, a second continuous drying unit could be included, separated from the first one by an appropriate load-lock system for vial transport. At the end of the continuous freeze-drying process, vials are removed from the vacuum conditions via another load-lock system and transferred to the final unit, where stoppering and capping of the processed vials in a controlled environment occurs.
During freeze-drying, the sublimated ice and desorbed water is collected using a cryogenic ice condenser. For this continuous freeze-drying concept, an appropriate condenser system is used for continuous removal of the generated water vapor. The vial throughput (i.e., scale-up) can be customized by adding parallel lines of the continuous freeze-drying technology modules (i.e., LEGOTM principle), as schematically shown in Figure 3.
In standard batch freeze-drying, all vials are packed upon the temperature controlled shelves. The only vials not completely surrounded by neighboring vials are positioned at the edge of the shelf, however, these vials are not representative for the rest of the batch as they suffer from the radiation effect.5 This set-up for batch freeze-drying does not allow real-time measurement and control of critical process parameters (CPPs) at the level of the individual vial. Therefore, most process analyzers applied during batch freeze drying monitor the entire batch, e.g., the methods based upon the monitoring of the gas composition in the drying chamber to determine the primary drying endpoint.12 Process analyzers which monitor at individual vial level during batch freeze-drying are often invasive, e.g., thermocouples and resistance temperature detectors, and not representative for the rest of the batch.13 An additional issue arises in how to implement the invasive PAT tools during production without compromising sterility.
In the continuous freeze-drying technology, all vials are freely rotating during both freezing (at relative high velocity) and drying (at low velocity for homogeneous energy transfer over the entire vial surface), without being surrounded by other vials. Hence, compared to batch freeze-drying, there is no vial-to-vial variability and process analyzers can be implemented for real-time monitoring of CPPs at the individual vial level. Even CPPs which were never able to be monitored correctly during batch freeze-drying are measurable during continuous freeze-drying of unit doses (e.g., temperature at sublimation front – see further). Several process analytical tools, i.e., near-infrared (NIR) spectroscopy and thermal imaging, were evaluated and considered very promising.8,14 Also, both techniques prove to be highly complementary. NIR spectroscopy can provide detailed in-line information about several quality attributes like residual moisture content, protein conformation or the solid state of diff erent components (e.g., mannitol). In turn, thermal imaging allows contactless, real-time and spatial monitoring of the product temperature at the sublimation interface, which should be kept below the collapse temperature during the entire primary drying step to ensure an acceptable cake appearance of the end product.
In the continuous drying module, each spin frozen vial is centered in front of a single radiator, hence allowing an optimal drying trajectory for each vial. Implementation of the thermal imaging process analytical tool in this set-up, led to the development of a feedback control loop allowing individual temperature-regulation for each spin frozen vial in the drying chamber, as illustrated in Figure 4. The real-time measured product temperature at the sublimation front for a specific vial during primary drying is compared with the critical collapse temperature. Depending on whether the measured sublimation front temperature tends to exceed the critical temperature or whether there is still a safe margin, the power of the corresponding radiator can be decreased or increased, respectively, avoiding the exceedance of the critical product temperature leading to cake collapse while maximizing the primary drying efficiency. This integrated approach strongly reduces the variability of CQAs and consistently guarantees the predefined quality of the end product, i.e., the cake appearance in this specific case. Hence, the continuous technology meets the recent QbD and PAT guidelines issued by the regulatory authorities, as opposed to the conventional batch freeze-drying process.
RheaVita and Ghent University have currently built two different types of prototypes based upon the continuous freeze-drying technology: 1) the single-vial continuous freeze-drying prototype and 2) the GMP-like pilot scale continuous freeze-drying prototype. The single-vial prototype allows the imitation of the continuous freeze-drying process for one single vial, as identical process conditions can be obtained similar to the industrial-scale continuous freeze-dryers (Figure 5). In case only a very limited amount of drug product material is available, i.e., at early stages during development, this prototype allows early development and optimization of the process and the (drug) formulation. In addition, the ability to process small amounts of drug product makes the single-single vial prototype suitable for the production of personalized medicines.
From a commercial point of view, the one-vial prototype is the perfect tool for R&D laboratories within pharmaceutical companies to gain experience regarding the continuous freeze-drying process. In the GMPlike pilot scale continuous freeze-drying prototype, all process modules are integrated and freeze-drying is executed in continuous fashion (Figure 6). This prototype is engineered around the implementation of needs to create and keep a sterile environment by choosing the proper materials and design principles. Further, both prototypes have the relevant PAT tools, i.e., NIR spectroscopy and thermal imaging, builtin and the practical implementation of mechanistic models leading to optimal process conditions via individual temperature-regulation of the IR heaters.
References
- Burns, L. (2009). The biopharmaceutical sector’s impact on the US economy: analysis at the national, state and local levels. www.archstoneconsulting.com/biopharmapdf/report.pdf
- Gieseler, H. (2012). Insights in lyophilization 2012. Current best practices & research trends. Antwerp, 2012.
- Kasper J., Friess W. The freezing step in lyophilization: physico-chemical fundamentals, freezing methods and consequences on process performance and quality attributes of biopharmaceuticals. European Journal of Pharmaceutics & Biopharmaceutics. 2017; 78: 248-263.
- Van Bockstal P.J., Mortier S.T.F.C., Corver J., Vervaet C., Nopens I., Gernaey K.V., De Beer, T. Quantitative risk assessment via uncertainty analysis in combination with error propagation for the determination of the dynamic Design Space of the primary drying step during freezedrying. European Journal of Pharmaceutics & Biopharmaceutics. 2017; 121: 32-41.
- International Conference on Harmonization (ICH) of technical requirements for registration of pharmaceuticals for human use, Topic Q8(R2): Pharmaceutical Development, Geneva, 2009
- United States Food and Drug Administration (FDA), Guidance for industry PAT – A framework for innovative pharmaceutical manufacturing and quality assurance, FDA, 2004.
- United States Food and Drug Administration (FDA), Pharmaceutical CGMPs for the 21st century – a risk based approach, FDA, 2004.
- Van Bockstal P.J., De Meyer L., Corver J., Vervaet C., De Beer T. Noncontact infrared-mediated heat transfer during continuous freeze-drying of unit doses. Journal of Pharmaceutical Sciences. 2017; 106(1): 71-82.
- Van Bockstal P.J., Mortier S.T.F.C., De Meyer L., Corver J., Vervaet C., Nopens I., De Beer T. Mechanistic modelling of infrared mediated energy transfer during the primary drying step of a continuous freeze-drying process. European Journal of Pharmaceutics & Biopharmaceutics. 2017; 114: 11-21.
- Van Bockstal P.J., Corver J., De Beer T. (2017) A continuous and controlled pharmaceutical freeze-drying technology for unit doses. European Pharmaceutical Review, Issue 6
- Van Bockstal P.J., Corver J., De Beer T. (2018) New Approach Suggests Continuous Lyophilization is Possible, PDA Letter, February issue
- Patel S., Pikal M. Process Analytical Technologies (PAT) in freeze-drying of parenteral products. Pharmaceutical Development and Technology, 2009; 14: 567–587.
- Nail S., Tchessalov S., Shalaev E., Ganguly A., Renzi E., Dimarco F., Wegiel L., Ferris S., Kessler W., Pikal M. Recommended Best Practices for Process Monitoring Instrumentation in Pharmaceutical Freeze Drying—2017. AAPS PharmSciTech, 2017; 18: 2379−2393.
- Van Bockstal P.J., Corver J., De Meyer L., Vervaet C., De Beer T. Thermal Imaging as a Noncontact Inline Process Analytical Tool for Product Temperature Monitoring during Continuous Freeze-Drying of Unit Doses. Analytical Chemistry, 2018; 90: 13591−13599.