Yintai: Lyophilization Process Validation (Part I)
Introduction
The process validation of lyophilized products can be divided into two categories: prospective validation for new products and retrospective validation for established products. Prospective validation requires a series of validation tests at normal production scale to confirm the applicability and reproducibility of design or pilot-scale technical parameters in large-scale production. Retrospective validation involves summarizing, inducing, analyzing, and technically evaluating the original records of technical data related to the product and the quality retention data of batch production, which can objectively reflect the actual production status of the product. The conclusion is drawn whether the lyophilization process parameters can consistently produce products that meet quality requirements within the specified range. During the validation process, the batch size of the product should be the same as regular production, and the number of batches should not be too few, otherwise the credibility of the statistical data will be low, affecting the reliability of the validation conclusion.
Implementation Guidance
The process validation of lyophilized drug production generally includes two aspects: the process validation before the product enters the lyophilizer and the validation of the lyophilization process after the product enters the lyophilizer.
A. Product Lyophilization Process Validation
Due to the large number of lyophilization process parameters for the product, selecting the appropriate and sufficient process parameters for examination in the validation test is crucial for the confidence in the validation results. Since the crystallization process may involve a large amount of organic solvents, understanding and controlling the content of organic solvents in the lyophilization process and the residual amount of organic solvents after the completion of the lyophilization process will become an important part of the process validation for lyophilized products. For example, the crystallization speed of the solution is closely related to the crystallization temperature, and the freezing speed has a significant impact on the size and uniformity of the crystal nuclei of the solution crystallization. Although technical parameters have been provided for each specific product, laboratory, or pilot process, many factors affect the above parameters in the large-scale production process. Therefore, this content should be included as a key project in the validation plan. The process validation documents should include the analysis and evaluation of the production process data and products for each batch. Production process data include records of controlled parameters such as lyophilization time, temperature, and pressure, especially the relationship between the changes in product temperature during the lyophilization process and the final results of the products, as well as the inspection results of each item in the product quality standard obtained at the end of the design and the statistical deviation from the quality standard.
Now, let's discuss the main contents of the process validation item by item:
(1) Process Water and Freezing Speed
① Process Water: The water or solvent used in the manufacturing process to prepare the drug solution, commonly known as process water. Process water accounts for most of the "drug solution" and helps to improve the accuracy of the dose. However, the composition of process water affects the structure of the drug solution freezing into ice crystals and the rehydration performance of the product. The degree of supercooling of the drug solution has a significant impact on the freezing quality (such as crystallization status). The lyophilization process must carefully control the freezing process of the drug solution to obtain the most ideal crystal structure. The supercooling temperature of the product solution during the freezing process is particularly susceptible to the influence of other components in the process water. Organic compounds (such as sugars, starches, etc.), inorganic compounds (such as sodium or potassium salts, etc.), and some other excipients have a significant impact on the thermodynamic properties of the drug solution, determining the degree of supercooling required for the drug solution to freeze and ultimately affecting the thermodynamic properties, crystalline state, and rehydration of the product. It can be seen that these other components in process water have a special impact on the structure and thermodynamic properties of the product, such as causing the destruction of the main drug activity, changes in pH value, increased rehydration time, and increased turbidity of the rehydration liquid. Therefore, the impact of process water on the freezing characteristics of the drug solution and the physicochemical properties of the product should be fully considered and examined as a key point in the validation test.
② Freezing Speed: Since the subject to be validated - the equipment capacity of the lyophilization equipment is fixed, but the actual load size of the equipment, seasonal (hot and cold) changes, and different states of equipment adjustment may cause changes in the refrigeration capacity of the freezing machinery, affecting the freezing speed of the product and thus affecting the freezing quality of the product. Therefore, the freezing speed of the product during the freezing stage should be confirmed through validation tests. For crystalline products, it is always hoped that the freezing speed is not too fast, so that the crystal nuclei are larger, which is conducive to the formation of large ice crystals and accelerates the sublimation speed. However, if the freezing speed is too slow, the crystals are too large, which may reduce the number of crystal nuclei and result in poor uniformity of the product crystallization. On the contrary, rapid freezing can quickly set the molecules of high molecular weight drugs with a random network structure in the drug solution, allowing the encapsulated organic solvents to escape smoothly under low pressure conditions. Therefore, it is necessary to verify the freezing speed of the lyophilized product solution to determine the cooling speed that meets the product forming process requirements.
(2) Product Temperature and Drying Time
The temperature of the product during the freezing and drying process is generally measured by temperature sensors placed directly in glass vials or trays and recorded in a multi-point full-process recorder. Although the product temperature can be directly measured, it is indirectly controlled by the temperature changes of the shelf. Therefore, the validation test should determine the relationship between the designed control values of the shelf temperature at different freezing and drying stages and the product temperature, as well as the impact of the temperature gradient changes of the shelf on the product temperature. It should also be determined through the validation test that the above temperature changes meet the technical requirements. The correct control of product temperature can optimize the drying process of the lyophilization process. During the drying process, when the product temperature control value is low, the intrinsic quality of the final product is easy to ensure, but it may lead to longer drying time and increased operating costs. When the product temperature control value is too high or out of control, the quality indicators of the final product will be affected. Therefore, an important content of process validation is to verify the control range of product temperature, ensure the quality of the final product within this range, and further optimize the parameters of product temperature control during the drying phase through validation tests, thereby reducing the drying time and conforming to the principles of industrial production.
The verification of product temperature is carried out in the following aspects:
① The product temperature during the sublimation drying phase mainly verifies whether the product temperature is in the temperature zone where sublimation is the fastest. The verification test of the product temperature and shelf temperature during the sublimation drying phase can be combined and carried out simultaneously.
② The product temperature during the secondary drying phase and the determination of the drying endpoint mainly verify whether the increased product temperature affects the product quality. If the product temperature is too high and lasts for a long time, it can cause serious decomposition or discoloration of the product. Since the product temperature usually rises gradually during the secondary drying phase and finally equals the shelf temperature, it is difficult to determine whether the drying is complete without an accurate method to determine whether the temperature meets the requirements. If the product dries quickly, continuing to dry will lead to unnecessary waste of energy and time, and extending the time the product is in the high-temperature area may affect the quality of the final product. On the other hand, if the moisture removal is slow due to insufficient time, the moisture residue will exceed the standard. Therefore, this phase should also determine whether the method of determining the drying endpoint is appropriate through the validation test. At present, the residual gas judgment method is mostly used to analyze and determine the endpoint of the lyophilization process. The operation method of the test is: at the estimated drying endpoint, cut off the passage between the drying chamber and the vacuum pump, and observe the speed of pressure change in the drying chamber. For water vapor, if the rate of pressure change is ΔH＜5 Pa/3 min, it can be roughly judged that the drying endpoint has been reached, and this method should also be determined by analyzing the residual moisture content of the product. Pressure rise test: The working principle of the pressure rise test is to separate the product chamber from the freezing chamber (stop water capture) and keep the tray temperature constant. At this time, the moisture in the product will continue to volatilize, and the pressure in the product chamber will rise.
When the pressure rise test fails to meet the acceptance criteria, the drying process can continue for a set duration until the final pressure rise test results are in compliance with the acceptance standards, corresponding to the moisture content of the final product meeting the requirements. If the test consistently fails, there may be a leak. The freeze-dryer may experience a decline in overall performance after being put into use, which could result in the final moisture content of the product not meeting the acceptance standards under the preset freeze-drying conditions; the pressure rise test for determining the endpoint of the freeze-drying process can precisely control the product moisture, which is a key indicator of the stability of freeze-dried products. The pressure rise test can be conducted at the end of both primary and secondary drying to determine the endpoint, but it is recommended that enterprises perform the pressure rise test to judge the process endpoint after secondary drying at a minimum. The pressure rise test can be automatically controlled or manually controlled, but when manually controlled, the operators of the freeze-dryer must have extensive operational experience to avoid exceeding the maximum pressure in the product chamber during the test, which could cause the freeze-dried cake to collapse.
The pressure rise test method can be used to determine whether drying has reached the endpoint by closing the main valve between the freeze-drying chamber and the condenser for a short period. If the pressure in the freeze-drying chamber rises slightly, it indicates that the product is fully dried; if the pressure rises significantly, further drying is required.
The impact of pressure control precision on the final product: During the sublimation process, to facilitate the smooth transfer of heat from the shelf to the bottom of the container, it is common to inject gas into the drying chamber to change the pressure and form convection for heat transfer. The effect of this air volume on pressure changes forms the control of pressure. Therefore, finding a pressure state that can create appropriate thermal convection while maintaining a uniform drying rate on the product surface is a goal to pursue. The appropriate pressure control precision is usually determined by the product's process conditions. Generally, when the ice block is thicker, the pressure control requirements are lower, and the control precision can be lower; when the ice block is thinner, the pressure control requirements are higher, and the control precision is also higher. The conclusion of the validation test for this item should be evaluated based on the final product's appearance analysis and quality inspection results.
Verification of the vacuum condenser temperature: From the perspective of modern vacuum technology, the freeze-drying process has adopted the condenser as a vacuum pump to remove water vapor, hence it is referred to as a vacuum condenser. For this special type of vacuum pump, in addition to thermodynamic performance indicators, the internal structure and piping of the vacuum condenser are also factors that must be considered in equipment selection. Generally, during the sublimation drying phase of the freeze-drying process, the working temperature of the vacuum condenser is between -75～-50℃, and the normal working temperature is generally controlled at around -60℃. At this time, the product's temperature is between -35～-10℃. When the temperature is at -76℃, the vapor pressure of ice is as low as about 0.1 Pa, therefore, operating the vacuum condenser below -76℃ does not have practical significance for increasing the drying rate. The working temperature of the vacuum condenser directly affects the pressure inside the drying chamber and the drying quality of the final product, thus it needs to be determined through validation.
