When To Use Dry Heat Vs. Rinsing For Medical Device Depyrogenation
How to reduce endotoxin levels
Depyrogenation is a process that removes pyrogens. The most prevalent and problematic pyrogens are the bacterial endotoxins found in the outer cell walls of gram-negative bacteria. Endotoxin removal methods (also known as depyrogenation processes) either destroy or remove endotoxins. When looking into how to reduce endotoxin levels, depyrogenation by dry heat or rinsing are the top methods for medical devices. Endotoxin removal methods for sterile liquid products, like injectables, utilize filtration.
What are pyrogens?
Pyrogens are any molecules or substances that cause a feverous reaction when they enter the human body. Endogenous pyrogens (such as the cytokine interleukin-1) are found naturally within the human body. Endogenous pyrogens create a fever-producing reaction when naturally produced by the body. Exogenous pyrogens are molecules located outside of the body, such as endotoxins from gram-negative bacteria or pyrogenic prions. For context, prions are misfolded proteins that can transmit their misfolded shape onto regular versions of the same protein. Prions are of most concern for medical products and devices that interact with brain or spinal tissues. Exogenous pyrogens either provoke endogenous pyrogen production to create a fever within the body or activate the body’s toll-like receptors (TLRs) to trigger a fever. Clinically, the fever produced by endogenous cytokines is indistinguishable from fever produced by exogenous pyrogens such as lipopolysaccharide (LPS). Thus, it is important for medical device companies to know how to reduce endotoxin levels during medical product manufacture and endotoxin removal methods, like depyrogenation by dry heat.
Why is depyrogenation needed?
What is depyrogenation by rinsing?
Depyrogenation by rinsing is an endotoxin removal method where bacterial endotoxins are destroyed through exposure to warm pressurized water. If you are looking for how to reduce endotoxin levels for manufacturing equipment or metals, depyrogenation by rinsing is a great option.
Medical devices and parenteral products must be sterile and pyrogen-free. Products can accumulate pyrogens from raw materials or other parts of the manufacturing process. The best endotoxin removal methods or destruction processes (depyrogenation) depend on the product. Standard depyrogenation methods are depyrogenation by dry heat, rinsing, and filtration. Comparisons between endotoxin removal through filtration versus dry heat or rinsing can be found HERE and HERE, respectively.
What is depyrogenation by dry heat?
Depyrogenation by dry heat is the process of destroying endotoxins through exposure to high temperatures. Depyrogenation by dry heat is another great option for how to reduce endotoxin levels for manufacturing equipment that isn’t heat sensitive.
How is depyrogenation by rinsing performed?
Rinsing is the best means to remove bacterial endotoxins on closures (such as elastomeric stoppers), medical devices, and other materials that are not compatible with dry heat depyrogenation. The depyrogenation rinsing process is completed using high-purity water at temperatures above 60°C. Multiple rinse cycles remove pyrogens and ensure that the high-purity water is not contaminated with bacterial endotoxins during processing. Since rinse water purity is key to successful pyrogen removal, the rinse water quality must meet a minimum standard of less than 0.25 Endotoxin Units (EU) per milliliter. Poor water quality for rinsing may add pyrogens to medical devices and products at the same rate it removes them. Further, drying the items directly after rinsing is imperative to prevent microbial proliferation and post-rinsing endotoxin increases. Water for Injection (or other highly pure water) held for more than three or four hours below 55°C and above 8° should be considered a microbial contamination risk unless the water is sterilized. Solvents other than water, such as caustic alkali or detergents, can be used for depyrogenation by rinsing. However, these solvents may leave harmful residuals on products. Thus, if solvents other than water are used for rinsing, the depyrogenation validation must prove no toxic residuals are left on depyrogenated products.
For depyrogenation by rinsing, critical factors to define and control during processing are:
- Solvent description (including detergent description if used and the source of high-purity water)
- Solvent temperature
- Solvent pressure
- The flow rate of the solvent through the system
- Solvent recirculation (justification if used)
When removing endotoxins from a product via rinsing, the established endotoxin limit must not be exceeded by the sum of the endotoxin components of the active pharmaceutical ingredient (API) and any contributions by the excipients or other additives for parenteral products, pharmaceuticals, and other formulation products. For depyrogenation, the goal is often a 3-log reduction in recoverable endotoxin for the product. However, where a 3-log reduction is not required, inherent endotoxin burden and calculated endotoxin contamination risk can be used to justify depyrogenation processes that provide less than a 3-log reduction in pyrogens. The log reduction is calculated using the following formula:
Log10 reduction = (log10 recoverable activity) − (log10 residual activity)
How is depyrogenation by dry heat performed?
Dry heat is commonly used for the depyrogenation of heat-stable materials. Dry heat depyrogenation depends upon time and temperature. Depyrogenation by dry heat processes are performed at temperatures ranging from 250°C up to about 400°C. Understanding the total thermal input makes it possible to predict the depyrogenation efficacy of dry heat processes at various times and temperatures. Since bacterial endotoxins are more resistant to the effects of dry heat than bacterial spores, depyrogenation methods also sterilize the materials they depyrogenate.
Dry heat depyrogenation uses air first to heat and then to cool items. Due to the heat capacity of dry air, loaded objects are slowly heated and cooled during dry heat treatment. Items in the dry heat ovens must be placed in the exact locations every time for depyrogenation cycles to be valid due to the limited heat capacity of air. Indeed, varying load mass and product distribution can result in dry heat processing variability. Dry heat depyrogenation, like dry heat sterilization, uses a combination of temperature sensors and thermocouples to regulate temperature and dwell time to the levels needed to kill the endotoxin load on incoming materials. By convention, the z-value for dry heat depyrogenation ranges from 45°–55°C. This z-value is the rate at which the depyrogenation destruction rate varies as a function of temperature.
The dosimetric measurement for dry heat depyrogenation processes is measured by FD units. An FD of one is defined as the depyrogenation effect achieved by one minute of heating at 250°C. FD calculations are used to compare dry heat depyrogenation effects during processes that have varying temperatures. A sum of the instantaneous temperature contributions over the entire depyrogenation process will calculate the overall process efficacy (in FD ). The process efficiency can be calculated using the process start time, process end time, temperature at each time increment, and the time interval between temperature measurements. Commercial software enables companies to calculate FD of processes by integrating the total FD accumulated during a treatment. FD calculations are used during validation, validation maintenance, and any change control to the process.
When to use depyrogenation by dry heat?
If the materials from a medical device or product can withstand high 250°C or above temperatures without losing functionality, dry heat depyrogenation is recommended.
When to use depyrogenation by filtration?
Depyrogenation filtration processes are for liquids (e.g., solutions, suspensions, emulsions, and the like). Nearly all solutions containing proteins and peptides have their pyrogens removed by filtration.
Summary
Depyrogenation is a process that removes pyrogens. Testing for pyrogens is an imperative safety metric for regulatory approval of a medical device or product. The most prevalent and problematic pyrogens are the bacterial endotoxins found in the outer cell walls of gram-negative bacteria. Depyrogenation by dry heat is the process of destroying bacterial endotoxins through high heat exposure. In contrast, depyrogenation by rinsing is the process of removing bacterial endotoxins through exposure to warm, pressurized water. Dry heat is commonly used for the depyrogenation of heat-stable materials, as depyrogenation processes are performed at temperatures ranging from 250°C up to about 400°C. As a bonus, dry heat depyrogenation also sterilizes the materials they depyrogenate. Rinsing is a common depyrogenation method for medical products and other materials that cannot undergo dry heat depyrogenation. All in all, ensure you choose a contract testing organization that can provide appropriate bacterial endotoxin testing and depyrogenation for your product needs.
Ethide Labs is a contract testing organization specializing in Bacterial Endotoxin Testing and Sterilization Validations. Ethide Labs also offers Microbiology Testing, Bioburden Testing, Sterility Testing, EO Residual Testing, Cytotoxicity Testing, Environmental Monitoring & Package Integrity Testing services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
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International Organization for Standardization. Sterilization of health care products- Moist heat- Part 1: Requirements for the development, validation, and routine control of a sterilization process for medical devices. Geneva (Switzerland): ISO; 2006. (ISO 17665-1:2006/(R)2016).
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
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