Sterilization and Sterility Notes

Sterilization and Sterility

Learning Outcomes

  • Understand the importance of controlling microbial contamination for pharmaceutical products.
  • List different types of sterile dosage forms.
  • Discuss different sterilization methods and their applications.
  • Discuss sterility testing and define the sterility assurance level.

Sterile Pharmaceutical Products

  • Microbial contamination poses risks:
    • Reduces product efficacy.
    • Causes adverse effects in patients, potentially fatal.
    • Alters product appearance or composition.
  • Products must be sterile if they directly contact internal body fluids or tissues. This means they must be essentially free from contamination with microorganisms like bacteria, viruses, yeasts, and molds.

Sterile Dosage Forms

  • Examples of sterile dosage forms include:
    • Injections (small and large volume parenterals).
    • Topical ophthalmic and nasal medications.
    • Irrigation solutions.
    • Dialysis fluids.
    • Topical wound healing products.
    • Sterile devices (e.g., implants, syringes).

Injection Categories

  • Sterile dosage forms for injections can be categorized as:
    • Solutions ready for injection.
    • Soluble solid products that require dissolving before administration.
    • Suspensions ready for injection.
    • Insoluble solid products requiring suspension before administration.
    • Emulsions ready for injection.
    • Liquid concentrates requiring dilution before administration.
    • Microspheres.
    • Liposomes.

Terminal Sterilization

  • Product containers are filled and sealed under high-quality environmental conditions.
  • The product is sterilized in its final container.
  • Terminal sterilization is the preferred method due to the lower risk.
  • The sterilization process must be compatible with all components: drug, container/closure, and excipient.

Aseptic Processing

  • Each component (drug, excipient, container, and closure) is sterilized separately.
  • Final products are filled and sealed in an extremely high-quality environment.
  • This process involves:
    • Sterilizing the drug substance.
    • Sterilizing the container.
    • Sterilizing the closure.
    • Sterilizing the excipient(s).
    • Aseptic processing to yield a sterile drug product in a sterile final product within a sterile container, using a sterile closure, and sterile excipient.

Production Facilities

  • Sterile product manufacturing occurs within:
    • Cleanrooms.
    • Isolators.

Sterilization Methods

  • Sterilization is defined as the complete destruction or elimination of microbial life.
  • The choice of method depends on:
    • Compatibility of the process with the preparation (no significant adverse effects).
    • Successful validation of the process (lethal to resistant spores).
    • Whenever possible, terminal sterilization by heat in the final container is preferred.

General Sterilization Methods

  • Common sterilization methods include:
    • Heat sterilization (steam/moist heat, dry heat).
    • Gas sterilization.
    • Radiation sterilization.
    • Filtration sterilization.

Heat Sterilization

  • Sterilization occurs through the destruction of essential proteins in the cell.
  • The death of microorganisms depends on temperature and duration of heat exposure.
    • Higher temperatures require less exposure time.
  • Moist heat is more effective than dry heat.
    • Example: B. subtilis var niger spores resist dry heat at 121°C much longer than moist heat at the same temperature (nearly 2000 times).

Steam Sterilization (Autoclave)

  • Uses steam under pressure to raise the temperature above 100°C.
  • The heat of condensation releases significant energy, killing microbes.
  • Steam quality is critical: pure steam with no air or non-condensable gases, saturated but not super-heated.

Steam Sterilization Applications, Advantages, and Disadvantages

  • Used for terminal sterilization of:
    • Aqueous injections.
    • Ophthalmic preparations.
    • Irrigations.
    • Dialysis solutions.
    • Equipment used in aseptic processing, etc.
  • Advantages:
    • Most common and effective method.
    • More efficient than dry heat due to the lethal action of water and heat.
    • No toxic contaminants remain.
  • Disadvantages:
    • Not suitable for anhydrous preparations or those not penetrated by moisture.
    • Not suitable for thermo-labile substances.
    • Does not destroy pyrogens.

Dry Heat Sterilization

  • Carried out in ovens or tunnels using filtered air.
  • Heat transfer occurs through radiation, convection, and conduction.
  • Fans aid in air circulation.

Dry Heat Sterilization Applications, Advantages, and Disadvantages

  • Used for:
    • Glassware and containers used in aseptic manufacture.
    • Non-aqueous thermo-stable powders and liquids (oils).
  • Advantages:
    • Suitable for substances harmed by moisture.
    • Can destroy pyrogens on materials like glass.
  • Disadvantages:
    • Less efficient, requiring higher temperatures and longer times.
    • Not suitable for aqueous solutions.
    • Accurate control of process parameters is more difficult than with saturated steam.

Gas Sterilization (Ethylene Oxide)

  • Sterilization via exposure to ethylene oxide gas, a potent and highly penetrating agent.
    • Flammable when mixed with air; usually mixed with inert gas (e.g., CO2 or N2).
  • Mechanism: denatures proteins by replacing functional groups with alkyl groups.
  • Sterilization occurs in a chamber similar to an autoclave.
  • Kills all living microbes and their spores.

Gas Sterilization Applications and Conclusions

  • Used for:
    • Powders with microbes on the surface (not embedded).
    • Equipment, instruments, and devices made from plastic, rubber, metal, and other materials.
  • General Conclusions:
    • Less reliable and more expensive than steam sterilization.
    • Should not be used when steam sterilization is practicable.
    • Minimize microbial contamination of objects to be sterilized to increase reliability.

Radiation Sterilization

  • Achieved by exposure to radiation:
    • Gamma rays (most penetrative and effective).
    • Beta particles (accelerated electrons, less penetrative).
    • Ultraviolet light (low energy, surface sterilization).
  • Mechanism: Directly damages essential molecules (e.g., DNA or enzymes) in microorganisms via ionization or electron excitation.
  • Gamma sterilization is overall the preferred method.
  • Applications are similar to gas sterilization (e.g., powders and objects made from plastic, rubber, metal, and other materials).

Radiation Sterilization Advantages and Disadvantages

  • Advantages:
    • Insignificant temperature rise (approximately 4°C).
    • Can be used after packaging (no aseptic handling needed).
    • Reliable and accurately controlled.
    • Can treat dry, moist, and frozen materials.
  • Disadvantages:
    • High running costs.
    • Radiation hazards require caution.
    • May damage medications or packaging.

Filtration Sterilization

  • Filters function by:
    • Sieving.
    • Entrapment.
    • Electrostatic attraction.
  • Factors affecting sterilization:
    • Type of microbes.
    • Type of filter.
    • Temperature.
    • Type of fluid.
    • Pressure/vacuum applied.
  • Physically removes microbes by passing liquid or gas through filters with small pores.

Filtration Sterilization in Sterile Production, Advantages, and Disadvantages

  • Filtration is used to:
    • Remove particulate contaminants (including microbes) from liquids, air, and gas.
    • Achieve a sterile product (pore size of 0.2µm or less).
    • Reduce bio-burden for sterilization.
  • Advantages:
    • Used to sterilize heat-sensitive materials (e.g., vaccines, enzymes, antibiotics).
  • Disadvantage:
    • Filters do not remove all possible microbial contamination.

Microbial Death Kinetics

  • The death rate of microbes during sterilization generally follows first-order kinetics (logarithmic order).
  • Bioburden: Initial number of microbes prior to sterilization (y-intercept of the microbial death kinetic plot).

Survival Probability of Microbes

  • Shows a graph of survival count vs. survival probability, illustrating a 90% (1 log) reduction in count.

Sterility Assurance Level (SAL)

  • SAL describes the probability of a single unit being non-sterile after sterilization.

Sterility Levels for Different Products

  • Sterilization level varies according to the item's use.
  • A pharmaceutical product is considered sterile if the probability of survival of a microorganism is less than:
    • For Parenterals: 1 on 1,000,000 (SAL of 10^{-6}).
    • For Topicals: 1 on 1000 (SAL of 10^{-3}).
    • Plus, two other conditions: endotoxins and biomechanical properties.
  • The importance of sterilization is to reduce the original microbial population (bioburden) to reach a SAL of 10^{-6}. Higher bioburden requires a longer cycle time or higher temperature/dose.

Sterility Testing

  • Every batch of sterile products in their final containers should be tested for sterility.
  • Sterility testing has statistical limitations:
    • Performed on small, random samples.
    • Assumes samples are representative of the entire batch.
    • The number of containers and quantity of product to be tested are defined in the Pharmacopoeia.

Sampling for Sterility Testing

  • Minimum number of product units to be tested depends on the batch size:
    • ≤ 100 containers: 10% or 4 containers (whichever is greater).
    • 100 ~ 500 containers: 10 containers.
    • ≥ 500 containers: 2% or 20 containers (whichever is less).

Sampling for Sterility Testing (Quantity)

  • Minimum amount of product to be tested depends on the quantity in each product unit:
    • < 1 mL: Whole content.
    • 1 ~ 40 mL: Half of the content, but not less than 1 mL.
    • 40 ~ 100 mL: 20 mL.
    • > 100 mL: 10% of the content, but not less than 20 mL.

Sterility Testing Facilities

  • Sterility testing should be carried out under aseptic conditions:
    • In a cleanroom or isolator.
    • Operators should wear sterile garments.
    • Operators should be appropriately trained and validated.
    • Appropriate cleaning, sanitization, and disinfection procedures should be in place.
    • Environmental monitoring should be conducted.

Interpretation of Sterility Test

  • The products meet requirements if all media vessels incubated with product samples reveal no microbial growth.
  • A test may be repeated only if it can be demonstrated that the test was invalid for causes unrelated to the product.
  • The repeat test uses the same number of samples as the first test.
  • Any contamination detected in the repeat test means the product does not comply.

Microbial Control

  • Sterility: Absence of viable microorganisms in the product.
  • Pyrogens: Substances that can cause transient fever and, in severe cases, death due to shock. The primary concern is endotoxin.

How to Test for Endotoxins?

  • Rabbit Pyrogen Test (FDA approved in 1941).
  • LAL Test:
    • Gel clot LAL test licensed as a biological product in 1973.
    • FDA issues final Guideline on LAL testing (kinetic LAL) in 1987.
    • USP, EP, JP issue harmonized document for LAL testing in 2000.