6.1 Terminal Sterilization vs. Sterilizing Filtration

Sterilization Modalities in Sterile Compounding

Sterilization is the validated process by which all viable microorganisms (including bacterial vegetative cells, fungal spores, and bacterial endospores) are eliminated or destroyed. USP General Chapter <797> treats terminal sterilization in the final sealed container as the preferred method whenever the CSP and its container-closure system can tolerate it. Sterilizing filtration is used for heat-labile preparations. Timing matters too: an injectable CSP that contains nonsterile components, or touches nonsterile devices, must be sterilized within 6 hours after preparation is completed, before bacteria can multiply and shed endotoxin. The SOPs must describe each cycle (temperature, pressure, duration, permitted loads), how biological indicators and endotoxin challenge vials are used, and how personnel are trained.

Sterility Assurance Level (SAL)

Sterilization processes are designed to achieve a Sterility Assurance Level (SAL) of $10^{-6}$, meaning that the theoretical probability of a single surviving viable microorganism in or on a sterilized unit is less than one in one million ($1\text{ in }1,000,000$).

Terminal Sterilization Methods

1. Saturated Steam Sterilization (Autoclaving)

• Mechanism: Denatures and coagulates essential cellular proteins and microbial enzymes through moist heat transferred via condensing saturated steam under pressure.
• Standard Cycle: Saturated steam at $121^\circ\text{C}$ ($250^\circ\text{F}$) and $15\,\text{psi}$, for a time proven with biological indicators. USP <797> gives 20 to 60 minutes as an example, depending on the volume or size of the CSP. Solutions are passed through a filter of 1.2 µm or smaller before filling, sealed containers must be able to generate steam internally, and a calibrated recorder documents each cycle.
• Physical Parameters:
  • $D$-Value (Decimal Reduction Time): The time in minutes required at a specific temperature to reduce a microbial population by 90% (1-log reduction).
  • $Z$-Value: The temperature change (in degrees Celsius) required to change the $D$-value by a factor of 10.
  • $F_0$ Value: The equivalent lethality delivered to a container at $121^\circ\text{C}$. A minimum $F_0$ of 8 to 15 minutes is standard for terminal autoclave cycles.
• Biological Indicator (BI): Spores of Geobacillus stearothermophilus ($10^6$ spores per carrier), an extremely heat-resistant thermophilic spore-former.

2. Dry Heat Sterilization & Depyrogenation

• Mechanism: Kills microbes through cellular protein oxidation and irreversible macromolecular desiccation.
• Depyrogenation: Endotoxins (lipopolysaccharides from Gram-negative bacterial walls) are remarkably heat-stable and survive standard autoclaving. Destroying endotoxins requires dry heat depyrogenation:
  • Typical Cycle: Depyrogenation ovens commonly run at about $250^\circ\text{C}$. Dry heat sterilization, which is a different goal, is usually done at $160^\circ\text{C}$ or higher.
  • Depyrogenation Standard: The cycle must be shown, initially and at least annually, to achieve at least a 3-log (99.9%) reduction in endotoxin using Endotoxin Challenge Vials (ECVs). It must be re-established whenever the load, duration or temperature changes. Items that cannot take the heat are depyrogenated by repeated rinsing with sterile, nonpyrogenic water.
• Biological Indicator for dry heat sterilization runs: Spores of Bacillus atrophaeus (formerly Bacillus subtilis), used with every run or load.

Sterilizing Filtration for Heat-Labile Preparations

When a drug is thermolabile (e.g., proteins, peptides, heat-sensitive antibiotics, ophthalmics), terminal heat sterilization causes chemical breakdown. In these cases, the solution must be passed through a sterilizing-grade membrane filter into a sterile container inside an ISO Class 5 PEC.

Filter Specifications under USP <797>

• Pore Size: Must possess a nominal pore rating of $0.22\,\mu\text{m}$ or $0.2\,\mu\text{m}$.
• Microbial Challenge Retention: Certified to retain a minimum challenge of $10^7\text{ microorganisms per square centimeter}$ of effective filter area of Brevundimonas diminuta (ATCC 19146), an unusually tiny bacterium ($0.3\,\mu\text{m} \times 0.9\,\mu\text{m}$).
• Membrane Compatibility: Filter membranes (PES, PVDF, PTFE, nylon) must be selected based on drug compatibility, low protein binding, and surfactant tolerance.

Mandatory Post-Filtration Filter Integrity Testing

A sterile filter membrane can rupture, deform under excessive differential pressure, or contain micro-pinholes invisible to the naked eye. Therefore, USP <797> establishes an absolute mandate:

The Bubble Point Test

1. The used filter membrane is wetted with the compounding liquid (or sterile water).
2. Upstream gas (air or nitrogen) pressure is gradually increased.
3. The Bubble Point Pressure is the specific pressure at which gas overcomes the surface tension of the fluid filling the membrane pores, producing a continuous stream of bubbles downstream.
4. Pass/Fail Standard: The measured bubble point must meet or exceed the minimum the filter manufacturer states for that membrane and wetting fluid. If several filters are used in one process, each must pass.

Protocol Following a Failed Filter Integrity Test

If the filter fails the bubble point test, sterility of the filtrate cannot be assured, and the CSP cannot be released as it is. USP <797> requires it to be discarded, or refiltered for sterilization no more than one additional time after the cause is investigated and an appropriate filter is chosen. The new filter must also pass integrity testing. If a CSP is known to carry heavy particulate matter, a larger-pore prefilter (for example, 1.2 µm) goes upstream of the sterilizing filter.

Thermal Lethality Kinetics: D-Value, z-Value, and F0 Modeling

Terminal moist heat sterilization cycles are engineered using mathematical kinetic models to ensure reproducible destruction of heat-resistant microbial endospores without degrading thermolabile formulation components:

Mathematical Principles of Microbial Inactivation

1. D-Value (Decimal Reduction Time): The time required in minutes at a specified temperature to reduce the surviving microbial population by 90% (a one-log reduction). For Geobacillus stearothermophilus in neutral aqueous media at 121°C, D-values typically range from 1.5 to 2.0 minutes.
2. z-Value: The temperature change (in degrees Celsius) required to change the D-value by a factor of 10. For steam sterilization, the standard compendial z-value is 10°C.
3. F0 Value (Equivalent Lethality): The integrated equivalent exposure time in minutes at 121.1°C (250°F). Cycles are designed around an F0 target, commonly 8 to 15 minutes, and then verified with biological indicators to support a Sterility Assurance Level (SAL) of $10^{-6}$.

Overkill vs. Bioburden-Based Sterilization Cycles

• Overkill Cycle: Delivers an F0 of at least 15 minutes, achieving an at least 12-log reduction of resistant endospores regardless of initial bioburden.
• Bioburden-Based Cycle: Used for heat-sensitive drug entities, calibrated to deliver an SAL of $10^{-6}$ based on pre-sterilization bioburden enumeration and resistance profiling.