ETO – Sterilization process for pharmaceutical products
Ethylene oxide (ETO) is a colorless liquid (it is liquid at temperatures below 10.8oC).ETO has been widely used as a low-temperature sterilant. It has high penetrating power and passes through and sterilizes large packages of materials including plastics. It is most commonly used to sterilize medical and pharmaceutical products that cannot support conventional high temperature steam sterilization or materials that may be deteriorated by radiation sterilization.
Mode of action – The microbicidal activity of ETO is considered to be the result of alkylation of protein, DNA, and RNA. Alkylation, or the replacement of a hydrogen atom with an alkyl group, within cells prevents normal cellular metabolism and replication.
ETO gas boiling point is 10.73 ºC at atmospheric pressure and is highly explosive when pure. Normally the ETO used for gas sterilization is mixed with diluents such as carbon dioxide or nitrogen in different proportions from 10 % ETO to 90 % ETO to eliminate explosive tendency.
Critical process parameters – Effective control of ethylene oxideconcentration, temperature humidity and exposure time .
Hazards of ETO
ETO gas is highly reactive, explosive, carcinogenic and mutagenic
In addition to eye pain and sore throat, exposure to ETO can cause difficult breathing and blurred vision.
Exposure can also cause dizziness, nausea, headache, convulsions and can result in vomiting and coughing.
ETO is a carcinogen that may cause leukemia and other cancers
ETO is also linked to spontaneous abortion, genetic damage, nerve damage, pheripheral paralysis, muscle weakness.
In liquid form ETO can cause severe skin irritation upon prolonged or confined contact.
Critical process parameters for ETO sterilization – Effectivecontrol of ethylene oxide concentration, temperature and moisture.
ETO Processing steps
1. Environmental Preconditioning
Most of the EO sterilization processes starts with conditioning of the products to be sterilized in the sterilization chamber or a separate room. Preconditioning is usually performed in a room which has been specially designed to heat and humidify the products to a stable internal temperature and moisture content prior to entering the chamber. This will assure that the sterilization process is reproducible regardless of external influences such as varying climatic conditions.
- Once preconditioning is complete, the products are placed in a heated chamber which has been designed to withstand the extreme pressures realized when delivering the sterilization process.
2. Initial Evacuation
To safely deliver the 100% ethylene oxide process, at least 97 percent of the air must be removed from the chamber. Two most common methods of accomplishing this requirement are (1) pulling a deep vacuum, or (2) performing a series of partial vacuums followed by a series of nitrogen injections. This combination, when performed using an adequate number of repetitions, will purge (remove) the air, thus allowing the process to be performed safely.
During the previous preconditioning step, heat and moisture were added to the product to a predetermined or stable condition. When the initial evacuation phase of the process is performed, the product can lose a significant amount of moisture. This moisture must be replaced prior to introducing the ethylene oxide. This is accomplished by adding humidity in the form of steam injections. The amount of steam required is calculated to yield a predetermined relative humidity. After the addition of steam, the product is allowed to dwell or soak for the amount of time required to replace the moisture lost from the evacuation phase.
4. Gas Injections and Gas Dwell
After the humidification phase, liquid ethylene oxide is first heated into a gaseous phase, then injected into the chamber. The amount of gas or gas concentration is dependent on two primary factors which are addressed during cycle design.
The most important factor is to assure that the minimum gas concentration required to achieve sterility within the product is attained. This minimum concentration must be balanced against the second factor, which is the maximum amount of gas that can be injected before difficulties arise due to high levels of post-sterilization EO residuals.
After the gas has been injected, the exposure phase of the process is performed. This is the phase in which the product is exposed to heat, relative humidity, and gas for a predetermined amount of time. As a rule of the thumb, the more difficult the product is to sterilize, the longer the exposure time. The amount of exposuretime is determined by the process design scientist, after careful analysis of the product, load configuration, and desired level of sterility. Preliminary laboratory experiments may be needed prior to validation execution.
5. Post exposure Gas Purge and Air Inbleed
After the exposure phase of the process, all gas must be removed from the chamber until the levels of EO fall below the flammable limit for the gas (3 percent or 30,000 ppm). This is accomplished by performing a series of post-vacuums, each followed with a nitrogen backfill (wash).
A maximum working pressure for the washes is selected by the process design scientist to assure that the products, which may have been softened during the exposure phase of the process, will not be damaged. An ample number of washes is performed to reduce product residues and facilitate safe handling of the product after processing.
To reduce the amount of residence time in the vessel, products after sterilization are usually placed in a heated room for additional removal of the residual gases. The rooms are maintained at elevated temperatures and the out gassed residues are continuously removed from the room and scrubbed. The aeration rooms help contain any airborne EO and continually reduce the in-product residues.
ETO Processing steps
Preconditioning/conditioning - Precondition to a specified RH and
Sterilization cycle - Exposure to ETO gas
Aeration - Dissipation of remaining gases
Typical ETO treatment conditions:
Temperature between 30°C and 60°C
Relative humidity above 30%
Gas concentration between 200 and 1000 mg/L
Exposure time of 2 to 10 hours
The validation of EO sterilization processes, which includes physical and microbiological performance qualification, is described in detail in ISO 11135 and European Norm (EN) 550.Following tests to be performed as a part validation.
1. Bioburden measurement
2. EO residual measurement
3. One fractional cycle
4. 3 Half cycle
5. One full cycle
Half Cycle Qualification
This method involves determination of the minimum time of exposure to Ethylene oxide, with all other process parameters except time remain constant, at which there are no survivors. Two further experiments should be performed to confirm the minimum time. All three trials should show no growth from the biological indicators. The specified exposure time should be at least double this minimum time. In each cycle biological indicators containing about 106 viable spores of Bacillus atrophaeus complying with AAMI/ISO-11138 shall be used. A minimum of 3 successful half – cycle tests shall be performed for the load configuration to show that the process and subsequent results are repeatable. The half cycle will demonstrate that the selected minimum time of exposure to ETO during the sterilization process will repeatedly achieve 100% kill of the BI located with the load. This time then will be double to produce the exposure time for routine processing of the product load.
The biological indicators shall be retrieved after the sterilization cycles and incubated at 30 – 350 celsius for 48 hours. 2 unprocessed biological indicators shall be incubated as positive controls. Sterility testing shall be carried out after each half cycle. The samples should pass the sterility test.
One full cycle of twice the exposure time of half cycle shall be conducted to confirm the exposure period. The biological indicators shall be retrieved after the sterilization cycles and incubated at 30 – 350 celsius for 48 hours. The samples should pass the sterility test.
One 1/6th cycle shall be performed to demonstrate that BI has resistance greater than or equal to the natural product resistance. The cycle is intended to achieve partial positive and partial negative results.
Advantages of ETO sterilization are:
- Low temperature
- High efficiency – destroys microorganisms including resistant spores
- Large sterilizing volume/ chamber capacity
- Non corrosive to: plastic, metal and rubber materials
- Excessively Long cycle
- Safety concerns - carcinogenic to humans
- Toxicity issues - toxic residues on surgical instruments andtubing
- Not recommended for flexible scope
- ETO is flammable
- Requires special room conditions, safety equipment and separate ventilation system
- Relatively high annual costs for maintenance and consumables
Facts to be remembered about ETO sterilization
ETO gas is highly reactive, explosive, carcinogenic and mutagenic
ETO reacts with cellular proteins and nucleic acids
B. atrophaeus is the recommended biological indicator ETO sterilization
Process variables for ETO sterilization include pressure, ethylene oxide concentration, temperature, humidity and exposure time.
Due to flammability ETO is often diluted with fluorocarbon gases or carbon dioxide.
ETO sterilization is the least aggressive form of the sterilization, often resulting in no change in properties or appearance
ETO sterilization is conducted at temperatures in the 50 -600C range.
ETO is absorbed by many materials. For this reason, following sterilization the item must undergo aeration to remove residual ETO.
As a part of ETO validation a microbial challenge must be performed to demonstrate adequacy of the processes to achieve the desired sterility assurance level.
In view of the known positive potential of ethylene oxide for genotoxic carcinogenicity, it is recommended that use is acceptable only when pharmaceutically absolutely necessary, and then residual ethylene oxide in the product should not exceed a limit of 1 ppm (EMEA guideline)
All BI from the half and full cycle should show no growth on incubation.
The BI used for positive control and some BIs in the fractional cycle should show positive growth.