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Stability test (pharmacy)
English: stability testing
A Stability test is used to determine the stability and shelf life of a drug. It is carried out as part of drug development as a standard.
Medicines are subject to chemical, physical and microbial decomposition. Decomposition leads to a decrease in the concentration of the active substance. This reduces the effectiveness.
A shelf life of 5 years is aimed for, within which the active ingredient content of the drug must not fall below 90% of the initial value.
The shelf life depends on the storage and environmental conditions and thus differs in different regions of the world. In order to standardize these, the International Committee of Harmonization (ICH) has defined four climate zones, which are characterized in the following table:
|Zone||characterization||average kinetic temperature in ° C||relative humidity in% rh|
Zone 1 corresponds to northern European countries and zone 2 to southern European countries and the USA. In the local latitudes, drugs for zone 2 are being developed. The division into climatic zones is relevant for carrying out the stability test, as these define the storage conditions.
During the storage period, different types of instabilities can occur which affect the shelf life:
Stabilization is usually achieved through the addition of auxiliaries (e.g. antioxidants, buffers, thickeners, preservatives and others).
In the context of drug development, it is not advisable to store the drug for the desired shelf life (5 years) and then to check the concentration of the active ingredient, as this would delay the approval process.
Therefore, a method was developed with which the desired minimum shelf life can be confirmed without the drug being stored for the entire period.
Short-term stability tests ("stress tests", duration of 4 months) and long-term stability tests (duration of 2 years) are carried out, whereby the results of the long-term tests can also be submitted after approval has been granted. The investigations take place at temperatures higher than those defined in the climatic zones, in order to then extrapolate them to the defined temperatures. The drug is therefore exposed to "stricter" conditions in order to compensate for the shorter storage period.
In stability tests, the active ingredient concentration acts as a measured variable, which, according to the definition, must not fall below 90% of the initial value during the shelf life. It is a measure of chemical and most physical instabilities.
The degradation reaction is viewed as a pseudo first order process. The time at which there is only 90% of the initial value in the drug can be calculated using the following equation (the derivation is analogous to the half-life):
|t90% = 0.105 / k|
| t90% = Time at which 90% of the initial concentration is undershot|
k = rate constant
In order to calculate the shelf life, the speed constant must be determined. The Arrhenius equation is used as the basis for this:
|k = A * e-EA./ R * T|
k = rate constant
E.A. = Activation energy
R = general gas constant
T = absolute temperature
The drug is measured at higher temperatures and extrapolated to a lower temperature.
The measurement takes place at four temperatures; the velocity coefficient is determined for each temperature by means of kinetic measurements. According to the Arrhenius equation, these speed coefficients are plotted in a graph of log (k) versus 1 / T. A regression line is determined, which is extrapolated to a temperature of 25 ° C (climate zone 2). The rate constant for a temperature of 25 ° C. is obtained, which in the above equation for t90% is used to determine the shelf life.
In the case of microbiological examinations, a culture is created in order to determine the microbial contamination.
There are also special stability tests:
- Rocking test: With some dosage forms (e.g. ointments, suppositories), warm temperatures cannot be used because they would melt. As part of the swing test, these are exposed to periodically changing temperatures (e.g. between 5 and 35 ° C)
- Oxidation-sensitive drugs can be irradiated with a xenon lamp in order to check the success of light protection measures (e.g. amber glass bottle) and the lightfastness.
- Emulsions and suspensions can be exposed to mechanical stress during storage. In the case of solid dosage forms, abrasion tests can be carried out.
5 follow-up stability
Even after approval has been granted, samples are regularly taken from different batches and stored and evaluated according to the procedure outlined above. This procedure is used to map and guarantee the stability of all batches.
Bauer, Frömmig, Führer: Pharmaceutical Technology. With an introduction to biopharmacy. 10th edition, Stuttgart 2017
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