Paul (stability testing and HPLC) Flashcards
Specifications
It is important that a product remains within these specifications:
- Physical
- Chemical
- Microbiological
- Therapeutic
- Toxicological
What is stability testing used for?
To:
- Provide evidence for how the quality of the product varies with time
- Establish a shelf-life for the product
- Determine recommended storage conditions
- Determine the suitability of containers
Factors influencing the chemical stability (rate of reaction)
- Temperature
- Light
- Oxygen
- Moisture content
- pH
- Ionic strength
- Excipients
Why is stability testing necessary?
- Chemical degradation of the product lowers the active concentration of the drug
- Toxic degradation products may be formed
- Economic considerations- longer half life is better for manufacture and storage
- Patient/pharmacist assurance
- Legal requirement
ICH (international’s council of harmonisation) guidelines
Long term testing: the ambient study must be continued for 12 months beyond 12 months of the shelf life
Accelerated stability testing: must be >15C above ambient storage conditions
Frequency of testing requirements
Accelerated stability testing
- Product development
- Predict the shelf life of a product by accelerating the rate of decomposition, by increasing the temperature
- Can predict the shelf life quickly
- Common variables are temperature, humidity, light
Temperature
To calculate the rate constant, half life, and t90 for a first order reaction (exponential)
- Plot log percentage remaining vs time
- Slope = -K1/2.303
- t50 (half life) = 0.693/K1
- t10 = 0.152 x t50 (allowed as long as the products aren’t toxic
Arrhenius equation
Explains the effects of temperature on a rate of reaction.
For every 10C rise in temperature the speed of reaction increases abut 2-3 times.
We can calculate K values (rate constants) at several higher temperatures.
Plot logK against 1/T
Determine K and hence shelf life at ambient temperature.
Gradient m = -Ea/2.303R
Intercept = logA
Estimation of k (rate constant)
Several accelerated storage temperatures e.g. 70C, 60C, 50C, 40C.
Concentrations of drug determined over time.
Graphs drawn/equations of line determined.
Order of reaction identified.
All first order reactions (straight lines).
K values determined from slopes (for each temperature).
Assays for stability testing
ICH guidelines 1998:
Validated quantitative methods must be used that can detect changes with time in the chemical, physical or microbiological properties of a drug substance and drug product, and that are specific so the contents of the active ingredients, degradation products and other components of interest can be accurately measured without interference.
Stability- indicating assay method (SIAM)
- Forced decomposition studies under a variety of conditions e.g. pH, light, oxidation, heat, humidity, and separation of drug from degradation products
- chromatographic assays- requirement for separation of multiple components during analysis of stability samples
- HPLC (high performance liquid chromatography) - widely used
Thin layer chromatography (TLC)
- Solid liquid chromatography.
- Apparatus: glass tank, TLC plate (polar), sorbent, origin line, (non-polar) solvent
- Identification based on distance travelled and compared to a reference
- Rf equivalent to k’
- Rf = a/b where a is distance travelled by solute and b is distance travelled by solvent
High Performance Liquid Chromatography (HPLC)
- Solid liquid chromatography with analyte stationary phase interaction
- Retention and elution depends on analyte’s interaction with both plates
- Apparatus: solvent reservoir, columns, detectors, high pressure pump, injector
- Low performance LC
- Poor system efficiency
- Large particle size
- Broad peaks
- High limit of detection
- Long separation time
- High performance LC
- Good system efficiency
- Small particle size
- Narrow peaks
- Low limit of detection
- Shorter separation time
- Polar phase and non-polar phase go through detector at slightly different times
Chromatograms
- Chromatographic peaks represent a separated compound
- The amount of time between the injection of the sample and its elution from the column is known as the retention time
- The retention factor (K’) is often used to describe the migration rate on a column.
- The retention factor for analyte A is defined as: k’A = tR / tM. Ideally this is between 1 and 5
Separation of 2 compounds
The resolution of a separation is a measure of how well two peaks can be differentiated. It is defined as the difference in retention times between two peaks, divided by the combined widths of the elution peaks
