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The Different Types of Pharmaceutical Impurities
Identifying and quantifying pharmaceutical impurities in active pharmaceutical ingredients (API’s) and drug substances is essential for drug safety and efficacy, as well as being a regulatory requirement for drug registration.
What are the different kinds of pharmaceutical impurities, how does their presence occur in a drug substance and how are they identified and quantified?
Organic Impurities
The first kind of impurities to consider are organic impurities, these impurities include unreacted intermediates, by-products, degradation impurities, residual reagents and organic reagents and catalysts.
Apart from residual reagents and catalysts, these kinds of impurities are structurally related to the drug substance. By-products and unreacted intermediates are formed during the synthesis of the drug substance through unwanted side reactions, or incomplete reaction, while degradation impurities are formed during storage of the drug substance.
In a chemical synthesis reaction, it is often the case that a reaction won’t fully go to completion, meaning that at the end of a reaction there is a mixture of the desired material, unwanted reaction products, unreacted starting materials and reagents. Work up and purification of the finished reaction mixture is designed to remove unreacted starting materials, unwanted products and any other unwanted reagents/solvents/catalysts leaving the desired product. However, it is not always possible to completely remove unwanted reaction products or unreacted intermediates, and these can get carried through to the next reaction or into the final product.
The extent of formation of by-products during a reaction, and the quantity of unreacted intermediates remaining after a reaction has been completed, is affected by changes in the conditions during a reaction, so it is essential that conditions are tightly controlled during a reaction.
Impurities present in raw materials can also be carried through the manufacturing process into the final product, so it is essential that drug manufacturers have strong control of the quality of bought in raw materials, and that the impurity specification of these bought in raw material is tightly controlled and monitored, this is where QC/QA testing plays a crucial role.
Furthermore, unreacted intermediates resulting from one reaction step can go on a react in the next step in the manufacturing route under the new reaction conditions giving the potential to form new process impurities.
The most common process-related impurities are listed in a drug’s monograph, which also contains methods for identifying and quantifying these impurities, usually by HPLC or GC against reference standards. During the drug development process, impurities are identified and quantified using mass spectrometry techniques.
Degradation impurities are formed during storage of the drug substance. The inherent stability of a drug substance will affect the formation of impurities. Stability testing studies are required to be carried out to identify the structures of the impurities being formed, and at what rate they are being formed.
Inorganic Impurities
Inorganic materials such as catalysts, reagents, heavy metals, salts and charcoal can find their way into a drug substance because of the manufacturing process.
Processes that use heavy metal containing catalysts in their synthesis route are of particular concern as these can leave residual heavy metals in the finished product. Inorganic salts can be used as catalysts in some reactions, as well as in the work up of a reaction, but these are water soluble and so should be relatively easy to remove.
Charcoal, used as a support media for some catalysts, as well as in decolourisation, can be present in residual amounts and is removed by filtration. All inorganic impurities can be identified and quantified by pharmacopeial standards.
Residual Solvents
As per ICH guidelines, solvents are classified as one of three classes with respect to their presence in a drug substance.
Class one is for those solvents that cannot be present at all, these include solvents with high toxicity or high carcinogenicity e.g. benzene or 1,2-dichloroethane.
Class two is for those solvents that have potential toxicity and the use of which should be limited. As such, there are limits placed on the presence of these in drug substances – e.g. methanol, a toxic solvent, has a concentration limit of 3,000ppm.
Class three is for solvents with no significant toxicity risks, although there are no limits placed on the concentration of these solvents in a drug substance, their presence should still be limited as much as possible.
Working with FB Pharmtech
Identifying and controlling pharmaceutical impurities is critical for ensuring drug safety and quality. Whether dealing with organic impurities, inorganic materials, or residual solvents, maintaining strict control throughout the synthesis and manufacturing process is essential.
If you need assistance in synthesising or sourcing pharmaceutical chemicals with precision, at FB Pharmtech, we are here to help. Contact us to discuss your requirements and how we can support your efforts to meet regulatory standards and deliver safe, effective products.