The terms Related Substances and Impurities are closely related but are not always synonymous. In pharmaceutical quality control, impurities is the broader term, while related substances refers to a specific subset of impurities, particularly those structurally related to the drug substance.
EP has defined it as,
"Related substances are known impurities which may be identified or unidentified. They include intermediates and by-products from synthetically produced organic substance, co-extracted substances from natural products and degradation products of the substance.
This definition does not include other Impurities i.e. residual solvent or water/moisture or inorganic impurities or residues from cells and micro-organism or culture media used in a fermentation process.
"Pharmaceutical Impurities" are the unwanted chemicals (known or unknown) that remain with the active pharmaceutical ingredients (APIs), and/or finished products, or dveloped during manufacturing of APIs and/or finished products.
Types of Impurities
Impurities can be classified into:
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Organic impurities
- Starting materials
- Intermediates
- By-products
- Degradation products
- Related substances
-
Inorganic impurities
- Reagents
- Catalysts
- Filter aids
- Heavy metals
- Salts
-
Residual solvents
- Organic solvents remaining after manufacturing
-
Elemental impurities
- Lead (Pb)
- Cadmium (Cd)
- Arsenic (As)
- Mercury (Hg)
-
Nitrosamine impurities
- Potential mutagenic impurities
It is correct to say that all related substance are impurities but all impurities are not related substances.
Read also: An Overview of Pharmaceutical Impurities
