An impurity specification is only as credible as the standards used to establish it. When a generic manufacturer files an Abbreviated New Drug Application (ANDA), or an API maker prepares a Drug Master File (DMF), the impurity profile of the drug substance and product must be identified, quantified and justified. Impurity reference standards make that possible. This article explains the types of impurities, the thresholds that drive the work, what characterisation a standard needs, and how standards are used in practice.
Types of pharmaceutical impurities
ICH Q3A, the guideline on impurities in new drug substances, groups impurities into three categories, and the same logic applies to generics:
- Organic impurities: starting materials, by-products, intermediates, degradation products, reagents, ligands and catalysts. These are the main focus of impurity standards.
- Inorganic impurities: reagents, ligands, catalysts, heavy metals or other residual metals, inorganic salts, filter aids. Elemental impurities are addressed by ICH Q3D.
- Residual solvents: controlled under ICH Q3C.
Organic impurities may be further described as process-related (arising from synthesis) or degradation products (arising during storage or formulation). Mutagenic impurities, including nitrosamines, fall under ICH M7 and much lower limits.
The thresholds that drive the work
ICH Q3A sets thresholds for drug substances based on maximum daily dose:
| Maximum daily dose | Reporting | Identification | Qualification |
|---|---|---|---|
| ≤ 2 g/day | 0.05% | 0.10% or 1.0 mg/day intake, whichever is lower | 0.15% or 1.0 mg/day intake, whichever is lower |
| > 2 g/day | 0.03% | 0.05% | 0.05% |
ICH Q3B sets analogous, dose-dependent thresholds for degradation products in drug products. Above the identification threshold, the structure of an impurity should be determined. Above the qualification threshold, its safety should be justified. For generics, an impurity observed at a similar level in the reference listed drug is generally considered qualified, which is one reason comparative impurity profiling against the reference product is so important.
What an impurity reference standard is
An impurity reference standard is a sample of the impurity with confirmed structure and known purity. Official pharmacopoeial impurity standards exist for many specified impurities, but they are often available only in milligram quantities, and many impurities, particularly degradation products and NDSRIs, have no official standard at all. Laboratories therefore rely on characterised standards obtained from specialist suppliers or made by custom synthesis.
Characterisation: what the data package should contain
The credibility of a standard rests on its characterisation. A typical package includes:
- ¹H NMR consistent with the structure, and often ¹³C NMR, DEPT or two-dimensional experiments (COSY, HSQC, HMBC) where assignment is not straightforward
- Mass spectrometry, preferably high resolution, confirming molecular formula
- IR spectrum for functional-group confirmation
- HPLC chromatographic purity, with the method and wavelength stated
- Water content by Karl Fischer or TGA, and residual solvents where relevant
- Assigned purity or potency, often by mass balance or quantitative NMR
- For chiral impurities, chiral HPLC or specific rotation
- Storage conditions and a retest date
Area-percent HPLC purity alone is not enough for quantitative use. If a standard is 97% by HPLC area but contains 3% water and 1% residual solvent, its content is closer to 93%, and using 97% would bias every result.
How impurity standards are used
Method development and specificity
Standards are injected individually and as mixtures to confirm that the analytical method separates each impurity from the API and from each other. Spiking studies show that the method can detect each impurity in the presence of the matrix.
Relative response factors
Many methods quantify impurities against the API peak using a relative response factor (RRF), which corrects for differences in detector response. RRFs are determined using characterised impurity standards, typically from the ratio of calibration slopes.
Validation
During validation, standards establish limit of detection and quantitation, linearity, accuracy by recovery from spiked samples, and precision near the specification limit.
Identification in routine testing
Standards or system suitability mixtures confirm peak identity by retention time and, where used, by spectral match.
Standards in ANDA and DMF submissions
An ANDA or DMF typically includes the specification for each specified impurity, the analytical procedure, validation data and a justification of limits. Reviewers expect to see how impurity standards were obtained and characterised. Common deficiencies include missing structural data for a specified impurity, unsupported purity values, or RRFs determined with poorly characterised material. Providing a clear COA and spectra for each standard reduces the risk of questions.
Stability and handling of impurity standards
Degradation products are, by definition, formed because something in the molecule is labile, so impurity standards can be less stable than the API. Store them as directed, often refrigerated or frozen and protected from light, allow containers to reach room temperature before opening to avoid condensation, and record the date of first opening. Prepare stock solutions fresh or establish their solution stability as part of method validation.
Choosing a supplier of impurity standards
- Ask for a sample COA and example spectra before ordering.
- Check that purity assignment accounts for water, solvents and inorganic content.
- Confirm the retest date and storage conditions, especially for unstable degradants.
- For custom syntheses, agree the characterisation package in advance.
- Where compendial tests are involved, qualify working standards against official pharmacopoeial standards.
FAQs
Are pharmacopoeial impurity standards required for ANDA filings?
Not always. Non-compendial characterised standards are widely used, provided their identity and purity are well documented. Compendial tests may require official standards.
What is a relative response factor?
A factor that corrects for the difference in detector response between an impurity and the API, allowing the impurity to be quantified against the API peak.
Why is HPLC area percent not enough for a standard?
It ignores water, residual solvents and non-UV-absorbing material. Quantitative use needs an assigned purity that accounts for these.