Pharmaceutical Impurity Risk Assessment

Impurities may arise during API synthesis, drug-product manufacture, storage, degradation or interaction with formulation and packaging components. ToxWriter provides scientific impurity assessments to evaluate available toxicological evidence, patient exposure, applicable regulatory thresholds and the justification for proposed impurity controls.

Our scope may include mutagenic impurities, conventional organic impurities, degradation products, nitrosamines, residual solvents, elemental impurities and extractables/leachables.

Reporting, identification, qualification and quantitation

• Reporting threshold: the level above which an impurity result should be reported under the applicable guideline.
• Identification threshold: the level above which the impurity should be structurally identified, subject to the guideline's scope and provisions.
• Qualification threshold: the level above which evidence should establish biological safety at the proposed level, unless the impurity is already adequately qualified.
• Limit of quantitation (LOQ/QL): the lowest amount that a particular analytical procedure can quantify with suitable accuracy and precision.

ICH Q3A and Q3B state that the analytical procedure's quantitation limit should be no greater than the applicable reporting threshold. The LOQ is method-specific; it does not set or replace the regulatory threshold.

ICH Q3A(R2): Impurities in new drug substances

ICH Q3A(R2) applies within its defined scope to impurities in new drug substances produced by chemical synthesis. Thresholds are based on the maximum daily dose of the drug substance.

Lower thresholds may be appropriate for unusually toxic impurities; higher reporting thresholds require scientific justification.

Q3A example

For an API with a maximum daily dose of 500 mg, the reporting threshold is 0.05%. The identification threshold is 0.10% (0.5 mg/day, lower than 1.0 mg/day). The qualification threshold is 0.15% (0.75 mg/day, lower than 1.0 mg/day). These are thresholds for regulatory actions, not automatic safe levels or mandatory specifications.

Official guideline: ICH Q3A(R2)

ICH Q3B(R2): Degradation products in new drug products

ICH Q3B(R2) addresses relevant degradation products in qualifying new drug products containing chemically synthesised drug substances. The thresholds depend on the maximum daily dose of the drug substance.

Reporting thresholds

Identification thresholds

Qualification thresholds

The absolute daily-intake limits and percentage limits must be compared where both are specified. Lower thresholds may be appropriate for unusually toxic degradation products; higher thresholds require scientific justification.

Q3B example

For a drug substance administered at 50 mg/day, the reporting threshold is 0.10%. The identification threshold is 0.2% or 2 mg/day, whichever is lower; 0.2% corresponds to 100 µg/day. The qualification threshold is 0.5% or 200 µg/day, whichever is lower; 200 µg/day corresponds to 0.4% of a 50 mg dose.

Official guideline: ICH Q3B(R2)

ICH M7(R2): Mutagenic impurity assessment

ICH M7(R2) addresses the assessment and control of DNA-reactive mutagenic impurities to limit potential carcinogenic risk. Assessments may include literature review, structural alerts, complementary (Q)SAR methodologies, experimental data and compound-specific toxicological evidence.

ICH M7 classifies impurities into five categories: known mutagenic carcinogens; known mutagens with unknown carcinogenic potential; impurities with structural alerts and insufficient data; impurities with alerts shared with the drug substance or related compounds supported by relevant negative data; and impurities with sufficient evidence of absence of mutagenicity or without relevant alerts. Classification must be supported by the evidence; a structural alert alone does not prove mutagenicity.

General TTC-based acceptable intakes

These values are applied within the ICH M7 framework; they are not automatically applicable to every impurity. Compound-specific acceptable intakes, the cohort of concern, relevant exceptions and current addenda should be considered where applicable.

• FDA — ICH M7(R2) final guidance.
• FDA — ICH M7(R2) addendum on compound-specific acceptable intakes.
• FDA — ICH M7(R2) Questions and Answers.

Nitrosamine impurity risk assessment

Nitrosamines, including nitrosamine drug substance-related impurities (NDSRIs), require specific assessment because some compounds are potent carcinogens and may have compound-specific acceptable intake recommendations.

Assessment may include review of API structure, amines and nitrosating agents, manufacturing and formulation conditions, excipients, packaging, analytical findings, carcinogenic potency categorisation, read-across and available compound-specific toxicological data. The appropriate acceptable intake should be selected from the current applicable regulatory recommendations rather than assigning a generic limit without justification.

• FDA — Control of Nitrosamine Impurities in Human Drugs.
• FDA — CDER Nitrosamine Impurity Acceptable Intake Limits: maintained recommendations and updates for identified nitrosamines and NDSRIs.
• EMA — Nitrosamine impurities guidance for marketing authorisation holders: Q&A, acceptable intake appendices and potency categorisation materials.

Other relevant impurity guidelines

• ICH Q3C(R9) — Residual solvents: solvent classification and permitted daily exposure/control approaches.
• ICH Q3D(R2) — Elemental impurities: risk assessment and control of elemental impurities using permitted daily exposures.
• ICH Q3E — Extractables and leachables (draft): developing framework for assessment and control; confirm its status before relying on it as a final guideline.
• ICH Q2(R2) and Q14 — Analytical validation and analytical procedure development: relevant to analytical method capability, including quantitation limit.
• ICH Q6A — Specifications: principles for specifications and acceptance criteria.
• ICH Q9(R1) — Quality Risk Management: systematic risk-based assessment and control principles.
• EMA — Chemistry of active substances: EU guidance on manufacture and control of active substances, including impurity considerations.

How ToxWriter conducts an assessment

1. Define scope: establish impurity identity/source, product type, dose, route, treatment duration and target market.
2. Select the framework: determine the relevance of ICH M7, Q3A, Q3B, Q3C, Q3D, nitrosamine guidance or other applicable requirements.
3. Review evidence: evaluate available literature, (Q)SAR results, experimental toxicology, analytical data and compound-specific recommendations.
4. Evaluate limits and exposure: calculate relevant thresholds or acceptable intakes and assess measured/predicted patient exposure.
5. Identify data gaps and controls: consider additional identification, testing, qualification, specifications or process controls where justified.
6. Report: document methods, rationale, assumptions, limitations, conclusions and recommendations.

Information needed to start

• API or drug-product name and impurity identity/structure, if available.
• Impurity type, likely source, impurity level and analytical results.
• Maximum daily dose, route of administration and treatment duration.
• Relevant manufacturing or formulation information.
• Available toxicology, literature, (Q)SAR results or previous reports.
• Current/proposed acceptance criteria and intended regulatory markets.
• Any agency comments or specific questions to address.

Frequently asked questions

Does exceeding a Q3A/Q3B threshold mean an impurity is unsafe?

No. It signals a regulatory action such as reporting, identification or qualification. The threshold itself is not a toxicological safety limit, and the available evidence and impurity type must be considered.

Is LOQ the same as the reporting threshold?

No. The LOQ is specific to the analytical procedure. ICH Q3A/Q3B state that the analytical quantitation limit should be no greater than the reporting threshold, but the two values serve different purposes.

Are Q3A/Q3B thresholds appropriate for nitrosamines or mutagenic impurities?

Not automatically. ICH M7 and current nitrosamine-specific recommendations may require separate assessment and compound-specific limits.

Does a toxicological assessment include laboratory testing?

Not automatically. Existing information may support the assessment; additional analytical or toxicological studies may be recommended if there is a specific evidence gap.

Thresholds summarised here reflect the cited ICH guidance and are general reference points, not a substitute for reviewing the full guideline, applicable exclusions, current regional implementation and impurity-specific evidence. Always verify current regulatory guidance before applying a limit to a product.