Articles

ICH M7 explained: assessing and controlling DNA-reactive (mutagenic) impurities

22 juillet 2026

5 min read

Every synthetic drug carries a shadow population of impurities — starting materials, intermediates, reagents, catalysts and degradation products. Most are harmless at trace levels. A few are not: some impurities are DNA-reactive, capable of causing mutations and, potentially, cancer. Deciding which impurities need controlling, and to what limit, is the job of ICH M7.

ICH M7(R2) — “Assessment and Control of DNA Reactive (Mutagenic) Impurities in Pharmaceuticals to Limit Potential Carcinogenic Risk” — gives industry a pragmatic, science-based framework. This article walks through how it works: the (Q)SAR-first workflow, the five impurity classes, the acceptable-intake limits, and where the Ames test comes in.

Key points

The question M7 answers

Which impurities are potentially mutagenic — and what daily limit keeps carcinogenic risk negligible.

(Q)SAR first

Two complementary in silico methodologies (expert rule-based + statistical) screen structures before any bench work.

Five classes

Impurities are sorted into Classes 1–5 that dictate the control strategy.

The TTC

A threshold of toxicological concern of 1.5 µg/day corresponds to a negligible lifetime cancer risk.

The logic of ICH M7

ICH M7 applies to new drug substances and products (and to significant changes to existing ones), covering impurities that are actual or potential, including degradation products. Its central idea is proportionality: focus control where the mutagenic hazard is real, and avoid over-testing where structure-based evidence already says an impurity is safe.

The framework is built on the observation that most DNA-reactive mutagens share recognisable “structural alerts” — reactive chemical features (for example, certain alkylating groups, aromatic amines, or epoxides). If a structure carries no such alert, and two independent computational methods agree, further testing is usually unnecessary.

The (Q)SAR-first workflow

Rather than testing every impurity, M7 starts in silico. Two complementary (Q)SAR (quantitative structure–activity relationship) methodologies are applied:

an expert rule-based system (encoding known structural alerts), and
a statistical system (learning patterns from large mutagenicity datasets).

If both predict negative, the impurity can be treated as non-mutagenic — no bench testing required. If either flags an alert, the impurity moves forward for expert review and, where appropriate, confirmatory bacterial mutagenicity testing (the Ames test, OECD 471).

The numbers that matter

1.5 µg/day

Threshold of Toxicological Concern (TTC) — the default acceptable intake for a mutagenic impurity

10⁻⁵

The negligible lifetime excess cancer risk the TTC is designed to represent

Impurity classes that route each structure to the right control strategy

Independent (Q)SAR methodologies required before an impurity is cleared

The five impurity classes

ICH M7 sorts impurities into five classes according to what is known about their mutagenicity and carcinogenicity:

Class 1 — known mutagens and carcinogens; control to compound-specific limits.
Class 2 — known mutagens with unknown carcinogenic potential; control to the TTC.
Class 3 — a structural alert unrelated to the drug substance, no mutagenicity data; treat as Class 2 (TTC) or test by Ames.
Class 4 — a structural alert shared with the drug substance (or a related compound) that has tested negative; treat as non-mutagenic.
Class 5 — no structural alert; treat as non-mutagenic.

Acceptable intakes: the TTC and its limits

For most mutagenic impurities, the Threshold of Toxicological Concern of 1.5 µg/day defines an acceptable intake corresponding to a negligible (10⁻⁵) lifetime cancer risk. For shorter exposures, less-than-lifetime (LTL) limits allow proportionally higher intakes.

The cohort of concern

A critical caveat: the TTC does not apply to an especially potent group of carcinogens — the “cohort of concern”: aflatoxin-like, N-nitroso, and alkyl-azoxy compounds. These require compound-specific acceptable intakes, usually far below the generic TTC. This is precisely where the nitrosamine problem meets M7 — and why nitrosamine impurities have demanded their own dedicated guidance and enhanced testing.

How the Ames test fits

When (Q)SAR flags a structural alert that cannot be dismissed by expert knowledge, the bacterial reverse mutation test (Ames, OECD 471) is the confirmatory step. A negative Ames result generally reclassifies the impurity as non-mutagenic (Class 5-equivalent for control purposes); a positive result triggers control to the TTC or a compound-specific limit. For nitrosamines specifically, enhanced Ames conditions are needed, because standard protocols can under-detect these potent mutagens.

Frequently asked questions

What is ICH M7?

ICH M7 is the international guideline for assessing and controlling DNA-reactive (mutagenic) impurities in pharmaceuticals, to limit potential carcinogenic risk. It defines a structure-based, (Q)SAR-first workflow and sets acceptable-intake limits.

What is the TTC in ICH M7?

The Threshold of Toxicological Concern is 1.5 µg/day for a mutagenic impurity, corresponding to a negligible (10⁻⁵) lifetime cancer risk. Higher limits apply for less-than-lifetime exposure; it does not apply to the “cohort of concern”.

How does (Q)SAR fit into ICH M7?

Two complementary (Q)SAR methodologies — one expert rule-based, one statistical — screen each impurity’s structure. If both are negative, no further testing is needed; if either flags an alert, the impurity proceeds to expert review and, where relevant, Ames testing.

How does ICH M7 relate to nitrosamine guidance?

Nitrosamines belong to the “cohort of concern”, for which the generic TTC does not apply. They require compound-specific limits and enhanced Ames testing, which is why they are addressed by dedicated nitrosamine guidance alongside the M7 framework.

Work with a genotoxicity partner

GenEvolutioN supports pharmaceutical developers through the full ICH M7 pathway — from (Q)SAR assessment of impurities to GLP-compliant Ames testing, including enhanced protocols for nitrosamines. The result: a control strategy that satisfies regulators and keeps your programme moving.

Read more on the enhanced Ames test for nitrosamines, revisit the genotoxicity fundamentals, or talk to our team.