Drug recalls because of genotoxic impurities were rare until 2018.
Then the nitrosamine contamination crisis hit valsartan, ranitidine, metformin, and dozens of other products, ultimately affecting hundreds of marketed drugs globally.
The lesson: many programs had never run a systematic genotoxic impurity control assessment because the process chemistry had never been audited through an ICH M7 lens. For small molecule programs today, there is no grace period for that oversight.
What ICH M7 Actually Requires
ICH M7 is the international guideline for the assessment and control of DNA-reactive mutagenic impurities in pharmaceuticals, implemented in 2015 and currently at M7(R2) as of 2023.
The guideline applies to reagents, intermediates, and byproducts from the synthetic route; potential degradation products; and residual solvents or catalysts with structural alerts. It sits alongside ICH Q3A, covering only the subset of impurities with mutagenic potential.
The Five-Class System
ICH M7 compliance starts by classifying each potential mutagenic impurity into one of five classes:
- Class 1: Known human mutagen and carcinogen. Requires a compound-specific acceptable intake based on carcinogenic potency data. These carry the strictest limits.
- Class 2: Known bacterial mutagen, carcinogenic potential unknown. Controlled at the TTC of 1.5 μg/day.
- Class 3: Structural alert, no Ames test data. Treated as Class 2 at the TTC unless experimental data downgrades the classification.
- Class 4: Structural alert but proven non-mutagenic by an adequate Ames test. Controlled under standard ICH Q3A/Q3B.
- Class 5: No structural alert or mutagenicity concern. Treated as an ordinary impurity.
Any impurity with an unresolved structural alert sits in Class 3 and is controlled at the TTC until Ames test data changes the classification. Running Ames tests to downgrade Class 3 to Class 4 is usually cheaper than maintaining TTC-level analytical controls throughout development.
The TTC Concept and the Cohort of Concern
The TTC (Threshold of Toxicological Concern) of 1.5 μg/day corresponds to a lifetime excess cancer risk of 1 in 100,000. This is the default limit for Class 2 and Class 3 impurities.
Shorter treatment durations allow higher limits. A drug dosed for 14 days can have a mutagenic impurity limit well above 1.5 μg/day because cumulative lifetime exposure is lower.
Some impurities cannot use the 1.5 μg/day default. The cohort of concern includes nitrosamines, aflatoxins, and azoxy compounds. For these, the default acceptable intake drops to 18 ng/day, or a compound-specific limit is calculated using the Carcinogenic Potency Categorisation Approach (CPCA). This is why the nitrosamine crisis was so disruptive: drugs carrying nitrosamine contamination at levels acceptable for ordinary impurities were orders of magnitude above the 18 ng/day threshold.
The Four Control Options
ICH M7 defines four control options for demonstrating a mutagenic impurity is below its acceptable limit:
- Option 1: Analytical testing of the drug substance to confirm the impurity is below the limit
- Option 2: Testing at an intermediate stage, combined with a process argument that the impurity is purged subsequently
- Option 3: A purge factor calculation supported by limited analytical verification
- Option 4: A purge factor calculation alone, no analytical testing at the final API stage
Purge factor calculation uses the chemical reactivity, solubility, volatility, ionizability, and polarity of the impurity to estimate how much remains in the final API after each downstream step.
If the calculated purge factor is sufficient, an Option 4 control strategy can be justified without analytical testing at the API stage. Amgen, AstraZeneca, Eli Lilly, Pfizer, and others have published case studies confirming regulatory acceptance of Option 4 for nitrosamines when purge calculations are scientifically sound.
Option 4 reduces analytical burden during development but requires robust physicochemical justification. Weak arguments or missing process knowledge create health authority challenges.
In Silico Assessment
For Class 3 impurities, in silico QSAR tools are the primary screening method. ICH M7 compliance requires at least two complementary QSAR models. The “two-model plus expert review” approach endorsed by regulators uses independent statistical and rule-based models in combination. When both predict non-mutagenicity and expert review confirms no overriding concern, the structural alert can sometimes be addressed computationally without an Ames test.
False negative rates below 5% have been reported for this approach when models are used within their validated chemical space. Outside the validated domain, the Ames test remains the definitive tool.
Nitrosamine Impurities in 2025-2026
Nitrosamine control is the most active area of GTI regulatory attention. The FDA issued revised nitrosamine guidance in September 2024 and extended the deadline for manufacturer progress reports to August 2025. The EMA updated its nitrosamine Q&A to Revision 23 in October 2025.
For any program involving secondary amines as reagents or synthetic intermediates, a nitrosamine risk assessment is now effectively mandatory, even for early-phase programs.
How LAXAI Integrates GTI Control Into Development Programs
LAXAI’s process chemistry and analytical teams conduct genotoxic impurity control assessments as a standard part of route scouting, not as a late-stage compliance exercise.
Structural alerts are evaluated at route selection. Class 2 and Class 3 impurities are assessed for purge before the route is locked. Where purge calculations support Option 4, the process is designed to deliver that outcome. Where Option 1 analytical controls are needed, LAXAI’s analytical team develops and validates LC-MS/MS or GC-MS/MS methods to the required detection limits.
For programs involving nitrosamine risk, LAXAI runs a dedicated nitrosamine risk assessment covering both drug substance and drug product pathways before any IND tox batch is manufactured.
Talk to LAXAI’s process chemistry team about GTI control in your program at bd@laxai.com
FAQs
What is the TTC for genotoxic impurities under ICH M7? The standard TTC is 1.5 μg/day for lifetime exposure, a 1-in-100,000 excess cancer risk. Shorter durations allow higher limits. Cohort of concern impurities like nitrosamines use 18 ng/day or a compound-specific intake.
What is the difference between Class 2 and Class 3 impurities? Class 2 have confirmed Ames test mutagenicity, unknown carcinogenic potential. Class 3 have a structural alert but no Ames data. Both are controlled at the TTC. An Ames test can confirm Class 2 status or downgrade a Class 3 to Class 4, removing the TTC requirement.
What is a purge factor calculation? A calculation using the physicochemical properties of a mutagenic impurity to estimate how much remains in the final API after downstream steps. Sufficient purging can justify process-only control strategies (ICH M7 Options 3 and 4) without analytical testing.
Why are nitrosamines a special case under ICH M7? Nitrosamines belong to the cohort of concern. Their carcinogenic potency makes the standard 1.5 μg/day TTC non-protective. The default acceptable intake is 18 ng/day. Any process using secondary amines or nitrosating conditions requires a dedicated nitrosamine risk assessment.
