The concept of Genotoxic Impurities (GI) and Potential Genotoxic Impurities (PGI) is defined in the ICH M7 (2014) and EMA (2006) guidelines. The focus of these guidelines are on DNA reactive substances – those when present at low levels have a potential to directly cause DNA damage and lead to mutations, thereby potentially causing cancer. For late stage compounds and marketed products, acceptable increased cancer risk is set at a theoretically calculated Threshold of Toxicological concern (TTC) level of 1.5 mg/day.
Genotoxic impurities are classified into two types; Actual and Potential. Actual impurities are formed during i) manufacturing ii) long time storage iii) reaction with primary and secondary packaging iv)degradation and v) stress studies. The actual impurities include those observed in the drug substance above the ICH Q3A reporting thresholds and are required to be identified.
Potential impurities in the drug substance can also include those present in starting materials, reagents, by-products and intermediates in the route of synthesis and impurities formed during long term storage conditions.
As stated in Q3A guideline, these impurities should be identified, based on
- a sound scientific appraisal of chemical reactions involved in the synthesis,
- impurities associated with raw materials that could contribute to the impurity profile of the new drug substance,
- possible degradation products.
Impurities are expected to be identified based on knowledge of chemical reactions and conditions involved. Potential genotoxic impurities should be identified guided by existing genotoxicity data or structural alerts. When a potential impurity contains structural alerts, additional genotoxicity testing typically a bacterial reverse mutation assay should be considered and the approval limit (TTC) defined. Studies using isolated impurities are much more appropriate for this purpose and highly recommended.
It is difficult to define a safe exposure level (zero risk concept) for genotoxic carcinogens without a threshold. Moreover, complete elimination of genotoxic impurities from drug substances is often unachievable. Hence, it becomes necessary to implement a concept of an acceptable risk level, i.e. an estimate of daily human exposure at and below which there is a negligible risk to human health. To determine the limits, possible mechanism of action and dose-response relationship is considered.
Table: Acceptable intakes of mutagenic impurities
| Duration of treatment | ≤1 month | >1-12 months | >1-10 months | >10 years to lifetime |
| Daily intake (mg/day) for individual impurity | 120 | 20 | 10 | 1.5 |
| Total daily intake (mg/day) for multiple impurities | 120 | 60 | 30 | 5 |
The concept of Threshold of Toxicological Concern (TTC) was developed to define an acceptable intake for any unstudied chemical that poses a negligible risk of carcinogenicity or other toxic effects. For application of a TTC in the assessment of acceptable limits of mutagenic impurities in drug substances and drug products, a value of 1.5 μg/day can be justified based on the lifetime exposure calculation.
Structure-based assessments are useful for predicting bacterial mutagenicity outcomes based upon the established knowledge. There are a variety of QSAR approaches to conduct this evaluation including a review of the available literature, and/or computational toxicology assessment.
Some structural groups were identified to be of such high potency that intakes even below TTC would theoretically be associated with potential carcinogenic risk. This group of high potency mutagenic carcinogens are referred to as the “cohort of concern”, and comprise of aflatoxin-like-, N-nitroso-, and alkyl-azoxy compounds. These functional groups are either avoided or require strict controls.
Control strategy
- Control strategy for mutagenic impurities should be developed during the process development of a drug substance
- Possible impurity structures should be proposed based on mechanistic pathway of reactions, by-products, and the fate of impurities from raw materials, intermediates, reagents and solvents in the reaction
- Impurities should be assessed QSAR based software like DEREK / TOPKAT for mutagenicity and carcinogenicity and segregated into class 1-5 as described in ICH M7 hazard classification
- Risk assessment should be carried out for mutagenic impurities as per ICH Q9 guidelines. Risk should be assessed based on physic-chemical properties and process factors like chemical reactivity, solubility, volatility, ability to ionize and purging factors of mutagenic impurities
- Process and degradation related mutagenic impurities should be well controlled by systematic process development using ICH Q11 & Q8 guidelines, and also include a test for mutagenic impurity specification at or below the acceptance limit. The fate of the mutagenic impurities in the process will be established by purge factor
- Control should be applied for raw materials, starting materials, intermediates by applying material specification test for mutagenic impurities generated from these materials
- During lifecycle management, as per ICH Q10, the mutagenic impurities and critical parameters influencing the formation of these impurities should be tracked using statistical tools like control charts, along with a ‘periodic verification test’
What we do at Piramal?
- Highly knowledgeable and skilled R&D scientists for implementation of ICH Q8-11 guidelines during process development
- Propose and characterize all plausible impurities during process development and synthesis.
- Implement concepts of ICH M7 for mutagenic impurities control during process development and tracking
- Highly skilled analytical team to establish LOD/LOQ of mutagenic impurities
- Continuous monitoring of critical parameters and statistical analysis
- DEREK software to analyze mutagenicity of all plausible impurities
Decision tree for Assessment of Acceptability of Genotoxic impurities
