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The responsibility for this step is likely to fall to the chemists and analysts responsible for the design and development of the API synthetic process, with additional input from formulation development scientist groups who can comment on issues arising from stability and degradation studies, as well as excipient compatibility.

An evaluation of the synthetic route, focused on starting materials, intermediates, reagents, catalysts, and solvents, is carried out to identify materials that could possibly survive the process and present in the API as impurities. It should also include consideration of other potential impurities that may arise from the synthetic route, particularly in the final stages. These could include related substances of the API or intermediates, through to materials derived from interactions between reagents and solvents.

It is recommended to focus on what could be considered “reasonably” predicted. This aspect of the risk assessment has been thrown into sharp focus by the issues surrounding N‐nitrosamines. This is examined in depth in Chapter 10 where specific issues surrounding N‐nitrosamines are examined and Chapter 11 that looks in detail at potential side reactions. Throughout the evolution of guidance pertaining to MIs, the scope in terms of what to include in MI risk assessments has been a topic of considerable debate/discussion. For example, the earlier European Medicines Agency (EMEA) guideline contains the following advice:

As stated in the Q3A guideline, actual and potential impurities most likely to arise during the synthesis, purification and storage of the new drug substance should be identified, based on sound scientific appraisal of the chemical reactions involved in synthesis, impurities associated with raw materials that could contribute to the impurity profile of the new drug substance, and possible degradation products. This discussion can be limited to those impurities that might reasonably be expected based on scientific knowledge of the chemical reactions and conditions involved.

ICH M7 [8] also addresses this within Section 5.1 of the guideline. The emphasis is focused on actual impurities and potential impurities likely to be present in the API/drug product (DP). Another important aspect of this section of the guideline is that it also looks to link the ICH M7 to ICH Q3A 6/Q3B [9, 10] reporting and identification requirements. For marketed assets, actual impurities are defined as those observed in the drug substance and drug product above the ICH Q3A and ICH Q3B reporting thresholds and identification of actual impurities is expected when the levels exceed the identification thresholds outlined by ICH Q3A and ICH Q3B. This confirms the primacy of ICH Q3A and ICH Q3B in terms of identification thresholds for marketed assets. While ICH Q3A and ICH Q3B do not apply to products within the clinical phases of development, the included identification levels for impurities can be a useful guide for later phase assets. In general for earlier phase products, other impurity identification thresholds have been proposed that recognize the controlled nature of clinical development and the knowledge that most toxicities are dose and dosing duration dependent [11].

PMIs that might be present in API generally fall into the following categories:

 Unreacted contributory materials or intermediates with alerting substructures that have survived processing (for example, an unreacted nitroaromatic functionality within an API due to incomplete hydrogenation or a positional isomer unable to cyclize).

 Substances derived from contributory materials, intermediates, or the API itself that contain an alerting structural motif.

 Unrelated substances formed by combinations of solvents and reagents with each other or with contributory materials or intermediates, N‐nitrosamines, and sulfonate esters are of course the most well‐known and studied examples of this. As highlighted above the issue of MIs generated by side reactions is examined in detail in Chapter 11.

It is important to remember that whatever the root cause of a potential MI, the consideration of factors such as dose, duration, and proximity remain important factors in establishing actual risk.

Such an approach is fully aligned with the key central tenets of ICH Q9 [7] that focuses on the probability of an event occurring, combined with an evaluation of the impact of the event occurring, leading to a consideration of the risk posed. The magnitude of the risk is therefore related to the probability of the PMI being present. The greatest risk is posed by those agents used in the late stages of the API synthesis that possess well‐established alerting structural motifs, as there are fewer opportunities for them to be removed during processing, and these should be the main focus of the evaluation.

At an appropriate point in the development of an API, the risk assessment should also include consideration of materials arising from degradation during manufacture or on long‐term storage of the API or its formulated product. This review may be based on a combination of factors including expert scientific knowledge and in silico predictions, e.g. Zeneth™, of the typical degradation pathways of the API and formulated product based on the chemical structure and literature precedent. The conduct of such assessments is described in detail in Chapter 14.

Having agreed a list of materials, which might comprise degradation products, specified impurities, probable process impurities, including intermediates, reagents, and raw materials as well as “reasonably predicted” impurities based on potential side reactions, these should then be subjected to a formal structural assessment for mutagenicity.