To understand “actual” purge for the alkyl chloride 8, the Stage 4b product crude GW641597X was checked by 1H NMR (Figure 3.6), which confirmed that the level of 8 was present at approximately 0.2% m/m (0.12% w/w11) and is in good alignment with the predicted purge factor of 100 for Stages 4a and b.12 While the observed level in this typical batch of crude GW641597X is already significantly below the required TTC for a monofunctional alkyl chloride of 0.5% w/w, checking the 1H NMR for GW641597X derived from the Stage 5 product confirmed that alkyl chloride 8 is no longer observed, giving confidence that it is present at levels <0.10% w/w. These observations fully justify an ICH M7 [8] Option 4 control for alkyl chloride 8.
Figure 3.6 1H NMR of Stage 4b product crude GW641597X.
The use of 1H NMR to confirm purging for both ethyl bromoisobutyrate 2 and alkyl chloride 8 helps to verify the purge predictions from the process. They are also useful examples of the expected additional “non‐trace” experimental data (solubility, reactivity, and volatility) recommended for both noncommercial and commercial API routes to support an ICH M7 control Option 4 scientific rationale as advocated by the pharmaceutical consortium within the Barber publication [19].
3.6.2 Proposed ICH M7‐aligned Potential Mutagenic Control Regulatory Discussion
Based on the initial purge calculations allied to the additional non‐trace analysis, a control summary table (Table 3.10) is presented below.
Table 3.10 Proposed high‐level control summary table for potential MIs ethyl bromoisobutyrate 2, hydroxylamine, and alkyl chloride 8.
| Impurity | Point of potential formation/introduction and summary of rationale for impurity purging | Required purge and predicted purge | Control |
|---|---|---|---|
|
|
Starting material in Stage 1a (2 eq.), four steps from drug substance (DS). Consumed to low level (<5%) in Stage 1b; reactive during processing (Stage 4); soluble in isolation solvents (Stages 4 and 5). | Required purge = 20 Predicted purge = 1.0 × 105 Purge ratio = 5000 | Option 4 – controlled through chemical reactivity and physical processing. |
| NH2OH | Reagent in Stage 2 (2.5 eq.), four steps from DS. Reactive during processing (Stages 2, 3, and 4), highly soluble in isolation solvents (Stages 2, 3, 4, and 5). | Required purge = 39 Predicted purge = 1.0 × 108 Purge ratio = 2.56 × 106 | Option 4 – controlled through chemical reactivity and physical processing. |
|
|
Starting material in Stage 4 (1.15 eq.), two steps from DS. Confirmed at low level (c. 0.2%) within Stage 4b product following additional reactivity with aqueous base used within the process and solubility within the isolation solvent. Additional solubility anticipated in Stage 5 isolation solvent. | Required purge = 30 Predicted purge =1000 Purge ratio = 33 Measured purge = 75 (Stage 4b) Measured purge ≥ 150 (Stages 4b and 5) | Option 4 – controlled through chemical reactivity and physical processing. |
Further options for control could be considered specifically in the case of chloromethyl oxadiazole 8 where an Ames test could be performed to assess whether or not it is mutagenic.
3.6.3 Case Study 2 – Candesartan
As shown within the previous case study (GW641597X), the normal process for a risk assessment would be to identify the potential impurities within the drug substance and subsequently establish where mutagenicity concerns exist. However, this can also be extended to identify component constituents that may react together to an impurity of concern. By applying ICH M7 [8] control principles to these reactive species, the MI risk and any necessary control strategies can be established.
Candesartan cilexetil was developed as an angiotensin‐II receptor antagonist for the treatment of hypertension. Following the discovery of N‐nitrosodimethylamine (NDMA) in batches of valsartan and subsequently in additional sartans (losartan and irbesartan), it became necessary for all sartan‐containing medications to evaluate the risk posed by nitrosamines, which form part of the cohort of concern, in drug products [24]. This case study evaluates the risk of nitrosamine formation and subsequent carryover to the API by examining the fate of the individual components required to generate a nitrosamine impurity.
Candesartan cilexetil is prescribed for chronic use and is therefore subject to lifetime TTCs for any impurities present within the drug product. However, regulatory guidance at the time also indicated that due to the potent mutagenic and carcinogenic potential of some nitrosamines, LTL limits for nitrosamines could not be used. Interim limits for the presence of nitrosamine impurities NDMA and N‐nitrosodiethylamine (NDEA) were set at 96 and 26.5 ng/day, respectively, based on extrapolation from the respective TD50s. For candesartan, which has a maximum daily dose of 32 mg, this equates to a final API impurity concentration of 3 ppm for NDMA and 0.83 ppm for NDEA.
The most common method by which nitrosamines are formed is through the reaction of secondary or tertiary amines with a nitrosating agent such as sodium nitrite in acidic media. The process used for candesartan incorporates the use of triethylamine and dimethyl formamide (DMF), which are known to contain as contaminants, or decompose into, diethylamine and dimethylamine, respectively. The process additionally utilizes sodium nitrite, thereby introducing a theoretical risk of nitrosamine formation (Figure 3.7). The risk assessment process therefore needs to address whether these amines or the parent compounds are likely to be present within the same step (under appropriate conditions) at a level of concern, and thereby identify the degree of risk present for nitrosamine formation and any subsequent removal if formed.
Figure 3.7 Nitrosamine formation pathways from Et3N and DMF.
The manufacturing process for candesartan involves a nine‐stage synthesis whereby triethylamine and DMF are introduced in Stage 2, whereas nitrite is not present until Stage 5 (Figure 3.8). As a result, the key questions related to assessing the risk of nitrosamines within the synthesis of candesartan are:
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