The U.S. Food and Drug Administration (FDA) recently issued revised guidance on the control and mitigation of nitrosamine impurities in human drugs. The guidance provides a comprehensive roadmap for manufacturers of finished drugs and active pharmaceutical ingredients (APIs) regarding steps they should take to detect and control nitrosamine impurities. Although this revised guidance was issued in final form, as with all FDA guidance, comments may be submitted at any time.

Background

For the uninitiated, nitrosamines (technically N-nitrosamines) are a class of chemical compounds characterized by a nitroso group (-N=O) bonded to an amine. Some are classified as probable or possible human carcinogens. They can be formed through a chemical reaction between nitrites and amines, which are commonly found in manufacturing processes. FDA’s guidance reviews in detail the chemistry behind nitrosamine formation and the conditions that lead to nitrosamine formation.

FDA’s guidance categorizes nitrosamines into two major types:

• Small-Molecule Nitrosamines: These are low molecular weight compounds, such as N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA). These can form in drug products through interactions between amines and nitrosating agents (e.g., nitrites) during manufacturing or storage.
• Nitrosamine Drug Substance-Related Impurities (NDSRIs): These are nitrosamines formed through the nitrosation of an active pharmaceutical ingredient (API) or an API fragment. Unlike small-molecule nitrosamines, NDSRIs are structurally related to the API itself.

Concerns about nitrosamines in pharmaceuticals were raised in 2018 when nitrosamine contamination was found in widely used blood pressure medications. Subsequent investigations revealed nitrosamines in other drug products, including antidiabetic medications, antacids, and smoking cessation products. These discoveries have led to several drug recalls and heightened scrutiny from regulatory bodies.

Setting Acceptable Intake Limits

A challenge FDA faced in this area is how to establish safe levels of exposure to the large number of nitrosamine impurities possibly present at low levels in drug products. Particularly in the case of NDSRIs (those associated with APIs), there is often a lack of substance-specific toxicological test data from which a safe level of exposure can be derived. In general, FDA defines acceptable intake limits as “a level that approximates an increased cancer risk of one additional case in 100,000 subjects based on a conservative assumption of daily exposure to a mutagenic impurity in drug substances and drug products over a lifetime” (borrowing this from guidance developed by the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH).[1] FDA has published separate guidance, Recommended Acceptable Intake Limits for Nitrosamine Drug Substance-Related Impurities (NDSRIs) (August 2023), known as the RAIL Guidance. The RAIL Guidance recommends three general approaches to determining the acceptable intake limit for NDSRIs: (1) assessments based on substance-specific safety data (when it exists), (2) assessments based on a robustly-tested surrogate that is similar in structure and reactivity to the NDSRI, and (3) an approach referred to as “predicted carcinogenic potency categorization” to assign a recommended AI limit to an NDSRI based on the NDSRI’s activating and deactivating structural features.

FDA has published recommended AI limits for numerous nitrosamines on its website (CDER Nitrosamine Impurity Acceptable Intake Limits | FDA) and recommends 26.5 ng/day when an acceptable intake limit cannot be determined based on the described approaches. FDA recommends that for drug products where multiple nitrosamines may be present, the total limit should not exceed the acceptable intake limit for the most potent nitrosamine impurity – unless acceptable intake limits for the individual nitrosamines vary greatly, in which case that may be impractical.

Risk Assessments, Testing, and Specifications

FDA recommends that drug manufacturers conduct risk assessments (i.e., evaluate whether nitrosamine impurities are likely to be present), followed by analytical testing if a risk of nitrosamine contamination is identified. Analytical testing should be performed using sensitive and appropriately validated test methods. If such testing shows a nitrosamine impurity is present above 10% of the acceptable intake limit for that nitrosamine, FDA recommends that a specification be established to ensure the nitrosamine level remains within the recommended acceptable intake limit. 

Mitigation Strategies

To reduce or prevent nitrosamine impurities, manufacturers can take several steps:

• Process Optimization: Manufacturers should design their manufacturing processes to avoid conditions that lead to nitrosamine formation. This includes avoiding secondary or tertiary amines, which can react with nitrites under acidic conditions to form nitrosamines.
• Use of Alternative Reagents: Nitrites, which are commonly used as reagents in certain chemical processes, should be replaced whenever possible.
• Supplier Audits: Raw materials, particularly solvents and catalysts, should be carefully audited for nitrosamine contamination. This includes both fresh and recovered solvents, which can carry over impurities from other processes.
• Storage and Packaging: Manufacturers should assess the role of packaging materials, which can leach nitrosamines into the drug product during storage. Proper packaging choices and storage conditions can mitigate this risk.

FDA has put a significant amount of work into the existing nitrosamine guidance and clearly takes this issue seriously. Watch for additional updates as new information becomes available and FDA’s understanding of nitrosamines in drugs evolves.

[1] Guidance for Industry | M7(R2) Assessment and Control of DNA Reactive (Mutagenic) Impurities in Pharmaceuticals To Limit Potential Carcinogenic Risk