Robust nitrosamine risk assessment and nitrosamine analysis are essential in safeguarding the continued supply of safe and effective medicines. In this article, we examine the emergence of small molecule nitrosamines and NDSRIs as compounds of concern, as well as some key nitrosamine formation mechanisms. We also discuss some promising strategies for the control of nitrosamine impurities in human drugs, and the importance of high quality reference standards in nitrosamine testing.

What is a nitrosamine (and other nitrosamine detection basics)?

A nitrosamine – also known as an n-nitrosamine or an n-nitroso compound - is a carcinogenic impurity formed when amines react with nitrosating agents like nitrites, typically under acidic conditions or at high temperatures. Widely present in the environment, they were discovered in the 1870s, and first linked to cancer in the mid-1950s when Magee and Barnes revealed the associations between N-Nitrosodimethylamine (NDMA) and malignant tumours in rats. In the 1970s, researchers found nitrosamine compounds in beverages, preserved food, personal care products, and tobacco, as well as chlorinated and contaminated water. It was also in this period that nitrosamines were linked to the development of numerous cancers - including those of the bladder, lung, liver, and stomach. In June 2018, the control of nitrosamine impurities in human drugs became a major issue for the first time when the US Food and Drug Administration (FDA) discovered that batches of the leading blood pressure drug valsartan had been contaminated with NDMA. Soon after, the European Medicines Agency (EMA) announced that NDMA and N-Nitrosodiethylamine (NDEA) had been found in more sartans products, while further nitrosamine impurity analysis later revealed unacceptable levels of NDMA in both the heartburn medicine ranitidine and the anti-diabetic metformin.

Key pathways in NDSRI and small molecule nitrosamine formation

At one stage, these small nitrosamine-related batch recalls threatened to cause a potentially “catastrophic loss of critical medicines" – and prompted radical changes to both FDA and EMA nitrosamine guidance that required manufacturers to:

1. carry out a comprehensive review of all human medicines for the possible presence of nitrosamines.

2. submit changes to their manufacturing processes where nitrosamines were detected above permitted levels.

The pharma companies eventually identified numerous ways that small nitrosamine impurities may occur throughout the lifecycle of a drug product. They included the purchase of impure raw materials and excipients, the use of solvents, reagents, and catalysts, chemical processes such as quenching and buffering, and the degradation of unsuitable packaging.

Although these potential contamination sources were many and various, they could largely be managed by companies improving their nitrosamine impurity risk assessment protocols – and so many scientists began to think that they were close to solving the nitrosamine problem.

However, it was at this point that the threat from NDSRIs – or Nitrosamine Drug Substance-Related Impurities – became apparent. It also quickly became clear that this second class of compounds was even more problematic, since the Active Pharmaceutical Ingredients (APIs) in many drugs derive their structures from secondary or tertiary amines, or tertiary ammonium salts. This not only means that drug products like these are inherently capable of producing nitrosamine impurities, but also makes manufacturers’ NDSRI risk assessment processes much more complex, as each NDSRI is unique to its individual API.

For more on nitrosamines, read our Nitrosamine impurities in medicines: History and challengesblog.

Guidance on controlling NDSRIs and nitrosamines

While we have already learned much about the manufacturing conditions that can promote small molecule nitrosamine formation – as well as strategies to limit their occurrence – recent studies using model amines and representative formulations are beginning to shed more light on the complex factors behind the creation and control of NDSRIs.

As a new guide from LGC Standards shows, conditions that promote NDSRI formation include the presence in an API of aromatic secondary amines, which nitrosate more readily than their aliphatic counterparts. Meanwhile, the use of certain excipients can also play a role in making APIs more susceptible to nitrosation - with the formaldehyde-rich filler hydroxypropyl methylcellulose (HPMC) a likely contributor to NDSRIs forming in extended release propranolol capsules, via formaldehyde-catalysed nitrosation.

Given that even small reductions in nitrite levels can significantly reduce NDSRI formulation, drug manufacturers should consider replacing excipients like sodium starch glycolate - which is known to contain nitrites as an impurity - with low-nitrite alternatives such as povidone. The use of nitrite scavengers, including antioxidants and amino acids, is another promising strategy for mitigating NDSRI formation, as scavengers reduce the potential for nitrosation by converting nitrite into less reactive species such as Nitric Oxide (NO). Moreover, since acidic conditions favour NDSRI formation, raising the pH of a drug formulation with an alkalising excipient like sodium carbonate could be another effective means of NDSRI control.

Authors

Andy Blizzard

Content Writer