[Background] Several years ago it was discovered that carcinogenic nitrosamines (NA) were contaminating some prescription drugs. Subsequent studies broadened the number and types of drugs found with NA contaminants.  Because there were no resistant species in animal tests, the most potent nitrosamines, e.g., nitrosodimethylamine (NDMA), were classified into IARC group 2A or 2B, probably/possibly carcinogenic to humans.  Evidence for their carcinogenic effects in humans is limited; the most studied is the tobacco-specific nitrosamine NNK (IARC Group 1).  The US FDA and other regulatory agencies worldwide are addressing potential human risk and ways to ameliorate NA presence in drugs, as a primary approach to cancer prevention.  Regulatory action has been guided by the ICH guideline M7(R2), Assessment and Control of DNA Reactive (Mutagenic) Impurities in Pharmaceuticals.

[Objectives].  In the US, effort is directed by the USFDA and the pharmaceutical industry toward 1. Understanding sources of contamination and reasons for nitrosamine formation, along with measures for remediation/mitigation.  2. Methods for identification and quantitation of contaminants. 3.  Determination of acceptable levels in drugs.  4. Mechanistic understanding of NA carcinogenic action, including small commonly studied NA, and larger NA formed as drug addition products, NDSRI’s (nitroso drug substance related impurities), for which there are no model compounds. 

[Methods].  1. Identification of the many pathways of NA formation in drug products, along with development of methods of remediation.  2. Updating of the bacterial mutagenicity assay (Ames), which failed to reliably detect nitrosamines in the past. Other short-term assays are also being assessed with NA and NDSRI’s, ideally without animal tests.  3.  Because it is often difficult to produce certain drugs with zero NA contaminants, development of criteria for determining acceptable levels of nitrosamines in drugs, based on potency and exposure.  4. Mechanistic studies have included in silico analyses, e.g., QSAR related to metabolism, and quantum mechanical analyses. 

[Results] 1. Studies on NA sources provided a surprising diversity of contaminating mechanisms, from routine synthetic pathways of generic drugs employing nitrosating environments, to nitrite contamination, drug packaging, shelf instability, internal drug structure reactions, and the presence of common precursor drug structures.  2. An acceptable bacterial mutagenicity screening assay for detection of nitrosamines as mutagens was developed by alteration of the standard Ames bacterial mutagenicity assay.  Other assays are still being assessed. 3. Methods for determining acceptable limits of NA in drugs include an animal potency calculator and dose-response assessments. 4. Sophisticated SAR related to metabolism, and quantum mechanical analyses have provided new insights into mechanistic action of NA.  Molecular analyses to search for NA signatures in human tumors appear not to have been undertaken, to date.

[Implications for policy]. These studies are impacting regulatory guidelines for assessment of NA contamination in drugs in both Europe and the US.  There is renewed interest in other environmental exposures to these carcinogenic agents, including in vivo formation in humans.  The studies are building on the information obtained several decades ago from scientists studying NA around the world, including in Germany, the UK, Japan, the US, and IARC.