N-Nitroso Hydrochlorothiazide in Hydrochlorothiazide drug substance

Can someone provide the reference for the acceptable criteria for the control of N-Nitroso Hydrochlorothiazide in Hydrochlorothiazide drug substance.

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Neither this impurity nor its degradation products are cohort of concern. This impurity is positive in the Ames assay with and without a metabolic activator (S9) which is not a characteristic feature of an N-Nitrosamine which requires metabolic activation for alpha-hydroxylation and subsequent generation of the alkyl diazonium cation.
To our current knowledge EDQM has accepted an interim limit of 1.5µg/day for this impurity pending in vivo mutagenicity (TGR) results.
Dr. @conudel has recently presented this information in the CPHI nitrosamine forum.


@Gaurav Gaurav, @Muzaffar ,

I can confirm that some manufacturers already obtained Hydrochlorothiazide CEPs with a limit of 15 ppm, based on the TTC of 1,5 µg/day and the MDD of HCTZ (100 mg/day).
kind regards


Do you have any reference or article for support purpose ??

This limit was accepted by EDQM on the basis of a position paper submitted by EFPIA (European Federation of Pharmaceutical Industries Associations). However, this document is strictly confidential and cannot be shared.

I can only suggest the following articles:
Mutagenicity of amine drugs and their products of nitrosation (Andrews 1984)
(which confirm the mutagenicity of N-nitroso HCTZ)
Pathologic Effects of Chronic Administration of Hydrochlorothiazide, with and without Sodium Nitrite, to F344 Rats
(which demonstrate that the addition of sodium nitrite with HCTZ in the diet of the rats do not increase the risk of cancer).


Claiming that nitrosamine is out of the cohort of concern based on the AMES test alone is overly optimistic. Especially in the case of nitrosohydrochlorothiazide, where the number of reverse mutants with S9 activation is about 10 times higher than without said activation. This rather suggests that, in addition to the carcinogenic mechanism typical of nitrosamines, there is an additional minor mutagenic effect, eg related to the hydrolysis of hydrochlorothiazide to formaldehyde.

Finally EFPIA made public on its website the official position paper on N-Nitroso Hydrochlorothiazide:

Hydrochlorothiazide Investigation into potential N-Nitrosation and the specific nature of Nitroso HCTZ

In this document it is clearly explained why Nitroso-HCTZ should be considered in the Cohort of Concern.


My understanding is that this document specifically describes “why N-Nitroso Hydrochlorothiazide should not be considered in the cohort of concern.”

It is concluded, based on the studies described that, if formed within HCTZ related products, NOHCTZ is intrinsically unstable at physiological pH and that therefore control of NO-HCTZ as a member of the cohort of concern is not warranted. Instead, this impurity should be understood as a source of formaldehyde, an aromatic amine and the thiatriazine, none of which belong to the cohort of concern and thus require no specific control as levels formed from traces of the Nitrosamine of HCTZ are well below the appropriate TTC for each species.


Moreover, formaldehyde is generated also by the metabolism of HCTZ itself.
in fact, 4-amino-6-chloro-1,3-benzenedisulphonamide and formaldehyde are hydrolysis products of HCTZ:

LC, LC-MS/MS Studies for the Identification and Characterization of Degradation Products of Hydrochlorothiazide and Establishment of Mechanistic Approach towards Degradation

4-amino-6-chloro-1,3-benzenedisulphonamide was clearly identified as metabolite of HCTZ:
Detection of urinary markers for thiazide diuretics after oral administration of hydrochlorothiazide and altizide-relevance to doping control analysis

As consequence, also formaldehyde is a metabolite of HCTZ.


Of course, the mutagenicity can be explained by the appearance of formaldehyde as a decomposition product. However, what raises my concerns is the significantly increased mutagenicity in the case of activation with the s9 fraction, which could suggest that there may be many mechanisms of mutagenicity and does not rule out the metabolic activation typical of nitrosamines, which is key to qualifying them as a “cohort of concern”.


I want to add @AndyT’s presentation in FDA and the Center for Research on Complex Generics (CRCG) hosted a public workshop. It explains details of degradations kinetics of nitroso hydrochlorothiazide. It’s excellent!!

And results of the transgenic study were negative, according to him. It can be classified as class 5 and controlled ICH Q3A /3B limits.

Dr. Grahek reported the data on the stability and degradation pathway of N-Nitroso-Hydrochlorothiazide. It is unstable at physiologically relevant aqueous conditions and decomposes to several degradation products without the formation of diazonium ion.