I would like to share a question with you and hear your opinion regarding the potential nitrosation of bilastine API, more specifically at the piperidine ring.
Do you believe that the nitrosamine known as 1-(2-ethoxyethyl)-2-(1-nitrosopiperidin-4-yl)-1H-benzo[d]imidazole could be formed through the direct nitrosation of the API itself, or do you think its formation would be more likely associated with process-related impurities/intermediates from the synthesis?
In addition, do you think that other nitrosamines derived from the API could potentially be formed? For example, could nitrosation followed by ring opening lead to a nitrosamine in which the nitroso group and an aldehyde group are present in the same final molecule?
I have been looking for papers and other relevant information on this topic, but I have not yet been able to reach a clear conclusion. I would really appreciate hearing your thoughts and any references you may have.
The manufacturing process should be available from the supplier, at least in a schematic form in the DMF. The impurities of each synthesis steps are normally listed and for a nitrosamine risk assessment their fate is followed in the subsequent steps. If you don’t have access to enough details of the manufacturing process you may want to check the patent literature. If there is a non substituted piperidine intermediate, it is likely to be present as an impurity in the API, at a significant level from nitrosamine risk perspective, even if it is not included as a controlled impurity in the specification.
On the other hand, nitrosation and degradation followed by nitrosation of the tertiary amine may also occur. Regarding its relative importance vs the nitrosation of the hypothetical piperidine impurity, there are publications on the comparative kinetics on the reactivity of secondary and tertiary amine, e.g., N-Nitrosamine Formation in Pharmaceutical Solid Drug Products: Experimental Observations, Moser; Ashworth, Scrivens et al., J. Pharm. Sci., 2023, showing that the tertiary amine to be significantly less reactive with nitrite.
Nitrosation of the N-substituted piperidine with ring opening generating a nitrosamine compound including an aldehyde group, while theoretically would be possible, it was not reported, as far as I know, therefore this is even less probable.
Direct nitrosation of the API itself cannot be completely excluded; however, formation through a process-related impurity or synthetic intermediate, particularly free piperidine, may represent a more plausible pathway. Free piperidine intermediate could act as a susceptible secondary amine under nitrosating conditions and undergo direct nitrosation to generate the corresponding N-nitroso-piperidine moiety.
An alternative pathway involving nitrosative dealkylation of the API or a process intermediate may also be considered. Therefore, distinguishing between direct nitrosation of the API and formation via process-related impurities/intermediates would require appropriate mechanistic and analytical evidence. A Nitrosation Assay Procedure (NAP) study on the API, together with evaluation of relevant process intermediates and potential carryover impurities, would provide supporting evidence to assess the likelihood of these proposed pathways and establish the most plausible source of the observed nitrosamine impurity.
To the best of our knowledge (as API and drug product manufacturers), and having carried out forced nitrosation studies: direct nitrosation of bilastine is possible under forced conditions (high nitrite concentration, acid medium and heating) and 1-(2-ethoxyethyl)-2-(1-nitrosopiperidin-4-yl)-1H-benzo[d]imidazole is the only nitrosamine detected. No non-substituted piperidine intermediate is used and no such related impurity (at ICH levels) can be detected in bilastine.
This is interesting as bilastine is a cyclic tertiary amine and considered difficult to be nitrosated as ring opening should be taken place.
Nevertheless, i would like to note that if the non-substituted piperidine intermediate was present at levels less than ICH, would be still a significant source for the formation of this nitrosamine. For example, if the LoD of the method is 0.1% (or 1000ppm), then a quantity of this impurity at levels of e.g. 900 ppm would be ‘‘not detected’’. If 0.5% of this quantity was converted to the respective nitrosamine (which is a usual conversion for an impurity) then, approximately 7 ppm of the nitrosamine is anticipated to be formed. This quantity is easily being identified with LC/MS.
Did you calculated the conversion factor of bilastine to the respective nitrosamine in your NAP tests?
Regarding non-substituted piperidine intermediate, according to route of synthesis it is highly unlikely and it has not been detected by LC/MS, thus IMHO bilastine dealkylation + nitrosation is the most (only?) plausible (and known) mechanism.
Furthermore, the amount formed during forced nitrosation studies of bilastine significantly exceeded 0.5% (well above that figure, as I recall), which effectively should rule out any other source that the API itself.
We carried out several assays with 1 equivalent of bilastine and two equivalents of sodium nitrite in water, acidified with formic or hydrochloric acid to pH 2 to 3.5 (optimal for nitrosation according to literature). Different times/temperatures were tested, up to five days at room temperature.
