Indeed Losartan Azide has been determined to be nonmutagenic in the in vivo comet assay after it was positive in the Ames test. This has now been confirmed also by CMDh and it is now accepted as a Class 5 impurity that can be controlled in accordance with ICH Q3A/B.
However, this is true for the azido impurity and currently this method is not accepted for nitrosamines by the health authorities. Namely, the comet assay is not considered by the HAs as a valid test for nitrosamines, and currently the only method to totally de-risk a nitrosamine (and consider it a Class 5 impurity) is by showing it to be negative in the Ames test followed by an in vivo transgenic gene mutation (TGR) assay. The Ames test, even using modified conditions for nitrosamines (pre-incubation, hamster S9, minimal DMSO), is not currently accepted alone in order to consider a nitrosamine as nonmutagenic.
The hope is that the HAs will eventually be “convinced” that a modified Ames protocol would suffice to de-risk a nitrosamine, or that an in vivo comet assay will be accepted valid for nitrosamines.
In the meantime, hold on tight and enjoy the ride 
6 Likes