Deriving safe limits for N-nitroso-bisoprolol by error-corrected next-generation sequencing (ecNGS) and benchmark dose (BMD) analysis, integrated with QM modeling and CYP-docking analysis

N-nitroso-bisoprolol (NBP) is a nitrosamine drug substance-related impurity (NDSRI) of bisoprolol, which is used to treat cardiac diseases since decades. To investigate the mutagenic potential of NBP, in vitro methods such as Enhanced Ames Test (EAT) and a mammalian cell gene mutation (HPRT) assay were used. To assess the in vivo mutagenicity, a 28-day repeat-dose study was conducted in wild-type NMRI mice, and liver and bone marrow samples were subjected to error-corrected next-generation sequencing (i.e., duplex sequencing) followed by benchmark dose analysis (BMD). NBP did not show mutagenic effects in Ames tests using 10 % and 30 % induced rat or 30 % uninduced hamster S9. However, relevant increases in mutation frequencies were observed in an EAT in the presence of 30 % induced hamster S9 in strains TA100 and TA1535, confirming that the most stringent conditions of the EAT are appropriate to detect the mutagenic activity of weak mutagens, such as NBP. In the HPRT assay conducted in V79 cells, nitroso-diethylamine (NDEA) relevantly induced the mutation frequency, but not NBP. The highly sensitive error-corrected Next-Generation Sequencing (ecNGS) method to detect mutations across the genome represents an appropriate in vivo mutagenicity investigation equally suitable as a TGR assay to assess the mutagenic potential of nitrosamines. A weak induction of mutation frequencies was detected by ecNGS in the liver and the bone marrow of mice. Using BMD analysis, new safe limits were calculated for NBP, which are higher than the published AI of 1.5 µg/person/day. Using the approach to calculate Permissible Daily Exposure (PDE) limits according to ICH Q3C, a lifetime PDE of 400 µg/person/day was derived. Based on the ICH M7 framework for derivation of Acceptable Intake (AI) limits, an AI of 64 µg/person/day was established. Consistent with regulatory emphasis on mechanistic interpretation, in vivo modeling was further supported by in silico calculations. Specifically, the validated Computer-Aided Discovery and RE-design (CADRE) tool was used to predict the potency of NBP and further differentiate its metabolic activity from the anchor nitrosamine NDEA via quantum mechanics (QM) calculations and CYP-binding predictions. Outcomes of this analysis were consistent with in vivo studies, while offering a deeper understanding of the fundamental biochemistry using a physics-led method. The integrated in vivo-in silico investigation provides a data-based determination of safe limits, suggesting that the AI based on structural considerations solely might be over-conservative and should not be capped at the TTC.