Mutagenic and carcinogenic potency determinations for NDMA support the cumulative dose assumption underpinning the less-than-lifetime Threshold of Toxicological Concern

The management of N-nitrosamine impurities challenges pharmaceutical development and regulation worldwide. Because most medicinal exposures are shorter than lifetime and absolute impurity exclusion is impossible, reliable approaches to define duration-specific intake limits are essential. On the premise that carcinogenic risk is proportional to cumulative dose, the 'Less-Than-Lifetime' (LTL) Threshold of Toxicological Concern (TTC) framework defines progressively lower intake limits for mutagenic impurities over longer exposures. However, N-nitrosamines are currently treated as a 'cohort of concern', necessitating compound-specific evaluation placing reliance on in vivo mutagenicity assays for impurity qualifications. To better understand durational potency relationships and the application domain of the LTL-TTC, we applied benchmark dose (BMD) modelling to cumulative-dose-scaled transgenic rodent (TGR), error-corrected sequencing and rodent carcinogenicity datasets for N-nitrosodimethylamine (NDMA) obtained from the published literature. For TGR, cumulative-dose scaling better resolved liver as the most sensitive organ and reduced interstudy variability: liver BMDs spanned ~ 80-fold in daily-dose units but only ~ 20-fold when scaled to cumulative dose. Among closely-matched mouse liver gavage studies, cumulative-dose BMDs only varied by ~ 2.5-fold across 1 to 28-day treatment regimens. Error-corrected sequencing also demonstrated parity, with acute-dose regimens producing mutation burdens near-identical (< 1.2-fold) to those cumulated from 28-day repeat-dose regimens. Comparable results were obtained from carcinogenicity datasets confirming proportionality-of-effect to cumulative dose. These findings empirically support the validity of the LTL-TTC concept. More broadly, they demonstrate that short-term in vivo mutagenicity assays can serve as reliable surrogates for lifetime carcinogenicity studies, strengthening the scientific and regulatory basis for duration-adjusted acceptable intakes for N-nitrosamine impurities.