Ordinal association between cytochrome P450 inhibition and hepatotoxicity severity in rats

Cytochrome P450s (P450s) are essential for xenobiotic metabolism, and their inhibition is associated with chemical-induced liver toxicity. While qualitative associations between P450 inhibition and hepatotoxicity have been reported, the quantitative relationship between the degree of inhibition and the severity of hepatotoxicity remains unclear. In this study, we explored the quantitative association between P450 inhibition and hepatotoxicity using lowest observed effect levels (LOELs) from rat repeated-dose toxicity (RDT) studies on 326 chemicals. Inhibitory activities against seven rat P450 isoforms were compared between compounds positive and negative for six liver-related group endpoints (gEPs). The results revealed that inhibitory activity against CYP1A1, CYP2B1, CYP2C6, and CYP3A2 was significantly higher in compounds positive for hepatocellular hypertrophy or dyslipidemia than in negative compounds. Although regression analyses did not show clear linear relationships between P450 inhibition and LOELs, nonparametric trend tests revealed modest monotonic associations, with increased P450 inhibition corresponding to lower LOELs. To identify structural factors influencing inhibition among highly toxic compounds, we compared molecular descriptors between those exhibiting strong or weak P450 inhibition. Descriptors related to aqueous solubility, Verhaar baseline toxicity, and structural complexity were consistently higher in the weak-inhibition group across multiple P450-gEP combinations. These findings suggest that inhibition of CYP1A1, CYP2B1, CYP2C6, and CYP3A2 partly contributes to the severity of hepatocellular hypertrophy and dyslipidemia, whereas highly toxic compounds with low P450 inhibition may exert their toxicity through P450-independent mechanisms.