Characterizing the Metabolism of Tire Rubber-Derived <i>p</i>-Phenylenediamine Quinones to Identify Potential Exposure Biomarkers in Humans

There is growing evidence of the frequent detection of tire rubber-derived contaminants <i>p</i>-phenylenediamine-derived quinones (PPD-Qs) (e.g., highly toxic <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>'-phenyl-<i>p</i>-phenylenediamine quinone (6PPD-Q)) in the environment and biota and the adverse impact on organisms. Hence, a better understanding of their biotransformation/metabolism in humans is essential. However, relevant data are lacking owing to recent discoveries. Herein, the biotransformation patterns of 6PPD-Q and other five commonly detected PPD-Qs were characterized via combined in vitro assay and maternal cord blood screening monitoring. Rapid metabolism was found for each PPD-Q incubated with human liver S9 fraction and microsomes, resulting in the formation of abundant phase I and phase II metabolites. The subsequent screening for potential PPD-Q metabolites in blood samples showed the presence of suspect metabolites. Three detected metabolites were confirmed by matching the mass spectra and retention times of in vitro metabolites. <i>N</i>-Dealkylated, carboxy, carbonyl, and reductive metabolites and glucose, cysteine, and methionine conjugates were observed for the first time. The semiquantitative concentrations of metabolites were higher than those of the parent PPD-Qs, and several metabolites such as carboxy products were proposed as candidate biomarkers of PPD-Q exposure to humans. 6PPD-Q and <i>N</i>,<i>N</i>'-diphenyl-<i>p</i>-phenylenediamine quinone were detected in maternal and/or cord whole blood samples for the first time. This study holds great importance in elucidating the potential risks and health effects of PPD-Qs.