Assessment of urinary 6-hydroxy-2,4-cyclohexadienyl mercapturic acid as a novel biomarker of benzene exposure

Assessing benzene exposure is a public health priority due to its deleterious health effects and ubiquitous industrial and environmental sources of exposure. Phenyl mercapturic acid (PhMA) is a commonly used urinary biomarker to assess benzene exposure. However, recent work has identified significant interlaboratory variation in urinary PhMA concentrations related to methodological differences. In this study, we present urinary 6-hydroxy-2,4-cyclohexadienyl mercapturic acid (pre-PhMA), a metabolite that undergoes acid-catalyzed dehydration to form PhMA, as a novel and specific urinary biomarker for assessing benzene exposure. We developed and validated the first quantitative liquid chromatography-tandem mass spectrometry assay for measuring urinary concentrations of pre-PhMA. The pH effect on the method of ruggedness testing determined that pre-PhMA is stable across the normal human urine pH range and that neutral conditions must be maintained throughout quantification for robust and accurate measurement of urinary pre-PhMA concentrations. The method exhibited below 2 ng/mL sensitivity for pre-PhMA, linearity over three orders of magnitude, and precision and accuracy within 10%. Urinary pre-PhMA concentrations were assessed in 369 human urine samples. Smoking individuals exhibited elevated levels of pre-PhMA compared to non-smoking individuals. Furthermore, the relationship between benzene exposure and urinary pre-PhMA levels was explored by examining the correlation of pre-PhMA with 2-cyanoethyl mercapturic acid, a smoke exposure biomarker. The urinary biomarkers exhibited a positive correlation (r = 0.720), indicating that pre-PhMA levels increased with benzene exposure. The results of this study demonstrate that urinary pre-PhMA is a rugged and effective novel biomarker of benzene exposure that can be widely implemented for future biomonitoring studies.