Phenanthrene tetraols as probes for the metabolic activation of carcinogenic polycyclic aromatic hydrocarbons in humans

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are believed to be among the causative agents for lung cancer in smokers. Benzo[a]pyrene (BaP) is a prototypic carcinogenic PAH, which requires metabolic activation to elicit its detrimental effects. The major end product of its diol epoxide metabolic activation pathway is r-7,t-8,9,c-10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (trans, anti-BaPT). Individual differences in exposure to, and metabolic activation of, carcinogenic PAH may influence cancer risk. Measurement of PAH metabolites in human urine could provide a direct way to assess individual differences in susceptibility to PAH-related cancer. We developed a sensitive and reliable method for quantitation of trans, anti-BaPT in human urine using gas chromatography-negative ion chemical ionization-tandem mass spectrometry (GC-NICI-MS/MS). A unique phenotyping strategy using [D10]PheT as probes have been developed, allowing direct assessment of metabolic activation of carcinogenic PAH in humans. We investigated [D10]phenanthrene metabolism in smokers after administration by inhalation in cigarette smoke or orally. Sixteen smokers received 10 µg [D10]phenanthrene in a cigarette or orally. Plasma and urine samples were analyzed for [D10]r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene ([D10]PheT), the major end product of the diol epoxide pathway, by gas chromatography-negative ion chemical ionization-tandem mass spectrometry. The ratios of [D10]PheT (oral dosing: inhalation) in 15 smokers were 1.03 ± 0.32 and 1.02 ± 0.35, based on plasma AUC0-∞ and total 48 hr urinary excretion, respectively. Overall there was no significant difference in the extent of [D10]PheT formation after the two different routes of exposure in smokers. Its application in clinical studies has approved, for the first time, that there is large inter-individual variation in PAH metabolic activation in smokers. One striking finding is that the formation of PAH diol epoxides, the ultimate carcinogen, occurs astonishingly rapidly in smokers, particularly after inhalation of cigarette smoke. The pharmacokinetics parameters of PAH metabolites in humans have also been presented. The results have established the basics for using [D10]PheT as a biomarker for PAH metabolic activation in future large-scale risk assessment studies in smokers.