Simultaneous quantification of ethylpurine adducts in human urine by stable isotope dilution nanoflow liquid chromatography nanospray ionization tandem mass spectrometry

• Analysis of 3-ethyladenine and 7-ethylguanine in human urine.
• Adduct enrichment by a polymeric reversed phase solid-phase extraction column.
• Sensitive analysis by stable isotope dilution nanoflow LC-NSI/MS/MS.

Ethylating agents contained in cigarette smoke can damage DNA producing ethylated DNA adducts, including N3-ethyladenine (3-EtAde) and N7-ethylguanine (7-EtGua). In this study, a highly specific and sensitive assay based on stable isotope dilution nanoflow liquid chromatography nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) was used to measure 3-EtAde and 7-EtGua in human urine. These urinary adducts were enriched by a polymeric reversed phase solid-phase extraction column before the nanoLC-NSI/MS/MS analysis. The on-column detection limits (S/N ≥ 3) of 3-EtAde and 7-EtGua were 15 fg (92 amol) and 10 fg (56 amol), respectively, while the lower quantification limits of 3-EtAde and 7-EtGua were 930 and 840 amol, respectively. Urinary concentrations of 3-EtAde and 7-EtGua in 21 smokers were 68.6 ± 29.4 and 18.7 ± 13.8 pg/mL, respectively. In 20 nonsmokers, concentrations of 3-EtAde and 7-EtGua were 3.5 ± 3.8 and 2.4 ± 3.0 pg/mL, respectively. The urinary concentrations of 3-EtAde and 7-EtGua were statistically significantly higher in smokers than in nonsmokers (p < 0.0001). Moreover, 3-EtAde and 7-EtGua concentrations are significantly correlated with the number of cigarettes smoked per day and with the smoking index. This highly specific and sensitive assay based on stable isotope dilution nanoLC-NSI/MS/MS assay should be clinically valuable in assessing the possibility of measuring urinary ethylpurines as noninvasive biomarkers for smoking-related cancers in humans.