Quantitative determination of urinary N3-methyladenine by isotope-dilution LC-MS/MS with automated solid-phase extraction

A rapid, highly specific and sensitive liquid chromatographic-tandem mass spectrometric (LC-MS/MS) method was developed for measuring urinary N3-methyladenine (N3-MeA). With the use of an isotope internal standard (d 3-N3-MeA) and on-line solid-phase extraction (SPE), urine samples can be directly analyzed within 12 min without prior sample purification. The detection limit of this method was estimated as 0.035 ng/mL on-column (11.7 fmol). Intra- and inter-day imprecision (CV) were <7.4% and <10.8%, respectively. Mean recovery of N3-MeA in urine was 97-101%. This method was further applied to study the methylating agents exposure arising from cigarette smoke. Eighty-six volunteers were recruited including 41 regular smokers and 45 nonsmokers. The results showed that the urinary levels of N3-MeA observed in smokers (10.9 ± 13.2 ng/mg creatinine) were significantly higher than those in nonsmokers (4.1 ± 4.3 ng/mg creatinine; P < 0.001). It was further noted that the urinary level of N3-MeA was found to be highly associated with cotinine for smokers (Spearman correlation coefficients, r = 0.69, P < 0.001), implying that cigarette smoking resulted in increased DNA methylation, followed by depurination and excretion of N3-MeA in urine. As a result of the on-line SPE system, this method is capable of routine high-throughput analysis and accurate quantification of N3-MeA, and would be a useful tool for the surveillance of methylating agent exposure and its associated cancer risk.