Detection and Quantitation of N′-Nitrosonornicotine in Human Toenails by Liquid Chromatography-Electrospray Ionization-Tandem Mass Spectrometry

Evaluation of tobacco carcinogen uptake and metabolism in individuals, as well as prediction of potential health risks in people who are exposed to tobacco products, is theoretically possible through the use of biomarkers of tobacco carcinogen exposure. The most practical and extensively used biomarkers that have provided important information about tobacco carcinogen dose and metabolism are urinary metabolites of a tobacco-specific lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (1, 2). The sum of the 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone metabolites 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronides, called total NNAL, was analyzed in several studies involving smokers, smokeless tobacco users, and nonsmokers exposed to secondhand tobacco smoke (3-7). Recently, we developed a method for quantitation of total NNAL in human toenails (8, 9). Use of toenail biomarkers offers several advantages over urine analysis, with potential evaluation of long-term cumulative exposure due to slow growth being the most appealing one (10). Among other advantages are ease of collection, unlimited stability, and availability in some large-scale epidemiologic studies. We developed sensitive mass spectrometric methods for analysis of total nicotine, its major metabolite cotinine, and total NNAL in toenails (8) and, in a separate study, provided essential validation data for the use of toenail biomarkers in the investigation of the role of chronic tobacco smoke exposure in human cancer (9).

Another recent development in the field of tobacco carcinogen biomarkers was detection and quantitation of a biomarker of human exposure to N'-nitrosonornicotine (NNN; ref. 11). NNN, like 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, is formed during tobacco processing via nitrosation of tobacco alkaloids and is present in both unburned tobacco and cigarette smoke (12-14). Based on its occurrence in tobacco products and cigarette smoke and on its carcinogenic activity in laboratory animals, NNN is believed to be a cause of esophageal cancer in smokers (1, 15). The sum of unchanged NNN and its pyridine-N-glucuronide (NNN-N-Gluc), called total NNN, was quantified in the urine of smokers and smokeless tobacco users (11).

The purpose of this study was to detect and quantify NNN in human toenails by using a highly sensitive and selective method: liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). The structures of the biomarkers discussed here are shown in Fig. 1.

NNN is carcinogenic and mutagenic and should be handled with extreme care using appropriate protective clothing and ventilation at all times.

NNN was purchased from Toronto Research Chemicals. [¹³C₆]NNN was purchased from Cambridge Isotope Laboratories. Nicotine, [CD₃]nicotine, cotinine, and [CD₃]cotinine were obtained from Sigma.

Toenail samples from 17 active smokers were available from our previous study (8). The smokers were originally recruited from several smoking studies conducted at the Transdisciplinary Tobacco Use Research Center as described previously (8). The entrance criteria of these studies required subjects to smoke at least 10 cigarettes per day for at least 1 year. All studies were approved by the University of Minnesota Research Subjects' Protection Programs Institutional Review Board Human Subjects Committee.

Before digestion, toenail clippings (40-100 mg) were weighed into 5 mL polypropylene tubes and washed in CH₂Cl₂ and dried as described previously (8).

Toenails were digested at 50°C overnight in 2 mL 1 N NaOH. The next day, the pH of the digests was adjusted to 6 to 8 by adding 1 N HCl. [¹³C₆]NNN (10 pg) was added as internal standard. The mixture was applied to a 5 mL ChemElut cartridge (Varian) and eluted with 2 × 8 mL CH₂Cl₂ into a 15 mL glass centrifuge tube, and the combined eluants were concentrated to dryness. The dry residue was redissolved in 1 mL H₂O and adjusted to pH 2 to 3 by adding 100 μL 1 N HCl, and the mixture was applied to a 60 mg Oasis MCX cartridge (Waters) activated with 5 mL CH₃OH and equilibrated with 10 mL H₂O. The cartridges were washed with 5 mL 1 N HCl, 5 mL CH₃OH, and 5 mL H₂O/CH₃OH/NH₄OH (90:5:5) and these washings were discarded. Then, NNN and [¹³C₆]NNN were eluted from the Oasis MCX cartridges with 5 mL H₂O/CH₃OH/NH₄OH (45:50:5) and the eluant was concentrated to dryness. Residues were dissolved in 0.5 mL CH₂Cl₂ and loaded on 100 mg Bond-Elut Silica cartridges (Varian) preequilibrated with 1 mL CH₂Cl₂. The cartridges were washed with 1 mL CH₂Cl₂ and 1 mL CH₂Cl₂/ethyl acetate (50:50). NNN and [¹³C₆]NNN were eluted with 2 mL ethyl acetate and the eluants were concentrated to dryness. The dry residues were transferred to autosampler vials with CH₃OH, dried, and stored at −20°C until analysis by LC-ESI-MS/MS. Before analysis, samples were redissolved in 10 μL of 2% CH₃OH in H₂O, and 5 μL were injected.

LC-ESI-MS/MS was carried out with a Finnigan TSQ Quantum Discovery Max instrument (Thermo Electron) interfaced with an Agilent Model 1100 capillary high-performance liquid chromatography system and a Model 1100 micro autosampler (Agilent). The high-performance liquid chromatograph was fitted with a 150 × 0.5 mm ZORBAX SB C18 RR 3.5 μm column (Agilent) eluted isocratically with 35% methanol in H₂O for 20 min at a flow rate of 10 μL/min. The column was maintained at 25°C. MS/MS variables were as follows: positive ion electrospray mode with selected reaction monitoring for m/z 178 → m/z 148 ([M + H]⁺ → [M + H - NO]⁺) for NNN and m/z 184 → m/z 154 for [¹³C₆]NNN at 0.5 amu scan width. The collision gas was Ar at a pressure of 1 mTorr, with collision energy of 12 eV. The quadrupoles were operated at a resolution of 0.7 amu.

Nicotine, cotinine, and NNAL in toenail clippings were analyzed as described previously (8).

The accuracy of the assay was determined by spiking toenail samples from two nonsmokers with 2, 5, and 10 pg NNN. Mean recovered NNN in these samples was 2.5, 5.8, and 12.3 pg, respectively (r = 0.99). Analysis of NNN in four aliquots of a smoker's digested toenail sample produced a coefficient of variation of 2.7%. A typical chromatogram of a toenail sample from a smoker is illustrated in Fig. 2B. The detection limit of the method is 0.02 pg/mg toenail in a 50 mg sample or 5 fmol/sample.

Toenails of 17 smokers from our previous study (8) were analyzed for total NNN. The results of these analyses as well as NNAL data for these samples from our previous study are summarized in Table 1. Mean total NNN level in these samples was 4.63 ± 6.48 (SD) fmol/mg toenail and correlated with the previously reported total NNAL (r = 0.96; P < 0.0001), total nicotine (r = 0.48; P < 0.05), and total cotinine (r = 0.87; P < 0.0001; ref. 8).

NNN levels found in nonsmokers' toenails were significantly lower than those in smokers (P = 0.03) and averaged 0.35 ± 0.16 fmol/mg. Also, toenails of all nonsmokers analyzed here contained nicotine and cotinine, averaging 0.76 ± 0.22 and 0.044 ± 0.020 pmol/mg toenail, respectively. Toenail NNN positively correlated with cotinine in these subjects (r = 0.58; P = 0.05) but not nicotine (r = 0.34; P = 0.29).

In this study, we report the presence of a tobacco-specific carcinogen, NNN, in human toenails and establish a LC-ESI-MS/MS method for its quantitation. Two recent previous studies conducted by our research group led to the discovery of two new biomarkers of human tobacco carcinogen exposure, that is, urinary NNN (11) and toenail NNAL (8). These findings, along with the availability of highly effective sample preparation techniques and highly sensitive and selective LC-ESI-MS/MS instrumentation, encouraged us to investigate the possibility of analyzing NNN in human toenails.

The method described here was developed based on our reported procedure for NNAL analysis in toenails (8, 9) and on a modified procedure for NNN analysis in urine (16). In addition to extraction of the digested and neutralized toenail sample on ChemElut and MCX cartridges, as reported for the toenail NNAL assay (8), we introduced an additional step (normal phase extraction) in the sample preparation procedure to produce cleaner samples and thus increase our chance of detecting and quantifying NNN. The method is characterized by a low detection limit for NNN and excellent accuracy and precision. Similar to the NNAL analysis in toenails, a disadvantage of the proposed method is that we are not able to distinguish between free NNN and NNN-N-Gluc. Because the analysis involves digestion in 1 N NaOH at the initial step, any NNN-N-Gluc, if present in human toenails, would be converted to its aglycone; thus, total NNN is measured. However, previous experiments with β-glucuronidase indicated that NNAL-O-Gluc was not present in human toenails (8). Moreover, for the purpose of general evaluation of human NNN uptake, total NNN would be sufficient.

Analysis of samples collected from smokers showed that amounts of NNN in smokers' toenails are generally higher than those of NNAL (Table 1). The ratio of toenail NNN to NNAL averaged 2.8, whereas the ratio between the levels of these biomarkers in smokers' urine was 0.1 (11). This could be due to differences in NNN and NNAL pharmacokinetics in humans. The biological half-life of NNAL in laboratory animals is generally slightly longer than that of NNN (17-21); however, there are no published data on NNN pharmacokinetics in humans. Differences in the chemical structure and polarity of the NNN and NNAL molecules, with NNAL being the more polar, should be considered. Despite the large number of studies on drug incorporation into nails (reviewed in ref. 10), the mechanism of this process has not been established yet, and it is unclear how the physicochemical characteristics of different compounds affect their interaction with the nail matrix. Studies involving the antifungal drug fluconazole, for instance, have shown that low oral doses of this drug produce high nail concentrations compared with other antifungal preparations probably due to the hydrophilicity of fluconazole, which is responsible for its high bioavailability and its high plasma concentrations (22). On the other hand, nail concentrations of cocaine generally exceed those of its metabolites, suggesting that lipophilicity is an important factor that influences xenobiotic affinity for keratin (23).

The strong positive correlation of toenail NNN with toenail NNAL and cotinine observed in smokers is similar to that reported for urinary biomarkers (11). Moreover, in our previous study, we showed that toenail NNAL and cotinine positively correlated with the levels of these biomarkers in plasma and urine (9). These considerations indicate that toenail NNN promises to be a useful biomarker of human chronic exposure to NNN. In fact, our long-term goal is to use toenail biomarkers in epidemiologic studies of the role of secondhand smoke exposure in human cancer. In this study, all nonsmoker toenails had low but detectable levels of total NNN as well as nicotine and cotinine. The presence of nicotine and cotinine in toenails of nonsmokers who report no exposure to secondhand smoke was shown in other studies (8, 24). Further investigations are necessary to evaluate the applicability of toenail NNN as a biomarker of tobacco carcinogen exposure in nonsmokers.

In summary, we have developed an assay for total NNN in human toenails. This new biomarker may be useful in epidemiologic studies of the role of smoking in esophageal cancer. Investigation of the role of secondhand smoke exposure in human cancer is another potential application. Further studies will be focused on the development of a method that will allow simultaneous determination of total NNAL and total NNN in human toenails.

We thank Bradley Lieberman for outstanding technical assistance, Dr. Peter W. Villalta for help with mass spectrometry techniques, and Bob Carlson for editorial assistance.