Simple high-throughput analytical method using ultra-performance liquid chromatography coupled with tandem mass spectrometry to quantify total 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol in urine

Application of an assay for 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) in urine for the assessment of tobacco-related harm

BACKGROUND

The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is formed from nicotine and related compounds during tobacco curing and is classified as a human carcinogen. Its major metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), is thought to be useful in the assessment of cigarette smoking harm minimisation strategies.

METHODOLOGY

Urine samples were collected from 24 current Caucasian smokers participating in a smoking cessation study; before and four weeks after a quit attempt. Samples were spiked with NNAL-d3 internal standard, extracted with ethyl acetate using liquid-liquid extraction, reconstituted with deionised water and analysed using ultra-performance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS). Both free (unconjugated) and total NNAL was measured, with totals determined by cleavage of NNAL-glucuronide using standard enzymatic hydrolysis methods.

RESULTS

Free NNAL levels (193.5±158.2pg/mL [mean±standard deviation]) measured in urine samples obtained at baseline (during ad lib smoking) were indicative of biological exposure to NNK. Free NNAL levels were significantly reduced four weeks after the quit attempt (64.5±77.6pg/mL; p<0.002). Assay performance met acceptance criteria with mean recovery of 59±23%, intra-day accuracy was 3.7%, 3.7 and 3.6% and precision was 11.3%, 5.1% and 5.3% at 5pg/mL, 20pg/mL and 100pg/mL levels respectively. Enzymatic hydrolysis of NNAL-glucuronide proved unreliable with complete loss of NNAL aglycone in some subject samples following the hydrolysis procedure.

CONCLUSION

Liquid-liquid extraction with UPLC-MS/MS is a convenient approach to measure low levels of NNAL in urine as a marker of biological NNK exposure, which can be used to assess the effectiveness of smoking reduction harm minimisation strategies. Enzymatic hydrolysis of NNAL-glucuronide and measuring the aglycone is not recommended.

Stereospecific Metabolism of the Tobacco-Specific Nitrosamine, NNAL

Among the most potent carcinogens in tobacco are the tobacco-specific nitrosamines (TSNAs), with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) being the most potent as well as one of the most abundant. NNK is extensively metabolized to the equally carcinogenic 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). Of the two NNAL enantiomers, (S)-NNAL not only appears to be preferentially glucuronidated and excreted in humans but also exhibits higher stereoselective tissue retention in mice and humans and has been shown to be more carcinogenic in mice than its (R) counterpart. Due to the differential carcinogenic potential of the NNAL enantiomers, it is increasingly important to know which UGT enzyme targets the specific NNAL enantiomers for glucuronidation. To examine this, a chiral separation method was developed to isolate enantiomerically pure (S)- and (R)-NNAL. Comparison of NNAL glucuronides (NNAL-Glucs) formed in reactions of UGT2B7-, UGT2B17-, UGT1A9-, and UGT2B10-overexpressing cell microsomes with pure NNAL enantiomers showed large differences in kinetics for (S)- versus (R)-NNAL, indicating varying levels of enantiomeric preference for each enzyme. UGT2B17 preferentially formed (R)-NNAL-O-Gluc, and UGT2B7 preferentially formed (S)-NNAL-O-Gluc. When human liver microsomes (HLM) were independently incubated with each NNAL enantiomer, the ratio of (R)-NNAL-O-Gluc to (S)-NNAL-O-Gluc formation in HLM from subjects exhibiting the homozygous deletion UGT2B17 (*2/*2) genotype was significantly lower (p = 0.012) than that with HLM from wild-type (*1/*1) subjects. There was a significant trend (p = 0.015) toward a decreased (R)-NNAL-O-Gluc/(S)-NNAL-O-Gluc ratio as the copy number of the UGT2B17*2 deletion allele increased. These data demonstrate that variations in the expression or activity of specific UGTs may affect the clearance of specific NNAL enantiomers known to induce tobacco-related cancers.