Disposition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) in bile duct-cannulated rats: Stereoselective metabolism and tissue distribution

Abstract

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) is a chiral compound, and the primary metabolite of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a major carcinogen in tobacco smoke. The goal of the present work was to study the pharmacokinetics and stereoselective metabolism and tissue retention of NNK and NNAL in the rat. Groups of rats were dosed with [5-(3)H]NNK (n = 3) or racemic [5-(3)H]NNAL (n = 3) at a target dose of 8.45 micromol/kg and were killed at selected time points for tissue collection. Separate groups of rats (n =5 per group) received the same dose of either NNK or NNAL and serial sampling of blood, bile and urine was carried out over 24 h. All samples were analyzed by C(18) reversed-phase HPLC with gradient elution and radioflow detection. A gas chromatograph-thermal energy analyzer (GC-TEA) was used to separate the (R)-/(S)-NNAL enantiomers. Racemic NNAL and NNK had large volumes of distribution (321 +/- 137 ml for NNK and 2772 +/- 1423 ml for NNAL) and similar total body clearances (12.8 +/- 2.0 ml/min for NNK and 8.6 +/- 2.6 ml/min for NNAL). The results indicated that the enantiomers of NNAL are stereoselectively metabolized and excreted. The glucuronide of (R)-NNAL, ((R)-NNAL-Gluc) was identified as the major metabolite in the bile after administration of either NNK or NNAL. (R)-NNAL was the major NNAL enantiomer in the bile or urine samples. At 24 h after racemic NNAL administration, NNAL comprised an average of 75.4% of total radioactivity in the lung with an (S)-/(R)-ratio of >20. The stereoselective localization of (S)-NNAL to lung tissue may contribute to the lung selectivity of NNK carcinogenesis. The present studies suggest a need to look beyond metabolic activation as the sole mechanism for lung carcinogenesis.

Analysis of Pyridyloxobutyl and Pyridylhydroxybutyl DNA Adducts in Extrahepatic Tissues of F344 Rats Treated Chronically with 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone and Enantiomers of 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol

“…Previous studies indicated that NNK was preferentially converted to ( S )-NNAL in rats (6, 7). ( S )-NNAL had a large volume of distribution, and was selectively retained in the lung, while ( R )-NNAL was extensively glucuronidated and excreted in the urine (10, 11). Both NNK and NNAL can be metabolically activated by cytochrome P450-catalyzed α-hydroxylation, and POB- and PHB-DNA adducts are formed exclusively from their corresponding precursors, as shown in Scheme 1.…”

“…Our results in this study fully supported this hypothesis, and indicate that the sequestration of ( S )-NNAL not only occurs in the lung, but also in other target tissues of NNK. In fact, Wu et al (10) showed the presence of ( S )-NNAL in liver and kidney 24 h after NNK or NNAL administration, although higher concentrations were found in the lung. Our results suggest extensive distribution and retention of ( S )-NNAL, possibly at receptor sites, in various extra-hepatic tissues including pancreas, nasal and oral mucosa.…”

“…NNAL can be subsequently glucuronidated and excreted in the urine. While ( R )-NNAL is extensively conjugated in this way, ( S )-NNAL tends to be retained in the lung and reconverted back to NNK (10, 11). In smokers and smokeless tobacco users, the ratios of ( S )/( R )-NNAL and their glucuronides in the urine were significantly higher 7 days after cessation than at baseline, indicating a stereoselective retention of ( S )-NNAL (12).…”

Precancerous model of human breast epithelial cells induced by NNK for prevention

“…NNK is considered one of the most potent lung carcinogens in tobacco products [18,19]. Wu et al [20] have revealed a novel persistent activity of NNK with an interchangeable conversion between NNK and its cell metabolites. Chhabra et al [21] have shown that gavage of rodents with NNK results in the formation of DNA-adducts in the mammary gland.…”

Comparative Metabolism of the Tobacco-Specific Nitrosamines 4-(Methylnitrosamino)-1-(3-Pyridyl)-1-Butanone and 4-(Methylnitrosamino)-1-(3-Pyridyl)-1-Butanol by Rat Cytochrome P450 2a3 and Human Cytochrome P450 2a13

“…Considering that NNK is rapidly converted to NNAL in vivo and that NNAL has similar carcinogenic potency to NNK in the rat lung (Hecht, 1998), an investigation of the kinetics of P450 2A3-and 2A13-mediated NNAL metabolism was warranted. Finally, the metabolism of individual NNAL enantiomers by P450s 2A3 and 2A13 was investigated because the metabolism and disposition of NNAL in animals and humans is profoundly affected by the absolute stereochemistry of the carbinol carbon (Hecht, 1998;Hecht et al, 2002;Wu et al, 2002). Thus, the effect of carbinol-carbon stereochemistry on the regioselectivity of metabolism by these P450s may provide important information about active-site architecture.…”