Metabolism of (plus or minus)-cotinine-2-14C in the rat

Characterization of multiple products of cytochrome P450 2A6-catalyzed cotinine metabolism

The primary metabolite of nicotine in smokers is cotinine. Cotinine is further metabolized to trans-3'-hydroxycotinine, the major urinary metabolite of nicotine in tobacco users. It was recently reported that cytochrome P450 2A6 catalyzes the conversion of cotinine to trans-3'-hydroxycotinine. In this work, we report that P450 2A6 metabolizes cotinine not only to trans-3'-hydroxycotinine but also to 5'-hydroxycotinine, norcotinine, and a fourth as yet unidentified metabolite. The products of baculovirus-expressed P450 2A6 [methyl-(3)H]cotinine metabolism were analyzed by radioflow HPLC. Three (3)H-labeled metabolites were detected and were present in approximately equal amounts. The identities of two of the metabolites were confirmed to be 5'-hydroxycotinine and trans-3'-hydroxycotinine by LC/MS/MS and LC/MS analysis and comparison to standards. The third product was not identified. A fourth product of P450 2A6-catalyzed cotinine metabolism was detected by LC/MS. It was identified by cochromatography with a standard and MS and MS/MS data to be norcotinine. An attempt was made to further characterize the unidentified (3)H-labeled metabolite by comparison to the cotinine metabolites generated by hamster liver microsomes. Hamster liver microsomes contain a P450, 2A8, which is closely related to P450 2A6, and have previously been shown to metabolize cotinine to three hydroxylated products, trans-3'-hydroxycotinine, 5'-hydroxycotinine, and N-(hydroxymethyl)norcotinine. We were unable to confirm that N-(hydroxymethyl)norcotinine was the unidentified cotinine metabolite generated by P450 2A6.

Stereoselective metabolism of nicotine and tobacco-specific N-nitrosamines to 4-hydroxy-4-(3-pyridyl)butanoic acid in rats

The carcinogenic tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are believed to play a role in cancers associated with the use of tobacco products. Urinary metabolites of NNK and NNN could be used as biomarkers for an individual's ability to metabolically activate or detoxify these nitrosamines. While several metabolites of NNK can be quantified in human urine, no assay is available to determine human urinary levels of NNK and NNN metabolites resulting from the critical alpha-hydroxylation metabolic activation pathways. The major urinary metabolites resulting from alpha-hydroxylation of NNK and NNN in rodents are 4-oxo-4-(3-pyridyl)butanoic acid (keto acid) and 4-hydroxy-4-(3-pyridyl)butanoic acid (hydroxy acid). The major obstacle to the use of these metabolites as biomarkers of metabolic activation is the fact that they are also metabolites of nicotine, which is present at levels 1400-13000 times greater than those of the nitrosamines in cigarette smoke. However, the chirality of hydroxy acid could be useful in overcoming this problem. If different enantiomers of hydroxy acid were formed from nicotine versus the nitrosamines, and if the overall yield of hydroxy acid from nicotine were substantially smaller than that from the nitrosamines, then hydroxy acid might be useful as a urinary biomarker of NNK and NNN alpha-hydroxylation. To these ends, F-344 rats were administered either [5-3H]NNK, [5-3H]NNN, [5-3H]keto acid, or [2'-14C]nicotine. The levels of urinary hydroxy acid were determined by HPLC analysis. Its stereochemistry was determined by conversion to its methyl ester, reaction with (S)-(-)-alpha-methylbenzyl isocyanate, and separation and quantitation of the resulting diastereomers by HPLC. Urinary hydroxy acid accounted for 12% of the NNK dose and 31% of the NNN dose, but only 1 and 0.1% of the dose of keto acid and nicotine, respectively. Furthermore, metabolism of NNK produced mainly (S)-hydroxy acid in the urine, while metabolism of keto acid and nicotine gave predominantly (R)-hydroxy acid. Both enantiomers were present in the urine of NNN-treated rats. Therefore, in the rat, it is possible to distinguish the hydroxy acid derived from nicotine from that derived from the nitrosamines. If similar pathways occur in humans, (S)-hydroxy acid could potentially be developed as a urinary biomarker of NNK and NNN alpha-hydroxylation in smokers.

Radiometric-high-performance liquid chromatographic assay for nicotine and twelve of its metabolites

A sensitive, reproducible radiometric-high-performance liquid chromatographic assay has been developed to measure concentrations of nicotine and twelve of its metabolites in biological fluids. Following administration of nicotine ([2-14C]pyrrolidine) to rats, the assay was used in a pharmacokinetic investigation.

Biotransformation of primary nicotine metabolites. II. Metabolism of [3H]-S-(-)-cotinine in the guinea pig: determination of in vivo urinary metabolites by high-performance liquid-radiochromatography

1. The metabolism of [3H]-S-(-)-cotinine in vivo has been investigated in the guinea pig. The quantitive determination of urinary metabolites has been carried out using a high-performance liquid radiochromagraphic procedure. Metabolite analysis of C- and N-oxidation products, and N-methylcotininium ion, were carried out on two chromatographic systems: (a) Partisil-10 SCX cation-exchange chromatography, and (b) Partisil-10 ODS reverse-phase chromatography. 2. 3-Hydroxycotinine was the major urinary metabolite of cotinine in the guinea pig, accounting for 57% of the total radioactivity in the urine. Cotinine-N-oxide constituted 18% of total metabolites in the urine, whereas neither nicotine nor the N-methylcotininium ion were detected as urinary metabolites of [3H]-S-(-)cotinine. S-(-)-Cotinine was extensively metabolized in the guinea pig; total recovery of radioactivity in 24 h urine was very high (greater than 95%) and very little cotinine was detected (less than 1%) in the urine. 3. Two unidentified metabolites of [3H]-S-(-)-cotinine were detected which collectively constituted approximately 20% of total urinary tritium.

The metabolism of [14C] nicotine in the cat

The metabolism of [2'-(14)C]nicotine given as an intravenous injection in small doses to anaesthetized and unanaesthetized cats has been studied. A method is described for the quantitative determination of [(14)C]nicotine and [(14)C]cotinine in tissues and body fluids. Nanogram amounts of these compounds have been detected. After a single dose of 40mug. of [(14)C]nicotine/kg., 55% of the injected radioactivity was excreted in the urine within 24hr., but only 1% of this radioactivity was unchanged nicotine. [(14)C]Nicotine is metabolized extremely rapidly, [(14)C]cotinine appearing in the blood within 2.5min. of intravenous injection. [(14)C]Nicotine accumulates rapidly in the brain and 15min. after injection 90% of the radioactivity still represents [(14)C]nicotine. Metabolites of [(14)C]nicotine have been identified in liver and urine extracts. [(14)C]Nicotine-1'-oxide has been detected in both liver and urine.