Phenotyping of N-acetyltransferase type 2 and xanthine oxidase with caffeine: when should urine samples be collected?
OBJECTIVE
Individual activities of N-acetyltransferase 2 (NAT2) and of xanthine oxidase (XO) can be assessed using ratios of urinary caffeine metabolites. We investigated how ratios changed over time and which urine collection interval would be the best for NAT2 and XO activity assessments.


METHODS
On two occasions separated by 14 days, 16 healthy male Caucasians collected urine before and 0-2, 2-4, 4-6, 6-8, 8-12, 12-16 and 16-24 h after a dose of 150 mg caffeine given in the framework of a phenotyping cocktail study. The metabolites 5-acetylamino-6-formylamino-3-methyluracil (AFMU), 5-acetylamino-6-amino-3-methyluracil (AAMU), 1-methylxanthine (1X), and 1-methylurate (1U) were quantified with LC-MS/MS. The molar ratio (AFMU + AAMU)/(1X + 1U + AFMU + AAMU) was used as a NAT2 metric, while the ratio 1U/(1X + 1U) served as XO metric.


RESULTS
The NAT2 ratios were stable in the intervals 4-24 h after caffeine dosing. Mean intra-individual coefficients of variation were 11-23% starting 4 h post-dose, while inter-individual variability reached 37-75%. The XO ratios increased gradually by 14% from the 2-4 to the 16-24 h interval. The mean intra- and inter-individual coefficients of variation of XO activity were 3-18 and 7-10% respectively. No significant differences between study occasions were observed.


CONCLUSIONS
Any sampling interval at least 4 h after caffeine dosing is suitable for NAT2 and XO activity assessments. XO activities can only be compared between volunteers and studies if the same urine collection schedule has been respected. The low intraindividual variability allows for sample sizes of 16 and 6 participants in crossover interaction studies of NAT2 and XO activity respectively.
Jetter
Alexander
A
Department of Pharmacology, Clinical Pharmacology, University Hospital Cologne, Köln, Germany. alexander.jetter@usz.ch
Kinzig
Martina
M
Rodamer
Michael
M
Tomalik-Scharte
Dorota
D
Sörgel
Fritz
F
Fuhr
Uwe
U
D000107:Acetylation; D000328:Adult; D001191:Arylamine N-Acetyltransferase / Q000235:genetics* / Q000378:metabolism / Q000652:urine; D002110:Caffeine / Q000008:administration & dosage* / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000008:administration & dosage / Q000494:pharmacology; D006801:Humans; D008297:Male; D010641:Phenotype; D013997:Time Factors; D014498:Uracil / Q000031:analogs & derivatives / Q000652:urine; D014527:Uric Acid / Q000031:analogs & derivatives / Q000652:urine; D044465:White People / Q000235:genetics; D014969:Xanthine Oxidase / Q000235:genetics* / Q000378:metabolism / Q000652:urine; D014970:Xanthines / Q000652:urine
D000697:Central Nervous System Stimulants; D014970:Xanthines; D014527:Uric Acid; C030530:1-methyluric acid; D002110:Caffeine; D014498:Uracil; C053085:1-methylxanthine; C036692:5-acetylamino-6-formylamino-3-methyluracil; D014969:Xanthine Oxidase; D001191:Arylamine N-Acetyltransferase; C478900:NAT2 protein, human; C054188:5-acetylamino-6-amino-3-methyluracil