When should urine samples for NAT2 phenotyping with caffeine be collected?

Abstract

For N-acetyltransferase type 2 (NAT2) phenotyping, urinary ratios of caffeine metabolites are determined after a caffeine test dose. It is not known how these ratios change with postdose time, and which urine collection interval is best for phenotyping. Therefore, 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 150 mg caffeine test dose. The test was repeated after 10 days. Concentrations of 5-acetylamino-6-formylamino-3-methyluracil (AFMU), 5-acetylamino-6-amino-3-methyluracil (AAMU), 1-methylxanthine (1X) and 1-methylurate (1U) were measured with LC-MS/MS (LOQ, 100 ng/mL). As a NAT2 metric, the molar ratio (AFMU+AAMU)/(1X+1U+AFMU+AAMU) was determined. In both periods, urinary ratios were stable from 4 until 24 hours postdose. The arithmetic means for the collection periods ranged from 0.152 to 0.169 and from 0.163 and 0.186, respectively, the CV was <6 % in all cases. Intraindividual CVs were between 9 and 16% starting 4 hours postdose, while interindividual variability reached values between 58 and 66%, respectively. There was no statistically significant difference in CV between the respective collection periods or between study periods. Hence, urine for NAT2 phenotyping may be collected at any time between 4 and 24 hours after caffeine dosing. The low intraindividual CVs enable the conductance of interaction studies assessing the effect of drugs on NAT2 activity with a sample size as low as n=12 subjects. Clinical Pharmacology & Therapeutics (2004) 75, P69–P69; doi: 10.1016/j.clpt.2003.11.259