THE USE OF CAFFEINE AS A METABOLIC PROBE FOR HUMAN DRUG-METABOLIZING-ENZYMES

1. Caffeine (CA) is metabolized extensively and at least 17 metabolite s arising from primary and secondary biotransformation pathways are fo und in urine following CA ingestion. The enzymes responsible for the f ormation of most of the metabolites derived from CA have been identifi ed. 2. Given the near ubiquitous consumption of CA, this compound pote ntially constitutes a useful substrate probe for assessment of certain xenobiotic metabolizing enzyme activities in vivo. Indeed, various ra tios of CA metabolites excreted in urine (urinary metabolic ratios; MR s) are now utilized widely for the population screening of enzyme acti vities. 3. Excretion of the acetylated secondary metabolite 5-actylami no-6-formylamino-3-methyluracil (AFMU) is dependent on the activity of the polymorphic N-acetyltransferase (NAT2), and certain MRs incorpora ting AFMU may be used for NAT2 phenotyping. 4. The conversion of 1-met hylxanthine (1-MX), another secondary metabolite of CA, to 1-methyluri c acid (1-MU) is catalyzed by xanthine oxidase (XO), and the urinary 1 -MU to 1MX ratio reflects XO activity. 5. N3-demethylation to form par axanthine (PX), a reaction mediated by cytochrome P4501A2 (CYP1A2), is the dominant primary metabolic pathway of CA. CA NS demethylation act ivity may be used as a measure of human hepatic CYP1A2 in vitro. 6. Pl asma CA clearance is considered to reflect CYP1A2 activity in vivo. Al though a number of MRs are based on the excretion of PX metabolites (P X derived from CA is employed for the assessment of CYP1A2 activity in vivo), factors other than enzyme activity may affect these ratios.