Urinary metabolic ratios

The relative safety of caffeine has lead to its widespread use as an in vivo probe for CYP1A2 activity in man. The primary route of caffeine metabolism is via N-demethylation to paraxanthine, theophylline, and theobromine. The major route of caffeine clearance in man is to paraxanthine (57). The N-3-demethylation of caffeine to paraxanthine has been shown to be mediated by CYP1A2 (58). However, paraxanthine is further metabolized to a number of different products, and as a consequence urinary metabolic ratios are often used to describe an individual CYP1A2 phenotype. 

Such approaches have been used successfully to demonstrate the induction of CYP1A2 by smoking (42). In addition, this study showed that oral contraceptives produce a small but significant inhibition of CYP1A2. Urinary metabolic ratios have also been used to show that oral AUC of clozapine was correlated with caffeine N-3-demethylation (59), a finding supported by some recent in vitro data, which has shown that clozapine N-demethylation is mediated by CYP1A2 (60). 

Rasmussen BB, Bosen K (1996) Determination of urinary metabolites of caffeine for the assessment of cytochrome P4501A2, xanthine oxidase and N-acetyltransferase activity in humans. Ther Drug Monitor 18 254-262 Rostami-Hodjegan A, Nurminen S, Jackson PR, Tucker GT (1996) Caffeine urinary metabolic ratios as markers of enzyme activity a theoretical assessment. Pharmacogenetics 6 121-149... 

The urinary metabolic ratio (hydroxytolbutamide plus carboxytolbutamide)/tolbutamide is generally used. There is a large interindividual variability in... 

Rostami-Hodjegan A, Nurminen S, Jackson PR, Tucker GT (1996) Caffeine urinary metabolic ratios as markers of enzyme activity a theoretical assessment. Pharmacogenetics 6 121-149... 

Somc i AA, Reinhard HiA Bochner F. Effects of omepmzole and cimetidine on the urinary metabolic ratio of TOguanil in healthy volunteers. EurJ CUn Pharmacol (1996) 50,417-19. 

Figure 1 Distribution of the urinary debrisoquine/4-hydroxydebrisoquine metabolic ratio (MR) in 695 Chinese and 1011 Swedish healthy subjects. The arrows indicate MR = 12.6, the antimode between EM and PM established in Caucasians. A line is drawn at MR= 1. Most Chinese EM have MR> 1, while most Swedish EM have MR< 1. Source From Ref. 17.

The phenotype for human CYP1A2 activity can be conveniently measured in vivo using caffeine as a probe drug and measuring urinary or salivary metabolite profiles. The caffeine metabolic ratio is increased in populations of smokers (31,92) and wide ranges of CYP1A2 activity have been measured in many different human studies (Table 2). 

Rost KL, Roots I. Accelerated caffeine metabolism after omeprazole treatment as indicated by urinary metabolite ratios coincidence with plasma clearance and breath test. Clin Pharmacol Ther 1994 55 402 111. 

Dapsone (4,4 -diaminodiphenylsulfone) has been widely used for phenotyping with respect to acetylation by NAT-2 however, the drug is also N-hydroxylated. Formation of the hydroxylamine metabolite by human liver microsomes was found to be selectively mediated by CYP3A (286) this led to the development of a zero- to eight-hour urinary metabolic recovery ratio approach [dapsone hydroxylamine (dapsone + dapsone hydroxylamine)] to quantitatively assess this pathway of metabolism (287,288). Subsequently, the trait measure has been applied as part of a cocktail approach (35) in a number of studies investigating the putative role of CYP3A as a risk factor in cancer (289-291) and other disease states (288,292,293). 

Tassaneeyakul W, Birkett DJ, Pass MC, et al. Limited value of the urinary phe-nytoin metabolic ratio for the assessment of cytochrome P4502C9 activity in vivo. Br J Clin Pharmacol 1996 42 774-778. 

Metabolic ratios (MR) used are plasma 17X/137X and urinary (1U + IX + AFMU)/17U. 

Fuchs P, Haefeli WE, Ledermann, Wenk M (1999) Xanthine oxidase inhibition by allopurinol affects the reliability of urinary caffeine metabolic ratios as markers for N-acetyltransferase2 and CYP1A2 activities. Eur J Clin Pharmacol 54 869-876... 

Labbe L, Sirois C, Pilote S, Arseneault M, Robitaille NM, Turgeon J et al. Effect of gender, sex hormones, time variables and physiological urinary pH on apparent CYP2D6 activity as assessed by metabolic ratios of marker substrates. Pharmacogenetics 2000 10 425-38. 

Figure 1-3. Frequency distribution of dextromethorphan metabolic ratios (MRs) in Asians, African Americans, Caucasians, and Mexican Americans. The MR is the log10 of the ratio between urinary dextromethorphan and one of its main metabolites, dextrophan. Lower MRs indicate more active enzyme activities.

Urinary excretion data of human subjects indicate that 4-hydroxylation of methamphetamine is much more extensive than that of amphetamine the metabolic ratio (total hydroxymethamphetamine/methamphetamine) in urine averaged about 15 with individual variations of approximately fiftyfold (Shimosato 1988), suggesting considerable importance of CYP2D6 polymorphism in the fate of methamphetamine. On the other hand, there seems to be no information on the further metabolism of p-hydroxymethamphetamine as is available for p-hydroxyamphetamine. 

In 21 healthy subjects, allopurinol 300 mg daily for 8 days altered the levels of urinary caffeine metabolites of a single 200-mg dose of caffeine. In particular, the metabolic ratio used to determine whether people are fast or slow acetylators was substantially changed. Thus, allopurinol may invalidate the results of phenotyping with the urinary caffeine test. In addition, the caffeine metabolite ratio used to express the activity of the cytochrome P450 isoenzyme CYP1A2 was not stable when allopurinol was used. This interaction is of relevance to research rather than clinical practice. 

---