Evaluation of caffeine as a test drug for CYP1A2, NAT2 and CYP2E1 phenotyping in man by in vivo versus in vitro correlations

Pharmacogenetics. 1996 Apr;6(2):159-76. doi: 10.1097/00008571-199604000-00003.


Caffeine is used to phenotype subjects in vivo for the cytochrome P450 isoforms CYP1A2 and CYP2E1, and for N-acetyltransferase type 2 (NAT2). However, how much of the variation in phenotyping parameters may be attributed to variations in CYP1A2 and CYP2E1 activities has not been determined. Therefore, this study intraindividually compared enzyme activities and/or content in liver samples with pharmacokinetic parameters of caffeine in vivo after administration of a test dose in 25 patients undergoing hepatectomy. Parameters measured in vitro were the high affinity components of caffeine 3-demethylation and phenacetin 0-deethylation, microsomal CYP1A2 and CYP2E1 immunoreactivity, and cytosolic sulfamethazine N-acetylation. Caffeine parameters in vivo included caffeine clearance from plasma and/or saliva, paraxanthine/caffeine ratios in plasma and saliva, plasma theophylline/caffeine ratio, and several metabolite ratios from spot urine sampled 6 h postdose. Correlations between parameters were determined using weighted linear regression analyses. Caffeine clearance and paraxanthine/caffeine ratios correlated most highly to intrinsic clearance of caffeine 3-demethylation and to CYP1A2 immunoreactivity (r= 0.584-0.82), whereas urinary CYP1A2 ratios correlated less strongly with CYP1A2 parameters in vitro. Assignment of acetylator phenotype by urinary NAT2 ratios was concordant with sulfamethazine-N-acetylation in vitro. In contrast to CYP1A2 parameters in vitro, CYP2E1 immunoreactivity was not related to the theophylline/caffeine plasma ratio. CYP1A2 activity, thus, is the major determinant of caffeine clearance and the paraxanthine/caffeine ratios in vivo, of which the saliva ratio 6 h postdose appears as the most advantageous parameter. The results confirm that phenotyping using caffeine provides valid estimates of CYP1A2 and NAT2 activity.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aged
  • Arylamine N-Acetyltransferase / genetics
  • Arylamine N-Acetyltransferase / metabolism*
  • Caffeine / blood
  • Caffeine / metabolism*
  • Caffeine / urine
  • Cytochrome P-450 CYP1A2 / genetics
  • Cytochrome P-450 CYP1A2 / metabolism*
  • Cytochrome P-450 CYP2E1 / genetics
  • Cytochrome P-450 CYP2E1 / metabolism*
  • Female
  • Genotype
  • Humans
  • In Vitro Techniques
  • Liver / metabolism
  • Male
  • Metabolic Clearance Rate
  • Middle Aged
  • Phenotype
  • Saliva / metabolism
  • Theophylline / blood
  • Theophylline / metabolism


  • Caffeine
  • Theophylline
  • Cytochrome P-450 CYP2E1
  • Cytochrome P-450 CYP1A2
  • Arylamine N-Acetyltransferase
  • NAT2 protein, human
  • 1,7-dimethylxanthine