title: 1-Methylxanthine derived from caffeine as a pharmacodynamic probe of oxypurinol effect.
abstract: OBJECTIVE
  In the present study we have investigated the use of caffeine, administered in the form of instant coffee, as a prodrug for 1MX to validate the use of the 1MU:1MX ratio following caffeine administration as a pharmacodynamic measure of oxypurinol effect on xanthine oxidase.
  
  
  METHODS
  Five healthy volunteers took caffeine 75 mg 8 hourly administered as instant coffee over a 7 day period. They were given allopurinol 600 mg on day 4. Urine was collected in 8 h aliquots from day 1-day 7. The ratio of 1-methyluric acid (1MU) to 1-methylxanthuric (1MX) was determined.
  
  
  RESULTS
  The relationship between the plasma oxypurinol (the active metabolite of allopurinol) concentration at the midpoint of each caffeine dosage interval and the decrement in the urinary 1MX to 1MU ratio fitted well by a sigmoid Emax model. Mean (+/-s.d.) values of the oxypurinol EC50(3.9 +/- 1.4 mg l-1), EC90(8.7 +/- 1.8 mgl-1) and the exponent, n (3.0 +/- 1.2) were similar to those obtained previously following either the direct administration of 1MX or the use of theophylline as a prodrug for 1MX.
  
  
  CONCLUSIONS
  These data indicate that the use of caffeine as a source of 1MX could provide a simple and ethically acceptable method for monitoring oxypurinol effect in patients taking allopurinol for the treatment of gout.
authors.affiliation: Department of Clinical Pharmacology, Flinders Medical Centre, Bedford Park, Adelaide, Australia.
authors.forename: D J
authors.initials: DJ
authors.lastname: Birkett
authors.forename: J O
authors.initials: JO
authors.lastname: Miners
authors.forename: L
authors.initials: L
authors.lastname: Valente
authors.forename: K J
authors.initials: KJ
authors.lastname: Lillywhite
authors.forename: R O
authors.initials: RO
authors.lastname: Day
mesh_terms: D000328:Adult; D002110:Caffeine / Q000493:pharmacokinetics*; D005260:Female; D006801:Humans; D008297:Male; D015335:Molecular Probes; D010117:Oxypurinol / Q000097:blood / Q000737:chemistry / Q000494:pharmacology* / Q000652:urine; D014527:Uric Acid / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D014970:Xanthines / Q000493:pharmacokinetics*
chemical_list: D015335:Molecular Probes; D014970:Xanthines; D014527:Uric Acid; C030530:1-methyluric acid; D002110:Caffeine; C053085:1-methylxanthine; D010117:Oxypurinol