Reduction in non-glomerular renal clearance of the caffeine metabolite 1-methylxanthine by probenecid.
OBJECTIVE
Urinary caffeine metabolic ratios used to quantify the activity of numerous drug-metabolizing enzymes are an established component of cocktail approaches for metabolic phenotyping. Because in vitro evidence suggests that 1-methylxanthine (1-MX), a major caffeine metabolite, is actively secreted into urine by organic anion transporters (hOATs), coadministration of renal hOAT inhibitors like probenecid may impair these procedures.


METHODS
In a randomized, placebo-controlled, double-blind crossover design, single oral doses of 300 mg caffeine with oral coadministration of placebo or 500 mg probenecid 3 times daily for 2 days were administered to 7 healthy men. The plasma and urine concentrations of caffeine and its major metabolites 1,7-dimethylxanthine (1,7-DMX) and 1-MX were determined by high-performance liquid chromatography.


RESULTS
Coadministration of probenecid resulted in a 34% reduction of the renal clearance of 1-MX (mean +/- SD 190 +/- 42 versus 290 +/- 83 ml min(-1), 95% CI on difference 0.2, 200, p = 0.04) with a 41% reduction in its estimated non-glomerular clearance. The renal clearances of caffeine and 1,7-DMX and the area under the plasma concentration-time curves of all substances were not significantly changed.


CONCLUSIONS
1-MX undergoes renal tubular secretion which is substantially reduced by probenecid, possibly due to inhibition of renal hOATs. This inhibition may explain the influence of probenecid on urinary caffeine metabolic ratios and, thus, its impact on the assessment of enzyme activities. It also suggests that 1-MX might serve as a model substrate for the renal tubular transport of organic anions.
Rengelshausen
J
J
Department of Internal Medicine VI, Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Heidelberg, Germany.
Göggelmann
C
C
Burhenne
J
J
Riedel
K D
KD
Mikus
G
G
Walter-Sack
I
I
Haefeli
W E
WE
D000328:Adult; D019540:Area Under Curve; D001191:Arylamine N-Acetyltransferase / Q000378:metabolism; D002110:Caffeine / Q000493:pharmacokinetics*; D000697:Central Nervous System Stimulants / Q000493:pharmacokinetics*; D002851:Chromatography, High Pressure Liquid; D003404:Creatinine / Q000097:blood / Q000652:urine; D018592:Cross-Over Studies; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D003864:Depression, Chemical; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D007668:Kidney / Q000378:metabolism*; D008297:Male; D011339:Probenecid / Q000494:pharmacology*; D012076:Renal Agents / Q000494:pharmacology*; D013056:Spectrophotometry, Ultraviolet; D014969:Xanthine Oxidase / Q000378:metabolism; D014970:Xanthines / Q000378:metabolism*
D000697:Central Nervous System Stimulants; D012076:Renal Agents; D014970:Xanthines; D002110:Caffeine; C053085:1-methylxanthine; D003404:Creatinine; D019388:Cytochrome P-450 CYP1A2; D014969:Xanthine Oxidase; D001191:Arylamine N-Acetyltransferase; C478900:NAT2 protein, human; D011339:Probenecid