Detection of allopurinol and oxypurinol in canine urine by HPLC/MS-MS: Focus on veterinary clinical pharmacokinetics.

Allopurinol is the most commonly used drug for the treatment of hyperuricemia in people, and in view of the risks of fatal hypersensitivity in patients with renal dysfunction, doses based on the glomerular filtration rate are proposed. In veterinary medicine, allopurinol is used in the treatment of canine leishmaniasis (CanL) caused by Leishmania infantum owing to the drug action of inhibiting the parasite's RNA synthesis. However, renal dysfunction frequently ensues from disease progression in dogs. The purpose of the present study was to standardize and validate a sensitive high-performance liquid chromatography-mass spectrometric (HPLC-MS/MS) method to determine the concentration of allopurinol and its active metabolite oxypurinol in canine urine for clinical pharmacokinetic investigation. Urine samples of eleven (11) dogs with naturally occurring CanL and in the maintenance phase of the treatment with alopurinol were used. For the chromatographic analysis of urine, the mobile phase consisted of a solution of 0.1 % formic acid (88 %) in 10 mM ammonium acetate. Separation of allopurinol and oxypurinol occurred in a flow of 0.8 mL/min on a C8 reverse phase column 5 µm, and acyclovir was the internal standard. The HPLC-MS/MS method was validated by reaching the limits of detection and quantification, reproducibility and linearity. The lower limit of quantification achieved by the method was 10 µg/mL for both allopurinol and oxypurinol. Calibration curves were prepared in blank urine added with allopurinol at concentrations of 10-1000 µg/mL, and oxypurinol at 10-200 µg/mL. Coefficients of variation of less than 15 % between intracurrent and intercurrent accuracy values were observed for both allopurinol and oxypurinol. Urine test samples remained stable after being subjected to freeze-thaw cycles and remaining at room temperature for 4 h. The method proved to be adequate to quantify allopurinol and oxypurinol in urine samples from dogs under treatment.

Determination of allopurinol and oxypurinol in human plasma and urine by liquid chromatography-tandem mass spectrometry.

Allopurinol is used widely for the treatment of gout, but its pharmacokinetics is complex and some patients show hypersensitivity, necessitating careful monitoring and improved detection methods. In this study, a sensitive and reliable liquid chromatography-tandem mass spectrometry method was developed to determine the concentrations of allopurinol and its active metabolite oxypurinol in human plasma and urine using 2,6-dichloropurine as the internal standard (IS). Analytes and the IS were extracted from 0.5ml aliquots of plasma or urine using ethyl acetate and separated on an Agilent Eclipse Plus C18 column using methanol and ammonium formate-formic acid buffer containing 5mM ammonium formate and 0.1% formic acid (95:5, v/v) as the mobile phase (A) for allopurinol or methanol plus 5mM ammonium formate aqueous solution (95:5, v/v) as the mobile phase (B) for oxypurinol. Allopurinol was detected in positive ion mode and the analysis time was about 7min. The calibration curve was linear from 0.05 to 5µg/mL allopurinol in plasma and 0.5-30µg/mL in urine. The lower limit of quantification (LLOQ) was 0.05µg/mL in plasma and 0.5µg/mL in urine. The intra- and inter-day precision and relative errors of quality control (QC) samples were ≤11.1% for plasma and ≤ 8.7% for urine. Oxypurinol was detected in negative mode with an analysis time of about 4min. The calibration curve was linear from 0.05 to 5µg/mL in plasma (LLOQ, 0.05µg/mL) and from 1 to 50µg/mL in urine (LLOQ, 1µg/mL). The intra- and inter-day precision and relative errors were ≤7.0% for plasma and ≤9.6% for urine. This method was then successfully applied to investigate the pharmacokinetics of allopurinol and oxypurinol in humans.

Mechanism of allopurinol induced TPMT inhibition.

Up to 1/5 of patients with wildtype thiopurine-S-methyltransferase (TPMT) activity prescribed azathioprine (AZA) or mercaptopurine (MP) demonstrate a skewed drug metabolism in which MP is preferentially methylated to yield methylmercaptopurine (MeMP). This is known as thiopurine hypermethylation and is associated with drug toxicity and treatment non-response. Co-prescription of allopurinol with low dose AZA/MP (25-33%) circumvents this phenotype and leads to a dramatic reduction in methylated metabolites; however, the biochemical mechanism remains unclear. Using intact and lysate red cell models we propose a novel pathway of allopurinol mediated TPMT inhibition, through the production of thioxanthine (TX, 2-hydroxymercaptopurine). In red blood cells pre-incubated with 250 µM MP for 2h prior to the addition of 250 µM TX or an equivalent volume of Earle's balanced salt solution, there was a significant reduction in the concentration of MeMP detected at 4h and 6h in cells exposed to TX (4 h, 1.68, p=0.0005, t-test). TX acts as a direct TPMT inhibitor with an apparent Ki of 0.329 mM. In addition we have confirmed that the mechanism is relevant to in vivo metabolism by demonstrating raised urinary TX levels in patients receiving combination therapy. We conclude that the formation of TX in patients receiving combination therapy with AZA/MP and allopurinol, likely explains the significant reduction of methylated metabolites due to direct TPMT inhibition.

A repeated oral administration study of febuxostat (TMX-67), a non-purine-selective inhibitor of xanthine oxidase, in patients with impaired renal function in Japan: pharmacokinetic and pharmacodynamic study.

BACKGROUND: Allopurinol has been widely used for treatment of hyperuricemia, however, it may be associated with various adverse effects. Febuxostat has been identified as a potentially safe and efficacious alternative. OBJECTIVES: A multicenter, open-label, parallel, between-group comparative study was conducted to investigate the effects of renal function on the pharmacokinetics, pharmacodynamics, and safety of febuxostat, a novel inhibitor of uric acid synthesis. METHODS: Based on creatinine clearance (Ccr), 29 subjects were assigned to 3 groups: normal renal function (Ccr ≥ 80 mL/min), mild renal dysfunction (80 mL/min > Ccr ≥ 50 mL/min), or moderate renal dysfunction (50 mL/min > Ccr ≥ 30 mL/min). Febuxostat was repeatedly orally administered at a dose of 20 mg/d for 7 days. RESULTS: Impaired renal function caused a slight increase in systemic exposure to unchanged febuxostat and its oxidative metabolites, but the exposure did not increase through repeated administration. Moreover, renal impairment did not markedly reduce the effects of febuxostat on plasma uric acid levels. There were no clinically significant adverse events even in patients with impaired renal function. CONCLUSIONS: Febuxostat is considered an inhibitor of uric acid synthesis that could be used in patients with mild to moderate renal impairment without dose adjustment.

Effects of bovine milk ingestion on urinary excretion of oxypurinol and uric acid.

OBJECTIVE: Although allopurinol is a xanthine oxidase inhibitor, its overall effect may be due to the action of oxypurinol, a metabolite of allopurinol and another xanthine oxidase inhibitor, since the biological half-life of oxypurinol is longer than that of allopurinol. Oxypurinol shares a renal transport pathway with uric acid and ingestion of bovine milk increases the urinary excretion of uric acid. Therefore, we investigated whether its ingestion promotes the urinary excretion of oxypurinol. SUBJECTS/METHODS: Bovine milk (15 ml/kg body weight) was administered to 6 healthy subjects who took allopurinol (300 mg) 12 h prior to ingestion. In addition, a control experiment was performed with the same subjects using the same protocol, except for the ingestion of water instead of bovine milk. Blood and urine samples were collected before and after bovine and water ingestion. RESULTS: In the bovine milk ingestion experiment, the urinary excretion values of oxypurinol and uric acid were increased by 18% and 38%, respectively, and the fractional excretion values of oxypurinol and uric acid were increased by 20% and 40%, respectively, whereas those did not change in the control experiment. In addition, the concentration of alanine and sum of concentrations of amino acids were increased by 16% and 20%, respectively, in the bovine milk ingestion experiment. CONCLUSION: These results suggest that bovine milk ingestion promotes the urinary excretion of oxypurinol as well as uric acid by increasing amino acid concentration.

Pharmacokinetic and pharmacodynamic interaction between allopurinol and probenecid in patients with gout.

OBJECTIVE: To investigate the pharmacokinetic and pharmacodynamic interaction between probenecid and oxypurinol (the active metabolite of allopurinol) in patients with gout. METHODS: This was an open-label observational clinical study. Blood and urine samples were collected to measure oxypurinol and urate concentrations. We examined the effects of adding probenecid to allopurinol therapy upon plasma concentrations and renal clearances of urate and oxypurinol. RESULTS: Twenty patients taking allopurinol 100-400 mg daily completed the study. Maximum coadministered doses of probenecid were 250 mg/day (n = 1), 500 mg/day (n = 19), 1000 mg/day (n = 7), 1500 mg/day (n = 3), and 2000 mg/day (n = 1). All doses except the 250 mg daily dose were divided and dosing was twice daily. Estimated creatinine clearances ranged from 28 to 113 ml/min. Addition of probenecid 500 mg/day to allopurinol therapy decreased plasma urate concentrations by 25%, from mean 0.37 mmol/l (95% CI 0.33-0.41) to mean 0.28 mmol/l (95% CI 0.24-0.32) (p < 0.001); and increased renal urate clearance by 62%, from mean 6.0 ml/min (95% CI 4.5-7.5) to mean 9.6 ml/min (95% CI 6.9-12.3) (p < 0.001). Average steady-state plasma oxypurinol concentrations decreased by 26%, from mean 11.1 mg/l (95% CI 5.0-17.3) to mean 8.2 mg/l (95% CI 4.0-12.4) (p < 0.001); and renal oxypurinol clearance increased by 24%, from mean 12.7 ml/min (95% CI 9.6-15.8) to mean 16.1 ml/min (95% CI 12.0-20.2) (p < 0.05). The additional hypouricemic effect of probenecid 500 mg/day appeared to be lower in patients with renal impairment. CONCLUSION: Coadministration of allopurinol with probenecid had a significantly greater hypouricemic effect than allopurinol alone despite an associated reduction of plasma oxypurinol concentrations. Australian Clinical Trials Registry ACTRN012606000276550.

Measurement of urinary oxypurinol by high performance liquid chromatography-tandem mass spectrometry.

Oxypurinol is the active metabolite of allopurinol which is used to treat hyperuricaemia associated with gout. Both oxypurinol and allopurinol inhibit xanthine oxidase which forms uric acid from xanthine and hypoxanthine. Plasma oxypurinol concentrations vary substantially between individuals and the source of this variability remains unclear. The aim of this study was to develop an HPLC-tandem mass spectrometry method to measure oxypurinol in urine to facilitate the study of the renal elimination of oxypurinol in patients with gout. Urine samples (50 microL) were prepared by dilution with a solution of acetonitrile/methanol/water (95/2/3, v/v; 2 mL) that contained the internal standard (8-methylxanthine; 1.5 mg/L), followed by centrifugation. An aliquot (2 microL) was injected. Chromatography was performed on an Atlantis HILIC Silica column (3 microm, 100 mm x 2.1mm, Waters) at 30 degrees C, using a mobile phase comprised of acetonitrile/methanol/50 mM ammonium acetate in 0.2% formic acid (95/2/3, v/v). Using a flow rate of 0.35 mL/min, the analysis time was 6.0 min. Mass spectrometric detection was by selected reactant monitoring (oxypurinol: m/z 150.8-->108.0; internal standard: m/z 164.9-->121.8) in negative electrospray ionization mode. Calibration curves were prepared in drug-free urine across the range 10-200 mg/L and fitted using quadratic regression with a weighting factor of 1/x (r(2) > 0.997, n=7). Quality control samples (20, 80, 150 and 300 mg/L) were used to determine intra-day (n=5) and inter-day (n=7) accuracy and imprecision. The inter-day accuracy and imprecision was 96.1-104% and <11.2%, respectively. Urinary oxypurinol samples were stable when subjected to 3 freeze-thaw cycles and when stored at room temperature for up to 6h. Samples collected from 10 patients, not receiving allopurinol therapy, were screened and showed no significant interferences. The method was suitable for the quantification of oxypurinol in the urine of patients (n=34) participating in a clinical trial to optimize therapy of gout with allopurinol.

Plasma concentrations and urinary excretion of purine bases (uric acid, hypoxanthine, and xanthine) and oxypurinol after rigorous exercise.

To investigate the effects of exercise on the plasma concentrations and urinary excretion of purine bases and oxypurinol, we performed 3 experiments with 6 healthy male subjects. The first was a combination of allopurinol intake (300 mg) and exercise (VO2max, 70%) (combination experiment), the second was exercise alone (exercise-alone experiment), and the third was allopurinol intake alone (allopurinol-alone experiment). In the combination experiment, exercise increased the concentrations of purine bases and noradrenaline in plasma, as well as lactic acid in blood and the urinary excretion of oxypurines, whereas it decreased the urinary excretion of uric acid and oxypurinol as well as the fractional excretion of hypoxanthine, xanthine, uric acid, and oxypurinol. In the exercise-alone experiment, exercise increased the concentrations of purine bases and noradrenaline in plasma, lactic acid in blood, and the urinary excretion of oxypurines, whereas it decreased the urinary excretion of uric acid and fractional excretion of purine bases. In contrast, in the allopurinol-alone experiment, the plasma concentration, urinary excretion, and fractional excretion of purine bases and oxypurinol remained unchanged. These results suggest that increases in adenine nucleotide degradation and lactic acid production, as well as a release of noradrenaline caused by exercise, contribute to increases in plasma concentration and urinary excretion of oxypurines and plasma concentration of urate, as well as decreases in urinary excretion of uric acid and oxypurinol, along with fractional excretion of uric acid, oxypurinol, and xanthine. In addition, they suggest that oxypurinol does not significantly inhibit the exercise-induced increase in plasma concentration of urate.

Effect of the angiotensin II receptor antagonist losartan on uric acid and oxypurine metabolism in healthy subjects.

OBJECTIVE: The acute effects of the angiotensin II receptor antagonist losartan on uric acid and oxypurine metabolism were evaluated. METHODS: Losartan (50 mg) was administered orally to 6 healthy males. Blood and urine samples for uric acid and oxypurine were collected before and up to 6 hours after losartan administration. The same examinations were performed later using enalapril (5 mg). RESULTS: Losartan decreased the serum uric acid concentration (from 5.9 +/- 0.9 to 5.2 +/- 1.0 mg/dl) and increased its fractional clearance, which reached a maximum after 2 hours, while enalapril did not. Losartan also induced an increase in the plasma concentration of hypoxanthine, peaking in the fourth hour, and a decrease in its urinary clearance, while the plasma xanthine concentration and its urinary clearance were unchanged. The extent of uric acid excretion was much greater than that of the oxypurines. CONCLUSIONS: Losartan, which has a high affinity for the urate/anion exchanger, has a transient uricosuric effect. Our data indicate that losartan induces a significant decrease in the urinary excretion of hypoxanthine without changes in xanthine.

Effects of angiotensin II infusion on renal excretion of purine bases and oxypurinol.

The effect of angiotensin II infusion on the renal transport of purine bases and oxypurinol (a metabolite of allopurinol) was investigated in 5 healthy subjects who were orally given allopurinol (300 mg) 9 hours prior to the study. Angiotensin II was intravenously administered at 8 ng/min/kg for 2 hours. The fractional clearances of uric acid, xanthine, and oxypurinol were significantly decreased during angiotensin II infusion; however, that of hypoxanthine did not change. The urinary excretion levels of uric acid, xanthine, and oxypurinol were also significantly decreased during angiotensin II infusion. These results suggest that angiotensin II infusion affected the renal clearances of uric acid, xanthine, and oxypurinol through direct tubular transport and/or hemodynamic changes. Accordingly, the hypouricemic effect of allopurinol may be exaggerated in hypertensive gout patients with an enhanced renin-angiotensin system, since an increased biological half-life of oxypurinol is expected in these patients.

Effect of fenofibrate on plasma concentration and urinary excretion of purine bases and oxypurinol.

OBJECTIVE: To investigate whether fenofibrate increases the clearance of purine bases (hypoxanthine, xanthine, uric acid) and oxypurinol. METHODS: We administered fenofibrate (150 mg) 3 times a day for 3 days, and then allopurinol (300 mg) 4 h after the last administration of fenofibrate, to 5 healthy subjects. Ten hours later, a clearance study was done. RESULTS: Following 3 day administration of fenofibrate, fractional clearance of xanthine, uric acid, and oxypurinol increased by 41% (p < 0.05), 101% (p < 0.01), and 51% (p < 0.01), respectively, compared to baseline values, while the respective plasma concentrations decreased by 46% (p < 0.05), 46% (p < 0.05), and 19% (p < 0.05). CONCLUSION: Our results suggest that fenofibrate, fenofibric acid, or fenofibrate derivatives can increase fractional clearance of xanthine, uric acid, and oxypurinol by acting on their common renal pathways. It is suggested that the hypouricemic effect of combination therapy using allopurinol and fenofibrate may be less than additive.

Effect of norepinephrine on the urinary excretion of purine bases and oxypurinol.

To examine whether norepinephrine affects the plasma concentrations and urinary excretion of purine bases and oxypurinol, we orally administered allopurinol (300 mg) to 5 healthy subjects and 9 hours later intravenously administered norepinephrine (12 to 20 microg/kg body weight), which causes a more than 10 mm Hg increase in diastolic pressure for 2 hours. Norepinephrine decreased the urinary excretion of uric acid by 33% (P <.01), oxypurinol by 32% (P <.01), and xanthine by 51% (P <.01), as well as the fractional clearance of uric acid by 32% (P <.01), oxypurinol by 24% (P <.05), and xanthine by 21% (P <.05) when measured 1 to 2 hours after administration. These results indicate that norepinephrine decreases the urinary excretion of uric acid, oxypurinol, and xanthine, probably via hemodynamic change. It is also suggested that the hypouricemic effect of allopurinol may be more potent than that expected in gout patients with enhanced sympathetic tone, such as in salt-sensitive hypertension.

Identification of a new point mutation in the human xanthine dehydrogenase gene responsible for a case of classical type I xanthinuria.

A 60-year-old Japanese man was diagnosed as having hypouricemia at an annual health check-up. The routine laboratory data was not remarkable except that the patient's hypouricemia and plasma levels of xanthine and hypoxanthine were much higher than those of normal subjects. Furthermore, the patient's daily urinary excretion of xanthine and hypoxanthine was markedly increased compared with reference values. The xanthine dehyrogenase activity of the duodenal mucosa was below the limits of detection. Nevertheless, allopurinol was metabolized to oxypurinol in vivo. Based on these findings, a subtype of classical xanthinuria (type I) was diagnosed. The xanthine dehyrogenase protein was detected by Western blotting analysis. Sequencing of the cDNA of the xanthine dehyrogenase obtained from the duodenal mucosa revealed that a point mutation of C to T had occurred in nucleotide 445. This changed codon 149 from CGC (Arg) to TGC (Cys), a finding that has not been previously reported in patients with classical xanthinuria type I.

Effect of furosemide on renal excretion of oxypurinol and purine bases.

To examine whether furosemide affects the plasma concentration and urinary excretion of purine bases and oxypurinol, we administered allopurinol (300 mg) orally to 6 healthy subjects and then administered furosemide (20 mg) intravenously 10 hours later. Furosemide (20 mg) decreased the urinary excretion of uric acid by 40% (P < .01), oxypurinol by 39% (P < .05), and xanthine by 43% (P < .05) and the fractional clearance of uric acid by 45% (P < .01) and oxypurinol by 34% (P < .05) when measured 1 to 2 hours after administration. Moreover, furosemide increased the plasma concentration of uric acid by 6% at 1.5 hours after administration. These results indicate that furosemide may decrease the urinary excretion of uric acid and oxypurinol by acting on their common renal transport pathway(s). In addition, it is suggested that the effect of furosemide on oxypurinol is clinically important, since the hypouricemic effect of allopurinol may become more potent as a result.

Effect of losartan potassium, an angiotensin II receptor antagonist, on renal excretion of oxypurinol and purine bases.

OBJECTIVE: To examine whether losartan affects the plasma concentrations and urinary excretion of purine bases and oxypurinol. METHODS: We administered allopurinol (300 mg) and then 9 h later losartan potassium (100 mg) to 5 healthy subjects. RESULTS: The urinary excretion of uric acid increased by 3.9- and 2.6-fold, and that of oxypurinol by 2- and 1.8-fold, at 1 to 2 h and at 2 to 3 h, respectively, after administration of losartan potassium. The fractional clearance of uric acid was increased by 4.3- and 3.2-fold, oxypurinol by 2.3- and 2.1-fold, and xanthine by 1.32- and 1.26-fold, at 1 to 2 h and at 2 to 3 h, respectively, after administration of losartan potassium. The plasma concentrations of uric acid decreased by 8% and 16%, oxypurinol by 7% and 11%, and xanthine by 42% and 45%, at 1.5 and 2.5 h, respectively, after oral administration. CONCLUSION: These results suggest that losartan potassium could increase urinary excretion of uric acid, xanthine, and oxypurinol by acting on their common renal transport pathways, since it was found that uric acid may share a renal transport pathway with oxypurinol and xanthine. It is also suggested that the effect of losartan potassium on oxypurinol and uric acid is clinically important, since the hypouricemic effect of a combination therapy using allopurinol and losartan potassium may be less than additive, while the uricosuric effect of losartan potassium may increase the frequency of calculi in the urinary tract.

Pharmacokinetics and pharmacodynamics of allopurinol in elderly and young subjects.

AIMS: The prevalence of hyperuricaemia and gout increases with age as does the incidence of adverse effects to allopurinol, the major uric acid lowering drug. The present study was performed to compare the disposition and effects of allopurinol and its active metabolite oxipurinol in elderly and young subjects without major health problems. METHODS: Ten elderly (age range 71-93 years) and nine young subjects (24-35 years) received an oral dose of 200 mg allopurinol in an open, single dose, cross sectional design. Four of these individuals were additionally dosed with 200 mg allopurinol intravenously. Plasma and urine concentrations of allopurinol, oxipurinol, hypoxanthine, xanthine, and uric acid were measured by h. p.l.c. RESULTS: Total clearance of allopurinol was not different in elderly (15.7+/-3.8 ml min-1 kg-1, mean+/-s.e. mean) and young subjects (15.7+/-2.1), whereas total clearance of oxipurinol was significantly reduced in the aged (0.24+/-0.03) compared with young controls (0.37+/-0.05) as was the distribution volume of oxipurinol (0.60+/-0.09 and 0.84+/-0.07 l kg-1, respectively). Oxipurinol was eliminated primarily by the kidneys, allopurinol by metabolism. Fractional peroral bioavailability of allopurinol was 0.81+/-0.16 (n=4, two elderly and two young subjects). Although maximal plasma concentrations of oxipurinol were significantly higher in elderly (5. 63+/-0.83 microgram ml-1 ) than in young persons (3.75+/-0.25) as was the area under the oxipurinol plasma concentration-time curve, AUC (260+/-46 and 166+/-23 microgram ml-1 h, respectively), the pharmacodynamic effect of oxipurinol was smaller in elderly than young subjects (time-dependent decrease of plasma uric acid 83+/-30 microgram ml-1 h in elderly compared with 176+/-21 in young controls). Oxipurinol increased the renal clearance of xanthine, suggesting inhibition of tubular xanthine reabsorption by oxipurinol. CONCLUSIONS: Although allopurinol elimination is not reduced in the aged, that of its active metabolite oxipurinol is because of an age-dependent decline in renal function. Xanthine oxidase inhibition by oxipurinol appears to be reduced in old age. In addition to its uricostatic action, oxipurinol has a xanthinuric effect which is also diminished in the elderly.

Two siblings with classical xanthinuria type 1: significance of allopurinol loading test.

Two brothers with classical xanthinuria who lacked xanthine dehydrogenase activity were encountered. Their hypouricemia was caused by underproduction of uric acid. In their duodenal mucosa, no xanthine dehydrogenase (oxidase) activity was detected. The patients had no symptoms except for duodenal ulcer in one case. The conversion of allopurinol to oxipurinol during an allopurinol loading test for determining the type of classical xanthinuria revealed that the patients had classical type 1 xanthinuria, because aldehyde oxidase activity was present. Furthermore, the allopurinol loading test was conducted to determine the optimal examination times and specimens required for this test.

Effect of glucagon on renal excretion of oxypurinol and purine bases.

OBJECTIVE: To investigate whether glucagon increases the urinary excretion of oxypurinol and purine bases. METHODS: We administered 1 mg glucagon intravenously to 5 healthy subjects taking 300 mg allopurinol orally, and determined plasma concentrations and urinary excretion of oxypurinol and purine bases. RESULTS: Glucagon increased the urinary excretion and fractional clearances of uric acid, xanthine, and oxypurinol, together with an increase in creatinine clearance, while it decreased plasma concentrations of xanthine and hypoxanthine. CONCLUSION: Glucagon-induced increases in urinary excretion of uric acid, xanthine, and oxypurinol were attributable to increases in the fractional clearances of uric acid, xanthine, and oxypurinol in addition to an increase in glomerular filtration rate. It is suggested that glucagon affects the renal common transport pathway of uric acid, xanthine, and oxypurinol by stimulating the release of a liver derived renal vasodilator.

1-Methylxanthine derived from caffeine as a pharmacodynamic probe of oxypurinol effect.

AIMS: 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.