[A case of Xanthinuria in a patient with marked hypouricemia]

Xanthinuria is a rare autosomal recessive disorder associated with a deficiency of xanthine oxidoreductase (XOR), which normally catalyzes the conversion of hypoxanthine to uric acid. The effects of this deficit are an elevated concentration of hypoxanthine and xanthine in the blood and urine, hypouricemia, and hypouricuria. The deficit in XOR can be isolated (type I xanthinuria) or associated with a deficit in aldehyde oxidase (type II xanthinuria) and sulfite oxidase (type III xanthinuria). While the first two variants have a benign course, are often asymptomatic (20%), and clinically indistinguishable, type III xanthinuria is a harmful form that leads to infant death due to neurological damage. The clinical symptoms (kidney stones, CKD, muscle and joint pain, peptic ulcer) are the result of the accumulation of xanthine, which is highly insoluble, in the body fluids. We describe a case of type I xanthinuria in a 52-year-old woman who presented with hypouricemia, hypouricuria and kidney stones. The diagnosis was based on purine catabolite levels in urine and serum measured by 3 nonroutine methods: high-pressure liquid chromatography, mass spectrometry, and magnetic resonance imaging. To identify the type of xanthinuria the allopurinol test was used. We believe that these tests will facilitate the diagnosis of xantinuria especially in asymptomatic patients without the need for a biopsy of the liver or intestines, which is useful only for scientific purposes.

How reliable is the allopurinol load in detecting carriers for ornithine transcarbamylase deficiency?

The allopurinol test aims to distinguish carriers and noncarriers for ornithine transcarbamylase (OTC) deficiency. We have evaluated the reliability of the test in at-risk females of known genotype. Results based on urine orotidine and/or orotic acid measurement were compared in terms of sensitivity and specificity. Retrospectively, we analysed the results of allopurinol tests in 42 women (22 confirmed heterozygotes and 20 noncarriers) from 23 pedigrees at risk of being carriers for OTC deficiency. Using a cut-off of 2 standard deviations above the mean of controls, the highest sensitivity (91%) was given by orotidine alone or in combination with orotic acid, but specificity was only 70% and 65%, respectively. We conclude that the value of the allopurinol test for detecting OTC carriers in at-risk females is limited. This needs to be recognized when counselling families. The test still has a role as a safe, quick, noninvasive screen of individuals at risk, but test results in possible carriers should be interpreted with caution. In the absence of other supportive evidence, confirmation by mutation analysis is required.

Determination of allopurinol by micelle-stabilised room-temperature phosphorescence in real samples

A very simple, rapid and highly sensitive method has been developed for the determination of allopurinol. The method is based on the room temperature phosphorescence of allopurinol in sodium dodecylsulphate (SDS) micelles, with thallium (I) providing the external heavy atom and sodium sulphite acting as the oxygen scavenger. Under the optimum experimental conditions, the range of application is 0.25-7.0 microg ml(-1) and the limit of detection is 0.014 microg ml(-1). The most relevant characteristic of this method is its great selectivity, e.g. allopurinol can be determined in the presence of its metabolite, oxypurinol. The results of the analysis of several pharmaceutical preparations were satisfactory. The clinical applicability of this procedure has been tested by analysing allopurinol in urine samples.

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.

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.

Xanthine oxidase inhibition by allopurinol affects the reliability of urinary caffeine metabolic ratios as markers for N-acetyltransferase 2 and CYP1A2 activities

OBJECTIVES

To evaluate the in vivo effect of xanthine oxidase (XO) inhibition by allopurinol on the determination of polymorphic N-acetyltransferase 2 (NAT2) and cytochrome P450 1A2 (CYP1A2) with urinary caffeine metabolic ratios.

METHODS

In an open, prospective study involving 21 healthy subjects (eight fast, 13 slow NAT2 acetylators) allopurinol (300 mg perday) was administered orally on trial days 1-8, followed by a wash-out period of 8 days. Urinary caffeine tests (200 mg caffeine p.o.) were performed repetitively. Urine was collected for 8 h and venous blood samples for the determination of allopurinol, oxypurinol and uric acid were drawn. The urinary caffeine metabolites 1-methyluric acid (1MU), 1-methylxanthine (1MX), 1,7-dimethyluric acid (17MU), 1,7-dimethylxanthine (17MX), 5-acetylamino-6-formylamino-3-methyluracil (AFMU), plasma allopurinol and oxypurinol were analysed using high-performance liquid chromatography (HPLC).

RESULTS

During XO inhibition by allopurinol, the formation of 1MU from 1MX and therefore the XO ratio 1MU/1MX decreased to 15.9 (1.2)% [mean with (SEM)] of baseline values (P < 0.005). The NAT2 ratio AFMU/1MX decreased likewise to 56.7 (6.3)% (P < 0.005). AFMU/(AFMU + 1MX + 1MU), an alternative NAT2 ratio, remained constant, but the CYP1A2 ratio (AFMU + 1MX + 1MU)/17MU, used to express CYP1A2 activity, transiently increased to 167 (13)% (P < 0.005). The NAT2 phenotype did not influence CYP1A2 and XO ratios or plasma oxypurinol pharmacokinetics.

CONCLUSIONS

Several caffeine metabolic ratios are commonly used to express the activities of NAT2, CYP1A2 and XO both in healthy volunteers and in polymedicated patients, although their reliability has not been evaluated thoroughly during concurrent drug administration. The findings of this study suggest that NAT2 phenotyping should be performed using the ratio AFMU/(AFMU + 1MX + 1MU) if an XO inhibitor may be present. It also shows that the determination of CYP1A2 activity with caffeine as a metabolic probe is considerably altered under these conditions. Thus, concomitant drug administration may impair the robustness of multiple pathways of the complex caffeine test. This points to the need for alternative probes, designed to assess only the activity of a single enzyme because, in contrast to healthy volunteers, in patients known or unknown drug interactions may often be present.

Pharmacodynamics of oxypurinol after administration of allopurinol to healthy subjects

1. Eight healthy subjects received 50, 100, 300, 600 and 900 mg allopurinol daily for 1 week each, in random order with 1 week separating each treatment period. The pre-dose plasma concentration of oxypurinol, the extent of inhibition of xanthine oxidase, plasma urate concentration and urine urate excretion rate were assessed on the last 2 days of each treatment week. 2. The ratio of 1-methyluric acid (1MU) over 1-methylxanthine (1MX) in the urine, following a dose of 50 mg 1MX infused intravenously over 20 min, was used to measure the inhibition of xanthine oxidase. 3. The steady-state plasma concentration of oxypurinol increased linearly with increasing dose of allopurinol between 50 mg to 600 mg day-1, with a weak indication of saturation at the higher 900 mg day-1 dose rate. 4. The relationships between plasma oxypurinol concentration and xanthine oxidase inhibition (1MU/1MX ratio), plasma urate concentration and urine urate excretion rate were fitted to an inhibition sigmoid Emax model and the C50 values for oxypurinol were 26.38 +/- 4.87, (mean +/- s.d.) 36.58 +/- 8.36 and 24.61 +/- 9.08 microM, respectively. 5. 1MU/1MX ratio appeared to be a reliable index of xanthine oxidase activity in vivo as the C50 for oxypurinol observed for 1MU/1MX ratio, plasma urate concentration and urine urate excretion rate were similar. 6. The concentration of oxypurinol required for inhibition of xanthine oxidase, as indicated by C50, was lower than those often observed in clinical practice.

Effect of BOF-4272 on the oxidation of allopurinol and pyrazinamide in vivo. Is xanthine dehydrogenase or aldehyde oxidase more important in oxidizing both allopurinol and pyrazinamide?

Allopurinol or pyrazinamide was administered to rats treated with BOF-4272 (a potent xanthine oxidase inhibitor) to investigate to what degree xanthine dehydrogenase participates in the oxidation of these agents. BOF-4272 markedly decreased the plasma concentration and the urinary excretion of both oxypurinol and 5-hydroxypyrazinamide. It also decreased the sum of the urinary excretion of allopurinol and oxypurinol and that of pyrazinamide and its metabolites, although it did not affect the sum of the plasma concentrations of allopurinol and oxypurinol at 105 min after administration of allopurinol or the plasma concentration of pyrazinamide during the period after the administration of pyrazinamide. These results suggested that BOF-4272 almost completely inhibited the oxidation of allopurinol and pyrazinamide and had some effect on the excretion and/or the tissue incorporation of these two compounds. Since the in vitro study demonstrated that BOF-4272 did not inhibit the activity of aldehyde oxidase, which oxidized both allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide, the results suggested that xanthine dehydrogenase was the more important enzyme in converting allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide.

Simultaneous determination of allopurinol and oxipurinol in human plasma and urine by high-performance liquid chromatography

The uricostatic drug allopurinol (CAS 315-30-0) is used for treatment of hyperuricaemia and is mainly bio-transformed to the active metabolite oxipurinol (CAS 2465-59-0) in humans. A new assay was developed for the simultaneous determination of both compounds in plasma and urine using ultrafiltration and ion exchange purification steps for plasma and urine, respectively. Reversed-phase high-performance liquid chromatography with ultraviolet detection was applied for the separation and quantitation of both compounds. The limit of detection was 0.1 microgram/ml for both compounds in plasma and 0.2 and 0.5 microgram/ml for allopurinol and oxipurinol, respectively, in urine. Within-run and day-to-day precision of 3-5% and 5-7% was determined for plasma and 6-8% and 8-10% for urine analysis. The assays were further validated using liquid chromatography with photodiode array detection and by comparison with methods using protein precipitation as the purifying step. The high analytical recoveries, selectivity, sensitivity, accuracy and reproducibility were adequate for the measurement of both compounds in pharmacokinetic studies and for drug monitoring in patients on allopurinol therapy.

Bioequivalence of allopurinol preparations: to be assessed by the parent drug or the active metabolite?

Allopurinol is converted almost completely into a single active metabolite, oxipurinol, which has the same therapeutic pattern but a much longer elimination half-life than the parent compound. Therefore both allopurinol and oxipurinol were evaluated in our bioequivalence study in healthy volunteers comparing two allopurinol brands. Bioequivalence determination was based on the 90% confidence intervals (CI) of the area under the plasma concentration time curve from time zero to infinity (AUC0-infinity), of the area from time zero to the last measurable plasma concentration (AUC0-t (last)), and Cmax. Because of the lack of compound-specific criteria we used conventional limits for the bioequivalence range. Under these conditions the brand chosen as test preparation was judged to be bioequivalent to the reference form with respect to the extent of bioavailability, AUC0-infinity, and AUC0-t (last) of the parent drug. The CI of Cmax of allopurinol slightly exceeded the upper limit of 130%, so that bioequivalence was not confirmed with regard to the rate of bioavailability of the parent compound. The CI values of both AUC and Cmax of the active metabolite were tighter than those of allopurinol. In addition, the CI values of Cmax of oxipurinol were smaller than those of the corresponding AUC. As a consequence the test drug can clearly be accepted as bioequivalent, based on metabolite data. Since the active metabolite is of greater therapeutic significance than the parent drug, assessment of the bioequivalence of allopurinol preparations needs to be based on oxipurinol rather than allopurinol.(ABSTRACT TRUNCATED AT 250 WORDS)

A case of xanthinuria: a study on the metabolism of pyrazinamide and allopurinol

A 74-year-old female was diagnosed as having xanthinuria by measurement of the uric acid level in plasma, purine bases in urine and activity of xanthine oxidase in the duodenal mucosa. The determination of the urinary excretion of purine bases in her family demonstrated a slightly increased urinary excretion of oxypurines in her younger brother, suggesting that he was a heterozygote. The pyrazinamide-loading test and allopurinol-loading test demonstrated that she could neither metabolize pyrazinoic acid into 5-hydroxypyrazinoic acid nor allopurinol into oxypurinol, although there was a slight metabolizing of prazinamide into 5-hydroxypyrazinamide. This suggested that she belonged to the subgroup which can neither metabolize pyrazinamide into 5-hydroxypyrazinamide, pyrazinoic acid into 5-hydroxypyrazinoic acid nor allopurinol into oxypurinol.

Pharmacokinetics and metabolism of allopurinol riboside

There are no safe and effective oral drugs to treat leishmaniasis and Chagas' disease. The safety, pharmacokinetics, and metabolism of single and multiple oral doses of allopurinol riboside, an investigational antiparasitic agent, were evaluated in a randomized, double-blinded, placebo-controlled study in 32 healthy male volunteers, at levels up to 25 mg/kg q.i.d. for 13 doses. No significant toxicity was detected. Allopurinol riboside peaks in plasma 1.6 hours after administration, has an elimination half-life of 3 hours, and steady-state concentrations in the therapeutic range. However, in contrast to preclinical studies in dogs (plasma levels proportional to oral doses up to 200 mg/kg), we found that plasma levels were unexpectedly low and did not rise with increasing dose. Furthermore, allopurinol and oxypurinol (unanticipated metabolites) were detected at levels proportional to the dose of allopurinol riboside. We present a model that includes incomplete absorption, metabolism of residual drug by enteric flora, and absorption of bacterial metabolites to explain these findings in humans.

[A study on treatment of hyperuricemia--effects and kinetics of allopurinol and oxipurinol]

In order to study the effects and pharmacokinetics of allopurinol (hereafter abbreviated to allo.) and oxipurinol (hereafter abbreviated to oxi.) six normal human subjects were given a single oral dose of either allo. (300mg) or oxi. (600mg), followed by serial determinations of serum and urinary levels of allo., oxi., uric acid, hypoxanthine (hereafter abbreviated to hx.) and xanthine (hereafter abbreviated x.) over a six-hour period. With a dose of 300mg of allo. or 600mg of oxi., the patterns of serum uric acid were similar. When 300mg of allo. was given, however, a reduction in the serum uric acid level occurred earlier. Additionally, it was found that urinary excretion of oxi. generally paralleled the plasma concentration. Allo. administration resulted in rises in plasma concentration of x., and urinary excretion of x. and hx. Oxi. administration, on the other hand, did not cause significant changes in the plasma content of x. or urinary excretory volume of hx. Only a slight increase was noted in the amount of x. excreted in the urine. When allo. was compared against oxi., pharmacokinetics of oxypurines, especially x. were found to differ markedly. The results suggested that differences in the reaction sites, varied intra- and extra-cellular distributions of allo. and oxi., and different effects on purine biosynthesis contribute to the aforementioned discrepancies.

Metabolism of pyrazinamide and allopurinol in hereditary xanthine oxidase deficiency

The metabolism of pyrazinamide and allopurinol was studied in three xanthinuric patients from two families with hereditary xanthinuria to determine whether both substrates were oxidized only by xanthine oxidase or by other oxidases as well. One xanthinuric patient could neither metabolize pyrazinamide into 5-hydroxypyrazinamide nor allopurinol into oxypurinol. Two xanthinuric patients could metabolize both pyrazinamide into 5-hydroxypyrazinamide and allopurinol into oxypurinol but could not oxidize pyrazinoic acid to 5-hydroxypyrazinoic acid. These findings suggest that xanthinuria comprises at least two subgroups.

Allopurinol kinetics after massive overdosage

A 15-year-old girl with normal renal function took 75 tablets of allopurinol 300 mg. She did not suffer any ill-effects. Samples obtained over 84 h showed prolonged elimination of allopurinol, with a half-life of 3.6 h, and oxipurinol, with a half-life of 26 h.

Liquid chromatography with multichannel ultraviolet detection used for studying disorders of purine metabolism

We used a reversed-phase "high-performance" liquid-chromatographic system equipped with a multichannel ultraviolet spectrometric detector and a micro-computer for analyzing urine samples from patients with disorders of purine metabolism. This system recorded a series of absorption-spectrum data from a single chromatographic run and stored them for subsequent analysis. Because the retention times and ultraviolet absorption spectra of the eluates were recorded simultaneously, identification of peaks was easy and quite accurate for simultaneous quantification of orotidine, adenine, hypoxanthine, uric acid, xanthine, allopurinol (4-hydroxypyrazolo[3,4-d]pyrimidine), oxypurinol (4,6-dihydroxypyrazolo[3,4-d]pyrimidine), inosine, and 2,8-dihydroxyadenine--compounds extremely difficult or even impossible to quantify simultaneously with a conventional single-wavelength spectrometer. We used this method to investigate purine metabolites in urines from a patient with hereditary xanthinuria, three patients with 2,8-dihydroxyadenine urolithiasis, and a gouty subject taking allopurinol.

Sustained reductions in oxipurinol renal clearance during a restricted diet

The renal clearance of oxipurinol, the major metabolite of allopurinol, was studied in six healthy subjects during normal and restricted (low protein and low calorie) diets. A 600 mg oral dose of allopurinol was administered after 7 days of a normal diet (100 mg protein/day) and again after 2 and 4 weeks of a restricted diet (19 gm protein/day). The renal clearance of oxipurinol was reduced from 19.6 +/- 1.5 ml/min during the normal diet to 10.9 +/- 0.8 and 12.0 +/- 0.9 ml/min (both P less than 0.001) during the restricted diet at 2 and 4 weeks, respectively. These changes in oxipurinol renal clearance paralleled changes in uric acid renal clearance. Furthermore, the plasma oxipurinol half-life was increased from 27.0 +/- 1.7 hours during the normal diet to 51.1 +/- 4.3 and 45.7 +/- 3.7 hours (both P less than 0.001) during the restricted diet at 2 and 4 weeks, respectively. We conclude that dietary protein and calorie restriction cause a sustained reduction in the elimination of oxipurinol.

Quantitative liquid chromatography of allopurinol and oxypurinol in human plasma and urine

A high performance liquid chromatographic (HPLC) assay is described for allopurinol and oxypurinol determination in human plasma and urine, in the range expected during therapy. The procedure involves addition of trichloroacetic acid to samples, followed by centrifugation. The supernatant is then neutralized and analyzed by reversed-phase HPLC. Characteristics of the method are reported, and data are presented on its application to the pharmacokinetics studies. Separation is optimal with an octadecylsilane (ODS) stationary phase and a sodium acetate mobile phase adjusted to pH 7.2 for plasma and pH 5 for urine.

Does benzbromarone in therapeutic doses raise renal excretion of oxipurinol?

Twenty male hyperuricaemic patients with normal kidney function were studied and it was found that the serum concentrations and excretions rates in the 24-hour urine of allopurinol and oxipurinol do not differ significantly after 9 days of oral treatment with either 300 mg allopurinol or a combination of 300 mg allopurinol and 60 mg benzbromarone daily. The sum of the excretion rates of the two pyrazolopyrimidines in the 24-hour urine represents 80.9% and 77.1%, respectively of the daily dose of allopurinol given alone or in combination with benzbromarone. As expected, the hypouricaemic effect of the combined therapy turned out to be stronger than that observed after monotherapy with allopurinol, due to the uricosuric component of benzbromarone. The difference was found to be highly significant.

High-performance liquid chromatography with polarographic and voltammetric anodic detection: simultaneous determination of allopurinol, oxipurinol and uric acid in body fluids

Allopurinol, oxipurinol and uric acid have been determined in human serum and urine by liquid chromatography with electrochemical detection. In particular the use of a polarographic detector operating in the oxidative mode, whose principle of detection is based on the property of allopurinol, oxipurinol and uric acid to form insoluble anodic films on mercury, is described. The performance of such a detector is compared with that of a glassy carbon wall-jet detector. Different procedures for sample pretreatment have been evaluated.

On the metabolism of allopurinol. Formation of allopurinol-1-riboside in purine nucleoside phosphorylase deficiency

Allopurinol-1-riboside, a major metabolite of allopurinol, is commonly thought to be directly synthesized by purine nucleoside phosphorylase (PNP) in vivo. As this enzyme is otherwise believed to function in vivo primarily in the direction of nucleoside breakdown, we have determined by high performance liquid chromatography and a conventional chromatographic method the urinary metabolites of allopurinol in a child deficient of PNP. In this patient approximately 40% of urinary allopurinol metabolites consisted of allopurinol-1-riboside, thus proving the possibility of indirect formation of allopurinol-1-riboside via allopurinol-1-ribotide in vivo, catalysed by hypoxanthine guanine phosphoribosyltransferase (HGPRT) and a phosphatase.

Determination of selected pyrimidines, purines and their metabolites in serum and urine by reversed-phase ion-pair chromatography

The qualitative and quantitative determination of selected pyrimidines, purines and azapurines and their metabolites by reversed-phase ion-pair high-performance liquid chromatography, using tetrabutylammonium hydroxide as pairing ion and isocratic chromatrographic conditions, is described. The method provides a selective and sensitive assay for these classes of compounds examined in complex biological fluids.

Two-dimensional thin-layer chromatography for the screening of disorders of purine and pyrimidine metabolism

A method is presented for the two-dimensional thin-layer chromatographic screening of purines, pyrimidines and their nucleosides in the urine. Prior to chromatography, isolation of these substances from the urine is performed by anion-exchange column chromatography. Purines and pyramidines are quantitatively eluted with formic acid 0.01 M and 4 M respectively. The results of recovery and stability experiments are given. Normal excretory patterns are presented. Also results in patients with various diseases are shown: ornithine transcarbamylase deficiency, adenosine deaminase deficiency, purine nucleoside phosphorylase deficiency, adenine phosphoribosyltransferase deficiency, xanthine oxidase deficiency and hypoxanthine-guanine phosphoribosyltransferase deficiency. Finally the pattern of a patient on treatment with allopurinol is given.

The determination of allopurinol and oxipurinol in human plasma and urine

A method is described for allopurinol and oxipurinol assay within human plasma and urine in the range expected during therapy. The method is based on high-performance ion-exchange chromatography following an efficient sample purification step using Chelex-100 resin in the Cu2+-form. Linear calibration curves are produced for allopurinol over the range 0.05-10 mumole/1 (0.068-1.36 mug/ml) in plasma and 0.005-1 mmole/1 (0.68-136 mug/ml) in urine and for oxipurinol 0.5-100 mumole/1 (0.076-15.2 mug/ml) in plasma and 0.1-2 mmole/1 (15.2-304 mug/ml) in urine.

Variations in allopurinol metabolism by xanthinuric subjects

1. The metabolism of allopurinol was studied in a newly discovered patient with xanthinuria.

2. No oxipurinol was detectable in the urine during allopurinol administration, in direct contrast to the results in a patient previously studied by one of us.

3. Other equally discrepant effects of allopurinol on urinary purine and pyrimidine excretion in these two cases are compared with those in the three other recorded cases.