Determination of allopurinol and oxipurinol in biological fluids by high-performance liquid chromatography

A simple method for quantification of allopurinol and oxipurinol in human serum by high-performance liquid chromatography with UV-detection

OBJECTIVES

Allopurinol is a uric acid lowering drug used in the treatment of gout and the prevention of tumor lysis syndrome. Allopurinol and its active metabolite oxipurinol inhibit xanthine oxidase, which forms uric acid from xanthine and hypoxanthine. Therapeutic drug monitoring is an important option for evaluation and optimization of allopurinol treatment in case of renal impairment, interaction with uricosuric drugs or to verify patient adherence. In this study we developed and validated a simple quantitative assay using reverse phased high-performance liquid chromatography (HPLC) with UV-detection as a method for quantification of allopurinol and oxipurinol in human serum in the presence of different frequently used drugs.

METHODS

The HPLC-UV method uses a mobile phase consisting of sodium acetate (0.02 M; pH 4.5), at a flow rate of 1.0 mL/min. Allopurinol and oxipurinol are detected by UV-absorption at 254 nm with a retention time of 9.9 min for oxipurinol and 12.3 min for allopurinol. Aciclovir is used as internal standard.

RESULTS

Validation showed for allopurinol lower and upper limits of quantification of 0.5 and 10mg/L and for oxipurinol 1 and 40 mg/L, respectively. The assay was linear over the concentration range of 0.5-10mg/L (allopurinol) and 1-40 mg/L (oxipurinol). Intra- and inter-day precision showed coefficients of variation <15% over the complete concentration range; accuracy was within 5% for allopurinol and oxipurinol. Endogenous purine-like compounds were separated from allopurinol, oxipurinol and aciclovir with a resolution factor >1.5. Exogenous purine-like compounds and co-medication frequently used by gout patients did not hinder the analysis due to the dichloromethane washing step or to low UV-absorpion at 253 nm. Serum levels of 66 patients prescribed allopurinol 300 mg/day were determined using this HPLC-UV method. Measured serum allopurinol and oxipurinol concentrations in clinical practice showed large variability with a range of <0.5-4.3 mg/L for allopurinol and <1.0-39.2 mg/L for oxipurinol, respectively.

CONCLUSION

We developed an easy-to-operate and validated HPLC-UV method for the quantification of allopurinol and oxipurinol in human serum. This method was proven to be valid for samples of gout patients frequently using concomitant medications.

Facile and rapid high-performance liquid chromatography method for simultaneous determination of allopurinol and oxypurinol in human serum

OBJECTIVE

To develop a rapid and sensitive assay for the simultaneous determination of allopurinol and oxypurinol in serum.

METHOD

High-performance liquid chromatography (HPLC) with UV-detection. Sample preparation consists of protein precipitation by an addition of trichloracetic acid.

RESULTS

Percentage recovery and intra-assay coefficient of variation (CV%) for allopurinol were 97.4-101.3 and 0.66-5.13, respectively, in the concentration range 0.5-5.0 microg/mL. For oxypurinol, the percentage recovery and the intra-assay CV% were 93.2-98.1 and 0.88-5.62, respectively, in the concentration ranges 0.4-20 microg/mL. There was no interference of endogenous compounds in this assay.

CONCLUSION

This method is useful for routine therapeutic drug monitoring of allopurinol in a clinical setting.

Simultaneous determination of allopurinol and oxypurinol by liquid chromatography using immobilized xanthine oxidase with electrochemical detection

A novel liquid chromatographic method using an immobilized xanthine oxidase reactor and an electrochemical detector was developed for the simultaneous determination of allopurinol and oxypurinol in rat plasma, intestinal wash and bile. Xanthine oxidase was immobilized on 5-microns aldehyde silica (prepacked into a 2 mm x 10 mm cartridge) in a simple procedure. Allopurinol eluted from an analytical column was converted to oxypurinol in the enzyme reactor with the eluent as the reaction medium and detected with high selectivity using an amperometric detector with a glassy carbon electrode at the applied potential of +0.85 V. High specificity of the enzymatic reaction combined with selectivity of the electrochemical detection eliminated the need for an extensive sample preparation. The assay was linear in the range 15-500 ng/ml of rat plasma, intestinal wash and bile with a low limit of detection of 10 pg on-column (signal-to-noise ratio = 4) for both allopurinol and oxypurinol.

Allopurinol kinetics in humans as a means to assess liver function: evaluation of an allopurinol loading test

A newly developed liver function test was performed on 18 apparently healthy individuals and 29 patients with liver disease. After intravenous injection of a low dose allopurinol (17.1 mumol/kg body mass), blood specimens were collected during 1 h. Plasma analyses of allopurinol and its metabolite oxipurinol were performed and the data were processed by means of a computer-based biodynamic model. This modelling approach makes it possible to estimate parameters, containing information about liver perfusion, hepatocyte membrane transport and hepatocyte cell mass. One parameter (kA31) showed complete discrimination between the reference sample group of healthy individuals and patients with severe liver dysfunction. In a reference sample group of patients with slightly to moderately reduced liver function, only a few patients (5/20) had a kA31 value over the decision limit. In this respect, the allopurinol loading test is superior to the conventional intravenous galactose tolerance test.

The effect of dietary protein on the clearance of allopurinol and oxypurinol

A decrease in dietary protein is known to depress renal plasma flow and creatinine clearance. Using a randomized crossover design, we investigated the pharmacokinetics of allopurinol and its principal metabolite, oxypurinol, after oral administration of 600 mg of allopurinol in six normal subjects receiving a high-protein (268 g per day) or low-protein (19 g per day) diet. For allopurinol, the area under the curve of plasma concentration versus time increased by a factor of 1.45 (P less than 0.02), the renal clearance decreased by 28 per cent (P less than 0.02), and the ratio of the clearance of allopurinol to that of creatinine (fractional excretion) was unchanged between the low-protein and high-protein diets. For oxypurinol, the area under the curve increased nearly three-fold (P less than 0.02), the renal clearance decreased by 64 per cent (P less than 0.02), the fractional excretion decreased by 49 per cent (P less than 0.02), and the plasma oxypurinol half-life increased nearly threefold from 17.3 +/- 1.5 (mean +/- S.E.M.) to 49.9 +/- 2.9 hours (P less than 0.02) during the low-protein diet, as compared with the high-protein diet. We conclude that with the low-protein diet, the absorption, metabolism, and excretion of allopurinol were minimally altered but the total-body clearance of oxypurinol was greatly reduced because of a large increase in the net renal tubular reabsorption of oxypurinol.

Kinetics of allopurinol after single intravenous and oral doses. Noninteraction with benzbromarone and hydrochlorothiazide

An high-pressure liquid chromatographic method was used to measure allopurinol and oxipurinol in plasma and urine in 6 healthy volunteers after a single intravenous or oral dose of allopurinol. The influence of coadministered benzbromarone and hydrochlorothiazide on the pharmacokinetics of allopurinol and oxipurinol was also investigated. After intravenous injection of allopurinol 300 mg the plasma disappearance was biexponential, with a mean distribution half-life of 2.32 +/- 1.08 min (mean +/- SD) and an elimination half-life of 47.8 +/- 10.6 min. The total clearance of allopurinol was 11.37 +/- 2.70 ml/min/kg, whereas its renal clearance was only 1.73 +/- 0.79 ml/min/kg. Oxipurinol disappeared monoexponentially from plasma with a mean half-life of 12.2 +/- 2.6 h. Its renal clearance was 0.42 +/- 0.091 ml/min/kg. After oral administration of allopurinol 300 mg the peak plasma concentration of 2.1 +/- 0.6 micrograms/ml (1.5 x 10(-5) M) was reached within 30 to 120 min. The peak level of oxipurinol of 5.8 +/- 1.5 micrograms/ml (3.8 x 10(-5) M) was found within 2 to 5 h after intravenous and oral allopurinol. The bioavailability of oral allopurinol computed from plasma data was 90.4 +/- 8.7%. The total recovery from urine was 77% (allopurinol 8%, oxipurinol 69%) after oral and 88% after i.v. administration. It was concluded that about 10% of the oral dose was not absorbed and that 12% was eliminated by an unknown mechanism, presumably as riboside. The pharmacokinetics of allopurinol and oxipurinol were not significantly influenced by coadministration of benzbromarone or hydrochlorothiazide.