Simultaneous determination of allopurinol and oxipurinol in biological fluids by mass fragmentography

The optimal use of allopurinol: an audit of allopurinol use in South Auckland

BACKGROUND

Gout is a common and challenging problem in South Auckland, New Zealand. Allopurinol is widely used but urate reduction remains unsatisfactory. Allopurinol dosing guidelines and a therapeutic range for plasma oxypurinol levels have been published.

AIMS

We aimed to determine the appropriateness of allopurinol dosing according to current guidelines and to assess the relationship between plasma creatinine, oxypurinol and urate. In addition, we assessed the clinical usefulness of the oxypurinol level.

METHODS

Thirty-one patients, on a stable dose of allopurinol for at least three weeks, had plasma creatinine, urate and oxypurinol measured as part of routine clinical assessment. Relationships between the various methods were examined using regression analysis. Fisher's exact test was used to test associations with categorical variables.

RESULTS

Fifty-five per cent of patients were on higher than recommended doses of allopurinol. There was a statistically significant relationship between calculated creatinine clearance and plasma oxypurinol level. Only 50% of patients with a plasma oxypurinol within the therapeutic range (30-100 micromol/L) had a plasma urate < 0.42 mmol/L and this did not increase significantly in the patients with an oxypurinol level > 100 micromol/L.

CONCLUSIONS

There is poor adherence to the current recommended dosing guidelines for allopurinol. Creatinine clearance rather than plasma creatinine needs to be used to predict the dose of allopurinol. The current role of the oxypurinol level is to identify non-compliers with allopurinol therapy. We need further research to clarify whether increasing the dose of allopurinol outside the recommended dose range to reach an oxypurinol level of close to 100 micromol/L may be of benefit in those who have not had sufficient urate reduction.

Kinetics of allopurinol and its metabolite oxypurinol after oral administration of allopurinol alone or associated with benzbromarone in man. Simultaneous assay of hypoxanthine and xanthine by gas chromatography-mass spectrometry

Allopurinol, oxypurinol, hypoxanthine and xanthine were assayed simultaneously using a highly specific method combining gas chromatography and mass spectrometry. Two hypo-uricaemic prescriptions were compared: i) 300 mg of allopurinol (AL); and ii) 100 mg of allopurinol plus 20 mg of benzbromarone (AL + BZB). When administered acutely, their effects on blood uric acid levels were similar. Analysis of the pharmacokinetic parameters of allopurinol and its metabolite after each treatment showed dose-linearity for the metabolite but not for the drug itself. The area under the concentration time curve for allopurinol was 40.3 +/- 9.3 mumol l-1 h after AL, against 8.4 +/- 3.9 mumol-1 h after AL + BZB, while for oxypurinol it was 948.0 +/- 125.4 mumol l-1 h after AL and 285.2 +/- 77.9 mumol l-1 h after AL + BZB. The difference in dosage form may partly account for this difference, but the benzbromarone also seems to be involved. Its role on the blood uric acid lowering action of the drug association is complex. Although benzbromarone appreciably favors the elimination of oxypurinol, which should result in a weakening of its hypo-uricaemic action, this is offset by enhanced elimination of hypoxanthine and xanthine. Renal clearance of xanthine was significantly increased under AL + BZB (173.1 +/- 65.6 ml/min against 112.2 +/- 32.9 ml/min after AL). Similarly, blood xanthine levels were proportionately higher in the presence of benzbromarone. The action of the two agents may thus be synergistic and not antagonistic, a pharmacological justification for the therapeutic use of this drug association.