Effect of ethanol on metabolism of the hypouricemic agents allopurinol and benzbromarone

Clinical pharmacokinetics and pharmacodynamics of allopurinol and oxypurinol

Allopurinol is the drug most widely used to lower the blood concentrations of urate and, therefore, to decrease the number of repeated attacks of gout. Allopurinol is rapidly and extensively metabolised to oxypurinol (oxipurinol), and the hypouricaemic efficacy of allopurinol is due very largely to this metabolite. The pharmacokinetic parameters of allopurinol after oral dosage include oral bioavailability of 79 +/- 20% (mean +/- SD), an elimination half-life (t((1/2))) of 1.2 +/- 0.3 hours, apparent oral clearance (CL/F) of 15.8 +/- 5.2 mL/min/kg and an apparent volume of distribution after oral administration (V(d)/F) of 1.31 +/- 0.41 L/kg. Assuming that 90 mg of oxypurinol is formed from every 100mg of allopurinol, the pharmacokinetic parameters of oxypurinol in subjects with normal renal function are a t((1/2)) of 23.3 +/- 6.0 hours, CL/F of 0.31 +/- 0.07 mL/min/kg, V(d)/F of 0.59 +/- 0.16 L/kg, and renal clearance (CL(R)) relative to creatinine clearance of 0.19 +/- 0.06. Oxypurinol is cleared almost entirely by urinary excretion and, for many years, it has been recommended that the dosage of allopurinol should be reduced in renal impairment. A reduced initial target dosage in renal impairment is still reasonable, but recent data on the toxicity of allopurinol indicate that the dosage may be increased above the present guidelines if the reduction in plasma urate concentrations is inadequate. Measurement of plasma concentrations of oxypurinol in selected patients, particularly those with renal impairment, may help to decrease the risk of toxicity and improve the hypouricaemic response. Monitoring of plasma concentrations of oxypurinol should also help to identify patients with poor adherence. Uricosuric drugs, such as probenecid, have potentially opposing effects on the hypouricaemic efficacy of allopurinol. Their uricosuric effect lowers the plasma concentrations of urate; however, they increase the CL(R) of oxypurinol, thus potentially decreasing the influence of allopurinol. The net effect is an increased degree of hypouricaemia, but the interaction is probably limited to patients with normal renal function or only moderate impairment.

Effect of ethanol on metabolism of purine bases (hypoxanthine, xanthine, and uric acid)

There are many factors that contribute to hyperuricemia, including obesity, insulin resistance, alcohol consumption, diuretic use, hypertension, renal insufficiency, genetic makeup, etc. Of these, alcohol (ethanol) is the most important. Ethanol enhances adenine nucleotide degradation and increases lactic acid level in blood, leading to hyperuricemia. In beer, purines also contribute to an increase in plasma uric acid. Although rare, dehydration and ketoacidosis (due to ethanol ingestion) are associated with the ethanol-induced increase in serum uric acid levels. Ethanol also increases the plasma concentrations and urinary excretion of hypoxanthine and xanthine via the acceleration of adenine nucleotide degradation and a possible weak inhibition of xanthine dehydrogenase activity. Since many factors such as the ALDH2*1 gene and ADH2*2 gene, daily drinking habits, exercise, and dehydration enhance the increase in plasma concentration of uric acid induced by ethanol, it is important to pay attention to these factors, as well as ingested ethanol volume, type of alcoholic beverage, and the administration of anti-hyperuricemic agents, to prevent and treat ethanol-induced hyperuricemia.

Ethanol as a xanthine dehydrogenase inhibitor

In the present study, we investigated whether ethanol inhibits the activity of xanthine dehydrogenase. Ethanol and/or inosine were administered to normal subjects, and plasma concentration and urinary excretion of purine bases were measured together with blood concentrations of lactic acid and pyruvic acid. In addition, ethanol and pyrazinamide were administered to these subjects, and plasma concentration and urinary excretion of pyrazinamide and its major metabolites were measured. Increases in plasma concentration and urinary excretion of xanthine induced by a combination of ethanol and inosine were greater than the sums of increases induced separately by ethanol and inosine, although increases in plasma concentration and urinary excretion of uric acid induced by the combination of ethanol and inosine were not different from the sums of increases induced separately by ethanol and inosine. Ethanol increased the ratio of blood lactic acid to blood pyruvic acid and decreased plasma concentration and urinary excretion of 5-hydroxypyrazinamide and 5-hydroxypyrazinoic acid. These results suggest that ethanol inhibits xanthine dehydrogenase presumably by an ethanol-induced increase in the cytosolic concentration of NADH in the liver.