Bioavailability and pharmacokinetics of intravenously and orally administered allopurinol in healthy beagles

OBJECTIVES: To determine bioavailability and pharmacokinetic parameters for allopurinol and its active metabolite, oxypurinol. ANIMALS: 6 healthy, reproductively intact female Beagles, 4.9 to 5.2 years old, and weighing 9.5 to 11.5 kg. PROCEDURE: In the first part of the study, allopurinol was administered IV at a dosage of 10 mg/kg of body weight to 3 dogs and 5 mg/kg to 3 dogs; the sequence was then reversed. In the second part of the study, allopurinol was administered orally at a dosage of 15 mg/kg to 3 dogs and 7.5 mg/kg to 3 dogs; the sequence was then reversed. In the third part of the study, allopurinol was administered IV (10 mg/kg), orally (15 mg/kg) with food, and orally (15 mg/kg) without food. Plasma samples were obtained at timed intervals, and concentrations of allopurinol and oxypurinol were determined. RESULTS: Maximal plasma allopurinol concentration and area under plasma allopurinol and oxypurinol concentration-time curves were 2 times greater when dogs were given 10 mg of allopurinol/kg IV, compared with 5 mg/kg, and when dogs were given 15 mg of allopurinol/kg orally, compared with 7.5 mg/kg. Allopurinol elimination half-life, time to reach maximal plasma oxypurinol concentration, and oxypurinol elimination half-life were significantly greater when dogs received 10 mg of allopurinol/kg IV, compared with 5 mg/kg, and when dogs received 15 mg of allopurinol/kg orally, compared with 7.5 mg/kg. CONCLUSIONS: Elimination of allopurinol is dependent on nonlinear enzyme kinetics. The bioavailability of allopurinol, and pharmacokinetic parameters of allopurinol and oxypurinol after oral administration of allopurinol, are not affected by administration with food. CLINICAL RELEVANCE: A dose threshold exists beyond which additional allopurinol would not substantially further inhibit xanthine oxidase activity. Oral administration of > 15 mg of allopurinol/kg to dogs would not be expected to result in greater reduction of plasma and urine uric acid concentrations. Also, allopurinol may be administered to dogs for dissolution or prevention of urate uroliths without regard to time of feeding.