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 E(max) model and the C50 values for oxypurinol were 26.38±4.87, (mean±s.d.) 36.5818.36 and 24.61±9.08 μM, respectively. 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. The concentration of oxypurinol required for inhibition of xanthine oxidase, as indicated by C50, was lower than those often observed in clinical practice.
- Xanthine oxidase