Induction of salicyluric acid formation in rheumatoid arthritis patients treated with salicylates.

Average steady-state serum salicylate concentrations and salicyluric acid (SU) formation rates were measured in 4 subjects with rheumatoid arthritis. After a salicylate washout period (1 month), the mean observed maximum formation rate of SU was determined by collecting frequent urine samples after a single oral dose of salicylate (35 mg/kg). The patients were then commenced on appropriate high dose salicylate therapy. Two and 5 weeks later, the mean observed maximum rates of SU formation were re-determined along with the average steady-state serum salicylate concentrations. Mean observed maximum SU excretion rates increased significantly between the single-dose study (0.96 +/- 0.22 mg/kg/h) and the last dose of the high dose therapy at 2 weeks (1.65 +/- 0.30 mg/kg/h; p less than 0.01); however, there was no further increase at week 5. Similar increases in the theoretical maximum rate of SU formation (Vmax) were observed between the single-dose study and after 2 weeks of high dose salicylate therapy. Average steady-state serum salicylate concentrations showed no decline between weeks 2 and 5. High dose salicylate therapy leads to acceleration of the rate of SU formation in patients with rheumatoid arthritis and this occurs largely during the first 2 weeks of therapy.