Salicyl phenolic glucuronide pharmacokinetics in patients with rheumatoid arthritis

The pharmacokinetics of salicyl phenolic glucuronide (SPG) and other salicylic acid (SA) metabolites were studied at three aspirin dosage regimens in eight patients with rheumatoid arthritis. Each patient received 1, 2 and 4 g enteric coated aspirin (ASA) daily in ascending order. At the end of each 2-week dosage period, plasma and urine were collected over a dosage interval for the estimation of various pharmacokinetic parameters. With increasing ASA dosage, mean clearance of SA to SPG was approximately constant (1.8 +/- 0.3, 1.7 +/- 0.2, and 1.5 +/- 0.2 ml/min at 1, 2 and 4 g/day, respectively) when related to plasma concentrations of total SA. The percentage of the ASA dosage recovered in urine as SPG increased from 5.2 +/- 1.1 to 7.1 +/- 1.1 to 10.5 +/- 1.7 at 1, 2 and 4 g/day, respectively. It was concluded, however, that the conversion of SA to SPG is saturable, since the mean clearance of SA to SPG decreased when calculated with respect to the plasma concentration of unbound SA (13.4 +/- 1.6, 11.0 +/- 1.4, and 6.6 +/- 1.9 ml/min at 1, 2 and 4 g/day, respectively). The kinetics of the formation and excretion of salicylurate and the excretion of gentisate were similar to those found in previous studies.