Factors affecting the kinetics of two benzothiazine non-steroidal anti-inflammatory medicines, piroxicam and isoxicam.

The pharmacokinetics of isoxicam and piroxicam were compared in 12 young adults (less than 40 years) and 12 elderly subjects (greater than 65 years). After a single oral dose of 200 mg isoxicam or 20 mg piroxicam blood samples were taken for 168 h and the plasma drug concentrations determined by HPLC. The elimination half life of piroxicam for the adults was 57.1 +/- 16.4 h (mean +/- SD; harmonic mean 52.9 h) and for the elderly subjects was 57.8 +/- 22.1 h (harmonic mean 52.1 h). The corresponding values after isoxicam were 34.3 +/- 13.6 h (harmonic mean 31.6) for the adults and 39.1 +/- 22.7 h (harmonic mean 33.5) for the elderly subjects. Similarly no differences were noted in either the AUC0-infinity after piroxicam (adults 154.1 +/- 52.2 micrograms . h/ml, elderly 163.6 +/- 99.1 micrograms . h/ml) and isoxicam (adults 642.7 +/- 241.9 micrograms . h/ml, elderly 787.9 +/- 613.1 micrograms . h/ml) or the apparent oral clearance of piroxicam (adults 2.39 +/- 0.80 ml/min, elderly 2.51 +/- 0.90 ml/min) and isoxicam (adults 5.84 +/- 2.04 ml/min, elderly 5.59 +/- 2.12 ml/min). One adult and two elderly subjects exhibited slower elimination of both medicines than the remainder of each group. However determination of the oxidation phenotype using sparteine metabolism showed that this was not a likely determinant of the reduced clearance.