Free plasma concentrations of piroxicam in patients with osteoarthritis: relation to age, sex and efficacy.

Steady state piroxicam plasma samples from 85 patients suffering from osteoarthritis of the hip and/or the knee, and treated with piroxicam 20 mg daily for at least four weeks, were obtained. Twenty-seven of these patients had a newly diagnosed osteoarthritis and had not been treated with a nonsteroidal antiinflammatory drug (NSAID) previously. The plasma samples were subjected to equilibrium dialysis, analyzed by HPLC and total and free drug concentrations measured and unbound fractions were calculated. In the 27 newly diagnosed patients with free piroxicam concentrations ranging from 0.004 to 0.117 micrograms/ml, there was no correlation between free concentration and change in any of the clinical response variables from pretrial to week 4. In the total patient population free concentrations were 0.057 +/- 0.038 micrograms/ml (mean +/- standard deviation). Females had a 79% higher free concentration than males (p < 0.0002) and there was a statistically significant (p < 0.01) increase of free concentration with increasing age in females. The unbound fraction was 0.87% +/- 0.36% (mean +/- SD). There was no difference in unbound fraction between the sexes, nor could we detect any change with increasing age.