Relationship between naproxen plasma concentration and its anti-inflammatory effect in experimental hepatitis

The relationship between the plasma concentration and the anti-inflammatory effect of naproxen (CAS 22204-53-1) after oral administration of a 6 mg.kg-1 dose was studied in rats with galactosamine-induced acute hepatitis and under control conditions. In control animals naproxen peak plasma levels of 35 +/- 0.4 micrograms.ml-1 were reached in 0.5 +/- 0 h. Concentration then decayed, half-life being 5.2 +/- 0.4 h. AUC was 131 +/- 5 micrograms.h.ml-1. In intoxicated rats peak plasma levels of 29 +/- 0.3 micrograms.ml-1 were reached in 0.7 +/- 0.1 h, half-life was increased to 11.1 +/- 1.3 h, and the AUC reached 259 +/- 21 micrograms.h.ml-1. In control rats the protective effect of naproxen against carrageenan-induced inflammation increased slowly, reaching a maximum of 38% in 4 h. The protective effect against plasma concentration curve exhibited a clear counterclockwise hysteresis, probably due to a slow naproxen transport from the circulation to its site of action. In animals with hepatitis, the protective effect remained quite constant at about 40% despite variations in plasma levels, probably because the maximal effect was reached. No clear hysteresis was observed in the effect-plasma concentration curve, suggesting that naproxen arrival to its site of action was faster. Results show that the relationship between naproxen plasma concentration and its anti-inflammatory effect is complex and therefore predictions on the pharmacological response in liver damage cannot be readily made by solely considering pharmacokinetic data.