Effects of naproxen on the in vivo synthesis of thromboxane and prostacyclin in man.

The effect of a single oral dose of 500 mg naproxen on the synthesis in vivo of thromboxane A2 and prostacyclin was studied in healthy volunteers. The synthesis of the prostanoids was assessed by measuring the urinary excretion of the metabolites 2,3-dinor-TxB2 and 2,3-dinor-6-keto-PGF1 alpha, respectively, using stable isotope dilution assays based on gas chromatography - mass spectrometry. Naproxen caused significant inhibition of the excretion of both metabolites for about two days. The reduction of the thromboxane metabolite was more pronounced (75% inhibition) than that of the prostacyclin metabolite (about 50% inhibition). The data support the idea that naproxen causes reversible inhibition of cyclooxygenase.