High-performance of liquid chromatographic determination of [3,4-di-(4-methoxyphenyl)-5-isoxazolyl]acetic acid and its metabolites in human plasma and urine.

A high-performance liquid chromatographic method is described for the simultaneous determination of the non-steroidal analgesic and anti-inflammatory agent [3,4-di-(4-methoxyphenyl)-5-isoxazolyl]acetic acid and its three metabolites in plasma and urine. Deproteinized plasma (with acetonitrile) or urine was applied to a Sep-Pak C18 cartridge, washed with distilled water and then eluted with methanol. The methanol eluate was reduced to dryness. The resulting residues from the plasma and urine were redissolved in methanol aqueous solution, respectively. Aliquots of each solution were chromatographed on a reversed-phase column using a mobile phase of methanol-20 mM potassium dihydrogenphosphate (pH 6.4) (linear gradient from 0 to 100% methanol at 3%/min with a flow-rate of 1.5 ml/min) on a liquid chromatograph equipped with an ultraviolet absorbance detector (254 nm). Detection was limited to 10 ng/ml in plasma and 100 ng/ml in urine for each compound. An accurate and sensitive assay for the determination of [3,4-di-(4-methoxyphenyl)-5-isoxazolyl]acetic acid and its metabolites was established.