Determination of nalidixic acid and its two major metabolites in human plasma and urine by reversed-phase high-performance liquid chromatography.

This paper describes a precise and sensitive method for analysis of nalidixic acid and its two major metabolites in plasma and urine following the oral administration of a therapeutic dose in humans. After addition of an internal standard (oxolinic acid), 1-ml samples of plasma or urine are extracted at acidic pH with chloroform. The extracts are purified by re-extraction with sodium hydroxide solution and then chloroform. The final extracts are evaporated to dryness, reconstituted in mobile phase and injected into a high-performance liquid chromatograph equipped with RP-8 column and UV detector operating at 254 nm. The limit of sensitivity of the method is lower than 0.5 micrograms/ml of plasma or urine for each compound. The applicability of the method to pharmacokinetic studies of nalidixic acid in humans is demonstrated.