Determination of doxycycline in human plasma and urine samples by high performance liquid chromatography. Application for drug monitoring in malaria chemoprophylaxis.

A reverse-phase high performance liquid chromatographic method with ultraviolet detection is described for the measurement of doxycycline in human plasma and urine. After liquid-solid extraction on a Bond Elut C18 cartridge, doxycycline and demeclocycline (internal standard) are separated on a Novapak C18 column by isocratic elution. The mobile phase consists of acetonitrile-oxalate buffer, pH 2.3 (25:75; v/v). The eluent is monitored with an ultraviolet detector at 355 nm. The lower limit of quantification in plasma is close to 25 ng/ml. No chromatographic interference can be detected from endogenous compounds, tetracycline group antibiotics or antimalarial drugs. The method is accurate and precision is good with inter- and intra-assay relative standard deviations lower than 6.7%. The chromatographic procedure takes 8 minutes and can be used for therapeutic drug monitoring, clinical and pharmacokinetic studies.