Simultaneous determination of amoxicillin and clavulanic acid in human plasma by HPLC with UV detection

A simple and accurate high-performance liquid chromatographic method with ultraviolet detection at 220 nm has been validated for the simultaneous determination of amoxicillin and clavulanic acid in human plasma. Plasma samples were pretreated by direct deproteinization with methanol. A good chromatographic separation between both compounds was achieved using a reversed phase C8 column and a mobile phase, consisting of acetonitrile-phosphate solution-tetramethyl ammonium chloride solution. The calibration curves were linear over the concentration range of 0.625-20 mg l(-1) for amoxicillin and 0.3125-10 mg l(-1) for clavulanic acid with determination coefficients > 0.998. The method is accurate (bias < 7%) and reproducible (intra- and inter-day R.S.D. < 15%), with a quantitation limit of 0.625 and 0.3125 mg l(-1) for amoxicillin and clavulanic acid, respectively. Analytical recoveries from human plasma ranged from 91 to 102% for both components. This fully validated method, which allows the simultaneous measurement of amoxicillin and clavulanic acid in biological samples, is rapid (total run time < 10 min) and requires only a 100 microl sample. This assay is suitable for biomedical applications and was successfully applied to a pilot pharmacokinetics study in healthy volunteers after a single-oral administration of amoxicillin/clavulanic acid combination (500/125 mg).