Determination of Metformin in Human Plasma Using Normal Phase High Performance Liquid Chromatography

A simple , selective, sensiti ve, accurate, and precise normal phase HPLC method coupled with UV detection has been developed and validated for the determination of metformin in human plasma.After protein precipitation with acetonitrile, Metformin was extracted with dichloromethane. The mobile phase consisted of acetonitrile and phosphate buffer (0.05M) (60:40 % v/v) pH 7.0, the stationary phase was a normal phase silica column (250X4.6 mm ID, 51-lm particl e size). Detection was carried out using a UV detector set at 235nm. The method was linear over the concentration range 0.016-2 .709 ug/ml (y = 0.7898X +0.0048) and gave a limit of quantitation of 16 ng/ml. Analytical recovery, measured over three days, averaged 94.88%. The interday precision ranged 4.1 to 11.8 CV (%) for four quality control samples includ ing LLOQ, low, medium, and high . Metformin was found to be stable in plasma and in working stand ard solutions during sample collection, storage , and processing as well as in five free ze thaw cycle s. The described HPLC method was successfully employed for the analysis of authentic samples collected from three bioequivalence studi es involving 32 volunteers each. The average concentration - time profiles were plotted from the three bioequivalence studies which involved three doses of 500 mg/tablet, 850 mg/tablet and 1000mg/tablet under fasting conditions. Slow GI absorption and linear pharmacokinetics characterized the disposition of metformin.