Ion-pair HPLC method for the quantification of metformin in human urine

Metformin is an oral antidiabetic drug that has been used in the treatment of non-insulin-dependent diabetes, which improves glycemic control by primarily inhibiting hepatic gluconeogenesis and glucogenolysis [1]. Metformin is slowly absorbed after oral administration, about 60% of an oral dose is excreted in the urine as unchanged drug within 24 h, and about 30% of the dose is nonabsorbed and eliminated unchanged in feces [2]. Previous studies have shown a plasma elimination half-life ranging from 2.0 to 6.0 h after oral administration of varying doses (0.5 to 1.5 g/dose) [3]. Urine provides a non-invasive sample collection method, and determination of drug levels in urine is comparatively less complex than plasma and other body fluids [4]. Several reports indicate that urinary excretion data can be used to arrive at bioequivalence decision of different drug formulations [4-6]. Chemically, it is 1,1-dimethyl biguanide (Figure 1). Metformin is a small highly polar molecule (pKa=2.8, 11.5, log P octanol:water= −2.6), which has great solubility in water and poor solubility in lipids; it is also possible to retain in reversed-phase (RP) columns using ion-pairing reagents. Numerous methods for the quantitation of metformin in urine have been utilized over years. They include capillary electrophoresis with contactless conductivity detection [7], voltammetric method [8], cation exchange high-performance liquid chromatography (HPLC) [9], reverse phase RP-HPLC [10-12] and liquid chromatography (LC)–mass spectrometry (MS) [13]. Most of the methods have been based on HPLC with spectrophotometric detection in the range of 230–240 nm (Table 1). Chemical derivatization of metformin has been used JOURNAL OF APPLIED BIOANALYSIS, January 2016, p. 16-24. http://dx.doi.org/10.17145/jab.16.004 (ISSN 2405-710X) Vol. 2, No. 1