Determination of plasma metformin by a new cation-exchange HPLC technique

Metformin is an oral antihyperglycemic agent used in the therapy of noninsu lin-dependent diabetic patients. This biguanide can induce dangerous compli cations such as lactic acidosis when its plasma concentration is too high. For this reason, the determination of plasma metformin should always be don e during treatment. We developed a new HPLC method, for the routine determi nation of plasma metformin, with good reliability, rapid execution, and low costs. Sample preparation involved precipitation of the plasma proteins co ntaining the internal standard buformin with a mixture of methanol, zinc su lfate, and ethylene glycol: the diluted supernatant was injected into a cat ion-exchange column. The mobile phase was potassium dihydrogenphosphate buf fer-containing acetonitrile. The eluent was monitored at 236 nm. The calibr ation curve is linear within the range of 20-4000 ng/mL; the within-day coe fficients of variation were lass than 2.2% for metformin and 1.5% for bufor min; the day-to-day coefficients of variation were less than 2.5% for metfo rmin and 1.9% for buformin. The mean recoveries obtained from supplemented samples were included between 99.4 and 104.2% for metformin, Many character istics make this method useful and easily accessible to all clinical labora tories equipped with HPLC instrumentation.