[Simultaneous determination of verapamil and its major metabolite in human plasma by high performance liquid chromatography].

A sensitive high-performance liquid chromatographic assay suitable for the simultaneous determination of verapamil (I) and its major active metabolite norverapamil (II) in human plasma is described. After adding internal standard ethmosine, plasma samples were extracted using a mixture of n-hexane and n-butyl alcohol (12:1) to give mean recoveries of > 92% of both I, and II. The extracts were chromatographed on a C18 reversed phase column with a mobile phase composed of methanol, water and triethylamine (67: 33: 0.4, pH 6.7), with UV (lambda, 279 nm) detection. The calibration curves were linear over a wide range of concentrations (25-1000 ng.ml-1), and the limits of determination was 2.5 ng.ml-1 for I and 5.0 ng.ml-1 for II. The method showed good precision: the within-day RSD were < 7.6% for both I and II; the day-to-day RSD were < 8.6% for both I and II. Using this assay, plasma concentrations of both I and II were simultaneously determined in six volunteers after a single oral dose of 120 mg of verapamil.HCl.