A simple and sensitive reversed phase high-performance liquid
chromatographic method was developed for simultaneous
quantification of diltiazem HCl and its major metabolite Ndemethyldiltiazem
in human plasma. The method involves one step
solvent extraction of diltiazem, N-demethyldiltiazem and the internal standard, verapamil with n-hexane and diethyl ether
(50:50 v/v). The mobile phase comprised 0.1 M ammonium
dihydrogen phosphate-acetonitrile (62:38 v/v) and triethylamine
(0.08%) was added before the pH was adjusted to 5.9 with 85%
phosphoric acid. Analysis was run at a flow rate of 1.0 ml/min at a
detection wavelength of 238 nm. The completion time for assay was
not more than 10 minutes and lower limit of quantification was 5
ng/ml. The calibration curve for diltiazem and its metabolite was
linear over a concentration range of 5-200 ng/ml and average
recovery was about 90%. The coefficient of variation and percent
error values of the assay method within and between days were all
less than 10%.