Liquid chromatographic determination of dihydralazine and hydralazine in human plasma and its application to pharmacokinetic studies of dihydralazine.

An analytical method is described for the concurrent determination of dihydralazine (1) and hydralazine (2) in human plasma as unchanged or apparent compounds. For the assay of the unchanged compounds, plasma samples were acidified with 0.02 M HCI and derivatized first with nitrous acid, and afterwards with sodium methylate. For the assay of the apparent compounds, plasma samples were acidified with 3 M HCI, incubated at 90 degrees C for 30 min and derivatized as above. The derivatives were extracted and chromatographed by reversed-phase mode on a C18 mu Bondapak column. The fluorescence of the compounds was measured (excitation wavelength = 230 nm, emission wavelength = 430 nm). The limits of quantitation were 0.5 ng/mL for the unchanged compounds and 1 ng/ml for the apparent compounds. After oral administration of 25 mg of 1 to 2 healthy volunteers, the mean areas under the plasma concentration-time curves were respectively 43.7 and 590 ng X h/mL for unchanged and apparent 1. The corresponding mean elimination half-lives were 1.03 and 3.9 h. The mean area under the curve measured for 2 amounted to 6.3% of that obtained for 1 for the unchanged compounds and to 10.3% for the apparent compounds.