Urinary free methyldopa determined by reversed-phase high-performance liquid chromatography.

We describe a chromatographic method, in which 3,4-dihydroxybenzylamine is used as the internal standard, for determining free methyldopa in human urine. The drug was adsorbed onto alumina, eluted, and the eluate directly injected onto a reversed-phase column (octadecyl-bonded silica stationary phase), with dilute acetate buffer (pH 2.7) as the mobile phase and ultraviolet detection at 280 nm facilitated. Methyldopa is well separated from other urinary biogenic amines present in the alumina extract, and other commonly used antihypertensives and diuretics do not interfere with the analysis. The sensitivity of the method is adequate to quantify 8.0 mg of methyldopa per liter in 30 ml of sample; the lower limit of detection is 25 ng. Analytical recovery for methyldopa varied from 95 to 102% with within-run and day-to-day coefficients of variation of 2.7 (n = 10) and 3.8% (n = 5), respectively. This procedure is readily adaptable for use in studies of the pharmacokinetics of methyldopa and to routine clinical laboratory use.