Simultaneous determination of L-dopa and carbidopa in plasma by HPLC and its pharmacokinetics in men

AIM: To establish an HPLC method for simultaneous determination of L-dopa and carbidopa in human plasma and to study pharmacokinetics of L-dopa and carbidopa in men. METHOD: The protein in plasma was precipitated with the mixture of HClO₄ and sodium metabisulfite was used as antioxidant. The drug was absorbed by Al₂O₃ on the condition of alkalescence and separated on the condition of acidity. Mobile phase was consisted of stock solution (2.3% citric acid, 1.66% NaOH): methol: acetic acid: H₂O (250 : 40 : 14 : 696, v/v/v/v) containing 4 mmol/L sodium heptanesulfonate. The column is C₁₈g, 5μm, 250 μm × 4.6 mm I.D. The polar potential was 0.8 V. The plasma concentration of L-dopa and carbidopa was determined in 20 healthy subjects after oral administration of single-dose and multi-dose. RESULT: The assay exhibited a linear range of 50-3200 ng/ml. The absolute recoveries of L-dopa and carbidopa are both larger than 70% and RSD within and between days were smaller than 15%. After oral administration of a single dose, the pharmacokinetic parameters of L-dopa were: t_1/22.02 ± 0.41 h, t_max 2.0 ± 0.9 h, c _max 1553.5 ± 350.6 ng/ml; the parameters of carbidopa were: t_1/21.77 ± 0.68 h, t_max3.8 ± 0.7 h, c _max 236.98 ± 62.03 ng/ml. And after muti-dose, the parameters were: L-dopa: t_max 1.8 ± 0.6 h, c_max 1203.0 ± 319.5 ng/ml, c_av 322.9 ± 62.5 ng/ml, AUC_ss 3874.9 ± 749.5 ng·h/ml, DF3.75 ± 0.84; carbidopa: t _max 2.8 ± 0.7h, c_max 225.94 ± 89.94 ng/ml, c_av 60.53 ± 20.66 ng/ml, AUCss 726.41 ± 247.88 ng·h/ml, DF3.68 ± 0.52. CONCLUSION: The method of HPLC-ECD we established can be used for clinical study of the co-administration of L-dopa and carbidopa.