MEASURING L-DOPA IN PLASMA AND URINE TO MONITOR THERAPY OF ELDERLY PATIENTS WITH PARKINSON DISEASE TREATED WITH L-DOPA AND A DOPA DECARBOXYLASE INHIBITOR

We have established a method for measuring L-dopa in plasma and urine, including the metabolites dopamine and L-dopac, using separation by i on-pair reversed-phase HPLC and quantification with an electrochemical detector. The assay was applied to the therapeutic monitoring of elde rly patients with established Parkinson disease being treated with L-d opa plus a dopa decarboxylase inhibitor. Plasma L-dopa was evaluated i n relation to dosage and postdose sampling time in 71 outpatients with Parkinson disease. L-Dopa concentrations were greatest in the patient s taking the highest dosages prescribed and decreased significantly wi th increasing time after postdose sampling. Comparison of plasma L-dop a concentrations with a published therapeutic range established by int ravenous administration Of L-dopa was helpful in assessing the suitabi lity of each patient's drug dosage, assessing patients' compliance, an d avoiding overdosage but was not useful in the overall clinical asses sment of progression of disease or of the long-term therapeutic respon se. Urine measurements confirmed the plasma concentrations but showed no further advantage. The recommended time for sample collection is be tween 1.5 and 3 h after the first morning dose. Plasma is the preferre d matrix but if blood sampling is difficult, particularly from elderly /infirm individuals, an untimed urine collection could be used.