After three months of orally administered levodopa, urinary excretion of vanilmandelic acid was moderately increased, but free and conjugated norepinephrine and epinephrine were not significantly affected in 14 patients with Parkinson's disease. The excretion of free and conjugated dopamine, 3,4-dihydroxyphenylacetic acid, and homovanillic acid (HVA) rose one hundred to one thousandfold depending on the daily intake of levodopa. Plasma levels of levodopa attained by 14 patients on similar dosage showed great variations. Low levels of DOPA associated with high levels of acidic metabolites in plasma indicated extensive catabolism of the drug in the alimentary canal or in peripheral organs. Lumbar cerebrospinal fluid levels of HVA measured after three months of treatment in 14 patients were related to the clinical improvement of the patient, and HVA was not detected in two patients who failed to respond.
Hinterberger H, Andrews CJ. Catecholamine Metabolism During Oral Administration of Levodopa: Effects of the Medication in Parkinson's Disease. Arch Neurol. 1972;26(3):245–252. doi:10.1001/archneur.1972.00490090071005
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