Value of plasma-dopa determinations in the management of Parkinson's disease

The on-off phenomenon

The on-off phenomenon is an almost invariable consequence of sustained levodopa treatment in patients with Parkinson's disease. Phases of immobility and incapacity associated with depression alternate with jubilant thaws. Both pharmacokinetic and pharmacodynamic factors are involved in its pathogenesis, but evidence is presented to indicate that the importance of levodopa handling has been underestimated and that progressive reduction in the storage capacity of surviving nigrostriatal dopamine terminals is not a critical factor. Re-distribution of levodopa dosage which may mean smaller, more frequent doses, or larger less frequent increments, may be helpful in controlling oscillations in some patients. Dietary protein restriction, the use of selegiline hydrochloride and bromocriptine may also temporarily improve motor fluctuations. New approaches to management include the use of subcutaneous apomorphine, controlled-release preparations of levodopa with a peripheral dopa decarboxylase inhibitor and the continuous intra-duodenal administration of levodopa.

Plasma levodopa, dopamine and therapeutic response following levodopa therapy of Parkinsonian patients

In a double blind placebo controlled study of 6 Parkinsonian patients given a single dose of oral levodopa, the plasma levodopa and dopamine as measured by a radioenzymatic method both correlated with clinical improvement. There was a closer correlation with plasma dopamine than with plasma levodopa and it is proposed that peripheral decarboxylation may reflect central decarboxylation.

Clinical pharmacokinetics of levodopa in parkinson's disease

Although levodopa has provided a major advance in the treatment of parkinsonism, its maximum benefits have not yet been realised, in part because of its complicated pharmacokinetics. This review summarises that available pharmacokinetic data involving levodopa, especially as it relates to therapeutic response of parkinsonian patients. A large number of factors, including protein intake, gastric emptying time, pyridoxine ingestion, and dopa decarboxylase activity, affect plasma levels of levodopa attained following oral administration of this drug. Other variables influence the rate of brain uptake of levodopa from the blood. Even so, plasma levodopa concentration correlates significantly with dosage size in a large parkinsonian population and also coincides with therapeutic response in many, but not all, patients. Therefore, in certain instances, valuable information may be derived by correlating clinical response with plasma levodopa concentration. Cerebrospinal fluid levels of homovanillic acid, a major metabolite of dopamine, may have some value in predicting clinical response to levodopa. This relationship, however, has not been firmly established. Concentration of homovanillic acid or levodopa in body fluids may also be closely related to certain adverse side-effects, including abnormal involuntary movements, gastric discomfort and psychiatric disturbances. Evidence indicates that a clearer understanding of levodopa pharmacokinetics may improve the clinical management of parkinsonism.