Motor fluctuations in levodopa treatment: clinical pharmacology.

Fluctuations in motor performance and dyskinesias are a common problem in the long-term management of Parkinson's disease. The pharmacokinetics and the pharmacodynamics of levodopa, loss of presynaptic dopamine terminals, alterations in postsynaptic dopamine receptor sensitivity, or changes in the modulatory influences of nondopaminergic transmitter system could play a pathogenic role. The short half-life of levodopa and its absorption and transport are important factors. The pharmacodynamic response to levodopa changes during long-term therapy. An absolute threshold in plasma levodopa level, below which the patients are off, appears. The patients show a therapeutic window for levodopa which becomes narrower with time. This therapeutic window can be widened by giving continuous infusion of dopaminergic drugs. Motor fluctuations can be ameliorated by a more physiological continuous stimulation of the receptor site and by avoiding repeated shifting in levodopa levels. Long-term complications are attenuated by an early combination of levodopa with dopamine agonist.