The pharmacokinetics of intravenous and oral levodopa in patients with Parkinson's disease who exhibit on‐off fluctuations.

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Abstract
We have studied the clinical effects and pharmacokinetics of levodopa infusions and oral therapy in seven patients with Parkinson's disease. They all showed on‐off fluctuations whilst receiving long‐term treatment with levodopa in combination with a peripheral decarboxylase inhibitor. Intravenous infusion at a constant rate for up to 16 h resulted in a smoother clinical response, and maintained plasma levodopa concentrations within narrower limits compared with conventional oral therapy. Following infusion rates of 32‐80 mg h‐1 (0.5‐1.3 mg kg‐1 h‐1) the plasma concentration associated with optimum therapeutic response lay between 0.3 and 1.6 mg l‐1. There was considerable variation in the oral absorption and elimination of levodopa, both within and between subjects. The concentration of 3‐OMe dopa in plasma hardly increased during each day's levodopa therapy. In all cases levels were greater than the maximum concentrations of levodopa, sometimes by as much as a factor of 10. In contrast to most previous reports on the pharmacokinetics of levodopa, the data presented here are consistent with a two‐compartment kinetic model. It is not known whether the difference in pharmacokinetics is due to chronic therapy or whether it is specific to those patients who show on‐ off phenomena, but such changes might be related in some way to the development of fluctuations in clinical response.