Effect of Carbidopa Dose on Levodopa Pharmacokinetics With and Without Catechol-O-Methyltransferase Inhibition in Healthy Subjects

Background and Objectives The treatment of Parkinson's disease (PD) is still symptomatic since disease-modifying treatments for PD are not available. Oral levodopa is the gold standard for the treatment of PD motor symptoms. However, incomplete and fluctuating plasma exposure of levodopa leads to suboptimal treatment of the symptoms. The main objective of this study was to investigate to what extent increased carbidopa doses (50 and 100 mg) increase the plasma levels of 100-mg immediate-release (IR) levodopa compared to a 25-mg carbidopa dose with and without co-administration of 200 mg entacapone. Methods A double-blind, placebo-controlled, randomized, crossover, phase I, pharmacokinetic study with 25 healthy volunteers was conducted. In addition, a semi-mechanistic pharmacokinetic model was built to theoretically evaluate the effect of inhibiting aromatic amino acid decarboxylase (AADC) and catechol-O-methyltransferase (COMT) mediated metabolism of levodopa on the exposure of levodopa. Results The effect of increased carbidopa doses 50 and 100 mg on the total exposure (AUC) of 100 mg IR levodopa was +29% and +36%, respectively, when entacapone was co-administered. Without entacapone, the corresponding increases were +13% and +17%. With entacapone co-administration, the increased carbidopa dose also clearly increased levodopa trough concentration. There was no significant effect on the peak concentrations of levodopa. Conclusions Increasing carbidopa doses significantly increased the exposure and reduced the fluctuation of IR levodopa in plasma during simultaneous COMT inhibition with entacapone. Theoretical pharmacokinetic simulations suggested that the plasma profile of oral IR levodopa can be even further improved by optimizing AADC and COMT inhibition.