Objectives 

Although commercially available levodopa (LD) formulations include carbidopa (CD) or benserazide for gastrointestinal l-aromatic amino acid decarboxylase inhibition, little is known how manipulating CD delivery affects the pharmacokinetics of LD. Our research systematically evaluated the peripheral and central pharmacokinetics of LD during continuous subcutaneous CD delivery.

Methods 

We conducted pharmacokinetic experiments in pigs, mice, and humans to characterize effects of continuous subcutaneous CD delivery co-administered with LD as compared with oral LD/CD administration on LD pharmacokinetics. The porcine and human studies compared peripheral LD pharmacokinetic parameters (area under the curves [AUCs], peak plasma concentrations [C_max], and plasma elimination half-life [t_1/2]) and the mouse studies compared brain LD and dopamine concentrations.

Results 

In the pig, supplementary subcutaneous CD delivery significantly increased the LD t_1/2 and AUC versus LD/CD alone and versus additional oral CD administration. In mice, administration of supplementary subcutaneous CD substantially increased mean plasma concentrations of both LD and CD versus oral LD/CD alone at all time points. These increases were mirrored by increased brain dopamine levels for at least the 7 hours of study. In healthy human subjects, continuous subcutaneous CD administration, 3.33 mg/h x24h, increased the plasma LD t_1/2, C_max, and AUC by 17.4%, 40.5%, and 22.3%, respectively (P < 0.003).

Conclusions 

This series of studies demonstrates that small continuous dosing of subcutaneous CD has an unexpected effect on LD pharmacokinetics greater than the extent of decarboxylase inhibition achieved by additional oral CD administration.