Continuous drug delivery in early- and late-stage Parkinson's disease as a strategy for avoiding dyskinesia induction and expression

The treatment of the motor symptoms of Parkinson's disease (PD) is dependent on the use of dopamine replacement therapy in the form of l-dopa and dopamine agonist drugs. However, the development of dyskinesia (chorea, dystonia, athetosis) can become treatment limiting. The initiation of dyskinesia involves a priming process dependent on the presence of nigral dopaminergic cell loss leading to alterations in basal ganglia function that underlie the expression of involuntary movements following the administration of each drug dose. Once established, dyskinesia is difficult to control and it is even more difficult to reverse the priming process. Dyskinesia is more commonly induced by l-dopa than by dopamine agonist drugs. This has been associated with the short duration of l-dopa causing pulsatile stimulation of postsynaptic dopamine receptors compared to the longer acting dopamine agonists that cause more continuous stimulation. As a result, the concept of continuous dopaminergic stimulation (CDS) has arisen and has come to dominate the strategy for treatment of early PD. However, CDS has flaws that have led to the general acceptance that continuous drug delivery (CDD) is key to the successful treatment of PD. Studies in both experimental models of PD and in clinical trials have shown CDD to improve efficacy, but reduce dyskinesia induction, and to reverse established involuntary movements. Two key clinical strategies currently address the concept of CDD: (1) in early-, mid- and late-stage PD, transdermal administration of rotigotine provides 24 h of drug delivery; (2) in late-stage PD, the constant intraduodenal administration of l-dopa is utilized to improve control of motor symptoms and to diminish established dyskinesia. This review examines the rationale for CDD and explores the clinical benefit of using such a strategy for the treatment of patients with PD.