Consequently, the dopa dose required to control the motor manifestations must, be gradually increased as the disease progresses.

It quickly became clear also that, of the two dopa isomers, only the levorotatory stereoisomer, levodopa, produced therapeutic benefits, and chemical means to separate the two isomers were developed. In practice, only levodopa is now used in the treatment of PD, resulting in an improved safety profile. Soon after came the recognition that some of the adverse effects associated with the drug were the result of peripheral – rather than central – conversion of levodopa into DA, which, unlike levodopa, Inhibitors,research,lifescience,medical has significant autonomic activity.1 Since DA does not cross the blood-brain barrier (BBB), any DA produced in the peripheral nervous system docs not contribute to the clinical benefits afforded Inhibitors,research,lifescience,medical by levodopa, and actually causes significant, adverse events, particularly gastrointestinal and other autonomic disturbances. The enzyme involved in the transformation of levodopa to DA, ie, l-amino acid decarboxylase (L-AAD, initially called dopa decarboxylase) is widespread in the body, with high Inhibitors,research,lifescience,medical concentrations in the liver. Two agents were developed that could inhibit, it, and both are still in use: carbidopa and benserazide. At. present, practically all patients

who require treatment with levodopa receive it as a fixed-dose combination with one of these inhibitors. Of course, it is essential Inhibitors,research,lifescience,medical that levodopa be converted

into DA in the brain, and so the L-AAD inhibitor should not cross the BBB. The inhibition of peripheral L-AAD has another result, which was initially unappreciated: it prolongs the biological half-life of levodopa (and therefore also of DA in the brain). This effect, is important, in advanced PD. Early on in PD, there is a dramatic beneficial effect of levodopa, Inhibitors,research,lifescience,medical described as the “honeymoon.” As the disease advances and additional DA neurons Suplatast tosilate arc being lost, there is a need to compensate for this by increasing the daily dose of levodopa. This is first, manifested by shortening of the duration of action of individual levodopa doses, called “end-of dose” effect or wearing off. Later on, other manifestations appear, including “peak of dose” dyskinesias and erratic responses to levodopa (so-called unexpected “on-off,” or yo-yoing) (Table I). While the exact mechanism responsible for this erratic response is still elusive, it is at least partly dependent upon pharmacokinetic factors such as plasma levels of levodopa. In particular, the phenomenon of wearing off, where the initial prolonged response to individual doses of levodopa is no longer maintained,2 limits the patients’ independence.