Strategies for Treating Motor Fluctuations in Parkinson’s Disease

MIAMI—Motor fluctuations in Parkinson’s disease can arise from more than one cause, and a clinician needs to consider a range of possibilities. Most commonly, motor fluctuations arise as a consequence of chronic levodopa therapy, though the progression of parkinsonism is a contributing factor, according to an overview presented at the Second Pan American Parkinson’s Disease and Movement Disorders Congress. The pharmacokinetics of levodopa provide the basis for studying most clinical patterns of motor fluctuations, and new pharmacologic strategies are under development to improve upon existing treatment options.

Peter A. LeWitt, MD

“In recent years, there have been some exciting and novel directions of Parkinson’s disease therapeutics for motor fluctuations,” said Peter A. LeWitt, MD, Director of the Parkinson’s Disease and Movement Disorder Program at Henry Ford Hospital in West Bloomfield, Michigan.

A Need to Improve Levodopa Delivery

Beyond irregular effects of levodopa, motor fluctuations may be intrinsic to Parkinson’s disease, said Dr. LeWitt. One problem experienced by some patients is freezing of gait, immobility that is often situation-specific irrespective of medication dosing, he added. The sleep-benefit phenomenon, stress-exacerbated tremors and dyskinesias, and end-of-day medication unresponsiveness are further examples. “But for the most part, most motor fluctuations tend to be closely linked to the variable delivery of levodopa to the brain, where, after a short delay, it undergoes conversion to dopamine. This neurotransmitter does not have long to carry out its intended signaling because enzymes and re-uptake mechanisms quickly dispose of it. So, consistent delivery is the key for averting dose-by-dose motor fluctuations.”

During its 50 years of service to the Parkinson’s disease patient, levodopa has revolutionized the identity of this disorder. It has improved longevity, disability, and overall quality of life, and it inspired neurotherapeutic approaches for treating many other disorders. Levodopa use is so pervasive that today it is difficult to follow the untreated natural history of Parkinson’s disease. While levodopa is well known for the range of benefits it offers, less understood is why most patients gradually transition from a long-duration response to relatively brief dose-by-dose effects (on a time-scale roughly mirroring the clearance half-life of levodopa). Levodopa combined with a decarboxylase inhibitor is routinely administered in pill form, but, as Dr. LeWitt noted, “the human gastrointestinal tract is not well suited for optimal uptake of either drug.”

Because the short-duration response pattern is associated with benefits as brief as two to three hours per oral immediate-release dose, the focus for improving levodopa has been the use of extension therapies. Blocking the breakdown of peripheral levodopa metabolism (the mechanism for catechol-O-methyltransferase inhibition) or slowing the central metabolism of dopamine (by inhibiting monoamine oxidase-type B) join extended-release carbidopa-levodopa preparations as ways to improve upon the immediate-release product. “While these strategies do provide some level of effectiveness, the problems of irregular responsiveness and up to several hours of daily ‘off’ time haven’t been solved. ‘Off’ time still imposes a major burden on many patients living with Parkinson’s disease,” said Dr. LeWitt. Like delayed onset of effect and rapid wearing-off, levodopa-induced dyskinesias present another challenge for understanding their origin and optimal control. While new mechanisms of blocking dyskinesia are being sought, a simpler solution can be more continuous levodopa delivery so that drug concentration peaks causing involuntary movements are averted.