By Jason Napodano, CFA
Parkinson's Disease
Parkinsons disease (PD) is a neurodegenerative brain disorder that results from the death of dopamine-generating cells in the substantia nigra region of the midbrain. PD is also characterized by the accumulation of a protein called alpha-synuclein into inclusions called Lewy bodies in neurons. The cause of PD is generally idiopathic, although some atypical cases have a genetic origin. The disease is named after the English doctor James Parkinson, who published the first detailed description in An Essay on the Shaking Palsy in 1817.
PD patients often exhibit marked reduction in motor control and an increase in parkinsonism (tremors, hypokinesia, rigidity, bradykinesia, and postural instability). However, as the disease progresses, patients often exhibit non-motor symptoms that include autonomic dysfunction, neuropsychiatric problems (mood, cognition, behavior or thought alterations, psychosis), and sensory and sleep difficulties. Parkinsons disease psychosis (PDP) is common in nearly 50% of PD patients a decade after initial diagnosis. Anxiety and depression are common co-morbidities. Initial signs of PD include shaking, loss of smell, difficulty writing, trouble sleeping, constipation, and poor posture. Diagnosis of a typical case is mainly based on symptoms, with tests such as neuroimaging used for confirmation.
There is no cure for PD. Instead, physicians attempt to manage the symptoms of the disease through a multidisciplinary approach that may include pharmacological, social, and surgical options. The most common pharmaceutical treatment options are those which look to increase the level of dopamine in the brain. These include dopamine replacement therapies (DRT) combined with dopa decarboxylase inhibitors, dopamine agonists, and MAO-B inhibitors. The treatment option is often tailored specifically for the patient based on the stage and severity of the disease and the balance between good symptom control and side-effects resulting from enhancement of dopaminergic function.
Despite these co-formulations, Levodopa carries significant risk of side-effects, including dyskinesia. As a result, despite its effectiveness in reducing motor symptoms associated with Parkinsons disease, physicians often attempt to delay Levodopa therapy until the disease progresses to a more moderate-to-severe stage. Most early-stage PD patients start out on MAO-B inhibitors and / or dopamine agonists, or low-dose Levodopa. However, PD is a progressive and degenerative disease, and patients typically progress to the point where starting Levodopa or increasing the Levodopa dose is necessary in five years after initial diagnosis. After a decade on therapy, almost all PD patients require high doses of Levodopa, as well as surgical options including deep brain stimulation (DBS). As the dose and use of Levodopa increases, the incidence of dyskinesia also increases.
Levodopa also has a relatively short half-life, requiring dosing averaging three to four times a day. Peak plasma concentrations of Levodopa occur 60 to 90 minutes after dosing. Unfortunately, this is also when peak side effects such as dyskinesia occur. The hefty dosing requirement of Levodopa creates compliance issues, especially at night when patients may sleep through their dose schedule dosing every six hours. The peaks and troughs associated with Levodopa create significant on and off treatment times for PD patients.
Levodopa-Induced Dyskinesia
Levodopa-Induced Dyskinesia (LID) is a major side-effect of Levodopa use. LID is characterized by hyperkinetic movements, including chorea (abnormal involuntary movement), dystonia (sustained muscle contraction, abnormal posture), and athetosis (involuntary convoluted movements). It is most common at times of peak L-DOPA plasma concentrations (peak-dose dyskinesia), although it may also occur when plasma concentrations of L-DOPA rise and fall (diphasic dyskinesia) or during off-time (off-period dystonia).
In the U.S., there are an estimated 500,000 to 1 million patients suffering from Parkinsons disease. There are no approved treatment options for PD-LID. Approximately 50% of PD patients will experience LID after 4 to 6 years on L-DOPA therapy. The number rises to 90% after 10 to 15 years on L-DOPA therapy.