Parkinson's disease (PD)

Levodopa coupled with a DOPA decarboxylase inhibitor is the most effective therapy for PD.² Carbidopa does not readily pass through the blood-brain barrier, thereby allowing levodopa to be converted to active dopamine preferentially in the central nervous system, because dopamine also does not cross the blood-brain barrier.²

Levodopa provides a significant improvement of symptoms, which can be tested in a pharmacological challenge and can be used to aid the diagnosis of PD.¹ Levodopa relates to the development of motor complications.¹ An extended-release carbidopa-levodopa formulation in order to reduce motor fluctuations has been developed and recently approved.¹

In advanced PD, when motor complications become disabling and are poorly managed with classical pharmacological therapy, levodopa can be administered directly into the duodenum by pump through a gastrostomy catheter as a levodopa-carbidopa gel; this formulation has been shown to reduce motor fluctuations significantly in advanced PD.¹ Subcutaneous infusion of the potent dopamine agonist, apomorphine, can also have an important effect on severe motor fluctuations.⁴

Dopamine agonists directly stimulate the postsynaptic dopamine D1-3 receptors in the striatum without the requirement for further metabolism within the dopaminergic neurons.¹ Dopamine agonists are not as effective in reversing motor symptoms as levodopa but are associated with a lower risk for dyskinesia; they may be used as monotherapy in earlier phases of PD or in conjunction with levodopa.¹ Pramipexole (e.g. Mirapex) is a synthetic, non-ergot, amino thiazoledopamine agonist with high relative in vitro specificity and full intrinsic activity at the dopamine D2 subfamily of receptors, binding with higher affinity to D3 than to D2 or D4 receptor subtypes.⁵ The relevance of D3 receptor binding in PD is unknown, but it is suggested that the preferential affinity of pramipexole for the D3 receptor subtype could contribute to efficacy in the treatment of psychiatric symptoms depression.⁵ Unlike the ergot dopamine agonists, pramipexole has little or no interaction with adrenergic or serotonergic receptors.⁵

Deep brain stimulation is a well-established treatment for the motor symptoms of PD.⁴ Several clinical trials suggested that deep brain stimulation of either the subthalamic nucleus or globus pallidus internus is effective in moderate-to- severe PD.⁴ Thalamic deep brain stimulation is also an option for treatment of tremor.⁴ Surgical therapy is an option when the parkinsonian motor features continue to respond to levodopa but motor fluctuations and dyskinesia become disabled.⁴ Specific non-motor features, including non-motor fluctuations, sleep-related symptoms, and behavioral abnormalities, can improve with deep brain stimulation.⁴ The time to surgical treatment is about 10–13 years after diagnosis of PD.⁴

Non-motor symptoms often have limited treatment options. However, a variety of treatments can effectively control or improve disability from non-motor symptoms, such as psychiatric symptoms, sleep disorders, autonomic dysfunction, and fatigue.⁴ Evidence supports the efficacy of tricyclic antidepressants, specifically desipramine and nortriptyline, for treatment of PD-related depression.⁴ In practice, selective serotonin reuptake inhibitors, including citalopram, escitalopram, fluoxetine, paroxetine, and sertraline, are commonly used medications to treat depression in PD.⁴ The extended release formulation of venlafaxine, a serotonin and norepinephrine reuptake inhibitor, and the dopamine agonist pramipexole have been shown to be effective in treating depression in PD patients.⁴

The motor and non-motor symptoms of late-stage of PD typically respond poorly to levodopa.⁴ Abnormalities in non-dopaminergic neurotransmitters (e.g. acetylcholine, glutamate, norepinephrine, and serotonin) also contribute to the symptoms of PD.⁴ Expression of these levodopa-resistant symptoms probably involves some of these other neurotransmitter systems.⁴ In addition, reduction in acetylcholine due to degeneration of cholinergic structures might be associated with dementia and falls in late-stage PD.⁴ Accordingly, the cholinesterase inhibitor rivastigmine is efficacious for the treatment of dementia in PD.⁵ A trial of donepezil for treatment of falls suggested that a rise in cholinergic tone might improve postural stability in PD.⁴

Complications of long-term dopaminergic treatment are features of advanced disease. Complications include motor and non-motor fluctuations, dyskinesia, and psychosis, which can limit function and reduce quality of life.⁴ Fluctuations and dyskinesia are believed to result, in part, from pulsatile stimulation of striatal dopamine receptors, which occurs later in the disease when intracerebral levodopa concentrations become more closely linked to plasma levodopa concentrations.⁴ Available pharmacological strategies to reduce the wide fluctuations in dopamine concentrations include the addition of a dopamine agonist, monoamine oxidase type B inhibitor, or catechol-O-methyltransferase inhibitor.⁴

Non-dopaminergic treatments such as amantadine and clozapine have effects on multiple neurotransmitter systems and can be effective in treating dyskinesia.⁵ Psychosis in PD is most effectively treated with clozapine; however, clozapine can be associated with potentially life-threatening agranulocytosis, an idiosyncratic adverse drug reaction.⁴

Cholinesterase inhibitors (e.g. rivastigmine) might reduce visual hallucinations and delusions in patients with PD who have dementia.⁴ The selective serotonin 5-HT2A inverse agonist pimavanserin has been shown to reduce positive psychotic symptoms without worsening motor function and is being considered for Food and Drug Administration (FDA) approval for this indication.⁴