Drugs for Parkinson’s Disease 1

Parkinson’s Disease Parkinson’s disease (PD) is a neurodegenerative disorder of the extrapyramidal system associated with disruption of neurotransmission in the striatum Characterized by dyskinesias and akinesia Proper function of the striatum requires a balance between the neurotransmitters dopamine and acetylcholine (ACh) Imbalance between dopamine and ACh results from degeneration of the neurons that supply dopamine to the striatum. 2

Parkinson’s Disease Affects more than 1 million Americans Second only to Alzheimer’s disease as the most common degenerative disease of neurons Symptoms generally appear in middle age and progress No cure for motor symptoms Drug therapy can maintain functional mobility for years (prolongs/improves quality of life). 3

Cardinal Symptoms of PD Dyskinesias Tremor at rest Rigidity Postural instability Bradykinesia (slowed movement) Tremor In addition to motor symptoms Autonomic disturbances Depression Psychosis and dementia 4

Dopamine/ACh Imbalance in Striatum Imbalance results from degeneration of the neurons that supply dopamine to the striatum. Without adequate dopamine, ACh causes excessive stimulation of GABA-releasing neurons. Overactivity of GABA neurons contributes to the motor symptoms of PD. Uncertain of cause of degeneration—may be alpha-synuclein. 5

Fig. 21-1. A model of neurotransmission in the healthy striatum and parkinsonian striatum. 6

Parkinson’s Disease Therapeutic goals Ideal treatment (reverse neuronal degeneration or prevent further degeneration) does not exist. Goal is to improve patient’s ability to carry out activities of daily life. Drug selection and dosages are determined by extent to which PD interferes with work, dressing, eating, bathing, and other activities of daily living. 7

Drug Therapy for Parkinson’s Disease Two major categories Dopaminergic agents By far the most commonly used drugs for PD Promote activation of dopamine receptors Levodopa (Dopar) Anticholinergic agents Prevent activation of cholinergic receptors Benztropine (Cogentin) 8

Drug Therapy for Parkinson’s Disease Levodopa (drug holidays recommended) Levodopa/carbidopa Dopamine agonists Pramipexole (Mirapex) Entacapone (Comtan) Amantadine (Symmetrel) Selegiline (Eldepryl, Carbex) 9

Dopaminergic Agents Mechanisms of action Levodopa: promotes dopamine synthesis Dopamine agonists: stimulate dopamine receptors directly Selegiline: inhibits dopamine breakdown Amantadine: promotes dopamine release COMT inhibitors: enhance effects of levodopa by blocking its degradation 10

Drug Selection: Initial Treatment Mild symptoms: MAO-B inhibitor Selegiline or rasagiline More severe symptoms: levodopa or a dopamine agonist Levodopa more effective than dopamine agonists, but long-term use carries a higher risk for disabling dyskinesias Management of motor fluctuations “Off” times (can be reduced with dopamine agonists, COMT inhibitors, and MAO-B inhibitors) Drug-induced dyskinesias 11

Fig. 21-2. Steps leading to alteration of CNS function by levodopa. 12

Fig. 21-3. Conversion of levodopa to dopamine. 13

Levodopa Only given in combination with carbidopa or carbidopa/entacapone Highly effective, but benefits diminish over time Orally administered, rapid absorption from small intestine Food delays absorption. Neutral amino acids compete with levodopa for intestinal absorption and for transport across blood-brain barrier. High-protein foods will reduce therapeutic effects. 14

Levodopa Adverse effects Drug holidays Nausea and vomiting Dyskinesias Cardiovascular effects Psychosis May darken sweat and urine Can activate malignant melanoma Drug holidays Drug interactions: first-generation antipsychotics, MAOIs, anticholinergics, pyridoxine Food interactions: protein and vitamins with pyridoxine 15

Carbidopa Advantages Levodopa/carbidopa (Sinemet, Paracopa) No adverse effects of its own Increases available levodopa in the CNS and allows for 75% decrease in levodopa dosage; therefore, reduces cardiovascular and GI adverse effects Effects come mainly from levodopa when given in combination. Levodopa/carbidopa (Sinemet, Paracopa) Carbidopa alone (Lodosyn) 16

Fig. 21-4. Fate of levodopa in the presence and absence of carbidopa. 17

Dopamine Agonists First-line drugs for PD Direct activation of dopamine receptors in striatum Comparison with levodopa Less effective than levodopa Not dependent on enzymatic conversion to be active Do not compete with dietary proteins Lower incidence of response failure and less likely to cause dyskinesias Two types of dopamine agonists Derivatives of ergot Nonergot derivatives 18

Nonergot Dopamine Agonists Pramipexole (Mirapex) Used alone in early PD and with levodopa in advancing PD Maximal benefits take several weeks to develop. Adverse effects Monotherapy – nausea, dizziness, daytime somnolence, insomnia, constipation, weakness, and hallucinations Combined – orthostatic hypotension and dyskinesias and increase in hallucinations Rare instances of pathologic gambling and other compulsive self-rewarding behaviors 19

COMT Inhibitors Inhibit metabolism of levodopa in the periphery No direct therapeutic effects of their own Two COMT inhibitors available Entacapone (safer and more effective) Tolcapone 20

Entacapone Selective, reversible inhibitor of COMT Only for use with levodopa Inhibits metabolism of levodopa in the intestines and peripheral tissues Prolongs time that levodopa is available to the brain Increases levodopa availability by inhibiting COMT, which decreases production of levodopa metabolites that compete with levodopa for transport Adverse effects: from increasing levodopa levels 21

Levodopa/Carbidopa/Entacapone Fixed-dose combinations sold as Stalevo More convenient than taking separate doses Costs a little less Disadvantage Available only in immediate-release tablets Available in only three strengths 22

MAO-B Inhibitors Considered second- and third-line drugs for treatment of PD Combination with levodopa – can reduce the wearing-off effect Selegiline 23

Selegiline (Eldepryl, Zelapar) Monotherapy or used with levodopa Modest improvement in motor function Causes selective, irreversible inhibition of type B monoamine oxidase (MAO-B) Can suppress destruction of dopamine derived from levodopa and prolong the effects of levodopa Adverse effects Monotherapy: insomnia Drug interactions: levodopa 24

Nonmotor Symptoms and Their Management 90% of patients develop nonmotor symptoms (autonomic disturbances, depression, dementia, and psychosis). Depression Amitriptyline: only effective drug TCA Anticholinergic effects that can exacerbate dementia Antiadrenergic effects that can exacerbate hypotension 25