Antiparkinson drug
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"Parkinson disease" AND "pharmacotherapy"
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Antiparkinson Drugs
Parkinson disease (PD) results from progressive degeneration of dopaminergic neurons in the substantia nigra, leading to dopamine depletion in the neostriatum. All current drugs offer symptomatic relief - none halt neuronal degeneration. The key principle is restoring the dopamine/acetylcholine balance in the basal ganglia.
Pathophysiology Basis
The loss of nigrostriatal dopamine leads to:
- Bradykinesia (most responsive to treatment)
- Rigidity
- Resting tremor
- Postural instability
Drug Classes
1. Levodopa + Carbidopa (Gold Standard)
Mechanism:
Dopamine itself cannot cross the blood-brain barrier (BBB). Levodopa, its immediate precursor, is actively transported across the BBB and converted to dopamine in residual nigrostriatal neurons.
- Carbidopa is a peripheral DOPA decarboxylase inhibitor that cannot cross the BBB. It blocks conversion of levodopa to dopamine in the periphery, reducing the required dose 4-5 fold and markedly reducing peripheral side effects (nausea, vomiting, arrhythmias, hypotension).
- Usual starting dose: carbidopa/levodopa 25 mg/100 mg three times daily.
Therapeutic uses:
- First-line for PD, most effective for bradykinesia; also reduces rigidity and tremor.
- About two-thirds of patients respond substantially for the first 3-5 years, then response declines.
Adverse effects:
- Peripheral: Nausea, vomiting, orthostatic hypotension, cardiac arrhythmias
- Central: Dyskinesias (choreoathetosis), "on-off" fluctuations, wearing-off phenomenon, hallucinations, confusion
- The on-off phenomenon: unpredictable switches between mobile ("on") and immobile ("off") states, related to short plasma half-life (1-2 hours)
Key drug interactions:
- Pyridoxine (B6) enhances peripheral metabolism of levodopa - avoided without carbidopa
- MAO inhibitors (non-selective): hypertensive crisis risk
- Antipsychotics: block dopamine receptors, worsen parkinsonism
(Lippincott Pharmacology, p. 515-517; Katzung, p. 780)
2. Dopamine Receptor Agonists
These directly stimulate dopamine receptors (D2 > D3) without requiring conversion in surviving neurons - useful in advanced disease where neuron count is very low.
| Drug | Type | Route | Notes |
|---|---|---|---|
| Bromocriptine | Ergot | Oral | Older; risk of fibrosis, vasospasm |
| Pramipexole | Non-ergot | Oral | D3 preferring; also for restless legs |
| Ropinirole | Non-ergot | Oral | Also for restless legs |
| Rotigotine | Non-ergot | Transdermal patch | Once-daily; steady levels |
| Apomorphine | Non-ergot | SC injection / sublingual | Rescue for acute "off" episodes |
Advantages over levodopa:
- Longer duration of action - fewer "on-off" fluctuations
- Fewer dyskinesias (especially in early therapy)
- May delay need for levodopa initiation in early PD
Adverse effects:
- Nausea, vomiting, orthostatic hypotension
- Hallucinations, confusion (more common than with levodopa)
- Impulse control disorders - compulsive gambling, hypersexuality, binge eating (up to 45% prevalence) - related to D2/D3 mesocorticolimbic activation
- Sudden-onset sleep ("sleep attacks") - especially pramipexole and ropinirole
- Ergot-specific: pulmonary/retroperitoneal fibrosis, erythromelalgia, cardiac valvulopathy (pergolide)
Contraindications: History of psychosis, recent MI, active peptic ulcer; ergot derivatives - avoid in peripheral vascular disease.
(Lippincott Pharmacology, p. 524-527; Katzung, p. 782-783)
3. MAO-B Inhibitors
Mechanism: Monoamine oxidase B selectively metabolizes dopamine in the brain. Inhibiting it raises dopamine levels in the striatum.
| Drug | Dose | Use |
|---|---|---|
| Selegiline (deprenyl) | 5 mg with breakfast + 5 mg with lunch | Adjunct to levodopa |
| Rasagiline | 0.5-1 mg/day | Monotherapy (mild disease) or adjunct |
| Safinamide | 50-100 mg/day | Adjunct in fluctuating patients |
- Selegiline is metabolized to amphetamine and methamphetamine (may cause insomnia - avoid evening doses).
- Rasagiline is more potent; used as monotherapy for early disease.
- Both may have modest neuroprotective effects (MPTP animal models - MAO-B converts MPTP to the toxic MPP+), but human evidence is inconclusive.
- At high doses, selegiline loses MAO-B selectivity and can cause hypertensive crisis with tyramine-rich foods ("cheese reaction").
(Katzung, p. 783-784)
4. COMT Inhibitors
Mechanism: Catechol-O-methyltransferase (COMT) metabolizes levodopa peripherally. Blocking COMT extends the half-life of levodopa, reducing "wearing-off."
| Drug | Site of action | Key feature |
|---|---|---|
| Entacapone | Peripheral only | Preferred - no hepatotoxicity; given with each levodopa dose |
| Tolcapone | Peripheral + central | Risk of fatal hepatotoxicity - requires LFT monitoring |
| Opicapone | Peripheral | Once-daily dosing |
- COMT inhibitors are always used as adjuncts to levodopa/carbidopa - never alone.
- Entacapone + carbidopa + levodopa is available as a fixed combination (Stalevo).
- Main side effects: diarrhea, orange discoloration of urine, dyskinesias (from enhanced levodopa effect), hepatotoxicity (tolcapone).
(Katzung, p. 579-585; Harrison's, 2025)
5. Amantadine
Originally an antiviral drug, accidentally found to have antiparkinsonian activity.
Mechanisms:
- Stimulates dopamine release from presynaptic terminals
- Blocks NMDA-type glutamate receptors (anti-dyskinetic effect)
- Mild anticholinergic activity
Clinical use:
- Mild early PD (monotherapy)
- Reducing levodopa-induced dyskinesias in advanced disease
- Less efficacious than levodopa; tolerance often develops within weeks
Adverse effects: Restlessness, hallucinations, confusion, livedo reticularis, peripheral edema, urinary retention, dry mouth. Abrupt withdrawal can cause acute confusional state and parkinsonism worsening.
(Katzung, p. 785; Lippincott, p. 528)
6. Adenosine A2A Receptor Antagonist
Istradefylline (analog of caffeine): Selective A2A antagonist. Taken 20-40 mg/day as adjunct to carbidopa-levodopa to reduce "off" time. Side effects include dyskinesias and hallucinations.
7. Anticholinergic (Antimuscarinic) Agents
Drugs: Benztropine, trihexyphenidyl, biperiden, procyclidine
Mechanism: In PD, dopamine depletion leaves acetylcholine relatively unopposed. Blocking muscarinic receptors corrects this imbalance.
Effects: Improve tremor and rigidity, but little effect on bradykinesia. Less effective than levodopa overall - used as adjuncts, especially in younger patients with prominent tremor.
Adverse effects: Classic anticholinergic - dry mouth, constipation, urinary retention, blurred vision, confusion (especially in elderly).
Contraindications: Glaucoma, prostatic hypertrophy, cognitive impairment.
Summary Table
| Drug Class | Key Drugs | Main Mechanism | Best Used For |
|---|---|---|---|
| Levodopa + carbidopa | Sinemet, Rytary | Dopamine precursor | All stages - first-line |
| DA agonists | Pramipexole, ropinirole | Direct D2/D3 agonism | Early disease (delay levodopa), fluctuations |
| MAO-B inhibitors | Selegiline, rasagiline | Block DA breakdown | Early monotherapy, adjunct |
| COMT inhibitors | Entacapone, tolcapone | Extend levodopa half-life | Wearing-off phenomenon |
| Amantadine | Amantadine | NMDA block, DA release | Mild disease, dyskinesias |
| Anticholinergics | Benztropine, trihexyphenidyl | Block ACh excess | Tremor, rigidity (adjunct) |
| A2A antagonist | Istradefylline | Block adenosine A2A | Reduce "off" time |
Complications of Long-term Levodopa Therapy
- Wearing-off ("end-of-dose" deterioration): PD symptoms return before next dose; managed by reducing dosing intervals, adding COMT inhibitor or MAO-B inhibitor.
- On-off phenomenon: Unpredictable motor fluctuations - managed by extended-release formulations, COMT inhibitors, or deep brain stimulation (DBS).
- Dyskinesias: Involuntary movements at peak dose - managed by dose reduction, amantadine, or DBS.
- Drug holiday (rarely used): Temporary withdrawal to restore levodopa responsiveness - significant risk of neuroleptic malignant-like syndrome.
Non-pharmacological Option
Deep brain stimulation (DBS) of the subthalamic nucleus or globus pallidus interna is indicated for patients with motor fluctuations or dyskinesias refractory to pharmacologic optimization.
Sources: Lippincott Illustrated Reviews: Pharmacology, p. 515-530; Katzung's Basic & Clinical Pharmacology 16e, p. 780-790; Goodman & Gilman's Pharmacological Basis of Therapeutics; Harrison's Principles of Internal Medicine 22e (2025)
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