Drugs Used in Parkinson's Disease

Parkinson's disease (PD) is a chronic neurodegenerative disorder caused by progressive loss of dopaminergic neurons in the substantia nigra pars compacta, producing a relative dopamine deficit and cholinergic excess in the striatum. Pharmacotherapy aims to restore the dopamine–acetylcholine balance — by increasing dopaminergic transmission or reducing cholinergic overactivity. This is one of the most repeatedly tested CNS pharmacology areas for NEET PG.

Pathophysiological basis of therapy

The nigrostriatal pathway uses dopamine acting on D1 (excitatory, direct pathway) and D2 (inhibitory, indirect pathway) receptors. Degeneration of these neurons unmasks relative cholinergic dominance in the corpus striatum. Symptoms appear when roughly 70–80% of nigrostriatal neurons are lost.

The cardinal motor features form the mnemonic TRAP:

• Tremor (resting, "pill-rolling", 4–6 Hz)
• Rigidity (cogwheel/lead-pipe)
• Akinesia/bradykinesia
• Postural instability

High-yield: The therapeutic strategy is twofold — increase dopamine (levodopa, dopamine agonists, MAO-B inhibitors, COMT inhibitors, amantadine) or decrease acetylcholine (central anticholinergics). Dopamine itself is useless because it does not cross the blood–brain barrier (BBB).

Classification of anti-parkinsonian drugs

Class	Prototype drugs	Mechanism
Dopamine precursor	Levodopa (with carbidopa/benserazide)	Converted to dopamine in CNS
Peripheral decarboxylase inhibitors	Carbidopa, benserazide	Block peripheral conversion of levodopa
Dopamine agonists (non-ergot)	Pramipexole, ropinirole, rotigotine, apomorphine	Directly stimulate D2/D3 receptors
Dopamine agonists (ergot)	Bromocriptine, cabergoline, pergolide	D2 agonist (largely abandoned)
MAO-B inhibitors	Selegiline, rasagiline, safinamide	Block dopamine breakdown
COMT inhibitors	Entacapone, tolcapone, opicapone	Block peripheral (± central) dopamine/levodopa breakdown
NMDA antagonist/dopamine releaser	Amantadine	Releases dopamine, blocks NMDA & reuptake
Central anticholinergics	Trihexyphenidyl (benzhexol), benztropine, biperiden	Block muscarinic receptors in striatum

Levodopa — the gold standard

Levodopa (L-DOPA) is the single most effective drug in PD and remains the cornerstone, especially for bradykinesia and rigidity. It is the immediate metabolic precursor of dopamine, and unlike dopamine it crosses the BBB via the L-aromatic amino acid (LNAA) transporter. Inside the brain it is decarboxylated by DOPA decarboxylase (AADC) to dopamine.

Why combine with carbidopa?

When given alone, more than 95% of levodopa is decarboxylated peripherally to dopamine, causing nausea, vomiting and hypotension, and only ~1% reaches the brain. Carbidopa and benserazide are peripheral DOPA decarboxylase inhibitors that do not cross the BBB.

High-yield: Adding carbidopa allows the levodopa dose to be reduced by ~75%, increases brain delivery, and markedly reduces peripheral side effects (nausea, cardiac arrhythmias, postural hypotension). The usual fixed ratio is levodopa : carbidopa = 4:1 (e.g., 100 mg : 25 mg). About 75–100 mg/day of carbidopa is needed to fully saturate peripheral decarboxylase.

Carbidopa does not reduce central side effects (dyskinesias, psychosis) — these may even worsen because more levodopa reaches the brain.

Adverse effects of levodopa

Early/dose-related: Nausea and vomiting (area postrema/CTZ stimulation — note domperidone is the antiemetic of choice as it does not cross BBB; avoid metoclopramide and prochlorperazine which are central D2 blockers and worsen PD), postural hypotension, cardiac arrhythmias (β1 effect of peripheral dopamine).

Late/chronic:

• Dyskinesias — abnormal involuntary movements (chorea, dystonia), occurring in the majority after 5–10 years; peak-dose dyskinesias are commonest.
• Behavioural — hallucinations, vivid dreams, psychosis, confusion (more in elderly).
• Motor fluctuations — wearing-off and on-off (see below).

High-yield: Pyridoxine (vitamin B6) enhances peripheral decarboxylation and abolishes the effect of plain levodopa — but this interaction is abolished when carbidopa is co-administered. Frequently tested.

Motor complications — wearing-off vs on-off

Feature	Wearing-off (end-of-dose)	On-off phenomenon
Timing	Predictable, before next dose	Unpredictable, sudden
Relation to dose	Clear (effect fades as level falls)	Often unrelated to dosing
Mechanism	Falling plasma levodopa + reduced striatal storage	Fluctuating receptor responsiveness, narrow therapeutic window
Duration of "off"	Gradual onset	Abrupt ("like a switch")
Management	More frequent dosing, add COMT inhibitor, MAO-B inhibitor, CR formulation, dopamine agonist	Continuous dopaminergic stimulation (infusions, agonists, apomorphine rescue)

Stepwise approach to levodopa motor complications: Optimise dose/frequency → add COMT inhibitor (entacapone) or MAO-B inhibitor to prolong action → add/switch to dopamine agonist → consider continuous infusion (intestinal levodopa gel, apomorphine) → deep brain stimulation (subthalamic nucleus/GPi).

Important contraindications & interactions

• Non-selective MAO inhibitors (phenelzine, tranylcypromine): combining with levodopa can precipitate a hypertensive crisis (accumulated peripheral dopamine/noradrenaline). Stop MAOIs at least 2 weeks before levodopa.
• Angle-closure glaucoma, active psychosis, recent myocardial infarction — caution/avoid.
• Avoid in melanoma (levodopa is a precursor of melanin) — relative caution.
• High-protein meals reduce levodopa absorption (competition at LNAA transporter).

High-yield: "Drug holiday" (temporary withdrawal of levodopa) was historically used to restore responsiveness, but it carries the risk of a neuroleptic malignant syndrome-like state and aspiration/DVT — largely abandoned now.

Dopamine agonists

Directly stimulate striatal dopamine receptors, bypassing the need for functioning nigral neurons and for enzymatic conversion. They have a longer half-life than levodopa, giving smoother (more continuous) stimulation and fewer dyskinesias / motor fluctuations — making them preferred initial monotherapy in younger patients (<60 years) to delay levodopa-related complications.

Non-ergot (preferred):

• Pramipexole — D3 > D2 selective; also used in restless legs syndrome; renally excreted.
• Ropinirole — D2/D3 agonist; metabolised by CYP1A2.
• Rotigotine — transdermal patch (continuous delivery).
• Apomorphine — potent D1/D2 agonist; subcutaneous rescue therapy for sudden "off" episodes; strongly emetogenic (pretreat with antiemetic — but not ondansetron + apomorphine, which can cause severe hypotension).

Ergot derivatives (largely abandoned): Bromocriptine, cabergoline, pergolide — risk of cardiac valvular fibrosis, retroperitoneal/pulmonary fibrosis (serotonin 5-HT2B mediated).

High-yield: Dopamine agonists are classically associated with impulse control disorders (pathological gambling, hypersexuality, compulsive shopping/eating), sleep attacks (sudden-onset somnolence — warn against driving), hallucinations, and ankle oedema. These are MORE common with agonists than levodopa.

MAO-B inhibitors

Selegiline (deprenyl) and rasagiline selectively and irreversibly inhibit monoamine oxidase-B, the enzyme that degrades dopamine in the brain, thus prolonging dopamine action. Safinamide is a newer reversible MAO-B inhibitor with glutamate-modulating action.

• Used as monotherapy in early/mild disease and as adjunct to reduce wearing-off.
• Selegiline is metabolised to amphetamine and methamphetamine → insomnia (avoid evening dosing), anxiety.
• At low doses MAO-B selectivity is maintained, so the "cheese reaction" (tyramine-induced hypertensive crisis) is minimal; at high doses selectivity is lost.

High-yield: MAO-B inhibitors + pethidine (meperidine), SSRIs, or other serotonergic drugs can precipitate a serotonin syndrome / hyperpyrexia. Rasagiline has a possible disease-modifying (neuroprotective) claim (ADAGIO trial) — frequently asked as a "putative neuroprotective" agent.

COMT inhibitors

Catechol-O-methyltransferase (COMT) methylates levodopa to 3-O-methyldopa (3-OMD), an inactive metabolite that also competes for BBB transport. Inhibiting COMT increases the proportion and duration of levodopa reaching the brain — useful for wearing-off.

Drug	Site of action	Key point
Entacapone	Peripheral only	Safe, no hepatotoxicity; combined as levodopa+carbidopa+entacapone (triple pill)
Tolcapone	Peripheral + central	More potent but causes fatal hepatotoxicity — needs LFT monitoring; reserved
Opicapone	Peripheral	Once-daily, long-acting

High-yield: COMT inhibitors are only effective with levodopa (no benefit alone). Harmless orange-brown discolouration of urine occurs with entacapone. Diarrhoea is a notable side effect. Tolcapone = hepatotoxicity ("Tolcapone = Toxic to liver").

Amantadine

Originally an antiviral (influenza A), amantadine has modest anti-parkinsonian activity through several mechanisms: it promotes dopamine release, inhibits dopamine reuptake, has anticholinergic action, and is an NMDA receptor antagonist.

High-yield: The NMDA-antagonist property makes amantadine the drug of choice for levodopa-induced dyskinesias. It is also useful in early mild disease.

Adverse effects: Livedo reticularis (classic, reddish-blue mottling of skin, often with ankle oedema), confusion/hallucinations, insomnia, anticholinergic effects. Renally excreted — reduce dose in renal impairment.

Central anticholinergics

Trihexyphenidyl (benzhexol), benztropine, biperiden, procyclidine block muscarinic receptors in the striatum, correcting the relative cholinergic excess.

• Best for tremor and rigidity; little effect on bradykinesia.
• Most useful in young patients with tremor-predominant disease and in drug-induced parkinsonism (antipsychotic-induced).
• Avoid in the elderly (confusion, memory impairment) and in prostatic hypertrophy, glaucoma.

High-yield: Anticholinergics are the drugs of choice for drug-induced (antipsychotic) parkinsonism and acute dystonias, but should not be used for tardive dyskinesia (which they worsen). They have minimal effect on bradykinesia.

Choosing initial therapy — exam logic

Younger patient (<60 yr), functionally significant disease → start with a dopamine agonist (delays levodopa dyskinesias). Tremor-predominant, young → anticholinergic. Elderly / cognitively impaired / functionally disabling disease → levodopa-carbidopa (most effective, fewer neuropsychiatric effects than agonists). Mild early disease → MAO-B inhibitor (selegiline/rasagiline) as monotherapy.

Flow for symptomatic worsening: Levodopa-carbidopa → add COMT/MAO-B inhibitor for wearing-off → add dopamine agonist → amantadine for dyskinesias → DBS for refractory motor fluctuations.

Drug-induced parkinsonism (key differential)

Caused by central D2 blockers: typical antipsychotics (haloperidol, chlorpromazine), metoclopramide, prochlorperazine, reserpine, tetrabenazine. It is usually symmetrical and reversible. Levodopa is ineffective (receptors are blocked); treat by withdrawing the offending drug + anticholinergics.

Drug	Best symptom controlled	Signature toxicity
Levodopa-carbidopa	Bradykinesia, rigidity	Dyskinesias, on-off, psychosis
Dopamine agonists	Overall (delays levodopa)	Impulse control disorder, sleep attacks, fibrosis (ergot)
MAO-B inhibitors	Wearing-off, early disease	Insomnia (selegiline), serotonin syndrome
COMT inhibitors	Wearing-off	Diarrhoea; tolcapone hepatotoxicity
Amantadine	Dyskinesias	Livedo reticularis, confusion
Anticholinergics	Tremor, rigidity	Confusion, dry mouth, urinary retention

Recently asked / exam angle

• Carbidopa mechanism — peripheral DOPA decarboxylase inhibitor that does not cross the BBB; reduces dose and peripheral side effects but not central ones.
• Drug of choice for levodopa-induced dyskinesias = amantadine (NMDA antagonism).
• Antiemetic of choice with levodopa = domperidone; metoclopramide is contraindicated (worsens PD).
• Apomorphine = subcutaneous rescue for "off" periods; potent emetic.
• Entacapone turns urine orange; tolcapone = hepatotoxicity.
• Selegiline metabolite = amphetamine/methamphetamine.
• Ergot agonists (cabergoline/pergolide) = cardiac valve fibrosis.
• Non-selective MAOI + levodopa = hypertensive crisis.
• Pyridoxine reverses plain levodopa effect (negated by carbidopa).
• Impulse control disorders / sleep attacks are linked to dopamine agonists.
• Most effective single drug for PD = levodopa.

Rapid revision

1. Levodopa is the most effective drug; always combined with carbidopa/benserazide (peripheral decarboxylase inhibitors that don't cross the BBB), ratio 4:1.
2. Carbidopa reduces nausea, hypotension and the levodopa dose, but not central effects (dyskinesia, psychosis).
3. Domperidone is the safe antiemetic; avoid metoclopramide/prochlorperazine in PD.
4. Pyridoxine abolishes plain levodopa's effect — negated once carbidopa is added.
5. Wearing-off = predictable end-of-dose fading; on-off = sudden unpredictable switches.
6. Amantadine (NMDA antagonist) is the DOC for levodopa-induced dyskinesias; causes livedo reticularis.
7. Dopamine agonists (pramipexole, ropinirole) preferred in the young to delay dyskinesias; cause impulse control disorders and sleep attacks.
8. Ergot agonists (bromocriptine, cabergoline, pergolide) → valvular/retroperitoneal fibrosis — largely abandoned.
9. Selegiline/rasagiline = MAO-B inhibitors; selegiline yields amphetamine metabolites; risk of serotonin syndrome with pethidine/SSRIs.
10. Entacapone (peripheral, urine orange), tolcapone (central + hepatotoxic), opicapone (once-daily) — only work alongside levodopa.
11. Anticholinergics (trihexyphenidyl) best for tremor/rigidity and drug-induced parkinsonism; useless for bradykinesia; avoid in elderly.
12. Non-selective MAOI + levodopa → hypertensive crisis; refractory motor fluctuations → deep brain stimulation (STN/GPi).